The Peripheral Nervous System

The Peripheral Nervous System

Overview

• The peripheral nervous system (PNS) includes all parts of the nervous system except the brain and spinal cord.
• Function: Communication between the central nervous system (CNS) and the rest of the body.
• Components:
  • Cranial nerves
  • Spinal nerves
• Function Types:
  • Somatic (voluntary) function
  • Autonomic (automatic, subconscious) function

Divisions

• Sensory (afferent) division:
  • Somatic and visceral sensory nerve fibers.
  • Function: Conducts impulses from receptors to the CNS.
• Motor (efferent) division:
  • Motor nerve fibers.
  • Function: Conducts impulses from the CNS to effectors (muscles and glands).
  • Autonomic Nervous System (ANS):
    • Visceral motor (involuntary).
    • Function: Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands.
    • Sympathetic division:
      • Mobilizes body systems during activity ("fight or flight").
    • Parasympathetic division:
      • Conserves energy.
      • Promotes "housekeeping" functions during rest ("rest and digest").
  • Somatic Nervous System:
    • Somatic motor (voluntary).
    • Function: Conducts impulses from the CNS to skeletal muscles.

Nerve Structure

• Peripheral nerves are made of neuron axons; one neuron can give rise to thousands of axons.
• Like muscles, the axons have layers of protective wrappings, as well as blood supplies.
• Protective Layers (from outer to inner):
  • Epineurium
  • Perineurium (surrounds fascicles)
  • Endoneurium

Spinal Nerves

• Spinal nerves come in pairs (right and left side of the body).
• There are 31 pairs of nerves that exit the central nervous system via nerve roots between vertebrae.
• Nerve roots represent the transition from the spinal cord (central nervous system) to the peripheral nervous systems' spinal nerves; they are part of the peripheral nervous system.

Neural Plexuses

• The 31 pairs of peripheral spinal nerves are organized into neural plexuses in the cervical, brachial, and lumbosacral regions of the body.
• Cervical Plexus: Comprised of cervical spinal nerves 1-4 (C01-C04).
• Brachial Plexus: Extends to the arm; comprised of cervical spinal nerves 5-8 and the 1st thoracic nerve (C05-T01).
• Lumbar Plexus: Includes the 12th thoracic nerve and lumbar nerves 1-4 (T12-L04).
• Sacral Plexus: Includes lumbar nerves 4 and 5, as well as all of the sacral nerves.
• There can be overlap or slight variation in which nerves are accounted for as part of the lumbar vs. the sacral plexus. Often, the lumbosacral plexus is referred to as a single entity.

Neural Plexuses and Peripheral Nerves

• Neural plexuses give rise to the peripheral nerves that continue into the body's periphery.

• Nerves arising from Plexuses:

  • Cervical Plexus
    • Phrenic nerve
  • Brachial Plexus
    • Axillary nerve
    • Median nerve
    • Radial nerve
    • Ulnar nerve
  • Lumbar Plexus
    • Femoral nerve
    • Obturator nerve
  • Sacral Plexus
    • Common fibular nerve
    • Sciatic nerve
    • Tibial nerve