The peripheral nervous system (PNS) sits outside the spinal cord and brain within the nervous system and makes up the central nervous system (CNS). Bundles of axons, or nerve fibers, transfer information to and from the CNS. Distributed through both nervous systems, the autonomic nervous system (ANS) innervates involuntary structures, including smooth muscle, heart, and glands.
Primarily, the PNS delivers information to the brain from the various senses and carries signals that enable muscle movements. The PNS also provides signals the brain employs to control breathing, heartbeat, digestion, and other unconscious, life-essential processes. Thoughts manage somatic nervous functions while autonomic functions run automatically.
The PNS is so critical because it filters stimuli from the outside world, through the senses, into information that the brain applies in directing action. Most peripheral nerves accomplish this through a route that runs through the spinal cord.
Branching directly from the CNS and spinal cord, spinal nerves begin as multiple rootlets, which combine to form two primary roots, the anterior and posterior. The roots carry motor fibers that form the spinal nerve proper and connect with various body parts. Spinal nerves exit the vertebral column via intervertebral foramina, which sits between successive adjacent vertebrae. These spinal nerves further divide into branches known as rami as they travel outward. The body’s longest nerve is the sciatic nerve, which originates the spine’s lumber region and has branches that extend to the toe tips. This gives it an adult length, on average, of more than a meter.
The PNS also encompasses cranial nerves that are distinct in connecting directly with the brain. They transmit signals from organs such as the ears, nose, and mouth to the brain, providing a sense of touch in the neck, head, and face.
All nerves contain axons, or cord-like fiber groupings, at the center of each nerve. Dendrite branches transfer electrical impulses, while the connective tissue layer endoneurium surrounds and protects the axons, as the perineurium surrounds axons known as fascicles. Finally, the epineurium makes up a connective tissue layer covering the nerve’s outer surface.
Since the PNS nerves have no protection, they become more vulnerable to trauma than CNS nerves. Injuries may occur at any point in the PNS that forms the connection between those nerves and the regions of the body they extend to, resulting in pain, numbness, loss of muscle function, and partial paralysis.
The type of neurons they possess and the direction of the information flow characterize nerves in the PNS. Neurons include motor, sensory, or mixed tasks. Afferent nerves receive sensory input, deliver it to the CNS, and encode the appropriate sensations, such as pain, touch, temperature, and proprioception (position in space).
Afferent nerves also deliver information about homeostasis or the organs’ inner state, processed at the subconscious level. This provides the brain with a continuous data flow regarding the stability and health of various systems without conscious monitoring. For example, these nerves provide the brain with vital information on the energy intake of specific organs.
By contrast, efferent nerves possess neurons that transmit signals from the CNS to the various muscles and organs in the periphery. For example, a signal by the brain to jump results in efferent motor neurons interacting with the skeletal muscles to execute a voluntary jump movement. In addition to skeletal muscles, efferent nerves also transmit information to effector organs, such as glands, and visceral organs, such as the heart and liver, and accomplish tasks such as gland secretion.
