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Peripheral Nervous System

The peripheral nervous system (PNS) refers those nerves (neurons) that are not part of the brain or spinal cord. Nerves inside the brain and spinal cord are part of the central nervous system (CNS). PNS nerves have their cell bodies (ganglia) inside the CNS but their length outside it. The PNS consists of neurons with both myelinated and unmyelinated axons. All the cranial nerves (the 12 pairs of nerves leaving the base of the braincase) are part of the peripheral nervous system although the second cranial nerve (the optic nerve) can be considered as part of the brain.

There are two kinds of nerve in the Peripheral Nervous System:

  1. Motor or efferent nerves. These are the nerves that connect to muscles in the limbs, torso, face and internal organs. They carry nerve signals from the CNS and make body parts move. The cell bodies of the motor nerves connect to CNS in the shaft of grey matter in the spine called the anterior horn. They terminate at a neuromuscular junction known as the motor plate or motor end plate. Neurotransmitters released by the motor nerves into the motor plate cause the muscle to contract.

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  3. Sensory or afferent nerves. These are nerves that connect to sensory organs and sensors in the skin, muscle and internal organs. They take relay nerve signals to the CNS. They convey heat, touch, position (proprioceptive), balance, sound and other information from the limbs and sense and internal organs. Sensory nerves terminate in the CNS at the root ganglion.
The peripheral nervous system is divided into two parts:
  1. The sensory-somatic nervous system (SNS).

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      The SNS is performs all our interactions with the physical world such as control of our limbs and receiving of conscious information from our senses. It consists of the 12 pairs of cranial nerves and the 31 pairs of spinal nerves.
  3. The autonomic nervous system (ANS).

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      The ANS consists of both motor and sensory nerves which run both to and from the central nervous system and fulfills "automatic" functions that we are seldom aware of such as heart activity and the release of hormones. The autonomic nervous system is further divided into the sympathetic nervous system and the parasympathetic nervous system.
Peripheral Nervous System links:
The Peripheral Level
The Nervous System (Peripheral Nervous System)
The Peripheral Nervous System
The Motor End Plate
Nervous System Lecture Notes
MS Glossary
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