Types of Neurons: How Neurons Are Classified and What They Do

Ask for a "neuron" and you get one word for several kinds of cell. Some sit in your skin and report pressure. Some run from the spinal cord to your toes. Some do nothing but pass a signal sideways to a neighbor. Knowing which is which is the difference between memorizing a diagram and understanding how a body works.

This guide sorts neurons two ways. The first is by the job they do. The second is by their shape. It then follows one signal from the moment it enters the body to the moment a muscle contracts.

Classification by Job: What the Neuron Does

Neurons are usually grouped into three types by function. The names describe the direction the information travels. That is why the same pair of words keeps appearing: afferent means arriving, and efferent means departing. The pattern is clearest in the spinal pathways. Cranial and autonomic pathways have their own arrangements.

Sensory Neurons: The Afferent Collectors

Sensory neurons, or afferent neurons, convert a stimulus into an electrical signal. That conversion is called sensory transduction. In the spinal pathways, their cell bodies do not sit in the spinal cord. They sit in swellings along the spinal nerves called the dorsal root ganglia. These are clusters of first-order neurons, gathered just outside the cord. From there, the afferent fibers carry information inward to the brain.

Sensory neurons transduce a stimulus into an electrical signal and carry it inward. Image: explorebiology, CC BY 4.0, via Wikimedia Commons

Sensory neurons are not one uniform group. They are specialized by the stimulus they detect. They are also grouped by where that stimulus comes from. Exteroceptors report from outside the body, and interoceptors report from inside it. The nociceptors are the clearest example. They respond to damaging or potentially damaging stimuli. They send "possible threat" signals to the spinal cord and brain. The brain then creates the sensation of pain. That draws attention to the body part, so the threat can be dealt with. This neural processing is called nociception. It is not the same as the experience of pain, which the brain produces.

Mechanoreceptors respond to mechanical pressure or distortion and turn it into an electrical signal. Thermoreceptors code changes in temperature. They are not a single cell type. Warm-sensitive fibers are commonly unmyelinated C fibers. Cold-sensitive fibers include C fibers and thinly myelinated A delta fibers. The A delta fibers conduct faster than the C fibers do. So cold and warmth are carried by a mix of fibers, not by two neatly separate systems. Some cold-sensitive fibers also respond to heat. That odd response is known as paradoxical heat.

Different receptors sit at different depths and answer different questions about the same touch. Image: Blausen.com staff, CC BY 3.0, via Wikimedia Commons

Motor Neurons: The Efferent Executors

Motor neurons, or efferent neurons, carry commands out to muscles and glands. Their cell bodies sit inside the central nervous system. They are found in the motor cortex, the brainstem and the spinal cord. Their axons project out to the effectors. This is the opposite of sensory neurons, whose cell bodies cluster in ganglia outside the cord.

Motor neurons carry commands outward from the central nervous system to the muscles. Image: explorebiology, CC BY 4.0, via Wikimedia Commons

Motor neurons divide into upper and lower. Upper motor neurons send axons down to the spinal cord. There they synapse mainly onto interneurons. In some pathways they also reach lower motor neurons directly. Lower motor neurons are the ones whose axons leave the cord and carry the signal to the muscle. The somatic lower motor neurons that drive skeletal muscle come as alpha, beta and gamma types. Beta motor neurons are the least well understood of the three.

The last step is a neuromuscular junction. This is a chemical synapse between a motor neuron and a muscle fiber. The sequence is ordered. The action potential arrives and calcium channels open. Calcium enters the terminal. It binds sensor proteins called synaptotagmins on the vesicles. That triggers the vesicles to fuse with the membrane and release their contents. In vertebrates the transmitter is acetylcholine. It crosses the gap and binds nicotinic acetylcholine receptors on the muscle fiber. Muscle needs this innervation. When a muscle loses it, its fibers degenerate and waste away.

Where a motor neuron hands acetylcholine to a muscle fiber. Image: Doctor Jana, CC BY 4.0, via Wikimedia Commons

One motor neuron and every muscle fiber it supplies form a motor unit. The concept was defined by Edward Liddell and Charles Sherrington in 1925. Groups of motor units work together as a motor pool to drive one muscle. The force of a contraction is set by how many units are recruited. Unit size varies enormously, from about five fibers to more than a thousand. Muscles that need fine control tend to have many small units. That lets them grade their force far more precisely.

Interneurons: The Internal Processors

Interneurons are everything that is neither sensory nor motor. They are the central nodes of neural circuits. A common description divides them into two kinds. Local interneurons have short axons and analyze small pieces of information in one neighborhood. Relay interneurons have long axons and connect circuits in one region to those in another. That split is a useful teaching shorthand rather than a strict system, because interneurons are very diverse.

