Neurons

The basic unit of the nervous system is the nerve cell, or neuron (Fig. 12-2). Not all neurons are the same, but all have the following features in common. Dendrites (DEN drytes), projections from the cell body, receive neural impulses, also called action potentials, from a stimulus of some kind. This impulse travels along the dendrite and into the cell body, which is the control center of the cell. This cell body contains the nucleus and surrounding cytoplasm.

From the cell body, the impulse moves out along the axon (AX on), a slender, elongated projection that carries the nervous impulse toward the next neuron. The terminal fibers result from the final branching of the axon and the site of the axon terminals that store the chemical neurotransmitters. In neurons outside the CNS, the axon is covered by the myelin (MY uh lin) sheath, which is a substance produced by Schwann (shvahn) cells that coat the axons.

From the axon’s terminal fibers, the neurotransmitter is released from the cell to travel across the space between these terminal fibers and the dendrites of the next cell. This space is called the synapse (SIN aps) (see Fig. 12-2). The impulse continues in this manner until its destination is reached.

Glia

These supportive, or stromal, cells are also called neuroglia (noo RAH glee ah). They accomplish their supportive function by physically holding the neurons together and also protecting them. One type of neuroglia, the astrocytes (AS troh sites), connect neurons and blood vessels and form a structure called the blood-brain barrier (BBB), which prevents or slows the passage of some drugs and disease-causing organisms to the CNS.

The Brain

The brain is one of the most complex organs of the body. It is divided into four parts: the cerebrum (suh REE brum), the cerebellum (sair ih BELL um), the diencephalon (dye en SEF fuh lon), and the brainstem (Fig. 12-3).

Cerebrum

The largest portion of the brain, the cerebrum, is divided into two halves, or hemispheres (Fig. 12-4). It is responsible for thinking, reasoning, and memory. The surfaces of the hemispheres are covered with gray matter and are called the cerebral cortex. Arranged into folds, the valleys are referred to as sulci (SULL sye) (sing. sulcus), and the ridges are gyri (JYE rye) (sing. gyrus). The cerebrum is further divided into sections called lobes, each of which has its own functions:

cortex = cortic/o

lobe = lob/o

1. The frontal lobe contains the functions of speech and the motor area that controls voluntary movement on the contralateral side of the body.

2. The temporal (TEM pur rul) lobe contains the auditory and olfactory areas.

3. The parietal (puh RYE uh tul) lobe controls the sensations of touch and taste.

4. The occipital (ock SIP ih tul) lobe is responsible for vision.

The Spinal Cord

The spinal cord extends from the medulla oblongata to the first lumbar vertebra (Fig. 12-5). It then extends into a structure called the cauda equina (KAH dah eh KWY nah). The spinal cord is protected by the bony vertebrae surrounding it and the coverings unique to the CNS called meninges (meh NIN jeez). The spinal cord is composed of gray matter, the cell bodies of motor neurons, and white matter, the myelin-covered axons or nerve fibers that extend from the nerve cell bodies. The 31 pairs of spinal nerves emerge from the spinal cord at the nerve roots.

Meninges

Meninges act as protective coverings for the CNS and are composed of three layers separated by spaces (Fig. 12-6). The dura mater (DUR ah MAY tur) is the tough, fibrous, outer covering of the meninges; its literal meaning is hard mother. The space between the dura mater and arachnoid membrane is called the subdural space. Next comes the arachnoid (uh RACK noyd) membrane, a thin, delicate membrane that takes its name from its spidery appearance. The subarachnoid space is the space between the arachnoid membrane and the pia mater, containing cerebrospinal fluid (CSF). CSF is also present in cavities in the brain called ventricles. Finally, the pia mater (PEE uh MAY tur) is the thin, vascular membrane that is the innermost of the three meninges; its literal meaning is soft mother.

ventricle = ventricul/o