Overview of the Peripheral Nervous System

Introduction

• Peripheral Nervous System (PNS): Comprises all the nerves outside the brain and spinal cord.
• Cranial Nerves: Nerves exiting the brain within the cranium.
• Spinal Nerves: Nerves exiting the spinal cord.
• Function: Enables bidirectional communication between the central nervous system (CNS) and the rest of the body.
• Components: 31 pairs of spinal nerves and 12 pairs of cranial nerves, totaling 43 paired nerves.

Classification of the PNS

Sensory System (Afferent)

• Function: Conveys impulses to the CNS.
• Types of Fibers:
  • Somatic Sensory Fibers: From skin receptors (pain, temperature, touch) and proprioceptors in muscles, joints.
  • Visceral Sensory Fibers: From internal organ receptors.

Motor System (Efferent)

• Function: Conveys impulses from the CNS to the periphery.
• Subdivisions:
  • Somatomotor System: Voluntary control over skeletal muscles.
  • Visceromotor (Autonomic Nervous System): Involuntary control over internal organs.

Autonomic Nervous System

• Branches:
  • Sympathetic Nervous System: "Fight or flight" response, originates from C8-L2.
  • Parasympathetic Nervous System: "Rest and digest" response, originates from S2-S4.
  • Enteric Nervous System: Controls digestive organs, includes myenteric and submucosal plexuses.

Clinical Facts on Motor Neurons

• Upper Motor Neurons: Located in the cerebral cortex.
  • Damage Effects: Spastic paralysis, hypertonia, hyperreflexia (e.g., cerebral palsy).
• Lower Motor Neurons: Located in anterior horns of the spinal cord and cranial nerve nuclei.
  • Damage Effects: Flaccid paralysis, hypotonia, hyporeflexia.

Spinal Nerve Structure

External and Internal Schemes

• Rootlets: Connect PNS to the spinal cord, merge to form the trunk.
• Trunk: Located outside the vertebrae.

Spinal Rootlets

• Anterior Horn: Somatomotor neurons.
• Posterior Horn: Somatosensory neurons.
• Lateral Horn: Visceromotor neurons.
• Intermediate Zone: Visceral sensory neurons.

Ganglia

• Spinal Ganglia: House cell bodies of afferent nerves.
• Chain (Paravertebral) Ganglia: Part of the sympathetic trunk.

Spinal Nerve Fibers

• Somatomotor Fibers: Regulate skeletal muscles.
• Visceromotor Fibers: Join sympathetic trunk via white rami communicants.
• Somatosensory Fibers: Collect sensory data from body surfaces.
• Viscerosensory Fibers: Collect sensory data from internal organs.

Branches of Spinal Nerve

Posterior Branch

• Function: Innervates back muscles, provides sensory innervation to skin.
• Examples: Suboccipital nerve, greater occipital nerve, superior clunial nerves.

Anterior Branch

• Function: Provides motor and sensory innervation to front body parts.
• Forms Plexuses:
  • Cervical Plexus (C1-C4)
  • Brachial Plexus (C4-T1)
  • Lumbar Plexus (T12-L4)
  • Sacral Plexus (L4-S4)

Conclusion

• Next steps involve detailed study of individual plexuses and their roles in the PNS.