Learn About the Peripheral Nervous System

Male Nervous System
Peripheral Nervous System. SCIEPRO/Science Photo Library/Getty Images

The nervous system consists of the brain, spinal cord, and a complex network of neurons. This system is responsible for sending, receiving, and interpreting information from all parts of the body. The nervous system monitors and coordinates internal organ function and responds to changes in the external environment. This system can be divided into two parts: the central nervous system (CNS) and the peripheral nervous system (PNS).

The CNS is composed of the brain and spinal cord, which function to receive, process, and send information to the PNS. The PNS consists of cranial nerves, spinal nerves, and billions of sensory and motor neurons. The primary function of the peripheral nervous system is to serve as a pathway of communication between the CNS and the rest of the body. While CNS organs have a protective covering of bone (brain-skull, spinal cord - spinal column), the nerves of the PNS are exposed and more vulnerable to injury.

Types of Cells

There are two types of cells in the peripheral nervous system. These cells carry information to (sensory nervous cells) and from (motor nervous cells) the central nervous system. Cells of the sensory nervous system send information to the CNS from internal organs or from external stimuli. Motor nervous system cells carry information from the CNS to organs, muscles, and glands.

Somatic and Autonomic Systems

The motor nervous system is divided into the somatic nervous system and the autonomic nervous system. The somatic nervous system controls skeletal muscle, as well as external sensory organs, such as the skin. This system is said to be voluntary because the responses can be controlled consciously.

Reflex reactions of skeletal muscle, however, are an exception. These are involuntary reactions to external stimuli.

The autonomic nervous system controls involuntary muscles, such as smooth and cardiac muscle. This system is also called the involuntary nervous system. The autonomic nervous system can further be divided into parasympathetic, sympathetic, enteric divisions.

The parasympathetic division functions to inhibit or slow down autonomic activities such as heart rate, pupil constriction, and bladder contraction. The nerves of the sympathetic division often have an opposite effect when they are located within the same organs as parasympathetic nerves. Nerves of the sympathetic division speed up heart rate, dilate pupils, and relax the bladder. The sympathetic system is also involved in the flight or fight response. This is a response to potential danger that results in accelerated heart rate and an increase in metabolic rate.

The enteric division of the autonomic nervous system controls the gastrointestinal system. It is composed of two sets of neural networks located within the walls of the digestive tract. These neurons control activities such as digestive motility and blood flow within the digestive system.

While the enteric nervous system can function independently, it also has connections with CNS allowing for the transfer of sensory information between the two systems.


The peripheral nervous system is divided into the following sections:

  • Sensory Nervous System - sends information to the CNS from internal organs or from external stimuli.
  • Motor Nervous System - carries information from the CNS to organs, muscles, and glands.
    • Somatic Nervous System - controls skeletal muscle as well as external sensory organs.
    • Autonomic Nervous System - controls involuntary muscles, such as smooth and cardiac muscle.
      • Sympathetic - controls activities that increase energy expenditures.
      • Parasympathetic - controls activities that conserve energy expenditures.
      • Enteric - controls digestive system activity.


Peripheral nervous system connections with various organs and structures of the body are established through cranial nerves and spinal nerves.

There are 12 pairs of cranial nerves in the brain that establish connections in the head and upper body, while 31 pairs of spinal nerves do the same for the rest of the body. While some cranial nerves contain only sensory neurons, most cranial nerves and all spinal nerves contain both motor and sensory neurons.