Nervous Systems in Animals: From Nerve Nets to Brains

A nervous system is the wiring that lets an animal sense its surroundings, decide what to do, and act on the decision, and it is the system that turns a collection of cells into an animal that behaves. A jellyfish has no brain, but its nerve net lets it pulse, feed and sting in the right order. An earthworm has no brain worth the name, but its ladder of nerve cords and ganglia lets it crawl, burrow and withdraw from danger. An octopus has a brain and a nervous system spread through its arms, and it can open a jar, change its colour to match a rock, and remember where it hid its food. A human has about eighty-six billion neurons, and they let us read, remember, speak and plan. The systems are different in size and in shape, but the unit is the same in every case: the neuron, a cell built to carry an electrical signal, and the synapse, the junction where one neuron passes its message to the next.

The reason a nervous system matters is speed. A chemical signal, a hormone travelling in the blood, can take seconds or minutes to reach its target, and it reaches everywhere at once, which is fine for slow jobs such as growth or reproduction. A nervous signal travels in milliseconds, and it goes exactly where it is needed, which is what an animal needs when it must escape a predator, catch prey, or keep its balance. The difference between a nervous system and a hormone system is the difference between a telephone call and a letter: both carry a message, but one is fast and directed, and the other is slow and general. Animals that move, hunt and escape need the telephone, and the nervous system is the telephone network, with neurons as the wires and synapses as the exchanges.

This article covers the whole subject at full depth: the neuron and how it works, the action potential that carries a signal along an axon, the synapse where the signal jumps from one cell to the next, the nerve nets of jellyfish and the first nervous systems, the radial systems of starfish, the ladder systems of flatworms and earthworms, the ganglia and mushroom bodies of insects, the extraordinary brains of octopuses, the vertebrate plan of a dorsal cord with a central and a peripheral division, the five divisions of the vertebrate brain, the reflex arc that acts before thought, cephalization and why heads evolved, how animals learn and remember, what happens when the system fails, and how scientists study a system they cannot see working. The science is explained in plain language, and the numbers are given where they matter, because the nervous system is a subject where the numbers, how fast a signal travels, how many neurons an animal has, are the point.

Nervous Systems at a Glance

A nervous system has three jobs, and every nervous system, from a jellyfish's net to a human brain, does all three. The first job is to sense: to detect light, touch, chemicals, temperature, sound and movement in the world outside and inside the body. The second job is to integrate: to take the information from the senses, compare it with memory, and decide what to do. The third job is to act: to send a command to a muscle or a gland, so that the animal moves, feeds, escapes or rests. Sense, decide, act: the three jobs are the same in every animal, and the differences between nervous systems are differences in how much deciding happens, and where it happens.

In the simplest animals, there is almost no deciding. A jellyfish's nerve net senses a touch and contracts the bell, and the response is direct, with no brain in between. In an earthworm, the deciding is spread out: each segment has a ganglion, a cluster of nerve cells, that can control its own segment, and the brain at the front only coordinates them. In an octopus, the deciding is shared: the central brain makes the big decisions, and each arm has its own nerve cord that makes the small ones, so that an arm can taste, reach and grasp on its own. In a mammal, the deciding is centralised: almost everything passes through the brain, and the spinal cord and the peripheral nerves are mostly messengers. The history of nervous systems is, in large part, the history of deciding moving from everywhere to one place, and then, in the octopus, partly moving back out again.

The second thing to understand is that nervous systems are built from the same parts everywhere. The neuron is the same kind of cell in a worm and in a whale: a cell body, with branches called dendrites that receive signals, and a long fibre called an axon that sends the signal on. The signal itself is the same: an electrical pulse, the action potential, that travels along the axon, and a chemical message, the neurotransmitter, that crosses the synapse to the next cell. The chemicals differ in detail, the speeds differ, and the arrangements differ enormously, but the parts are the same, which is why the same drugs that calm a human can sedate a fish, and why a venom that blocks a nerve signal can kill an insect and a mammal alike. The nervous system is an old invention, and its parts have been conserved for more than half a billion years.

The third thing is that not every animal has a nervous system at all. Sponges have no neurons: their cells react to touch and to chemicals individually, and they have no wiring to coordinate them, which is why a sponge cannot move away from danger or hunt. Placozoans, tiny flat animals that creep across the sea floor, also have no neurons. Every other animal group has neurons, and the first nervous systems, the nerve nets of jellyfish and their relatives, appeared more than six hundred million years ago, before brains, before backbones, and before most of the animals we know existed. The nervous system is one of the oldest systems in the animal kingdom, and its history is the history of animals becoming able to behave.

The Neuron: The Unit of the Nervous System

The neuron is a cell with a shape no other cell has, and the shape is the function. A typical neuron has a cell body, the soma, which holds the nucleus and the machinery of the cell, and two kinds of branches. The dendrites are short, tree-like branches that receive signals from other neurons, and a single neuron may have thousands of them, each one listening to a different neighbour. The axon is a single long fibre that carries the signal away from the cell body to the next neuron, or to a muscle, or to a gland, and it may be a fraction of a millimetre long, or, in a giraffe or a whale, more than a metre long. The axon ends in small swellings, the synaptic terminals, which release the chemical message that passes the signal on. The shape is a funnel: many inputs in, through the dendrites, and one output out, through the axon, and the cell body sits in the middle, adding up the inputs and deciding whether to fire.

Neurons come in three kinds, defined by their job, and every nervous system uses all three. Sensory neurons, also called afferent neurons, carry information from the sense organs and the tissues toward the central nervous system: they report light, touch, temperature, pain and the position of the body. Motor neurons, also called efferent neurons, carry commands from the central nervous system out to the muscles and glands: they make the animal move and secrete. Interneurons sit between the two, in the brain and the spinal cord, and they are the great majority of neurons in a vertebrate: they receive signals from sensory neurons, pass them to other interneurons, and send the result to motor neurons, and it is in the interneurons that the deciding happens. A reflex may use only a sensory neuron and a motor neuron, but a thought uses chains of interneurons, thousands of them, passing the signal back and forth until a decision emerges.

Most neurons cannot divide. A skin cell divides to replace itself, and a liver cell divides to repair damage, but a neuron, once it is made, is made for life, and if it dies it is usually not replaced. The reason is that a neuron's value is in its connections: each neuron is wired to thousands of others, in a pattern that carries the animal's memories and skills, and replacing a neuron would mean rewiring all of those connections, which the body cannot do. The price is that damage to the nervous system is hard to repair: a cut spinal cord does not heal the way a cut muscle does, because the axons cannot regrow across the gap, and the connections are lost. Some animals escape this price: a planarian flatworm can regrow its whole brain from a fragment of its body, and a zebrafish can repair its spinal cord, but a mammal cannot, and the difference is one of the great unsolved problems of medicine.

Neurons are supported by other cells, the glia, which are often ignored and are essential. In the vertebrate brain, the glia outnumber the neurons, and they do the jobs the neurons cannot: they hold the neurons in place, feed them, clean up after them, and insulate their axons. The oligodendrocytes of the central nervous system and the Schwann cells of the peripheral nervous system wrap axons in myelin, a fatty sheath that speeds the signal enormously. The astrocytes feed the neurons and mop up the chemicals they release. The microglia are the immune cells of the brain, clearing debris and fighting infection. A neuron without its glia is like a wire without its insulation and its maintenance crew: it can work for a while, and then it fails.

