Neurons are structurally classified into four categories based on how many processes (extensions) branch out from the cell body: multipolar, bipolar, unipolar (pseudounipolar), and anaxonic. Most anatomy and physiology textbooks focus on the first three, but current neuroanatomy references recognize all four as distinct structural types.
Multipolar Neurons
Multipolar neurons have one axon and two or more dendrites extending from the cell body. This gives them a branching, tree-like appearance and makes them easy to recognize under a microscope. They are the most common structural type in the central nervous system by a wide margin.
Both motor neurons and interneurons fall into this category. Every motor neuron that controls skeletal muscle is multipolar, with some extending axons from the spinal cord all the way down to the small muscles of the toes. Interneurons, which connect neurons to each other inside the brain and spinal cord, are also multipolar. Since interneurons are the most numerous class of neurons in the human nervous system, multipolar neurons dominate the overall count.
Bipolar Neurons
Bipolar neurons have exactly two processes: one axon and one dendrite, extending from opposite ends of the cell body. They are relatively small and much less common than multipolar neurons. You’ll find them almost exclusively in the special sense organs, where they relay information related to sight, smell, and hearing.
In the eye, bipolar neurons sit in the retina and help transmit visual signals toward the optic nerve. In the ear, they form the vestibular and auditory ganglia, connecting the sensory structures of the inner ear to targets in the brainstem. Their simple two-process design suits them well for carrying a sensory signal in one clear direction.
Unipolar (Pseudounipolar) Neurons
Unipolar neurons appear to have just one process leaving the cell body, but that single process quickly splits into two branches in a T-shaped junction. One branch travels out to the periphery (skin, muscles, joints) while the other heads into the spinal cord. Because of this split, most modern references call them pseudounipolar, meaning “falsely unipolar.” Older textbooks sometimes use “unipolar” alone, and both terms refer to the same cell type in vertebrates.
This design has a functional advantage. The cell body sits off to the side on the T-branch rather than in the direct path of the signal. That means electrical impulses traveling from the periphery to the spinal cord don’t have to pass through the cell body, which would slow them down. Most sensory neurons in the body are pseudounipolar. They span from their receptive fields in the skin or organs all the way to the dorsal horn of the spinal cord, carrying information about touch, pain, temperature, and body position.
Anaxonic Neurons
Anaxonic neurons are the least discussed of the four types. Despite the name (“no axon”), they do have processes extending from the cell body. The issue is that these processes are so small and similar-looking that you can’t distinguish which one is the axon and which are dendrites under a microscope. Any of the processes can function as an axon depending on conditions at a given moment.
Anaxonic neurons are found in the brain and retina. They play roles in local signal processing rather than transmitting information over long distances. Not every textbook includes them in the structural classification, which is why some sources list three categories and others list four.
How Structure Maps to Function
Structural type and functional role overlap in predictable ways. Motor neurons, which send commands from the central nervous system to muscles and glands, are multipolar. Sensory neurons, which carry information from the body to the spinal cord, are typically pseudounipolar. Interneurons, which process information and form circuits within the brain and spinal cord, are multipolar.
The bipolar neurons in sensory organs are a special case. They are technically sensory, but they serve the “special senses” (vision, hearing, smell, balance) rather than general body sensation like touch or pain. So while sensory neurons in the skin are pseudounipolar, sensory neurons in the retina or inner ear are bipolar. The structural design in each case reflects how far the signal needs to travel and how directly it needs to reach the central nervous system.
Quick Comparison by Process Count
- Multipolar: one axon, two or more dendrites (three or more total processes)
- Bipolar: one axon, one dendrite (two total processes)
- Pseudounipolar: one process that splits into two branches (appears as one process leaving the cell body)
- Anaxonic: multiple small processes, none clearly identifiable as an axon