Coordination is not carried by the wire. It happens in the middle of the wire, in the integrating center, and that is the piece the earlier version of this post left out.
The original ran to a single block of about seventy words. It named three parts: receptors, neurons, and effectors.
It said receptors receive stimuli from the environment, that transduction turns those stimuli into electrical impulses, and that neurons sit between the two ends.
That is a useful skeleton and it is worth keeping. What it never does is say how the three connect, and the answer to that is the whole subject. A receptor does not send anything to an effector. A receptor sends a signal to the central nervous system, the central nervous system decides something, and a different neuron carries the decision outward. The joining is the coordination.
The system has two halves, not three loose parts
The nervous system divides into a central nervous system and a peripheral nervous system.
The central nervous system is the brain and the spinal cord. It is where signals are compared against everything else the body knows, and it is the only place a response is decided.
The peripheral nervous system is everything outside that. It is mostly bundles of axons called nerves, and it divides twice.
The first division is somatic against autonomic. The somatic nervous system drives skeletal muscle. The autonomic nervous system runs the internal business of the body: heart, gut, glands, and blood vessels.
The autonomic part then splits again, into sympathetic, parasympathetic, and enteric systems.
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| The division that matters. The original three-part list has no place to put the central nervous system, which is where coordination actually happens. Laura Guerin for the CK-12 Foundation, CC BY-SA 3.0, via Wikimedia Commons |
The plumbing is regular in a way worth knowing. Twelve pairs of cranial nerves leave the brain. Thirty-one pairs of spinal nerves leave the spinal cord, one pair per segment, and the cord itself is about 45 centimeters long and roughly a centimeter and a half across.
Nerves leaving the spinal cord and entering it are given names by direction: sensory, or afferent, for signals coming in; motor, or efferent, for signals going out. The two run together in the same nerve, which is why a cut nerve usually takes both with it.
The reflex arc is the shortest example of coordination
A reflex arc is the pathway a signal follows to produce a reflex: a response that begins before the brain has finished working out what happened. It is the cleanest demonstration of the system because every link has to be there and nothing can be skipped.
The five steps are:
- A receptor detects the stimulus.
- A sensory, afferent neuron carries the signal to the spinal cord or the brainstem.
- An integrating center, where the neurons of the spinal cord or brainstem gray matter synapse with each other.
- A motor, efferent neuron carries the signal out from the front of the cord.
- An effector -- usually a muscle, sometimes a gland -- carries out the response.
Note what step three is. It is a synapse, not a wire. Something has to be compared with something else for the system to have made a decision rather than passed a message along.
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| The knee jerk, drawn in full. The tap stretches a muscle spindle inside the quadriceps, the sensory neuron runs into the spinal cord without going via the brain, and the motor neuron runs straight back out. Amiya Sarkar, CC BY-SA 4.0, via Wikimedia Commons |
The knee jerk, in detail
Tapping the tendon just below the knee stretches a muscle spindle buried in the quadriceps. The spindle fires. Its signal runs into the L3 and L4 roots of the spinal cord and synapses directly onto the motor neuron for the same muscle. The quadriceps contracts and the lower leg kicks forward.
The brain is not in that loop. It is told what happened afterwards, if it is told at all. This is the reason the reflex is fast, and it is the reason a reflex still works when the cord is injured above the level of the response.
There is a second part to it that the diagram usually leaves out. The same sensory input activates local interneurons that release glycine onto the motor neurons of the opposing muscle, the hamstring, switching them off. Relaxing the muscle that would oppose the movement is half of what makes the knee jerk clean.
This makes the knee jerk monosynaptic: one sensory neuron, one motor neuron, and a single chemical synapse. It is close to the rarest kind of pathway there is.
Everything else you reflexively do -- flinching, pulling your hand off a hot surface, blinking -- goes through at least one interneuron first. Those reflexes are polysynaptic, which is why they are slower.
Three kinds of neuron, and the cells that are not neurons
The original put "neurons" in the middle as though it were one thing. Neurons are divided into three classes by function, and the distinctions are what make an arc possible.