One common assumption needs correcting. Interneurons are usually described as living only inside the central nervous system. That is where the great majority sit. But the definition is functional, not geographic. A cell that passes a signal from one cell to the next is an interneuron by that logic, wherever it sits. The enteric nervous system, which runs through the gut, keeps its own local reflex circuits and its own interneurons. So "only in the central nervous system" is too absolute.

Gray matter holds cell bodies and synapses; white matter is mostly myelinated axons. Image: Nephron, CC BY-SA 3.0, via Wikimedia Commons

The gray matter of the central nervous system is where most of this processing happens. It holds neuronal cell bodies, dendrites, glial cells, capillaries and synapses, along with unmyelinated axons. White matter sits next to it. It contains most of the axons in the central nervous system, many of them myelinated, and relatively few cell bodies. That is why the two look different. In living tissue, gray matter is not really gray at all. It looks more pink than gray, because of the blood running through it.

Classification by Shape: How the Neuron Is Built

The functional split describes what a neuron does. The structural split describes what it looks like. The two schemes overlap rather than compete. A motor neuron is multipolar. A sensory neuron from the skin is pseudounipolar.

A multipolar neuron: one axon and many dendrites, so a great deal of information can be integrated. Image: explorebiology, CC BY 4.0, via Wikimedia Commons

Multipolar Neurons: Many Inputs, One Output

A multipolar neuron has a single axon and many dendrites. That makes it well suited to integrating large amounts of information from other neurons. Multipolar neurons are most neurons in the central nervous system. They include motor neurons and most interneurons. They are especially common in the cortex and the spinal cord. In the periphery they also turn up in the autonomic ganglia.

Bipolar Neurons: One In, One Out

A bipolar neuron has two processes on opposite sides of the cell body. These are usually described as one axon and one dendrite. These cells are mainly sensory. Examples include the olfactory receptor neurons of the olfactory epithelium, whose axons form the olfactory nerve. The spiral ganglion and the vestibular ganglion also contain bipolar neurons. In the retina the cells called bipolar cells are a different thing: they are retinal interneurons, not primary sensory neurons.

Olfactory receptor neurons are bipolar: one process picks up odor molecules, the other carries the signal to the bulb. Image: OpenStax, CC BY 4.0, via Wikimedia Commons

Olfactory receptor neurons have one more unusual property. Unlike most neurons, they are continually replaced through life. New ones grow from stem cells in the olfactory epithelium. The rate varies with age and circumstances. Even so, this is one of the few places in the body where worn-out neurons are routinely replaced.

Pseudounipolar Neurons: A Split Axon

A pseudounipolar neuron has only one extension from its cell body, because its axon has split into two branches. These cells start life as bipolar in shape and become pseudounipolar as they mature. That is where the name comes from. The "pseudo" marks the true bipolar arrangement as an earlier stage, not a different cell.

The single process splits, giving one branch to the receptor and one to the spinal cord. Image: explorebiology, CC BY 4.0, via Wikimedia Commons

This shape is a neat solution to a routing problem. A sensory neuron has to report from the skin back to the spinal cord. It does not want its cell body sitting in the middle of the nerve, where it would be exposed. Parking the body in a dorsal root ganglion solves that. One process runs out to the receptor, and one runs back to the cord.

Shape	Structure	Where you find it	Main role
Multipolar	Many dendrites, one axon	Cortex, spinal cord, autonomic ganglia	Integrating large amounts of input; most CNS neurons
Bipolar	Two processes on opposite sides	Olfactory epithelium, spiral and vestibular ganglia	Specialized sensing for smell, hearing and balance
Pseudounipolar	One process that splits in two	Dorsal root ganglia	Carries sensory information from the body to the cord
Unipolar	A true single projection	Discussed historically; most real examples are pseudounipolar	The simplest theoretical arrangement

Following One Signal From Touch to Movement

The classification makes more sense once one signal is traced end to end. Touch a hot surface, and here is which cell type does what:

• A sensory neuron in the skin detects the stimulus and transduces it into an electrical signal.
• That neuron is pseudounipolar, with its cell body parked in a dorsal root ganglion outside the cord.
• The afferent fiber carries the signal into the spinal cord.
• An interneuron receives it and relays it. This processing step can be skipped in a fast reflex.
• A motor neuron, a multipolar cell, carries the command out, usually to several muscle fibers at once as a motor unit.
• At the neuromuscular junction, acetylcholine is released and the muscle contracts, pulling the hand away.
• The signal also travels up to the brain, which is where the pain is actually felt, slightly after the withdrawal.

Note the division of labor. The withdrawal is fast because a short spinal circuit handled it. The pain comes later because it had to wait for the brain. These are not two separate reflexes. They are the same signal delivered to two places, and the circuit is built to serve the urgent one first.