Type of neuron	Direction of the signal	Job	Where it is found
Sensory neuron	From the senses toward the centre	Reports light, touch, temperature, pain, body position	Sense organs, skin, muscles, internal organs
Motor neuron	From the centre out to the body	Commands muscles to contract and glands to secrete	Spinal cord and brain, reaching out to muscles and glands
Interneuron	Between neurons, inside the centre	Connects, integrates and decides; carries memory and learning	Brain and spinal cord; the great majority of neurons in vertebrates

How a Signal Travels: The Action Potential

A neuron at rest is electrically charged, and the charge is the key to everything it does. Inside the cell, there is more potassium and less sodium than outside, and the imbalance is maintained by a pump in the cell membrane, the sodium-potassium pump, which pushes three sodium ions out for every two potassium ions it brings in, using energy to do it. The result is that the inside of the neuron is about seventy millivolts negative compared with the outside, a voltage called the resting potential, and the neuron sits at that voltage, charged like a battery, waiting for a signal. The pump never stops, because without it the charge would leak away and the neuron could not fire.

When a signal arrives, the charge changes in a wave. A stimulus opens channels in the membrane, and sodium ions rush into the cell, because they are plentiful outside and scarce inside, and the inside becomes less negative, then positive, rising to about forty millivolts, in a phase called depolarisation. Then the sodium channels close, potassium channels open, and potassium rushes out, and the inside becomes negative again, in repolarisation, and it overshoots slightly, becoming more negative than the resting potential, before the pump restores the balance. The whole event, the rise and the fall, takes about one or two milliseconds, and it is called the action potential. It is the electrical pulse that carries a message along an axon.

The action potential is all or none: it either fires fully or it does not fire at all. A weak stimulus that does not reach the threshold, about fifty-five millivolts negative, produces nothing, and a strong stimulus produces an action potential of exactly the same size as a moderate one. The neuron does not send a bigger signal for a stronger stimulus: it sends more signals, more often. A gentle touch produces a few action potentials a second, and a hard pinch produces many, and the brain reads the frequency, not the size, to judge the strength. The rule is simple, and it is the same in every neuron, from a worm to a whale: the message is carried in the rate of firing, not in the height of the pulse.

The speed of the signal depends on the axon. In a thin, unmyelinated axon, the action potential creeps along at about one metre per second, as the wave of depolarisation moves from one patch of membrane to the next. In a thick axon, it travels faster, because the resistance inside is lower, and the squid's giant axon, which is up to a millimetre across, carries a signal at about twenty-five metres per second. In a myelinated axon, the signal is far faster still, because the myelin sheath insulates the axon, and the action potential jumps from one gap in the myelin to the next, the nodes of Ranvier, in a process called saltatory conduction, from the Latin word for leaping. A large myelinated fibre in a mammal can carry a signal at up to one hundred and twenty metres per second, fast enough to cross the body in a fraction of a second. The squid's giant axon was the preparation in which Hodgkin and Huxley worked out the whole mechanism in the 1950s, work that won them the Nobel Prize in 1963, and it remains one of the great experiments in the history of biology.

After an action potential, the neuron cannot fire again immediately. For a brief period, the refractory period, the sodium channels are closed and will not reopen, and no stimulus, however strong, can trigger another pulse. The refractory period has two uses: it makes the signal travel in one direction only, because the membrane behind the wave cannot fire again, so the wave can only move forward, and it sets a limit on how fast a neuron can fire, about a thousand times a second at most, and usually far fewer. The limit is a real one: a neuron cannot send information faster than its refractory period allows, and the brain's speed is set, in part, by this simple fact about its wiring.

The Synapse: Where Neurons Talk

Neurons do not touch each other. Between the end of one neuron and the start of the next there is a gap, the synaptic cleft, about twenty nanometres wide, and the signal must cross it, and it does so chemically. When an action potential arrives at the end of an axon, it opens calcium channels, calcium flows in, and the calcium triggers small sacs, the synaptic vesicles, to release their contents into the cleft. The contents are neurotransmitters, chemical messengers, and they drift across the gap and bind to receptors on the next neuron, opening channels there and starting a new electrical signal. The electrical pulse becomes a chemical message, and the chemical message becomes an electrical pulse again, and the signal passes on.

There are many neurotransmitters, and they do different jobs. Acetylcholine is the messenger at the neuromuscular junction, where a motor neuron tells a muscle to contract, and it is also used in the brain, where it is involved in attention and memory. Glutamate is the main excitatory transmitter in the vertebrate brain, the one that makes the next neuron more likely to fire, and GABA is the main inhibitory transmitter, the one that makes it less likely to fire. Dopamine is involved in reward and in movement, and its loss causes the tremor of Parkinson's disease. Serotonin affects mood, sleep and appetite. Noradrenaline, also called norepinephrine, prepares the body for action, raising the heart rate and sharpening attention. A single neuron usually releases one main transmitter, but a single transmitter can have different effects in different places, because the receptors differ, and the same molecule can excite one cell and inhibit another.

Synapses are not fixed: they change, and the change is the basis of learning and memory. When two neurons fire together repeatedly, the synapse between them becomes stronger, so that the signal passes more easily next time, a rule first proposed by Donald Hebb in 1949 and summed up as cells that fire together wire together. When a synapse is rarely used, it weakens, and the connection fades. New synapses can form, and old ones can be pruned away, and the pattern of strengths across the whole network is, in a real sense, the animal's memory. The sea slug Aplysia, which has large neurons that are easy to study, showed the mechanism clearly: a simple learned response, withdrawing its gill when touched, changes the strength of a single synapse, and the change can last for days. Eric Kandel's work on Aplysia won the Nobel Prize in 2000, and it showed that learning is a physical change in a synapse, not a mysterious property of the mind.

Not all synapses are chemical. Some are electrical, where the two neurons are joined by gap junctions, channels that let the electrical signal pass directly from one cell to the next, with no chemical step in between. Electrical synapses are fast, because there is no delay while a transmitter is released and binds, and they are common in the nerve nets of cnidarians and in the escape circuits of fish and crustaceans, where speed matters more than flexibility. A crayfish's escape response, the tail flip that shoots it away from danger, uses electrical synapses to fire the giant motor neurons at once, and the whole escape takes only a few milliseconds. Chemical synapses are slower but far more flexible, because the transmitter can excite or inhibit, and the strength can change, and it is the chemical synapse that dominates the vertebrate brain, where flexibility matters more than raw speed.

Synapses are also where poisons and drugs act, which is why so many of them affect the nervous system. A venom that blocks acetylcholine receptors paralyses the muscles, because the motor neuron can no longer tell the muscle to contract, and many snake venoms work this way, by blocking the neuromuscular junction. A pesticide that blocks acetylcholinesterase, the enzyme that clears acetylcholine from the cleft, leaves the signal on permanently, and the insect's muscles spasm until it dies. Caffeine blocks the receptors for adenosine, a transmitter that makes the brain sleepy, and the brain, no longer hearing the sleep signal, stays awake. Alcohol strengthens the effect of GABA, the inhibitory transmitter, and weakens glutamate, which is why it slows the brain down. The synapse is the most delicate part of the nervous system, and it is the part that outside chemicals most easily reach.

Nerve Nets: The First Nervous Systems

The simplest nervous systems are nerve nets, and they are found in cnidarians, the jellyfish, sea anemones, corals and Hydra, and in ctenophores, the comb jellies. A nerve net is a diffuse mesh of neurons spread through the body wall, with no brain, no cord and no centre, and a signal can travel in any direction through the mesh, spreading outward from wherever it starts. Touch a Hydra on one side, and the signal spreads through the net, and the whole animal contracts. The system is simple, and it is enough: a jellyfish needs to pulse its bell, to feed, and to sting, and a net that spreads a signal everywhere can do all three.

The nerve net has a weakness, and the weakness is that it cannot do one thing at a time. Because the signal spreads in all directions, the whole animal responds at once, and it cannot contract one part of its body while relaxing another, or reach with one tentacle while holding still with the rest. A jellyfish cannot aim: it pulses, and it drifts, and it stings whatever it touches, and it has no way to steer toward food or away from danger with any precision. The net is a broadcast system, and a broadcast system cannot address a message to one part of the body alone. The limitation is the reason nerve nets were replaced, in most animals, by systems with cords and ganglia, where a signal can travel along a defined path to a defined place.

