Although the symptoms of a seizure may affect any part of the body, the electrical events that produce the symptoms occur in the brain. The location of that event, the extent of its reach within the tissue of the brain, and how long it lasts all have profound effects. These three factors determine the character of the seizure, its impact on the individual, and the social consequences involved. Any review of epilepsy therefore begins with a brief description of brain geography and function.
The brain is divided into parts, some overlaying one another in the order in which they have evolved.
The spinal cord sends and receives nerve impulses to and from the brain and the rest of the body. Immediately above the spinal cord is the brainstem, the oldest part of the brain. The brainstem controls eye movement, facial movement and sensations, sleeping and waking, breathing, taste, heartbeat and other bodily functions. At its base is the cerebellum, which coordinates balance and movement.
The upper part of the brainstem contains the thalamus and the hypothalamus. The thalamus regulates levels of consciousness and directs nerve impulses, including sensation, sound, vision, touch, and pain. The hypothalamus regulates hormone production. It is the site of the all-important pituitary gland that controls other endocrine glands such as the ovaries, testicles, thyroid and adrenal glands.
Overlying the brainstem is the upper brain, or cerebrum, composed of gray and white matter. The gray matter is made up largely of nerve cells (neurons) and supporting cells (glial cells). These together comprise the cerebral cortex. The white matter lies underneath the cerebral cortex and is composed of nerve fibers (axons and dendrites) connected to other parts of the brain, the spinal cord, the muscles of the body, and the glands.
The cerebrum is divided into two halves, or hemispheres. Each hemisphere controls physical action on the opposite side of the body. The left and right hemispheres are connected by a bridge of nerve fibres which carry electrical impulses back and forth from one side to the other, integrating the activity of the brain.
The nerve bridge, called the corpus callosum, has specific significance for epilepsy. In cases where seizure activity passes across the bridge to involve both hemispheres and produces devastating falls, treatment may involve surgical separation of the corpus callosum.