Although neurons vary considerably in size and shape, they generally have three principal regions: (1) a cell body, (2) dendrites, and (3) an axon (figs. 7.1 and 7.2). Dendrites and axons can be referred to generically as processes, or extensions from the cell body.

The cell body is the enlarged portion of the neuron that contains the nucleus. It is the "nutritional center" of the neuron where macromolecules are produced. The cell body also contains densely staining areas of rough endoplasmic reticulum known as Nissl bodies that are not found in the dendrites or axon. The cell bodies within the CNS are frequently clustered into groups called

Dendrites Axon hillock

Direction of conduction

Cell body



Direction of conduction

- Collateral axon


■ Figure 7.1 The structure of two kinds of neurons. (a) A motor neuron and (b) a sensory neuron are depicted here.

The Nervous System: Neurons and Synapses nuclei (not to be confused with the nucleus of a cell). Cell bodies in the PNS usually occur in clusters called ganglia (table 7.1).

Dendrites (dendron = tree branch) are thin, branched processes that extend from the cytoplasm of the cell body. Dendrites provide a receptive area that transmits electrical impulses to the cell body. The axon is a longer process that conducts impulses away from the cell body. Axons vary in length from only a millimeter long to up to a meter or more (for those that extend from the CNS to the foot). The origin of the axon near the cell body is an expanded region called the axon hillock; it is here that nerve impulses originate. Side branches called axon collaterals may extend from the axon.

Proteins and other molecules are transported through the axon at faster rates than could be achieved by simple diffusion. This rapid movement is produced by two different mechanisms: axo-plasmic flow and axonal transport (table 7.2). Axoplasmic flow, the slower of the two, results from rhythmic waves of contraction that push the cytoplasm from the axon hillock to the nerve endings.



Schwann cell nucleus



Schwann cell nucleus

Cell body

■ Figure 7.2 Parts of a neuron. The axon of this neuron is wrapped by Schwann cells, which form a myelin sheath.

Table 7.1 Terminology Pertaining to the Nervous System



Central nervous system (CNS)

Brain and spinal cord

Peripheral nervous system (PNS) Association neuron (interneuron)

Nerves, ganglia, and nerve plexuses (outside of the CNS) Multipolar neuron located entirely within the CNS

Sensory neuron (afferent neuron) Motor neuron (efferent neuron)

Neuron that transmits impulses from a sensory receptor into the CNS

Neuron that transmits impulses from the CNS to an effector organ, for example, a muscle


Cablelike collection of many axons, may be "mixed" (contain both sensory and motor fibers)

Somatic motor nerve

Nerve that stimulates contraction of skeletal muscles

Autonomic motor nerve

Nerve that stimulates contraction (or inhibits contraction) of smooth muscle and cardiac muscle and that stimulates glandular secretion


Grouping of neuron cell bodies located outside the CNS


Grouping of neuron cell bodies within the CNS


Grouping of nerve fibers that interconnect regions of the CNS

Table 7.2 Comparison of Axoplasmic Flow and Axonal Transport

Table 7.2 Comparison of Axoplasmic Flow and Axonal Transport

Axoplasmic Flow

Axonal Transport

Transport rate comparatively slow (1-2 mm/day) Molecules transported only from cell body

Bulk movement of proteins in axoplasm, including microfilaments and tubules Transport accompanied by peristaltic waves of axon membrane

Transport rate comparatively fast (200-400 mm/day)

Molecules transported from cell body to axon endings and in reverse direction Transport of specific proteins, mainly of membrane proteins and acetylcholinesterase Transport dependent on cagelike microtubule structure within axon and on actin and Ca2+

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