The Nervous system
The nervous system can be separated into three divisions, the central nervous system, the peripheral nervous system, and the autonomic nervous system. The Central Nervous system consists of the brain and the spinal cord. The spinal cord is a long, thick nerve trunk that rus from the base of the brain down through the spinal column, or backbone. The cord is composed of white matter. A cross sectional view of the spinal cord shows the gray matter as an H shaped area surrounded by white matter. Thirty one pair of spinal nerves leave the spinal cord. Each of these nerve trunks is attacked to the cord in two places. The root of the nerve that leaves the cord toward the from to fight body is called the ventral root or anterior root. The root that leaves toward the rear of the body is called dorsal root or posterior root. Sensory nerves enter the spinal cord at the dorsal root. Motor nerves leave the spinal cord at the ventral root. If the ventral root of a nerve is cut, the part of the vody to which the nerve goes cannot move, but it still has sensation. If the dorsal root of a nerve is cut, sensation disappears, but the body part can still move. Some nerve impulses entering the spinal cord are directed to the brain.
The Peripheral Nervous system consists of 12 pairs of cranial nerves and the 31 pair of spinal nerves. The Cranial Nerves come from the lower part of the brain. These neves control many sensations and actions including sight, smell, chewing, and swallowing. The spinal nerves leave the spinal cord at the small of the back. Each of the 5 pair of sacral nerves and the 1 pair of coccygeal nerves leave the spinal cord between the lowest vertebrae in the spinal cord.
The autonomic Nervous system regulates the inernal organs of the body. The fuctions it controls are involuntary. For example, you cannot deliberately control the rate of your heartbeat, though it may be influenced by your thoughts or emotions.
The nerves of the autonomic nervous system are connected to and regulated by the central nervous system are connected to and regulated by the central nervous sytem at a subconscious level. The nerve fibers have lettle or no myelin sheath. Most nerves in the autonomic system are effector speed of more than 300 per second. In the smaller fibers of the autonomic nervous system, the lowerst part of the brain. The membrane theory is the most commonly accepted theory of how