Neuron Anatomy: A Labeled 3D Motor Neuron and Its Parts

The anatomy of a neuron follows its job: it is an excitable cell built to carry information in one direction. Signals arrive on the dendrites, spread across the cell body, are turned into action potentials at the start of the axon, and travel down the axon to its terminals, which pass the message on at synapses. (OpenStax Anatomy and Physiology 2e, 12.2 Nervous tissue.)

The model is a vertebrate spinal motor neuron, the cell that makes skeletal muscles contract. Its cell body sits in the ventral (anterior) horn of the spinal cord's grey matter, and its axon leaves through the ventral root to run in a peripheral nerve to a muscle. (OpenStax Anatomy and Physiology 2e, 13.2 The central nervous system; OpenStax Biology 2e, 35.4 The peripheral nervous system.) It is the classic textbook neuron because it shows every part clearly: a large multipolar cell body, many dendrites, one long myelinated axon and terminal branches on muscle fibres.

Neurons share the organelles of other cells. What sets them apart are their dendrites and axons, specialised for receiving and sending electrical signals, and the connections they make with other neurons and with muscle. (OpenStax Biology 2e, 35.1 Neurons and glial cells.) The human brain alone holds about 86 billion neurons. (Herculano-Houzel (2009): The human brain in numbers, Frontiers in Human Neuroscience 3:31; OpenStax Biology 2e, 35.1 Neurons and glial cells.)

A labeled neuron diagram shows, from the input end to the output end: dendrites with their spines, the cell body with its nucleus, nucleolus and Nissl bodies, the axon hillock and initial segment, the axon wrapped in myelin sheaths separated by nodes of Ranvier, a side branch (collateral), and the terminal branches ending in synaptic boutons.

In the 3D model, switch on All labels to name every part at once, or click any part to zoom to it and read what it does. Drag the Myelin sheath slider down to see the bare axon underneath.

Every neuron has three main parts, dendrites, a cell body and an axon, plus specialised zones along the axon. This table gives each part, where it is, and its job.

The cell body (soma or perikaryon) is the neuron's metabolic centre. It contains the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria and other organelles common to most cells, and it makes almost all of the neuron's proteins. (OpenStax Biology 2e, 35.1 Neurons and glial cells.)

A large spinal motor neuron has a large, central nucleus surrounded by dark-staining Nissl bodies. These are stacks of rough endoplasmic reticulum with ribosomes; they extend into the dendrites but stop at the pale, Nissl-free axon hillock. (Clermont, Lalli & Bencsath-Makkai, Light-Microscopic Histology Atlas (McGill), 4.3 Neuron; Clermont, Lalli & Bencsath-Makkai, Light-Microscopic Histology Atlas (McGill), 4.1 Large pyramidal neuron; Kenhub: Parts of a neuron.) Because the axon has no rough ER of its own, proteins made in the cell body must be carried down it by axonal transport. Fast transport moves vesicles at about 250–400 mm a day, slow transport of cytoskeletal proteins only about 1 mm a day; a review gives 50–400 mm and under 8 mm a day. (BioNumbers 113074: fast and slow axonal transport rates in mammals (Brady et al., Basic Neurochemistry); Schenone et al. (2025): Basic pathological mechanisms in peripheral nerve diseases, International Journal of Molecular Sciences.) A neuron has over a thousand mitochondria to power all this. (OpenStax Anatomy and Physiology 2e, 3.2 The cytoplasm and cellular organelles.)

Dendrites are the receiving branches of a neuron. They are usually short, tapering and highly branched, and other neurons' axon terminals contact them at synapses. Excitatory inputs commonly land on dendrites, inhibitory inputs nearer the axon hillock. (OpenStax Anatomy and Physiology 2e, 12.2 Nervous tissue; OpenStax Anatomy and Physiology 2e, 12.5 Communication between neurons.) A single cerebellar Purkinje cell can receive contacts from up to about 200,000 other neurons. (OpenStax Biology 2e, 35.1 Neurons and glial cells.)

Dendritic spines are tiny protrusions that add surface area for synapses and receive input from excitatory axons. (OpenStax Biology 2e, 35.1 Neurons and glial cells; Rochefort & Konnerth (2012): Dendritic spines: from structure to in vivo function, EMBO Reports 13:699.) Spinal motor neurons have them too: in mice their spines peak around two weeks after birth and then thin out, leaving mostly short "stubby" spines on the outer dendrites, with an occasional mushroom-shaped one. (Kanjhan, Noakes & Bellingham (2016): Emerging roles of filopodia and dendritic spines in motoneuron plasticity, Neural Plasticity 2016:3423267.)

