| The peripheral nervous system is generally responsible for delivering messages from the CNS to the periphery and from the periphery to the CNS. The peripheral nervous system consists of 12 pairs of cranial nerves that originate from the brain, and 31 pairs of spinal nerves that originate from the spinal cord, and many branches of those nerves. Cranial nerves and spinal nerves together are called peripheral nerves. These nerves will be discussed in detail in other modules. Nerve fibers carrying information from the periphery to the CNS carry sensory information and are referred to as afferent nerves (these are really nerve fibers, but you will often hear them called nerves). Afferent nerves have receptors that detect different types of information, for example light, sound, touch, temperature, pain and stretch. These receptors are located primarily on the skin, but are also located in muscles, joints, and some organs to allow the brain to receive information about the location of various body parts (called proprioception) and the function of internal organs. Nerve fibers carrying information from the CNS to the periphery carry motor information to various effectors and are referred to as efferent nerves. Effectors are the structures that are altered by incoming information. The major effectors of the peripheral nervous system are skeletal muscle, cardiac muscle, smooth muscle of blood vessels and organs, and glands. Most nerves in the body carry both afferent and efferent nerve fibers, but some are exclusively motor or exclusively sensory. As soon as a nerve has entered the spinal cord, it is no longer considered part of the peripheral nervous system. Remember, the peripheral and central nervous systems are not actually separate systems, but rather two parts of one system.
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