Neurons in the mammalian CNS come in many different shapes and sizes. Most have the same parts as the typical spinal motor neuron illustrated in Figure 1. The cell body (soma) contains the nucleus and is the metabolic center of the neuron. Neurons have several processes called dendrites that extend outward from the cell body and arborize extensively. Particularly in the cerebral and cerebellar cortex, the dendrites have small knobby projections called dendritic spines. A typical neuron also has a long fibrous axon that originates from a somewhat thickened area of the cell body, the axon hillock. The first portion of the axon is called the initial segment. The axon divides into presynaptic terminals, each ending in a number of synaptic knobs that are also called terminal buttons or boutons. They contain granules or vesicles in which the synaptic transmitters secreted by the nerves are stored. Based on the number of processes that emanate from their cell body, neurons can be classified as unipolar, bipolar, pseudounipolar, and multipolar (Figure2).
Fig1. Motor neuron with a myelinated axon. A motor neuron is composed of a cell body (soma) with a nucleus, several processes called dendrites, and a long fibrous axon that originates from the axon hillock. The first portion of the axon is called the initial segment. A myelin sheath forms from Schwann cells and surrounds the axon except at its ending and at the nodes of Ranvier. Terminal buttons (boutons) are located at the terminal endings.
Fig2. Some of the types of neurons in the mammalian nervous system. A) Unipolar neurons have one process, with different segments serving as receptive surfaces and releasing terminals. B) Bipolar neurons have two specialized processes: a dendrite that carries information to the cell and an axon that transmits information from the cell. C) Some sensory neurons are in a subclass of bipolar cells called pseudounipolar cells. As the cell develops, a single process splits into two, both of which function as axons—one going to skin or muscle and another to the spinal cord. D) Multipolar cells have one axon and many dendrites. Examples include motor neurons, hippocampal pyramidal cells with dendrites in the apex and base, and cerebellar Purkinje cells with an extensive dendritic tree in a single plane. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA, Hudspeth AJ (editors): Principles of Neural Science, 5th ed. New York, NY: McGraw-Hill; 2013.)
The conventional terminology used for the parts of a neuron works well enough for spinal motor neurons and inter neurons, but there are problems in terms of “dendrites” and “axons” when it is applied to other types of neurons found in the nervous system. From a functional point of view, neurons generally have four important zones: (1) a receptor, or dendritic zone, where multiple local potential changes generated by synaptic connections are integrated; (2) a site where propagated action potentials are generated (the initial segment in spinal motor neurons, the initial node of Ranvier in cutaneous sensory neurons); (3) an axonal process that transmits propagated impulses to the nerve endings; and (4) the nerve endings, where action potentials cause the release of synaptic transmitters. The cell body is often located at the dendritic zone end of the axon, but it can be within the axon (eg, auditory neurons) or attached to the side of the axon (eg, cutaneous neurons). Its location makes no difference as far as the receptor function of the dendritic zone and the transmission function of the axon are concerned.
The axons of many neurons are myelinated, that is, they acquire a sheath of myelin, a protein–lipid complex that is wrapped around the axon. In the peripheral nervous system, myelin forms when a Schwann cell wraps its membrane around an axon up to 100 times. The myelin is then compacted when the extracellular portions of a mem brane protein called protein zero (P0 ) lock to the extracellular portions of P0 in the apposing membrane. Various mutations in the gene for P0 cause peripheral neuropathies; 29 different mutations have been described that cause symptoms ranging from mild to severe. The myelin sheath envelops the axon except at its ending and at the nodes of Ranvier, periodic 1-μm constrictions that are about 1 mm apart (Figure 1). The insulating function of myelin is discussed later in this chapter. Not all neurons are myelinated; some are unmyelinated, that is, simply surrounded by Schwann cells without the wrapping of the Schwann cell membrane that produces myelin around the axon.
In the CNS of mammals, most neurons are myelinated, but the cells that form the myelin are oligodendrocytes rather than Schwann cells. Unlike the Schwann cell, which forms the myelin between two nodes of Ranvier on a single neuron, oligodendrocytes emit multiple processes that form myelin on many neighboring axons. In MS, patchy destruction of myelin occurs in the CNS (Clinical Box 4–1). The loss of myelin is associated with delayed or blocked con duction in the demyelinated axons.