The axon is the neuron's single output cable. It starts at the axon hillock, a cone where the cell body tapers, continues as the initial segment, and runs to the terminals. The hillock and initial segment together form the trigger zone where action potentials begin; the initial segment has a high density of voltage-gated sodium channels. (OpenStax Anatomy and Physiology 2e, 12.2 Nervous tissue; OpenStax Anatomy and Physiology 2e, 12.4 The action potential.) A protein scaffold of ankyrin-G and βIV-spectrin anchors these channels and also acts as a filter that keeps the axon's contents separate from the dendrites'. (Jones & Svitkina (2016): Axon initial segment cytoskeleton: architecture, development, and role in neuron polarity, Neural Plasticity 2016:6808293.) How the action potential itself rises and falls is covered on our action potential diagram page.

A human motor axon can be about a metre long, from the base of the spine to the toes, yet only about 10 µm wide. (OpenStax Biology 2e, 35.1 Neurons and glial cells; Milo & Phillips, Cell Biology by the Numbers: How big is a human cell?.) Axons in general are 1–20 µm in diameter. (OpenStax Anatomy and Physiology 2e, 12.2 Nervous tissue.)

Axons can branch. A motor axon gives off a recurrent collateral inside the spinal cord that turns back and excites Renshaw cells, small interneurons that then inhibit the same motor neurons, a negative feedback loop called recurrent inhibition. The collateral releases acetylcholine onto nicotinic receptors, and one impulse in one motor axon can make a Renshaw cell fire. (Alvarez, Dewey, McMillin & Fyffe (1999): Distribution of cholinergic contacts on Renshaw cells in the rat spinal cord, Journal of Physiology 515:787; Enjin et al. (2017): Developmental disruption of recurrent inhibitory feedback in the Renshaw cell–motor neuron circuit, Journal of Neuroscience 37:5634.) In the muscle, the axon branches profusely so that one neuron supplies many muscle fibres; each branch ends in swollen boutons 1–5 µm across. (Slater (2017): The structure of human neuromuscular junctions: some unanswered molecular questions, International Journal of Molecular Sciences 18:2183.)

The myelin sheath is a fatty insulating wrap made of many layers of glial cell membrane. Schwann cells make it in the peripheral nervous system, each wrapping one segment of one axon; oligodendrocytes make it in the brain and spinal cord, each wrapping segments of several axons. (OpenStax Anatomy and Physiology 2e, 12.2 Nervous tissue; OpenStax Biology 2e, 35.1 Neurons and glial cells.) So the motor axon in the model is myelinated by an oligodendrocyte while it is still inside the spinal cord, and by Schwann cells once it leaves through the ventral root.

Between the myelin segments (internodes) are nodes of Ranvier, gaps about 1 µm long where the axon membrane is exposed and packed with voltage-gated channels; the signal is regenerated at each node and jumps from node to node. (OpenStax Biology 2e, 35.2 How neurons communicate.) Internodes are about 1–2 mm long, 100–1,000 times the axon's diameter. (OpenStax Anatomy and Physiology 2e, 12.2 Nervous tissue.) A review of Schwann cell sheaths gives maximal internodes of about 1 mm in mice and 2.4 mm in cows, and minimum lengths of 0.15–0.3 mm in all species. (Tricaud (2018): Myelinating Schwann cell polarity and mechanically-driven myelin sheath elongation, Frontiers in Cellular Neuroscience 11:414.)

Myelin and thickness together set the speed. The fastest axons, the thick myelinated Aα fibres that include alpha motor axons, are about 12–20 µm across and conduct at about 100 m/s. (e-Learning for Healthcare, e-Safe Anaesthesia: Nerve classification.) Measured in cats, alpha motor axons conducted at 71–91 m/s and the thinner gamma motor axons at 15–48 m/s. (Westbury (1982): A comparison of the structures of α- and γ-spinal motoneurones of the cat, Journal of Physiology 325:79.) Across all nerves, speeds range from about 100 m/s down to less than 0.1 m/s. (Byrne, Neuroscience Online (UTHealth), ch. 3: Propagation of the action potential.) When myelin is damaged, as in multiple sclerosis, current leaks out and conduction slows. (OpenStax Biology 2e, 35.2 How neurons communicate.)

A synapse is the junction where an axon terminal passes its signal to another cell. A motor neuron's synapse with a skeletal muscle fibre is called the neuromuscular junction, and it is the magnified close-up in the 3D model. (OpenStax Anatomy and Physiology 2e, 10.2 Skeletal muscle; OpenStax Anatomy and Physiology 2e, 12.5 Communication between neurons.)