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 here
Afferent	Carrying information toward the central nervous system.
Efferent	Carrying information away from the central nervous system.
Sensory transduction	Turning a physical stimulus into an electrical signal.
Dorsal root ganglion	A cluster of sensory neuron cell bodies just outside the spinal cord.
Exteroceptor	A sensory cell that reports a stimulus from outside the body.
Interoceptor	A sensory cell that reports a stimulus from inside the body.
Nociceptor	A receptor that responds to damaging or potentially damaging stimuli.
Nociception	The neural processing of a harmful stimulus, before it becomes pain.
Mechanoreceptor	A receptor that responds to pressure or distortion.
Thermoreceptor	A receptor that detects a change in temperature.
C fiber	A thin, unmyelinated nerve fiber that conducts slowly.
A delta fiber	A thinly myelinated fiber that conducts faster than a C fiber.
Upper motor neuron	A motor neuron whose axon stays inside the central nervous system.
Lower motor neuron	A motor neuron whose axon leaves the cord to reach a muscle.
Neuromuscular junction	The synapse where a motor neuron signals a muscle fiber.
Motor unit	One motor neuron and all the muscle fibers it supplies.
Motor pool	The group of motor units that drive a single muscle.
Interneuron	A neuron that passes a signal from one cell to the next.
Gray matter	Central nervous tissue rich in cell bodies, dendrites and synapses.
White matter	Central nervous tissue made mostly of long, often myelinated axons.
Multipolar neuron	A neuron with one axon and many dendrites.
Bipolar neuron	A neuron with two processes on opposite sides of the cell body.
Pseudounipolar neuron	A sensory neuron whose single process splits in two.

The words in this article that would otherwise send you to a dictionary.

Key Takeaways

• By function, neurons are sensory (afferent), motor (efferent) or interneurons, named for the direction information travels.
• In the spinal pathways, sensory neuron cell bodies sit in dorsal root ganglia outside the cord, while motor neuron cell bodies lie inside the central nervous system.
• Motor neurons split into upper neurons that stay in the central nervous system and lower neurons whose axons reach the muscle, ending at a neuromuscular junction that releases acetylcholine.
• A motor unit is one motor neuron plus the fibers it supplies. It was defined by Liddell and Sherrington in 1925, and its size ranges from about five fibers to more than a thousand.
• Interneurons are the central nodes of circuits. They are mostly, but not entirely, inside the central nervous system; the gut has its own local circuits.
• By shape, neurons are multipolar, bipolar or pseudounipolar. These categories overlap with the functional ones rather than replacing them.
• One signal can take a fast spinal route for reflex withdrawal and a slower route to the brain for the pain that follows.

Frequently Asked Questions

Are multipolar and motor neurons the same thing?

No. They answer different questions. Multipolar and bipolar describe shape, meaning how many processes leave the cell body. Motor and sensory describe function, meaning which direction the signal travels. The two overlap heavily. Most motor neurons are multipolar. But so are most interneurons, which are not motor neurons at all. A useful neuron is described by both, as in "a multipolar motor neuron."

Are interneurons only found in the brain and spinal cord?

Almost all are, and that is where textbooks place them. But the term is defined by what the cell does. It passes a signal from one cell to another, rather than receiving a stimulus or driving a muscle. It is not defined by where it sits. The enteric nervous system in the gut keeps local reflex circuits of its own. So the familiar claim is better stated as "almost entirely."

Why do cold and heat feel different in kind, not just in strength?

They are carried by different fibers, and the brain reads that difference as a difference in quality. Warm-sensitive fibers are commonly unmyelinated C fibers, which conduct slowly. Cold-sensitive fibers include C fibers and thinly myelinated A delta fibers, which conduct faster. So the two are not two clean systems, and the fastest fibers are not the same as the warm ones. Some cold-sensitive fibers also respond to heat, a puzzle known as paradoxical heat.

How is force graded in a muscle that only has all-or-nothing neurons?

Through numbers rather than force. An individual motor neuron either fires or does not. A muscle, though, contains many motor units. The nervous system varies force by recruiting more or fewer of them, and by firing them faster. That is why a muscle made of many small units can manage its force far more precisely than one built from a few large units.

Sources: the afferent and efferent arrangement, upper and lower motor neurons, and the neuromuscular junction follow NCBI Bookshelf (StatPearls) and standard physiology reviews. Nociception follows the review literature on nociceptors. The motor unit, its definition by Liddell and Sherrington, and motor-unit size follow peer-reviewed reviews of motor units. Gray and white matter follow the Queensland Brain Institute and standard histology accounts. Olfactory neuron replacement follows reviews of the olfactory epithelium. Figures are from Wikimedia Commons, with the author and license named in each caption.