But the nerve net is not as simple as it first looks. A jellyfish has sense organs, the rhopalia, which sit around the margin of the bell and contain eyespots that detect light, and statocysts that detect gravity and orientation, and the rhopalia set the rhythm of the bell's pulsing, acting as pacemakers. Cut the rhopalia away, and the jellyfish stops pulsing in a regular rhythm. The net, in other words, has local centres that do particular jobs, even though it has no brain. A sea anemone has a nerve net that lets it close its tentacles around food, open them again, and detach and move when conditions turn bad. The net is a working nervous system, and it has served cnidarians for more than six hundred million years, which is longer than any brain has existed.

Ctenophores, the comb jellies, have a nerve net of their own, and recent research has made them famous for a surprising reason: their neurons appear to have evolved independently from those of all other animals. A comb jelly's nerve net uses different transmitter molecules from the ones other animals use, and its neurons are built differently, with a different arrangement of synapses, which suggests that the nerve net evolved twice, once in the ancestors of cnidarians and other animals, and once in the ctenophores. If that is right, then the nervous system is not a single invention but two, and the comb jelly is a living example of evolution solving the same problem twice, in two different ways. The finding is still debated, but it has already changed how scientists think about the origin of the nervous system.

The Radial Plan: Echinoderms and the Nerve Ring

Echinoderms, the starfish, sea urchins, sea cucumbers and brittle stars, have a nervous system that matches their radial body plan. There is no brain and no head, because there is no front end: a starfish is symmetrical in all directions, and its nervous system is a ring around the mouth, with a radial nerve running out along each arm. The ring coordinates the arms, and each radial nerve controls its own arm, and the system works without a centre, because the animal has no direction to favour. A starfish can move in any direction, leading with any arm, and the nerve ring lets it do so, by letting any arm take the lead.

The echinoderm nervous system has two parts. The ectoneural system, on the outside of the body, is the sensory part: it receives information from the skin, from the tube feet, and from the eyespots at the tips of the arms, which are simple clusters of light-sensitive cells. The hyponeural system, deeper inside, controls the muscles and the internal organs. The two are connected, and together they let the starfish do what it does: creep across the sea floor on its tube feet, sense a mussel, open it, and evert its stomach into the shell to digest the animal inside. The coordination is slow and clumsy by vertebrate standards, but it is enough for a life that is lived slowly, on the sea floor, without hurry.

The most remarkable thing about the echinoderm nervous system is what it can do after damage. A starfish can regenerate a whole arm, including its radial nerve, and if it loses its central disc it can, in some species, regrow the whole animal from a single arm. The nerves regrow with the arm, and the new arm works as well as the old one. The ability comes from the same property that makes the system simple: because the nerves are spread out and not concentrated in a brain, losing a part does not lose the whole, and the part can be rebuilt. A vertebrate that loses its brain cannot regrow it, but a starfish that loses an arm can regrow the arm, nerves and all, and carry on as if nothing had happened.

The Ladder Plan: Flatworms and Annelids

The first brains appeared in the flatworms. A planarian, a small freshwater flatworm, has two clusters of nerve cells at its head end, the cerebral ganglia, joined by a commissure, and two longitudinal nerve cords running down the body, connected by cross branches at intervals, like the rungs of a ladder. The plan is called a ladder nervous system, and it is the first nervous system with a head end, a brain, and a cord, and it is the plan from which the nervous systems of most bilaterally symmetrical animals are derived. The planarian's brain is small, but it is a brain: it receives information from the eyespots and from the sensory cells in the head, and it sends commands down the cords to the muscles of the body.

The ladder plan has a great advantage over the nerve net: it can send a signal to one part of the body without sending it to all of them. A planarian can turn toward food with its head while its tail stays still, because the signal travels down one cord and not the other, and it can contract one side of its body while relaxing the other, because the cords are separate. The rungs of the ladder, the cross connections, let the two sides coordinate, so that the animal can crawl in a straight line or turn smoothly. The plan is a compromise between a net and a cord: it is spread out, so that each part of the body has its own wiring, and it is centralised, so that the head can coordinate the whole.

Annelids, the segmented worms, took the ladder plan further. An earthworm has a dorsal cerebral ganglion above the gut, the brain, and a ventral nerve cord running along the belly, with a ganglion in each segment, and the brain and the cord are joined by a ring of nerve tissue around the gut, the circumesophageal connectives. Each segmental ganglion controls its own segment: it receives sensory information from the skin of that segment, and it sends motor commands to the muscles of that segment, so that the worm can crawl by peristalsis, contracting one segment after another in a wave. The brain coordinates the segments, and it also receives information from the prostomium, the lobe above the mouth, which carries the worm's sense organs.

The earthworm's nervous system has a special feature: giant axons. Running along the ventral cord are three large fibres, one median and two lateral giant axons, which are much thicker than the other axons and carry signals much faster, and they are the worm's alarm system. Touch the worm sharply, and the giant axons fire, and the signal races down the cord, and the whole worm contracts and withdraws at once. The giant axon is a solution to the problem of speed: a thick fibre conducts faster than a thin one, and the worm, which has no myelin, uses thickness instead. The squid's giant axon, which Hodgkin and Huxley studied, is the same idea, on a larger scale: a thick fibre, built for speed, in an animal that lives or dies by how fast it can escape.

The ladder plan also shows the first signs of cephalization, the concentration of nerve cells and sense organs at the head end, which is the theme of the whole history of nervous systems. In a planarian, the brain is a small pair of ganglia. In an earthworm, the brain is larger, and it receives information from the prostomium, which carries touch and chemical receptors. In a leech, the brain is larger still, and it controls a more complex set of behaviours, including swimming and feeding. Each step up the ladder is a step toward a head that senses, decides and leads, and the rest of the body follows.

Arthropods: Segmental Ganglia and Mushroom Bodies

Arthropods, the insects, spiders, crabs and their relatives, have the most successful nervous system in the animal kingdom, measured by the number of species that use it: more than a million described species, and probably many more undescribed, all running on the same basic plan. The plan is the ladder plan, refined: a ventral nerve cord, with a pair of ganglia in each segment, and a brain at the front made of three fused pairs of ganglia, the protocerebrum, the deutocerebrum and the tritocerebrum, which receive information from the eyes, the antennae and the mouthparts. In many arthropods, the ganglia of the thorax are fused into a single mass, and the ganglia of the abdomen are fused too, so that the cord is a series of a few large ganglia rather than many small ones, and each ganglion controls the legs and the organs of its own part of the body.

The arthropod brain has parts that no other animal has. The mushroom bodies, paired structures in the protocerebrum, are the centres of learning and memory in an insect, and they are large in the animals that need to learn the most: in a honeybee, which must remember the way to a flower and the way home, the mushroom bodies are large and elaborately folded, and they grow as the bee learns its foraging routes. The central complex, a set of structures in the middle of the brain, controls walking and orientation, and it lets a desert ant keep track of where it is, counting its steps and watching the sun, so that it can walk in a straight line away from its nest and then walk straight back. The optic lobes, which process what the compound eyes see, are large in the animals that hunt by sight, such as dragonflies, and small in the animals that live in the dark.

The arthropod nervous system is also famous for its escape circuits, which are among the fastest in the animal kingdom. A crayfish's tail flip, which shoots it backwards away from danger, is driven by two giant neurons, the lateral giant and the medial giant, which run the length of the body and fire at once, contracting the abdominal muscles in a single, powerful stroke. The circuit uses electrical synapses, so that the signal passes with almost no delay, and the whole escape takes only a few milliseconds. A cockroach's escape, running away from a puff of air, is driven by giant interneurons in its ventral cord, which sense the air movement through hairs on its tail, the cerci, and fire the leg muscles before the animal has time to think. The escape circuits are reflexes, and they are the fastest things an arthropod does, because they have to be: an animal that hesitates is an animal that is eaten.