1. An action potential reaches the bouton and opens voltage-gated calcium channels; calcium entry makes synaptic vesicles fuse with the membrane. (OpenStax Anatomy and Physiology 2e, 12.5 Communication between neurons.)
2. Each vesicle, about 50 nm across, releases a packet of about 10,000 acetylcholine molecules at an active zone. (Slater (2017): The structure of human neuromuscular junctions: some unanswered molecular questions, International Journal of Molecular Sciences 18:2183.)
3. Acetylcholine crosses a cleft 50–100 nm wide, with a basal lamina running through its middle; a synapse between two neurons has a cleft of only about 20–30 nm and no basal lamina. (Zou & Pan (2022): Post-synaptic specialization of the neuromuscular junction: junctional folds formation, function, and disorders, Cell & Bioscience 12:93; Slater (2017): The structure of human neuromuscular junctions: some unanswered molecular questions, International Journal of Molecular Sciences 18:2183.)
4. It binds acetylcholine receptors packed at about 10,000 per µm² on the crests of the junctional folds, which open and depolarise the muscle membrane. Sodium channels deep in the folds then start the muscle action potential. (Zou & Pan (2022): Post-synaptic specialization of the neuromuscular junction: junctional folds formation, function, and disorders, Cell & Bioscience 12:93; Slater (2017): The structure of human neuromuscular junctions: some unanswered molecular questions, International Journal of Molecular Sciences 18:2183.)
5. Acetylcholinesterase in the cleft breaks acetylcholine down so it cannot act again. (OpenStax Anatomy and Physiology 2e, 10.2 Skeletal muscle.)

What happens next inside the muscle fibre is shown on the sarcomere diagram page.

Neurons are classified by structure according to how many processes leave the cell body: multipolar (one axon and two or more dendrites), bipolar (one axon and one dendrite), unipolar or pseudounipolar (a single process that splits in two) and anaxonic (no process recognisable as an axon). (OpenStax Anatomy and Physiology 2e, 12.2 Nervous tissue; OpenStax Biology 2e, 35.1 Neurons and glial cells.)

Textbooks disagree slightly on names. One calls human pseudounipolar sensory neurons "unipolar" and notes that true unipolar cells occur only in invertebrates; it also says "anaxonic" is a misnomer, because these cells do have axons that simply cannot be told apart from dendrites under the light microscope, so they count as multipolar. (OpenStax Anatomy and Physiology 2e, 12.2 Nervous tissue.) Another lists amacrine cells of the retina as neurons with no axon at all. (OpenStax Biology 2e, 35.1 Neurons and glial cells.)

By function there are three types of neurons. Sensory (afferent) neurons carry information from receptors toward the central nervous system; motor (efferent) neurons carry commands out to muscles and glands; and interneurons sit between them in the brain and spinal cord, relaying signals and forming circuits. (Queensland Brain Institute (University of Queensland): Types of neurons; OpenStax Biology 2e, 35.4 The peripheral nervous system.)

Sensory neuron cell bodies are grouped in the dorsal root ganglia; motor neuron cell bodies lie in the ventral grey matter and project through the ventral root. (OpenStax Biology 2e, 35.4 The peripheral nervous system.) See how these fit into the cord on our spinal cord anatomy guide, and how neurons are organised into the brain on the brain anatomy and cranial nerves guides.

Glial cells support neurons, maintain their surroundings and make myelin. There are six main types, four in the central nervous system and two in the peripheral nervous system. (OpenStax Anatomy and Physiology 2e, 12.2 Nervous tissue; OpenStax Biology 2e, 35.1 Neurons and glial cells.)

How many glia are there? Some textbooks still say the brain has about ten times as many glial cells as neurons. (OpenStax Biology 2e, 35.1 Neurons and glial cells.) Direct counts disagree: the adult human brain has about 86 billion neurons and 85 billion other cells, and a review of 150 years of counting found that histological data always supported a ratio of about one glial cell per neuron. (Herculano-Houzel (2009): The human brain in numbers, Frontiers in Human Neuroscience 3:31; von Bartheld, Bahney & Herculano-Houzel (2016): The search for true numbers of neurons and glial cells in the human brain, Journal of Comparative Neurology 524:3865; OpenStax Psychology 2e, 3.2 Cells of the nervous system.) For how neurons and glia make up nervous tissue, see our tissue types guide.

Sizes and speeds vary widely between neurons. These are the values for the motor neuron modelled here.

This is the text version of the 3D model: each labeled structure, grouped by region, with what it does. Use it as a study list for a neuron labeling worksheet.