October 11, 2026
There are two main classes of cells in the nervous system: nerve cells, or neurons, and glial cells, or glia.
A typical neuron has four morphologically defined regions: (1) the cell body, (2) dendrites, (3) an axon, and (4) presynaptic terminals.
The cell body or soma is the metabolic center of the cell. It includes the nucleus, which contains the genes of the cell, and the endoplasmic reticulum, an extension of the nucleus where the cell’s proteins are synthesized. The cell body usually gives rise to two kinds of processes: several short dendrites and one long, tubular axon. Dendrites branch out in tree-like fashion and are the main apparatus for receiving incoming signals from other nerve cells. The axon typically extends some distance from the cell body before it branches, allowing it to carry signals to many target neurons. An axon can convey electrical signals over distances ranging from 0.1 mm to 1 m.
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These electrical signals, or action potentials, are initiated at a specialized trigger region near the origin of the axon called the initial segment from which the action potentials propagate down the axon without failure or distortion at speeds of 1 to 100 m/s.
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The structure of a neuron.
Most neurons in the vertebrate nervous system have several main features in common. The cell body contains the nucleus, the storehouse of genetic information, and gives rise to two types of cell processes: axons and dendrites. Axons are the transmitting element of neurons; they vary greatly in length, some extending more than 1 m within the body. Most axons in the central nervous system are very thin (between 0.2 μm and 20 μm in diameter) compared with the diameter of the cell body (50 μm or more). Many axons are insulated by a sheath of fatty myelin that is regularly interrupted at gaps called the nodes of Ranvier.
The action potential, the cell’s conducting signal, is initiated at the initial segment of the axon and propagates to the synapse, the site at which signals flow from one neuron to another. Branches of the axon of the presynaptic neuron transmit signals to the postsynaptic cell. The branches of a single axon may form synapses with as many as 1,000 postsynaptic neurons. The apical and basal dendrites together with the cell body are the input elements of the neuron, receiving signals from other neurons.
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Nerve cells with similar properties can produce different actions because of the way they are interconnected.
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The amplitude of an action potential traveling down the axon remains constant at 100 mV because the action potential is an all-or-none impulse that is regenerated at regular intervals along the axon.
A key principle of brain function: the type of information conveyed by an action potential is determined not by the form of the signal but by the pathway the signal travels in the brain.
The brain thus analyzes and interprets patterns of incoming electrical signals carried over specific pathways, and in turn creates our sensations of sight, touch, taste, smell, and sound. To increase the speed by which action potentials are conducted, large axons are wrapped in an insulating sheath of a lipid substance, myelin. The sheath is interrupted at regular intervals by the nodes of Ranvier, uninsulated spots on the axon where the action potential is regenerated.
This historic tracing is the first published intra-cellular recording of an action potential.
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It was recorded in 1939 by Alan Hodgkin and Andrew Huxley from a squid giant axon, using glass capillary electrodes filled with sea water. The timing pulses (bottom) are separated by 2 ms. The vertical scale indicates the potential of the internal electrode in millivolts, the sea water outside being taken as zero potential.
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Near its end, the axon divides into fine branches that contact other neurons at specialized zones of communication known as synapses. The nerve cell transmitting a signal is called the presynaptic cell; the cell receiving the signal is the postsynaptic cell. The presynaptic cell transmits signals from specialized enlarged regions of its axon’s branches, called presynaptic terminals or nerve terminals. The presynaptic and postsynaptic cells are separated by a very narrow space, the synaptic cleft. Most presynaptic terminals end on the postsynaptic neuron’s dendrites, but some also terminate on the cell body or, less often, at the beginning or end of the axon of the postsynaptic cell. Some presynaptic neurons excite their postsynaptic target cells; other presynaptic neurons inhibit their target cells.
Ramóny Cajal deduced:
Neurons are thus classified into three large groups: unipolar, bipolar, and multipolar.
Neurons are classified as unipolar, bipolar, or multipolar according to the number of processes that originate from the cell body.
A. Unipolar cells have a single process emanating from the cell. Different segments serve as receptive surfaces or releasing terminals. Unipolar cells are characteristic of the invertebrate nervous system.
B. Bipolar cells have two types of processes that are functionally specialized. The dendrite receives electrical signals and the axon transmits signals to other cells.
C. Pseudo-unipolar cells, which are variants of bipolar cells, carry somatosensory information to the spinal cord. During development, the two processes of the embryonic bipolar cell fuse and emerge from the cell body as a single process that has two functionally distinct segments. Both segments function as axons; one extends to peripheral skin or muscle, the other to the central spinal cord.
D. Multipolar cells have a single axon and many dendrites. They are the most common type of neuron in the mammalian nervous system. Three examples illustrate the large diversity of these cells. Spinal motor neurons innervate skeletal muscle fibers. Pyramidal cells have a roughly triangular cell body; dendrites emerge from both the apex (the apical dendrite) and the base (the basal dendrites). Pyramidal cells are found in the hippocampus and throughout the cerebral cortex. Purkinje cells of the cerebellum are characterized by a rich and extensive dendritic tree that accommodates an enormous number of synaptic inputs.
Nerve cells are also classified into three major functional categories: Sensory neurons, Motor neurons, and Interneurons.
Sensory neurons carry information from the body’s peripheral sensors into the nervous system for the purpose of both perception and motor coordination. Some primary sensory neurons are called afferent neurons, and the two terms are used interchangeably. The term afferent (carried toward the central nervous system) applies to all information reaching the central nervous system from the periphery, whether or not this information leads to sensation. The term sensory designates those afferent neurons that convey information to the central nervous system from the sensory epithelia, from joint sensory receptors, or from muscle, but the concept has been expanded to include neurons in primary and secondary cortical areas that respond to changes in a sensory feature, such as displacement of an object in space, a shift in sound frequency, or the angular rotation of the head (via vestibular organs in the ear) or even something as complex as a face.
The term efferent applies to all information carried from the central nervous system toward the motor organs, whether or not this information leads to action. Motor neurons carry commands from the brain or spinal cord to muscles and glands (efferent information). The traditional definition of a motor neuron (or motoneuron) is a neuron that excites a muscle, but the designation of motor neuron now includes other neurons that do not innervate muscle directly but that command action indirectly.
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Sensory neurons supply the brain with data, whereas motor neurons convert ideation into praxis. Together they compose our interface with the world.
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