Peripheral Nervous System

The human peripheral system has two types of nerves based on location:

Spinal and cranial nerves can also be classified on the basis of function:

There are two types of autonomic nerves the parasympathetic and sympathetic nerves:

Most organs receive input from both sympathetic and parasympathetic nerves, usually with opposite effects on the organ. Most organs are continually receiving both sympathetic and parasympathetic stimulation. For example, sympathetic nerves signal the heart to speed up and parasympathetic stimulate it to slow down. Which dominates depends on the situation.

The Spinal Cord

The spinal cord is a pathway for signal travel between the peripheral nervous system and the brain. The cord is also the center for controlling some reflex actions. Sensory and motor neurons make direct reflex connections in the spinal cord. Spinal reflexes do not involve the brain.

The spinal cord (and also the brain) is covered with tough membranes, the meninges, and resides within the protection of the stacked vertebrae.

Signals move up and down the spinal cord in bundles of sheathed axons. The tracts located on the periphery of the spinal cord glisten because of their myelin sheaths and are called white matter. The central, butterfly-shaped area (in cross-section) consists of unsheathed axons, dendrites, and cell bodies and is called gray matter.

This animation (Audio - Important) describes the autonomic nervous system.