Arthropods also show how much behaviour a small brain can produce. A honeybee has about a million neurons, and it can learn the colour, the scent and the position of a flower, remember the way home over several kilometres, communicate the direction and the distance to its nest mates by dancing, and navigate by the sun and by the pattern of polarised light in the sky. A jumping spider, with a brain smaller than a pinhead, can plan a route to its prey, stalk it, and leap on it with precision, and it can learn from its mistakes. The lesson is that intelligence is not only about the number of neurons: it is about how they are wired, and a million well-wired neurons can do a great deal.

Molluscs and the Cephalopod Exception

Most molluscs have modest nervous systems. A snail has a set of paired ganglia, cerebral, pedal, pleural and visceral, joined by cords and commissures, and the ganglia control the foot, the mantle and the internal organs, and the animal moves slowly and senses little beyond light, touch and chemicals. A clam has even less: three pairs of ganglia, cerebral, pedal and visceral, and no eyes worth the name, and it lives by filtering water and closing its shell when disturbed. The molluscan plan is a ladder with the rungs spread out, and it is enough for animals that move slowly and live quietly.

Cephalopods, the octopuses, squid and cuttlefish, are the exception, and they are one of the most remarkable exceptions in the animal kingdom. An octopus has about five hundred million neurons, more than a dog, and only about a third of them are in the central brain: the other two-thirds are in the arms, in a nerve cord that runs down each arm, with ganglia at the base of each sucker. Each arm can taste, reach, grasp and explore on its own, and an arm that is cut off can still reach for food and avoid a painful stimulus for an hour or more, because the wiring is in the arm itself. The central brain gives the big commands, to go, to hide, to hunt, and the arms work out the details, which is a division of labour no other animal has.

The cephalopod brain itself is large and complex, with optic lobes that are bigger than the rest of the brain put together, because vision is the octopus's main sense, and with a vertical lobe that is involved in learning and memory, in the same way that the hippocampus is in a mammal. An octopus can learn to open a jar, to navigate a maze, to recognise individual humans, and to remember the solution to a problem for months. It can change the colour and the texture of its skin in a fraction of a second, using chromatophores, pigment cells that are controlled directly by nerves, so that the colour change is as fast as a thought, and it can match a rock, a coral, or a patch of sand, with a precision no other animal can match. The skin is, in effect, a display controlled by the brain, and the octopus is the only animal that can change its appearance at the speed of its nervous system.

The cephalopod nervous system is a case of convergent evolution: it evolved intelligence independently from the vertebrates, on a completely different body plan, and it reached a similar result by a different route. The octopus has no myelin, so its axons conduct slowly, but it has many of them, and it has a brain that is organised differently from a vertebrate's, with no cortex and no layered structure, yet it can do many of the things a vertebrate brain can do. The lesson is that there is more than one way to build a clever animal, and that the vertebrate way, with a large cortex and fast myelinated fibres, is not the only one. The octopus is the proof, and it is the reason cephalopods are studied as a second, independent example of how intelligence evolves.

The Vertebrate Plan: Dorsal Cord, CNS and PNS

Vertebrates turned the plan upside down, literally. Where an earthworm or an insect has a ventral nerve cord, running along the belly, a vertebrate has a dorsal nerve cord, running along the back, and the cord is hollow, a tube, where the invertebrate cord is solid. The dorsal hollow nerve cord is one of the defining features of the chordates, the group that includes all vertebrates, and it is the structure from which the brain and the spinal cord develop. The front end of the tube swells into the brain, and the rest becomes the spinal cord, and the two together make the central nervous system, the CNS, which is the part of the system that decides. Everything else, the nerves that run out to the body, makes the peripheral nervous system, the PNS, which is the part that senses and acts.

The peripheral nervous system has two divisions, and the division is by job. The somatic nervous system is the voluntary part: it carries sensory information from the skin and the muscles to the central nervous system, and it carries motor commands from the central nervous system to the skeletal muscles, the muscles that move the bones. It is the system you use when you decide to pick up a pen or to walk across a room. The autonomic nervous system is the involuntary part: it controls the smooth muscle of the gut and the blood vessels, the heart, and the glands, and it does so without conscious thought, adjusting the heart rate, the blood pressure and the digestion to the body's needs. The autonomic system has two branches that oppose each other: the sympathetic branch, which prepares the body for action, raising the heart rate and releasing glucose, and the parasympathetic branch, which restores the body to rest, slowing the heart and promoting digestion. The two work like the accelerator and the brake of a car, and the balance between them sets the state of the body.

The central nervous system is protected, because it is too important to leave exposed. The brain sits inside the skull, and the spinal cord runs inside the vertebral column, and both are wrapped in membranes, the meninges, and bathed in cerebrospinal fluid, which cushions them against shocks. The blood-brain barrier, a layer of tightly joined cells in the walls of the brain's blood vessels, keeps most chemicals in the blood from reaching the neurons, which protects the brain from poisons and from changes in the blood's chemistry, but also makes it hard to treat brain diseases with drugs, because the drugs cannot get in. The protection is necessary, because the central nervous system cannot repair itself: a cut in the spinal cord severs the axons, and the axons do not regrow, and the part of the body below the cut loses its connection to the brain permanently.

The number of nerves is regular enough to count. A human has twelve pairs of cranial nerves, which leave the brain directly and serve the head and the organs of the chest and the abdomen, including the vagus nerve, the longest of them, which runs from the brain to the gut and carries the parasympathetic signals that slow the heart and promote digestion. A human has thirty-one pairs of spinal nerves, which leave the spinal cord at intervals and serve the rest of the body, each one carrying sensory fibres in through its dorsal root and motor fibres out through its ventral root. The arrangement is segmental, a legacy of the segmented body plan of the vertebrates' ancestors, and it is the reason a damaged spinal nerve affects a particular band of skin and muscle, and no other.

The Vertebrate Brain: Five Divisions, One Job

The vertebrate brain has five divisions, and they appear in the same order in every vertebrate, from a fish to a human, although their sizes differ enormously. The telencephalon, the front part of the forebrain, becomes the cerebrum, the large, folded mass that dominates the mammalian brain and carries the cortex, the outer layer where thought, memory and language live. The diencephalon, the back part of the forebrain, holds the thalamus, which relays sensory information to the cortex, and the hypothalamus, which controls hunger, thirst, temperature and the hormones of the pituitary gland, and which links the nervous system to the endocrine system. The mesencephalon, the midbrain, holds the tectum, which in a fish or a frog is the optic lobe, the main visual centre, and the tegmentum, which carries the fibres that connect the forebrain to the hindbrain. The metencephalon, the front part of the hindbrain, becomes the cerebellum, which coordinates movement and balance, and the pons, which relays signals between the cerebellum and the rest of the brain. The myelencephalon, the back part of the hindbrain, becomes the medulla oblongata, which controls the vital functions, breathing, heart rate and blood pressure, and which must never stop, because if it stops, the animal dies.

The sizes of the divisions tell the story of each animal's life. A fish has large optic lobes, because it hunts by sight, and a small cerebrum, because it does little thinking. A frog has a small cerebrum and a large optic tectum, and it catches its prey by sight, snapping at anything small and moving. A bird has a large hyperpallium, the part of the forebrain that does the work of a cortex, and large optic lobes, and a large cerebellum, because flight demands precise coordination, and a songbird's forebrain holds the circuits that let it learn its song. A mammal has a large neocortex, the newest part of the cerebrum, which is folded into ridges and grooves to pack more area into the skull, and the size of the neocortex, relative to the rest of the brain, is the best single measure of how much an animal thinks. A human's neocortex is the largest, relative to body size, of any animal, and it is the part of the brain that lets us speak, plan and remember.

