Lecture on Nerves

Definition of a Nerve

• Enclosed, cable-like bundle of nerve fibers (axons).
• Part of the peripheral nervous system (PNS).
• Transmit electrical impulses (action potentials).

Function

• Provides pathways for nerve impulses:
  • Transmits to peripheral organs.
  • Sensory nerves transmit from periphery to the central nervous system (CNS).

Structure of a Peripheral Nerve

• Each axon is an extension of an individual neuron.
• Supported by Schwann cells which coat axons in myelin.
• Modular, wrapped in connective tissue similar to muscle fibers:
  • Endoneurium: Wraps individual axons.
  • Perineurium: Encloses bundles of axons known as fascicles.
  • Epineurium: Encloses entire nerve.
• Blood vessels (arteries and veins) travel within the nerve.

Types of Nerves

1. Cranial Nerves

   • 12 pairs originating from the brain.
   • Can be sensory, motor, or both.
   • Travel through foramen in the skull.

2. Spinal Nerves

   • Consistent anatomical structure and pattern.
   • Emerges from the spinal cord:
     • Dorsal Roots: Afferent, sensory, form a dorsal root ganglion.
     • Ventral Roots: Efferent, motor, no ganglion.
   • Merge to form spinal nerves which split into:
     • Dorsal Ramus: Innervates muscles, joints, and skin of the back.
     • Ventral Ramus: Innervates lateral, anterior trunk, and limbs.
   • Potential communicating rami with sympathetic nervous system axons.

Neuron Pathways

• Dorsal Root: Sensory, afferent ("arrives" at the CNS), pseudo-unipolar cells with cell bodies in dorsal ganglia.
• Ventral Root: Motor, efferent ("exits" the CNS), multipolar cells with cell bodies in the CNS.
• Interneurons: Illustrated in gray, within the CNS.

Mnemonics

• Afferent vs. Efferent:
  • Afferent Arrives (A A), Efferent Exits (E E).
  • SAME DAVE:
    • SAME: Sensory Afferent, Motor Efferent.
    • DAVE: Dorsal Afferent, Ventral Efferent.
• Sensory Line:
  • Pseudo-unipolar cells in dorsal ganglia.
• Motor Efferent Line:
  • Multipolar cells in CNS controlling skeletal muscles, exiting through ventral route.