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| A sensory neuron, with the cell body off to one side rather than at the end of the wire. That offset body is the anatomical signature of the afferent neuron, and it is why the cell body can sit outside the central nervous system entirely, in a dorsal root ganglion. explorebiology, CC BY 4.0, via Wikimedia Commons |
Sensory, or afferent, neurons respond to stimuli such as touch, sound, or light, and send signals to the spinal cord and on to the brain. The cell body sits off to one side of the fiber rather than at the end of it.
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| An interneuron: no receptor at one end, nothing to respond to directly. Its work is arithmetic on other neurons' signals, which is why most of the spinal cord is made of them. explorebiology, CC BY 4.0, via Wikimedia Commons |
Interneurons connect neurons to other neurons within the same region of the brain or spinal cord. They have no receptor at one end and no muscle at the other.
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| A motor neuron, with the cell body in the spinal cord and an axon running out to muscle. The longest of these reaches from the base of the spine to the toes, over a metre. explorebiology, CC BY 4.0, via Wikimedia Commons |
Motor neurons receive signals from the brain and spinal cord and control muscle contraction and glandular output. Their cell bodies are inside the central nervous system, at one end of a fiber that runs out to the muscle.
The longest motor axons are over a metre long in an adult human, running from the base of the spine to the toes. Sensory axons can be longer, running from the toes to the back of the spinal cord, over a metre and a half.
| Neuron type | Carries signals | Cell body sits |
| Sensory (afferent) | From a receptor to the CNS | Off to one side of the fiber |
| Interneuron | Between neurons, inside the CNS | Among its dendrites |
| Motor (efferent) | From the CNS to a muscle or gland | At the start of the fiber |
The three functional classes of neuron, by what they connect and where the cell body sits.
And the cells that are not neurons at all. The original described the nervous system as made of "highly specialized cells" and then named neurons, which reads as though neurons are all of it. They are not.
Glial cells are the other half of nervous tissue. They supply nutrients to neurons, hold them in place, and form myelin.
They also destroy pathogens, remove dead cells, and provide guidance cues that direct new axons to the targets they need to reach.
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| Glia, which are not decoration around the real thing. They make the myelin, supply the fuel, and clear up the damage. For half a century they were believed to outnumber neurons ten to one; the modern estimate is about one to one. explorebiology, CC BY 4.0, via Wikimedia Commons |
That correction is recent and worth stating plainly, because the older figure is still in most textbooks. The ten-to-one ratio was settled in 1949 and held for sixty years. Modern counting methods put glia at roughly the same number as neurons in the whole brain, with the proportion varying by region.
Receptors transduce, which is not the same as firing
The original said receptors "convert stimuli into the form of electrical impulses, a process called transduction". Half of that is right and half of it collapses two different things.
Transduction is the receptor's own work: turning light, pressure, stretch, temperature, or a chemical into a change in the voltage across its membrane. That change is a graded receptor potential. It is local, it is graded in size, and a weak stimulus produces a small one.
The electrical impulse that travels down an axon is a different event. It is an action potential, it is all-or-nothing, and it only starts if the graded potential reaches threshold. A stimulus too weak to cross threshold produces no impulse at all, and the receptor simply reports nothing.
So the nervous system is not a device that turns stimuli into impulses. It is a device that weighs stimuli, and only calls out when the weighing crosses a line.
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| Six endings in skin, each built to notice a different thing. None of them is a general-purpose stimulus receiver: what a receptor responds to is decided by its shape and by which ion channels sit in its membrane. BruceBlaus, CC BY 3.0, via Wikimedia Commons |
Not all receptors are watching the outside world
The original says receptors receive stimuli from the environment. Many of the most important ones do not.
The muscle spindle in the knee jerk above reads the length of a muscle. The vestibular apparatus in the inner ear reads head position and acceleration. The carotid body reads the oxygen content of blood as it passes. Baroreceptors read blood pressure. None of these is environmental information.