The hypothalamus deserves a closer look, because it is the link between the nervous system and the rest of the body. It monitors the blood's temperature, its saltiness, its glucose and its hormones, and it acts on what it finds: if the body is cold, it orders shivering and constricts the blood vessels of the skin; if the body is hot, it orders sweating and dilates them; if the blood is low on water, it orders thirst and the release of the hormone that makes the kidneys save water; if the day is short, it reads the signal from the eyes and sets the body's clock. When an animal hibernates, it is the hypothalamus that lowers the body's temperature and slows the heart and the breathing, and when it wakes in spring, it is the hypothalamus that brings the temperature back up. The hypothalamus is small, about the size of a pea in a human, and it does more jobs than any other part of the brain.

The cerebellum, the little brain at the back, is the coordinator. It receives information from the eyes, the ears, the muscles and the joints, and it compares what the body is doing with what the brain intended, and it corrects the difference, smoothly and automatically, without conscious thought. A bird in flight, a fish turning in the water, a cat landing on its feet, all depend on the cerebellum to keep their movements precise. Damage to the cerebellum does not paralyse an animal, but it makes its movements clumsy and uncoordinated, and it cannot walk in a straight line or reach for an object without overshooting. The cerebellum has more neurons than the rest of the brain put together, about half of all the neurons in the human brain, packed into a small, tightly folded structure, because coordination needs many small adjustments, and each adjustment needs a neuron.

Brain division	Main parts	Main jobs
Telencephalon (forebrain)	Cerebrum, cortex, olfactory bulbs	Thought, memory, learning, smell, voluntary movement
Diencephalon (forebrain)	Thalamus, hypothalamus, pituitary	Relay of sensory information; hunger, thirst, temperature, hormones
Mesencephalon (midbrain)	Tectum (optic lobes), tegmentum	Vision and hearing in fish and frogs; relay of signals in mammals
Metencephalon (hindbrain)	Cerebellum, pons	Coordination of movement and balance; relay to the cerebellum
Myelencephalon (hindbrain)	Medulla oblongata	Breathing, heart rate, blood pressure; the vital functions

The Reflex Arc: The Fastest Circuit

A reflex is a response that happens before thought, and the reflex arc is the circuit that makes it happen. The arc has five parts: a receptor, which senses the stimulus; a sensory neuron, which carries the signal to the central nervous system; an interneuron, which passes the signal on; a motor neuron, which carries the command out; and an effector, a muscle or a gland, which acts. Touch a hot surface, and the pain receptors in your skin fire, and the sensory neuron carries the signal to the spinal cord, and the interneuron passes it to the motor neuron, and the motor neuron tells the muscles of your arm to pull your hand away, and your hand is moving before you have felt the pain. The brain learns about it afterwards, and the feeling of pain arrives after the movement, because the reflex does not wait for the brain.

The simplest reflex has no interneuron at all. The knee-jerk reflex, which a doctor tests by tapping the tendon below the kneecap, uses only two neurons: a sensory neuron, which senses the stretch of the muscle, and a motor neuron, which tells the same muscle to contract, and the leg kicks. The reflex is monosynaptic, with a single synapse between the two neurons, and it takes only a few tens of milliseconds, which is why the leg kicks before you can stop it. The knee-jerk is a stretch reflex, and its job is to keep the muscle at the right length: if the muscle is stretched, it contracts, and the stretch is corrected, and the body stays upright. The same reflex, in the muscles of the legs, is what keeps a standing animal standing, correcting the small sways before they become falls.

Most reflexes are polysynaptic, with one or more interneurons in between, and the interneurons let the reflex do more than one thing at once. The withdrawal reflex, which pulls a limb away from pain, also uses interneurons to relax the opposing muscles, so that the limb can move freely, and to excite the muscles of the other side of the body, so that the animal keeps its balance while it withdraws. The interneurons also send a copy of the signal up to the brain, so that the animal learns what happened, and the brain can decide whether to do more, to run away, or to stay. The reflex is fast, and the brain is slow, and the two work together: the reflex saves the limb, and the brain decides what to do next.

Reflexes are not only for escape. The blinking reflex protects the eye, closing the lid when something approaches. The swallowing reflex moves food from the mouth to the stomach, in a sequence of muscle contractions that cannot be stopped once it starts. The pupillary reflex narrows the pupil in bright light and widens it in the dark. A newborn mammal has a suckling reflex, and a newborn bird has a gaping reflex, opening its mouth when the parent arrives. Reflexes are the oldest behaviours an animal has, and they are built into the nervous system before birth, ready to work the moment the animal is born, because an animal that had to learn to breathe or to swallow would not live long enough to learn.

Cephalization: Why Heads Evolved

Cephalization is the concentration of nerve cells and sense organs at the front end of the body, and it is the trend that runs through the whole history of nervous systems, from the flatworm's small brain to the human's large one. The reason is movement. An animal that moves forward meets the world head first, and the head is the first part to encounter food, danger and mates, so it pays to put the sensors there, and to put the wiring that interprets them there too. A jellyfish, which drifts and has no front end, has no head. A planarian, which crawls forward, has a head with eyespots and a brain. A fish, which swims forward, has a head with eyes, nostrils and a brain. A mammal, which runs forward, has a head with eyes, ears, nose and a large brain. The head is the end that meets the world, and the nervous system follows the head.

Cephalization is tied to bilateral symmetry, the arrangement of the body into a left and a right half, with a front and a back. A radially symmetrical animal, such as a starfish, has no front, and it has no head, and its nervous system is a ring. A bilaterally symmetrical animal has a front, and it has a head, and its nervous system has a brain. The two go together, because a front end needs a head, and a head needs a brain, and the animals that became bilaterally symmetrical, more than half a billion years ago, were the animals that developed brains. The flatworms were among the first, and their small cerebral ganglia are the first brains in the animal kingdom.

The trend did not stop with a small brain. In the vertebrates, the brain grew, and the cerebrum grew fastest of all, because the cerebrum is the part that learns, remembers and plans, and the animals that learned and remembered best survived best. In the cephalopods, the brain grew too, independently, because an octopus that could learn where food was hidden and remember how to open a shell survived better than one that could not. Cephalization is not a single event but a direction, and it has been followed, independently, by every group of animals that moves and hunts, because a head that senses and a brain that decides are worth their cost in every life that depends on finding food and avoiding danger.

Senses and the Nervous System

The nervous system begins with the senses, because a nervous system that senses nothing has nothing to decide about. Every sense is a specialised receptor, a cell that converts a physical signal, light, sound, touch, chemicals, temperature, into an electrical signal that a neuron can carry. A photoreceptor converts light into a signal. A mechanoreceptor converts pressure or vibration into a signal. A chemoreceptor converts a molecule into a signal. A thermoreceptor converts temperature into a signal. The conversion is called transduction, and it is the first step in every nervous process: the world is physical, the nervous system is electrical, and the receptor is the translator between them.

The senses differ enormously between animals, because different animals live in different worlds. A dog can smell what a human cannot, because it has far more olfactory receptors and a far larger olfactory bulb. A bat can hear what a human cannot, because its ears are tuned to ultrasound, and it navigates by echolocation, building a picture of the world from echoes. A pit viper can sense infrared radiation, the heat of a warm-blooded animal, through pits on its face that are wired directly to its brain. A platypus can sense the tiny electric fields that a shrimp's muscles produce, through receptors in its bill. A migratory bird can sense the Earth's magnetic field, through a mechanism that is still not fully understood, and it uses the sense to navigate across continents. The senses are the nervous system's windows on the world, and each animal's windows are different, because each animal lives in a different world, and the full story of animal senses is covered in our companion article on animal senses beyond human senses, which explores the senses that humans do not have, and the animals that use them.

The nervous system does not only sense the outside world: it senses the body itself. Proprioceptors, the receptors in the muscles, tendons and joints, report the position of the limbs and the tension in the muscles, and the brain uses the information to coordinate movement, so that an animal can walk without looking at its feet. The vestibular apparatus of the inner ear reports the position of the head and its acceleration, and the brain uses it to keep the animal upright and to stabilise the eyes when the head moves. The receptors in the gut, the blood vessels and the lungs report hunger, blood pressure and the level of carbon dioxide, and the brain uses them to control the body's internal state. The senses that look inward are as important as the senses that look outward, and an animal that cannot feel its own body cannot move it.

