The neuron is composed of dendrites, a cyton, and an axon (fig. 1-42). The dendrites are thin receptive branches, and vary greatly in size, shape, and number with different types of neurons. They serve as receptors, conveying impulses toward the cyton. The cyton is the cell body containing the nucleus. The single, thin extension of the cell outward from the cyton is called the axon. It conducts impulses away from the cyton to its terminal branches, which transmit the impulses to the dendrites of the next neuron.

Large axons of the peripheral nerves are commonly enclosed in a sheath, called neurilemma, composed of Schwann cells (fig. 1-42). Schwann cells wrap around the axon and act as an electrical insulator.

Figure 1-42.-The neuron and its parts.

The membranes of the Schwann cell are composed largely of a lipid-protein called myelin, which forms a myelin sheath on the outside of an axon. The myelin sheath has gaps between adjacent Schwann cells called nodes of Ranvier. Nerve cells without Schwann cells also lack myelin and neurilemma sheaths.

IMPULSE TRANSMISSION
When dendrites receive a sufficiently strong stimulus, a short and rapid change in electrical charge, or polarity, of the neuron is triggered. Sodium ions rush through the plasma membrane into the cell, potassium ions leave, and an electrical impulse is formed, which is conducted toward the cyton. The cyton receives the impulse and transmits it to the terminal filaments of the axon. At this point a chemical transmitter such as acetylcholine is released into the synapse, a space between the axon of the activated nerve and the dendrite receptors of another neuron. This chemical transmitter activates the next nerve. In this manner, the impulse is passed from neuron to neuron down the nerve line to a central area at approximately the speed of a bullet.

Almost immediately after being activated, the chemical transmitter in the synapse is neutralized by the enzyme acetylcholinesterase, and the first neuron returns to its normal state by pumping out the sodium ions and drawing potassium ions back in through the plasma membrane. When these actions are completed, the nerve is ready to be triggered again. Aparticularly strong stimulus will cause the nerve to fire in rapid succession, or will trigger many other neurons, thus giving a feeling of intensity to the perceived sensation.