This is a substantive correction rather than a wording one. A nervous system that only watched the outside world could not hold itself upright, keep breathing, or know that its own blood was starved of oxygen. Between them, internal receptors supply most of the continuous background correction that keeps a body running.
How the signal actually travels
The membrane and the action potential
Everything so far has described a route. What makes it work in milliseconds is a membrane holding a voltage.
Nearly all animal cell membranes keep the inside negative relative to the outside. In a typical animal cell this is about -70 millivolts, and in a neuron at rest it is the same figure. That gradient is held up by ion pumps, and it is stored energy waiting to be spent.
A stimulus that reaches the axon hillock raises that voltage. The hillock is the point where the axon leaves the cell body.
The hillock carries the highest density of voltage-gated sodium channels anywhere in the cell, which is what makes it the easiest place to excite. If the voltage there crosses threshold, near -55 millivolts, those channels open and sodium flows in. The interior goes positive, the polarity reverses, and the reversal is what "depolarization" names.
Then potassium channels open and potassium flows back out, and the gradient is restored. On the way the voltage overshoots below resting level.
The whole up-and-down event in a mammalian neuron lasts a few thousandths of a second and peaks around +40 millivolts. It is the same size whether the stimulus was gentle or violent.
That last point is the one that matters for coordination. The action potential is all-or-nothing, so it cannot be half as big for a weak stimulus.
The size of a stimulus is carried instead by how many spikes arrive and how fast. That is why nervous signaling is described as a frequency code.
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| The gap between two cells, and the apparatus on either side of it. By the time a signal crosses, the electrical form has become a chemical one, which is what lets a synapse amplify, inhibit, and delay as well as pass on. Looie496 and Il Clooth, public domain, via Wikimedia Commons |
Myelin, and why speed matters more than size
Oligodendrocytes in the central nervous system and Schwann cells in the peripheral system wrap axons in layers of a fatty substance called myelin. It insulates the axon electrically and lets the signal jump from gap to gap instead of travelling continuously.
The effect is large. Unmyelinated axons conduct at roughly 0.5 to 10 metres per second. Myelinated ones reach up to about 150 metres per second. That is the difference between a signal you feel eventually and one you feel at once.
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| Axon diameter and myelination against conduction velocity. Myelin is worth more than diameter, which is why the fastest fibers are both wide and wrapped. WillowW, CC BY 3.0, via Wikimedia Commons |
The jumps happen at the nodes of Ranvier, the short uninsulated gaps between myelin segments, so the impulse appears to leap rather than run. This is saltatory conduction.
The synapse is where the arithmetic happens
Signals do not run through neurons. They cross from one to the next at a synapse, and that crossing is a chemical step, not a wire.
When an action potential reaches an axon terminal, it opens voltage-gated calcium channels. Calcium entry triggers vesicles to release neurotransmitter into a gap of 20 to 40 nanometers. The transmitter diffuses across and binds receptors on the next cell.
This costs time. A chemical synapse introduces a synaptic delay of roughly half a millisecond to one millisecond between the action potential arriving at one side and the response on the other. A pathway with several synapses adds that delay several times over.
The delay is not a cost to be minimised. It is the reason a nervous system can do arithmetic at all.
The same transmitter can be released in different amounts and at different rates, and it can land on cells that respond differently. So a synapse can add to a signal, subtract from it, or hold it below threshold.
Not every synapse is chemical. Electrical synapses connect cells directly, with no gap to cross and no delay. They are the exception in most of the nervous system and the rule in some, and they are why not every reflex is described this way.
Inhibition, which the original never mentions
The knee jerk above switches off the hamstring as well as switching on the quadriceps. That is inhibition, and it is not a detail.
Sensory input arriving at the spinal cord simultaneously excites the muscle that caused the stimulus and, through interneurons, inhibits the muscle that would oppose the response. Without the inhibition the two muscles would fight each other and the movement would be a twitch rather than a clean jerk.
So coordination is not mostly about turning things on. A large part of it is about arranging that several things happen in the right order, and that some of them do not happen at all.
