Anatomy of the Brain
The brain is the hub of the central nervous system and controls all bodily functions and processes. It weighs about three pounds and is surrounded by protective bone called the skull or cranium. The brain, which has the texture of gelatin, is held together by three layers of membranes called the dura, pia, and arachnoid.
Between the pia and arachnoid membranes is the subarachnoid space, through which a network of arteries and veins carries blood to and from the heart. Injury to these blood vessels can lead to blood clots, which can exert damaging pressure against the brain's delicate tissue. The brain is surrounded by a cushioning reservoir of cerebrospinal fluid (CSF).
The lower part of the brain (called the brain stem) passes through a hole at the base of the skull and merges with the spinal cord and the rest of the nervous system. The brain stem can be compared to a telephone cable with thousands of individual wires (nerve fibers) that carry signals to and from all parts of the body.
The brain stem also regulates such body functions as consciousness, fatigue, heart rate, and blood pressure. Damage to the stem can cause loss of consciousness, or concussion of the brain.
Behind the brain stem is the cerebellum, a curved mass of nerve tissues that regulates balance and coordinates fine motor skills. It enables us to move quickly and smoothly, thread a needle, or throw a dart with accuracy.
The cerebral cortex is the largest part of the brain and is shaped like a large, wrinkled, walnut divided in half (the right and left cerebral hemispheres) and joined at the center. The right hemisphere controls the left side of the body; the left hemisphere controls the right side.
In most people, the left hemisphere regulates language and speech and the right hemisphere controls nonverbal, spatial skills such as the ability to draw or play music. If the right side of the brain is damaged, movement in the left arm and leg, vision to the left, or hearing in the left ear, may be affected. An injury to the left side of the brain affects speech and movement on the right side of the body.
The cerebral cortex is further divided into several areas called lobes. Of these:
- The left and right frontal lobes, located behind the forehead, control intellectual activities, such as the ability to organize, and figure prominently in personality, behavior, and emotional control.
- The temporal lobes, situated immediately behind and below the frontal lobes and just behind the ears, control memory, speech and comprehension.
- The parietal lobes located at the back of the head and above the ears, control the ability to read, write, and understand spatial relationships.
- The areas between the frontal and parietal lobes regulate movement and sensation.
- The occipital lobes, situated at the back of the head, control sight.
In the middle of the cerebral cortex are several small white nuclei, or nerve centers, called the diencephalon. Among these is the pea-sized hypothalamus, which regulates appetite, thirst, temperature, and some aspects of memory and controls sexual arousal. Another is the limbic system, which is associated with the control of emotions and moods.
Damage to these areas can result in impairment to the functions they regulate.