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 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 transmitter chemical 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. A particularly 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.
Figure 3-39.—The Central nervous system.
CENTRAL NERVOUS SYSTEM
The central nervous system consists of the brain and spinal cord (fig. 3-39). The brain is almost entirely enclosed in the skull, but it is connected with the spinal cord, which lies in the canal formed by the vertebral column.
Figure 3-40.—Sheath of a neuron.
Figure 3-41.—Functional areas of the brain.
Brain
The brain has two main divisions, the cerebrum and the cerebellum. The cerebrum is the largest and most superiorly situated portion of the brain. It occupies most of the cranial cavity. The outer surface is called the cortex. This portion of the brain is also called gray matter because the nerve fibers are unmyelinated (not covered by a