Learning and Memory: Beyond Reflex

A reflex is fixed, but learning changes the nervous system, and the change is physical. The simplest form of learning is habituation, in which an animal stops responding to a stimulus that is repeated and harmless: a sea slug that is touched gently again and again stops withdrawing its gill, because the synapse between the sensory neuron and the motor neuron weakens. The opposite is sensitisation, in which a painful stimulus makes the animal respond more strongly to the next touch, because the synapse strengthens. Both are changes in the strength of a synapse, and both can last for days, and both were worked out in Aplysia, the sea slug with large neurons that made the mechanism visible.

More complex learning needs more neurons. Classical conditioning, in which an animal learns that one stimulus predicts another, needs a circuit that can associate two signals, and it is found in flatworms, in snails, and in every vertebrate. Operant conditioning, in which an animal learns that an action has a consequence, needs a circuit that can link a behaviour to a reward, and it is found in insects, in octopuses, and in every vertebrate. A honeybee can learn the colour and the scent of a rewarding flower in a single visit, and it remembers the lesson for days. An octopus can learn to open a jar by watching another octopus do it, which is learning by observation, a skill once thought to belong only to vertebrates. The circuits differ, but the principle is the same: the synapses that are used together grow stronger, and the animal's behaviour changes.

Memory is stored in the pattern of synaptic strengths across a network, and the storage has two forms. Short-term memory is a temporary change, a strengthening that lasts for minutes or hours, and it depends on the chemicals already present at the synapse. Long-term memory is a lasting change, and it needs new proteins to be made, and new synapses to grow, and it can last for a lifetime. In Aplysia, the difference was shown clearly: a single training trial produces a short-term change, and repeated trials produce a long-term change, with new synaptic connections growing between the neurons. In a mammal, the hippocampus, a structure deep in the temporal lobe, is the centre that turns short-term memories into long-term ones, and damage to it leaves an animal able to remember the past but unable to form new memories, as the famous patient H.M. showed, after his hippocampi were removed to treat his epilepsy.

The most remarkable thing about learning is how much a small brain can do with it. A fruit fly has about a hundred thousand neurons, and it can learn to avoid an odour that was paired with a shock, and to prefer an odour that was paired with sugar, and it remembers the lesson for a day. A honeybee has about a million neurons, and it can learn a route to a flower, remember the time of day when the flower opens, and communicate the route to its nest mates by dancing. The lesson is that learning does not need a large brain: it needs a circuit that can change, and a change that can last, and even a hundred thousand neurons, wired the right way, can learn.

When the System Fails: Damage, Toxins and Repair

The nervous system is delicate, and when it fails, the failure is visible. A cut in the spinal cord severs the axons, and the part of the body below the cut loses its connection to the brain, and the loss is permanent, because the axons of the central nervous system do not regrow. A stroke, in which a blood vessel in the brain is blocked or bursts, kills the neurons that the vessel supplied, and the functions those neurons carried are lost, and the recovery, if it happens, comes from other neurons taking over the work, not from the dead neurons coming back. A disease such as Alzheimer's destroys the synapses, and the memories stored in them are lost, and the loss is gradual and irreversible. The nervous system has no spare parts, and its failures are the hardest to repair.

Many poisons work by attacking the nervous system, because the nervous system is the easiest system to break. Venomous animals use neurotoxins that block the synapse: a cobra's venom blocks the acetylcholine receptors at the neuromuscular junction, so the muscles cannot contract, and the victim is paralysed, and a cone snail's venom blocks the calcium channels that trigger the release of neurotransmitter, so the signal never crosses the synapse at all. The tetrodotoxin of the pufferfish blocks the sodium channels that carry the action potential, so the neurons cannot fire, and the victim is paralysed while remaining conscious. The poisons work because the nervous system depends on a small number of precisely tuned mechanisms, and a molecule that blocks one of them stops the whole system.

Some animals can repair what a mammal cannot. A planarian can regrow its entire brain from a fragment of its body, because its stem cells, the neoblasts, can make new neurons and wire them correctly. A zebrafish can repair a cut spinal cord, because its glial cells form a bridge across the gap, and the axons grow across it. A salamander can regenerate a whole limb, nerves and all. The ability comes from the same property that makes these animals simple: their nervous systems are less centralised, and their cells retain the ability to divide and to differentiate, which mammalian neurons have lost. Scientists study these animals in the hope of learning how to switch the ability back on in mammals, and the work is one of the most active areas of neuroscience.

How Scientists Study Nervous Systems

Scientists study nervous systems by listening to them, and the listening began with the squid. In the 1950s, Alan Hodgkin and Andrew Huxley inserted electrodes into the giant axon of a squid, which is up to a millimetre across, large enough to hold an electrode, and they recorded the action potential as it travelled, and they worked out the whole mechanism: the sodium channels that open, the potassium channels that follow, and the currents that carry the signal. The work won the Nobel Prize in 1963, and it is still the foundation of everything we know about how neurons fire. The squid's giant axon was the right preparation at the right time, and it is a reminder that a simple animal, studied carefully, can reveal a mechanism that applies to every animal, including us.

The tools have grown more precise since then. The patch clamp, developed by Erwin Neher and Bert Sakmann, who won the Nobel Prize in 1991, lets a scientist record the current through a single ion channel, the tiny pore in the membrane through which sodium or potassium flows, and it showed that the channels open and close in discrete steps, like switches. The electroencephalogram, the EEG, records the summed electrical activity of the brain from electrodes on the scalp, and it shows the rhythms of sleep and waking. Functional magnetic resonance imaging, fMRI, shows which parts of the brain are active during a task, by detecting the changes in blood flow that follow neural activity. Optogenetics, the newest tool, lets a scientist switch a single set of neurons on or off with light, by inserting a gene for a light-sensitive protein into those neurons, and it has let researchers test, directly, what a particular group of neurons does.

The most complete nervous system ever mapped belongs to a tiny worm, Caenorhabditis elegans, a roundworm one millimetre long, whose hermaphrodite has exactly three hundred and two neurons, and every connection between them has been traced, in a map called the connectome, which was completed in 1986 and remains the only complete wiring diagram of any animal's nervous system. The worm's simplicity makes it a model: with only three hundred and two neurons, every behaviour can be traced to a circuit, and the circuits can be tested, neuron by neuron, by killing a single cell with a laser and seeing what changes. The fruit fly's brain, with about a hundred thousand neurons, has now been mapped too, and the mouse's brain is being mapped, neuron by neuron, in a project that will take years. The maps are the beginning of a new kind of neuroscience, in which the whole wiring of a brain can be known, and the behaviour can be read from the wiring.

Nervous Systems Across the Animal Kingdom

The table below brings the whole subject together, showing how each major group of animals is wired, and it is worth reading as a summary of the article. The pattern is clear: the simplest animals have nets, the bilaterally symmetrical animals have cords and ganglia, the arthropods and the cephalopods have brains that rival the vertebrates in what they can do, and the vertebrates have the most centralised system of all, with a dorsal cord, a brain of five divisions, and a peripheral system that reaches every part of the body. The pattern is also a history: the first nervous systems were nets, the first brains were small ganglia at the head end, and the large brains of octopuses, birds and mammals evolved independently, by different routes, to do the same jobs.