The numbers worth remembering
| Figure | What it is |
| 86 billion | neurons in the human brain, from 2009 counting work |
| 61 to 99 billion | the honest range around that figure |
| about 1 to 1 | glia to neurons, not the older 10 to 1 |
| 100 trillion | synaptic connections, widely estimated |
| 850,000 km | combined length of axons and dendrites in one brain |
| over 1 metre | length of the longest motor axon, spine to toes |
| 0.5 to 10 m/s | conduction speed of an unmyelinated axon |
| up to 150 m/s | conduction speed of a myelinated axon |
| -70 mV | resting membrane potential in a typical animal cell |
| about -55 mV | threshold that has to be crossed to fire |
| +40 mV | peak of the action potential |
| 0.5 to 1 ms | delay added by each chemical synapse |
| 20% | of the body's energy used by the brain |
| 31 pairs | spinal nerves, one pair per segment |
| 12 pairs | cranial nerves |
Nervous system figures, from the sources listed at the foot of this article.
The energy figure is the one that reframes the whole organ. The brain is about 2 percent of body weight. It uses close to a fifth of the body's energy budget.
Most of that goes on maintaining the voltage gradients that let a spike happen at all.
Where the textbook picture stops holding
The reflex arc is a clean chain, and it is the right place to start. Four things it leaves out are worth knowing.
First, most reflexes are polysynaptic. Only the most simple ones use a single synapse. Everything else has at least one interneuron in the middle, which is both why it is slower and why it can be adjusted or inhibited.
Second, autonomic reflexes do not all run through the cord. Some are mediated more by the brain, and the enteric system, which controls the gut, is a network of some 100 million neurons largely independent of both.
Third, there is feedback. An effector is not only a destination. The result of the response is sensed and fed back in, so the response can be corrected while it is happening rather than only before it.
Fourth, some of the wiring is not neural at all. Neuroglia do not only support neurons, and the astrocyte's calcium signaling has turned out to have a role in synaptic transmission. The boundary between "neuron signaling" and everything else is blurrier than the three-part list implies.
And it is not always the brain
Nervous tissue is not unique to animals with brains, and it is not unique to animals at all. Sponges, placozoans, and mesozoans have no nervous system, while jellyfish and hydra manage with a diffuse nerve net and no central cord whatsoever.
Nervous systems arose roughly 550 to 600 million years ago in wormlike organisms. The structural plan common to almost all animals with one -- a brain, a central cord, and nerves radiating out -- dates from the Ediacaran period.
Invertebrate reflex circuits are the experimental model most of this field was built on, and they are often far simpler than a mammalian one. The squid giant axon, several centimeters long and visible to the naked eye, is the preparation on which much of what is known about how axons conduct was measured directly.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means |
| Receptor | A cell or ending that turns a stimulus into a change in membrane voltage. Works by transduction. |
| Transduction | The conversion of a stimulus into an electrical signal. Done by the receptor. |
| Graded potential | A local change in membrane voltage whose size depends on the stimulus. Not an impulse. |
| Action potential | An all-or-nothing spike that travels along an axon once threshold is crossed. |
| Threshold | The membrane voltage that has to be reached before an action potential starts. About -55 mV. |
| Sensory neuron | Carries signals from a receptor to the central nervous system. Also called afferent. |
| Motor neuron | Carries signals from the central nervous system to a muscle or gland. Also called efferent. |
| Interneuron | A neuron that connects to other neurons only. Where most of the processing happens. |
| Integrating center | Where signals are compared and a response decided. The spinal cord or brainstem in a reflex. |
| Effector | The organ that carries out the response. Usually a muscle or a gland, not a single cell. |
| Reflex arc | The pathway a signal follows to produce a reflex. |
| Monosynaptic | A reflex with one sensory and one motor neuron and a single synapse. The knee jerk. |
| Synapse | The junction where one cell signals the next. |
| Synaptic delay | The half-millisecond or so a chemical synapse adds to the journey. |
| Myelin | The fatty insulation wrapped around axons, made by oligodendrocytes and Schwann cells. |
| Saltatory conduction | Signal jumping between the uninsulated nodes of Ranvier, rather than running continuously. |
| Glia | The non-neuronal cells of nervous tissue: roughly as many as neurons, and not merely support. |
| Synaptic inhibition | A synapse that reduces a signal instead of raising it. Half of what makes a movement clean. |
Terms used above, in roughly the order they first appear.
