Peripheral nervous system (PNS): Nerves, ganglia and plexuses

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The peripheral nervous system (PNS) is composed of all the nervous structures outside the brain and spinal cord. It is functionally divided into somatic (voluntary) and autonomic (involuntary) nervous systems, with the latter being further subdivided into sympathetic, parasympathetic and enteric nervous systems. 

The PNS consists of all the nerves, ganglia and plexuses of the body. A nerve consists of a bundle of many nerve fibers (i.e., axons), their connective tissue coverings and nourishing blood vessels. Nerve fibers that carry information from the periphery towards the CNS are called afferent or sensory, while the ones transmitting impulses from the CNS to the periphery are known as efferent or motor. 

Nerves are also classified according to the place where they exit the CNS. Cranial nerves emerge from the brain while spinal nerves emerge from the spinal cord. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves. 

A ganglion is a group of neuron cell bodies outside the CNS. There are two main types of ganglia: sensory ganglia, which contain cell bodies of neurons responsible for sending information from the periphery to the CNS, and autonomic ganglia, which include sympathetic (close to the spinal cord) and parasympathetic (near or within the viscera) ganglia.

Finally, a plexus is a branching network of interconnected nerves. There are two main types of plexus: the spinal nerve plexuses, which are formed by the intersecting subdivisions of the anterior rami of the spinal nerves, and the autonomic plexuses which can be found in the thorax, abdomen, and pelvis where they are responsible for regulating the activity of visceral organs.

Learn more about nerves, ganglia and plexuses of the peripheral nervous system in this video!

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Review the structures of the PNS with the image galleries below. The first gallery reviews the structure of a nerve. The second gallery focuses on the types of nerves and ganglia. The third gallery displays the spinal plexuses.

Summary