Animal group	Type of nervous system	Brain	Distinctive feature
Sponges	None	None	No neurons at all; cells react individually
Cnidarians	Diffuse nerve net	None; rhopalia act as pacemakers	Signal spreads in all directions; whole body responds at once
Flatworms	Ladder: cerebral ganglia and two cords	First true brain, a pair of ganglia	First cephalization; can turn toward food
Annelids	Ladder: dorsal brain, ventral cord, segmental ganglia	Cerebral ganglion above the gut	Giant axons for fast escape; each segment controls itself
Arthropods	Ventral cord, segmental ganglia, fused brain	Three fused ganglia; mushroom bodies for learning	Fastest escape circuits; honeybee navigates by dance and sun
Molluscs (snails, clams)	Paired ganglia joined by cords	Small cerebral ganglia	Slow and simple; enough for a life lived quietly
Cephalopods	Central brain plus nerve cords in each arm	Large brain with huge optic lobes and a vertical lobe	About 500 million neurons, two-thirds in the arms; learns, remembers, camouflages
Echinoderms	Nerve ring plus radial nerves	None	No front end, no head; any arm can lead; regenerates nerves with arms
Vertebrates	Dorsal hollow cord; CNS and PNS	Five divisions: forebrain, midbrain, hindbrain	Most centralised system; myelinated fibres up to 120 metres per second

Key Takeaways

• A nervous system senses, integrates and acts, and every nervous system, from a jellyfish's net to a human brain, does all three jobs.
• The neuron is the unit of the nervous system, with dendrites that receive signals, a cell body that adds them up, and an axon that sends the signal on.
• Sensory neurons carry information in, motor neurons carry commands out, and interneurons, the great majority in vertebrates, do the deciding in between.
• The resting potential is about seventy millivolts negative, maintained by the sodium-potassium pump, which pushes three sodium ions out for every two potassium ions it brings in.
• An action potential is all or none: a stronger stimulus produces more signals, not bigger ones, and the brain reads the frequency to judge the strength.
• Myelin speeds conduction by saltatory conduction, with the signal jumping between the nodes of Ranvier, and a large myelinated fibre can carry a signal at up to one hundred and twenty metres per second.
• Neurons communicate across the synaptic cleft with neurotransmitters, including acetylcholine, glutamate, GABA, dopamine and serotonin, and the signal changes from electrical to chemical and back again.
• Learning is a physical change in a synapse: synapses that fire together grow stronger, and the pattern of strengths across the network is the animal's memory.
• Cnidarians have diffuse nerve nets with no brain, and a signal spreads in all directions, so the whole animal responds at once.
• Echinoderms have a nerve ring around the mouth and a radial nerve along each arm, with no brain and no head, because they have no front end.
• Flatworms have the first true brain, a pair of cerebral ganglia, and a ladder nervous system with two cords and cross connections.
• An earthworm has a ventral nerve cord with a ganglion in each segment, and giant axons that carry the alarm signal for fast escape.
• Insects have mushroom bodies for learning and memory, and a honeybee, with about a million neurons, can learn a route, remember it, and communicate it by dancing.
• An octopus has about five hundred million neurons, two-thirds of them in its arms, and each arm can taste, reach and grasp on its own.
• Vertebrates have a dorsal hollow nerve cord, divided into the central nervous system, the brain and spinal cord, and the peripheral nervous system, which senses and acts.
• The vertebrate brain has five divisions: telencephalon, diencephalon, mesencephalon, metencephalon and myelencephalon, and their sizes reflect each animal's life.
• The hypothalamus, about the size of a pea in a human, links the nervous system to the hormones, controlling hunger, thirst, temperature and hibernation.
• A reflex arc has five parts, receptor, sensory neuron, interneuron, motor neuron and effector, and the knee-jerk reflex uses only two neurons and takes a few tens of milliseconds.
• Cephalization, the concentration of sense organs and nerve cells at the head end, follows movement: the end that meets the world first gets the sensors and the brain.
• Sponges and placozoans have no neurons at all, and every other animal group does, which makes the nervous system one of the oldest systems in the animal kingdom.
• C. elegans, a roundworm with exactly three hundred and two neurons, has the only complete wiring diagram, the connectome, of any animal's nervous system.

Short Questions

1. What is a neuron?

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A neuron is a cell built to carry an electrical signal, and it is the unit of the nervous system. It has a cell body, which holds the nucleus, and two kinds of branches: dendrites, short tree-like branches that receive signals from other neurons, and a single long fibre, the axon, that carries the signal away to the next neuron, or to a muscle, or to a gland. A neuron may have thousands of dendrites, each listening to a different neighbour, and the cell body adds up the inputs and decides whether to fire. Neurons come in three kinds: sensory neurons, which carry information in; motor neurons, which carry commands out; and interneurons, which sit between them and do the deciding, and which are the great majority of neurons in a vertebrate.

2. How does an action potential travel along an axon?

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An action potential travels as a wave of electrical change along the axon's membrane. At rest, the inside of the neuron is about seventy millivolts negative, a charge maintained by the sodium-potassium pump. When a stimulus reaches the threshold, sodium channels open, sodium rushes in, and the inside becomes positive, in depolarisation; then the sodium channels close, potassium channels open, potassium rushes out, and the inside becomes negative again, in repolarisation. The wave moves along the axon because each patch of membrane, as it depolarises, triggers the next patch to do the same. In a myelinated axon, the signal jumps between the gaps in the myelin, the nodes of Ranvier, in saltatory conduction, and it can travel at up to one hundred and twenty metres per second. The refractory period, during which the neuron cannot fire again, makes the wave travel in one direction only.

3. Why do jellyfish not need a brain?

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Jellyfish do not need a brain because their lives are simple, and a nerve net is enough for them. A jellyfish needs to pulse its bell, to feed, and to sting, and a diffuse net of neurons, spread through the body wall, can do all three: a touch anywhere spreads through the net, and the whole animal contracts. The jellyfish also has rhopalia, sense organs around the margin of the bell that contain eyespots and statocysts, and the rhopalia set the rhythm of the pulsing, acting as pacemakers. A brain would let the animal do one thing at a time, aiming and steering, but a jellyfish does not need to aim: it pulses, and it drifts, and it stings whatever it touches, and the net, which has served cnidarians for more than six hundred million years, is enough.

4. Why is an octopus called the most intelligent invertebrate?

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An octopus is called the most intelligent invertebrate because it has about five hundred million neurons, more than a dog, and a brain that can learn, remember and solve problems. It can open a jar, navigate a maze, recognise individual humans, and remember a solution for months. Two-thirds of its neurons are in its arms, in a nerve cord that runs down each arm, so that each arm can taste, reach and grasp on its own, and the central brain makes the big decisions while the arms work out the details. It can change the colour and texture of its skin in a fraction of a second, using chromatophores controlled directly by nerves, matching a rock or a coral with a precision no other animal can match. Its intelligence evolved independently from the vertebrates, on a completely different body plan, which makes it a second, independent example of how a clever animal can evolve.

5. What is a reflex arc?

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A reflex arc is the circuit that produces a reflex, a response that happens before thought. It has five parts: a receptor, which senses the stimulus; a sensory neuron, which carries the signal to the central nervous system; an interneuron, which passes the signal on; a motor neuron, which carries the command out; and an effector, a muscle or a gland, which acts. Touch a hot surface, and the pain receptors fire, and the signal travels to the spinal cord, and the motor neuron tells the muscles of the arm to pull the hand away, and the hand is moving before the brain has felt the pain. The simplest reflex, the knee-jerk, uses only two neurons, a sensory neuron and a motor neuron, with a single synapse between them, and it takes only a few tens of milliseconds. Reflexes are built into the nervous system before birth, because an animal that had to learn to breathe or to swallow would not live long enough to learn.

6. What is the difference between the central and peripheral nervous systems?

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The central nervous system, the CNS, is the brain and the spinal cord, and it is the part that decides. The peripheral nervous system, the PNS, is all the nerves that run out from the centre to the body, and it is the part that senses and acts. The peripheral system has two divisions: the somatic system, which is voluntary, carrying sensory information from the skin and muscles to the centre and motor commands from the centre to the skeletal muscles, and the autonomic system, which is involuntary, controlling the smooth muscle, the heart and the glands without conscious thought. The autonomic system has two branches that oppose each other, the sympathetic, which prepares the body for action, and the parasympathetic, which restores it to rest. The central system is protected inside the skull and the vertebral column, because it cannot repair itself, while the peripheral nerves can regrow after damage.

