Section 6. Brain, Spinal Cord, and Nerve Disorders

Chapter 76. Biology of the Nervous System

Topics: Introduction | Brain | Spinal Cord | Peripheral Nerves | Effects of Aging

Peripheral Nerves

The peripheral nervous system consists of more than 100 billion nerve cells that run throughout the body like strings, making connections with the brain, other parts of the body, and often with each other. Peripheral nerves consist of bundles of nerve fibers. Nerves conduct impulses at different speeds depending on their diameter.

The peripheral nervous system has two parts: the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of nerves that connect the brain and spinal cord with muscles controlled by conscious effort (voluntary or skeletal muscles) and with sensory receptors in the skin. (Sensory receptors are specialized endings of nerve fibers that detect information in and around the body.)

See the figure Typical Structure of a Nerve Cell.

The autonomic nervous system connects the brain stem and spinal cord with internal organs and regulates internal body processes that require no conscious effort. Examples are the rate of heart contractions, blood pressure, the rate of breathing, the amount of stomach acid secreted, and the speed at which food passes through the digestive tract. The autonomic nervous system has two divisions: the sympathetic and the parasympathetic. These divisions work together, usually with one activating and the other inhibiting the actions of internal organs. The main function of the sympathetic division is to prepare the body for stressful or emergency situations--for fight or flight. The main function of the parasympathetic division is to prepare the body for ordinary situations. For example, the sympathetic division increases pulse, blood pressure, and breathing rates, and the parasympathetic system decreases each of them.

Cranial and Spinal Nerves: Nerves that connect the brain with the eyes, ears, nose, and throat and with various parts of the head, neck, and trunk are called cranial nerves; there are 12 pairs of them (see Section 6, Chapter 96). Nerves that connect the spinal cord with other parts of the body are called spinal nerves. The brain communicates with most of the body through the spinal nerves. There are 31 pairs of them, located at intervals along the length of the spinal cord (see Section 6, Chapter 93). Several cranial nerves and most spinal nerves are involved in both the somatic and autonomic parts of the peripheral nervous system.

Spinal nerves emerge from the spinal cord through spaces between the vertebrae. Each nerve emerges as two short branches (called spinal nerve roots): one at the front of the spinal cord and one at the back. Nerves at the front, which are motor nerves, carry commands from the brain and spinal cord to other parts of the body, particularly to skeletal muscles. Those at the back, which are sensory nerves, carry sensory information (about body position, light, touch, temperature, and pain) to the brain from other parts of the body. Each sensory nerve carries information from a specific area of the body, called a dermatome (see Section 6, Chapter 93).

After leaving the spinal cord, some spinal nerves form networks of interwoven nerves, called nerve plexuses. In a plexus, nerve fibers from different spinal nerves are sorted and recombined so that all fibers going to or coming from one area of a specific body part are put together in one nerve (see Section 6, Chapter 95).