brain, spinal cord, and nerve disorders
Nerves
Autonomic nervous system
The autonomic system connects the brain stem and spinal cord with internal organs and regulates internal body processes that require no conscious effort and that people are thus usually unaware of (see Overview of the Autonomic Nervous System ). Examples are the rate and strength of heart contractions, blood pressure, the rate of breathing, and the speed at which food passes through the digestive tract. The autonomic nervous system has 2 divisions: Sympathetic division:Its main function is to prepare the body for stressful or emergency situations—for fight or flight. Parasympathetic division:Its main function is to maintain normal body functions during ordinary situations. These divisions work together, usually with one activating and the other inhibiting the actions of internal organs. For example, the sympathetic division increases pulse, blood pressure, and breathing rates, and the parasympathetic system decreases each of them. A nerve cell (neuron) consists of a large cell body and nerve fibers—1 elongated extension (axon) for sending impulses and usually many branches (dendrites) for receiving impulses. The impulses from the axon cross a synapse (the junction between 2 nerve cells) to the dendrite of another cell. Many large nerve cells have axons that are surrounded by oligodendrocytes in the brain and spinal cord and by Schwann cells in the peripheral nervous system. The membranes of these cells consist of a fat (lipoprotein) called myelin. The membranes are wrapped tightly around the axon, forming a multilayered sheath. This myelin sheath resembles insulation, such as that around an electrical wire. Nerve impulses travel much faster in nerves with a myelin sheath than in those without one. 