Frequently Asked Questions

What is a nervous system?

A nervous system is the wiring that lets an animal sense its surroundings, decide what to do, and act on the decision. It is built from neurons, cells that carry electrical signals, and synapses, the junctions where one neuron passes its message to the next. It has three jobs: to sense, through receptors that detect light, touch, chemicals, temperature and movement; to integrate, by comparing the information with memory and deciding what to do; and to act, by sending commands to muscles and glands. The nervous system is fast, carrying a signal in milliseconds, and directed, sending it exactly where it is needed, which is what an animal needs when it must escape a predator, catch prey, or keep its balance.

Do all animals have a nervous system?

No, not all animals have a nervous system. Sponges have no neurons at all: their cells react to touch and to chemicals individually, and they have no wiring to coordinate them, which is why a sponge cannot move away from danger or hunt. Placozoans, tiny flat animals that creep across the sea floor, also have no neurons. Every other animal group has neurons, and the first nervous systems, the nerve nets of jellyfish and their relatives, appeared more than six hundred million years ago, before brains, before backbones, and before most of the animals we know existed. The nervous system is one of the oldest systems in the animal kingdom.

How fast do nerve signals travel?

The speed depends on the axon. In a thin, unmyelinated axon, the action potential creeps along at about one metre per second. In the squid's giant axon, which is up to a millimetre across, it travels at about twenty-five metres per second. In a large myelinated fibre in a mammal, the signal jumps between the gaps in the myelin, the nodes of Ranvier, in saltatory conduction, and it can travel at up to one hundred and twenty metres per second, fast enough to cross the body in a fraction of a second. Speed matters most where escape is at stake: the giant axons of earthworms, crayfish and squid are all built for speed, because an animal that hesitates is an animal that is eaten.

Which animal has the largest brain relative to its body?

Among vertebrates, the animals with the largest brains relative to body size are the small primates and the dolphins, and the human brain, at about one thousand three hundred to one thousand four hundred grams, is the largest of any land animal relative to its body, with about eighty-six billion neurons. Among invertebrates, the octopus has the largest brain, with about five hundred million neurons, more than a dog, and two-thirds of them in its arms. Relative brain size is not the whole story, however: a honeybee has only about a million neurons, and it can learn a route, remember it, and communicate it by dancing, which shows that intelligence depends on how neurons are wired, not only on how many there are.

Can an octopus arm think for itself?

In a real sense, yes. An octopus has about five hundred million neurons, and only about a third of them are in the central brain: the other two-thirds are in the arms, in a nerve cord that runs down each arm, with ganglia at the base of each sucker. Each arm can taste, reach, grasp and explore on its own, and an arm that is cut off can still reach for food and avoid a painful stimulus for an hour or more, because the wiring is in the arm itself. The central brain gives the big commands, to go, to hide, to hunt, and the arms work out the details. No other animal divides its nervous system this way, and it is the reason an octopus can do eight things at once, with each arm working independently.

Do insects feel pain?

The honest answer is that scientists do not yet know for certain. Insects certainly detect damage: they have nociceptors, receptors that sense harmful stimuli, and they withdraw from heat, from crushing, and from chemicals that damage their tissues, and the withdrawal is driven by reflex circuits. What is not settled is whether they experience pain as a feeling, in the way a mammal does, because pain, as a feeling, needs a brain that can integrate the signal with memory and emotion, and an insect's brain is small. Recent studies have found that insects show some behaviours that look like a pain state, such as guarding an injured limb and preferring a shelter after injury, but the evidence is not yet conclusive, and the question remains open. What is certain is that insects sense damage and react to it, and that the reaction is fast, because a slow reaction to damage is a fatal one.

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MCQs

1. What is the basic unit of the nervous system?

• A. The nephron
• B. The neuron
• C. The alveolus
• D. The osteocyte

Answer: B

Explanation: The neuron is the cell built to carry an electrical signal, with dendrites that receive signals, a cell body that adds them up, and an axon that sends the signal on, and it is the unit from which every nervous system, from a jellyfish's net to a human brain, is built.

2. What carries the signal along an axon?

• A. An action potential
• B. A hormone travelling in the blood
• C. The flow of blood through the axon
• D. A muscle contraction

Answer: A

Explanation: An action potential is the electrical pulse that travels along an axon, a wave of depolarisation followed by repolarisation that takes about one or two milliseconds, and it is all or none: it either fires fully or it does not fire at all.

3. Which animal has a diffuse nerve net and no brain?

• A. An earthworm
• B. An octopus
• C. A Hydra
• D. A frog

Answer: C

Explanation: Hydra, a cnidarian, has a diffuse nerve net spread through its body wall, with no brain, no cord and no centre, and a signal can travel in any direction through the mesh, so the whole animal contracts at once when it is touched.

4. What is the function of myelin?

• A. It produces neurotransmitters at the synapse
• B. It stores oxygen for the neuron
• C. It protects the brain from infection
• D. It insulates the axon and speeds conduction by saltatory conduction

Answer: D

Explanation: Myelin is a fatty sheath wrapped around axons by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system, and it lets the action potential jump between the nodes of Ranvier, so that a large myelinated fibre can carry a signal at up to one hundred and twenty metres per second.

5. In an octopus, most neurons are found...

• A. In the central brain only
• B. In the arms
• C. In the mantle
• D. In the eyes

Answer: B

Explanation: An octopus has about five hundred million neurons, and only about a third of them are in the central brain: the other two-thirds are in the arms, in a nerve cord that runs down each arm, so that each arm can taste, reach and grasp on its own.

6. What is the knee-jerk reflex?

• A. A monosynaptic reflex with only a sensory neuron and a motor neuron
• B. A reflex controlled by the cerebrum
• C. A reflex that must be learned
• D. A reflex with no receptor

Answer: A

Explanation: The knee-jerk reflex uses only two neurons, a sensory neuron that senses the stretch of the muscle and a motor neuron that tells the same muscle to contract, with a single synapse between them, and it takes only a few tens of milliseconds, which is why the leg kicks before you can stop it.

7. Which part of the vertebrate brain controls balance and coordination?

• A. The medulla oblongata
• B. The thalamus
• C. The cerebellum
• D. The olfactory bulb

Answer: C

Explanation: The cerebellum, the little brain at the back, receives information from the eyes, the ears, the muscles and the joints, and it compares what the body is doing with what the brain intended, and it corrects the difference, smoothly and automatically, and it holds about half of all the neurons in the human brain.

8. In which direction does the nerve cord run in an earthworm?

• A. Dorsal, along the back
• B. Lateral, along the side
• C. Inside the gut
• D. Ventral, along the belly

Answer: D

Explanation: An earthworm has a ventral nerve cord running along the belly, with a ganglion in each segment, and a dorsal cerebral ganglion above the gut, joined by a ring of nerve tissue around the gut, the circumesophageal connectives, which is the ladder plan of the annelids.

9. What is cephalization?

• A. The growth of the tail end of the body
• B. The concentration of sense organs and nerve cells at the head end
• C. The loss of the brain in simple animals
• D. The division of the body into segments

Answer: B

Explanation: Cephalization is the concentration of nerve cells and sense organs at the front end of the body, and it follows movement: an animal that moves forward meets the world head first, so it pays to put the sensors and the wiring that interprets them at the head, and the trend runs from the flatworm's small brain to the human's large one.

10. How many neurons does the roundworm C. elegans have?

• A. Exactly 302
• B. About a million
• C. About 86 billion
• D. None, it has no neurons

Answer: A

Explanation: The hermaphrodite of Caenorhabditis elegans, a roundworm one millimetre long, has exactly three hundred and two neurons, and every connection between them has been traced in the connectome, the only complete wiring diagram of any animal's nervous system.