Key Takeaways
- Coordination happens in the integrating center, not in the wire between receptor and effector. That center is the part the original version of this post left out.
- The route is receptor, sensory neuron, integrating center, motor neuron, effector, and every step is done by a different cell type.
- Transduction is the receptor converting a stimulus into a graded potential. It is not the same as an action potential, which needs threshold to be crossed first.
- Many receptors report on the body, not the environment. Muscle spindles, the vestibular apparatus, and baroreceptors are among them.
- Effectors are organs, usually muscles and glands, rather than individual cells.
- The action potential is all-or-nothing. Stimulus strength is carried by frequency, not by spike size.
- Myelin is worth more than diameter: about 0.5 to 10 m/s unmyelinated against up to 150 m/s myelinated.
- Each chemical synapse adds about half a millisecond, and that delay is what makes summation and inhibition possible.
- The brain holds about 86 billion neurons and roughly as many glia, and uses about a fifth of the body's energy.
Frequently Asked Questions
If every response went through the brain, why is a reflex faster?
Because it does not. In a spinal reflex the signal reaches the cord and turns around there.
Routing it up to the brain and waiting for a decision would add the whole length of that journey, and the delay of the synapses inside it.
The brain is informed in parallel, and often afterward. That is why you know you touched something a moment after you already moved.
Why is the knee jerk tested, if it does not involve the brain?
Because the cord is a real piece of anatomy with real pathways in it, and the response depends on them. An abnormal or absent knee jerk points at a specific spinal level and often at a specific nerve root.
It also depends on the spinal cord being intact at that level. That is why an improper patellar reflex is treated as a sign of a problem with the central nervous system, not just with the leg.
Are receptors always on the outside of the body?
No, and this is the error in the original version of this post that mattered most. Receptors inside the body read muscle length, joint position, blood pressure, blood oxygen, blood glucose, and the distension of internal organs.
Between them these supply most of the continuous background correction that keeps a body upright and running. Without them you would not notice bleeding until you collapsed.
What is an effector, exactly?
An effector is the structure that carries out the response, and it is nearly always an organ rather than a cell.
A muscle is an effector, made of many muscle cells. A gland is an effector, made of many secretory cells. A single motor neuron innervates one such organ, and the individual cells inside it are not the effector.
Why does a weak stimulus not produce a weak impulse?
Because the impulse is not a measurement of the stimulus. It is a signal that the threshold was crossed.
Above threshold, the spike has the same size every time. The nervous system records how strong something was by how many impulses it sent and how close together they were.
Why does an impulse only travel in one direction?
Because the refractory period follows it. For a short time after an action potential, the membrane is either unresponsive or inactivated, so the region just travelled through cannot be excited again.
The impulse therefore cannot reverse. It runs from the axon hillock to the axon terminal and stops there, at a synapse.
Is it really true that glia are as many as neurons?
Yes, on present evidence, and the change of mind is recent. The figure that glia outnumber neurons ten to one was published in 1949 and went unchallenged for sixty years.
Modern counting methods put the ratio at close to one to one across the whole brain, with the proportion varying by region. Older textbooks still carry the ten-to-one figure.
Key references
• Reflex arc
• The nervous system, divided
• Neuron: structure and the three functional classes
• Action potential
• Spinal cord anatomy and the 31 nerve pairs
• Neuroanatomy of the spinal cord
• Myelination and conduction velocity
• Synaptic transmission and synaptic delay
• How many neurons are in the human brain?
• Neurons and glia in the human brain, a review
Images: Wikimedia Commons, with the author and license named in each caption. Figures follow the sources above.
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