Chapter 1: Introduction to the Peripheral Nervous System

Week Nine Lecture Notes

Overview of Lecture

  • Week nine is influenced by Labor Day, leading to a reduced schedule with only one lecture.
  • Focus areas are:
    • Plexuses and peripheral nerves
    • Stretch reflex

Structure of Lecture

  • Divided into two main sections:
    • Section One: Plexuses and Peripheral Nerves
    • Section Two: Stretch Reflex
  • Importance of Familiarizing with Specific Learning Outcomes
    • Specific learning outcomes relate directly to examination content.
    • Essential for preparing for assessments and final exams.

Specific Learning Outcomes

  1. Understanding the plexuses and the peripheral nerves derived from them.
  2. Identifying sensory and motor functions of the peripheral nerves.
  3. Study of the stretch reflex.
  4. Familiarity with lesions or pathologies related to peripheral nerves, specifically R equina syndrome.
  5. Classification of sensory and motor neurons in peripheral nerves.

Section One: Plexuses and Peripheral Nerves

Peripheral Nervous System

  • Focus on peripheral nerves, which consist of many axons/fibers grouped together.
  • A nerve is structured with fibers surrounded by protective membranes:
    • Epineurium: outer membrane surrounding the entire nerve.
    • Perineurium: membrane surrounding bundles of fibers (fascicles).
    • Endoneurium: membrane surrounding individual fibers/axons.

Types of Nerve Fibers

  • Sensory (afferent) and Motor (efferent) fibers classified histologically.
  • Classification based on:
    1. Diameter of the axon.
    2. Myelination.
  • Conduction velocity is affected by both the diameter and level of myelination.

Classification Schemes

  • Motor Neurons Classification (Erlinger and Gasser, 1930s-40s):
    • A fibers (Alpha, Beta, Gamma, Delta): Myelinated.
    • B fibers: Less myelinated.
    • C fibers: Unmyelinated and slowest conduction velocities.
  • Sensory Neurons Classification (Lloyd, 1940s):
    • 1A, 1B, 2 (for muscle spindles and Golgi tendon organs), 3, 4 (for pain and crude touch).

Hierarchy of Conduction Velocity

  • Higher myelination and larger diameter yield higher conduction velocity.
  • Velocity classification:
    • A-alpha: Fastest conduction (e.g., motor to skeletal muscle).
    • A beta: Touch and pressure receptors.
    • A delta: Pain and temperature (slow).
    • C fibers: Slow pain, temperature (unmyelinated).

Structure of Peripheral Nerves

  • Nerves can be:
    • Mixed (sensory and motor).
    • Purely sensory or purely motor.
  • Cranial Nerves: Some are pure motor, some pure sensory, others mixed.
  • Most spinal nerves are mixed except cervical nerve C1 (purely motor).

Spinal Nerves and Plexuses

  • Spinal Nerve Formation:
    • Entry through intervertebral foramina, splits into dorsal root (sensory) and ventral root (motor).
    • Dorsal ramus supplies the back; ventral ramus forms plexuses (except in thoracic region).

Major Plexuses

  1. Cervical Plexus: C1-C4 (includes contribution from C5 to brachial plexus).
  2. Brachial Plexus: C5-T1.
  3. Lumbar Plexus: L1-L4.
  4. Sacral Plexus: L4-S4.
  5. Coccygeal Plexus: S5 and coccygeal.

Detailed Analysis of Cervical Plexus

  • Roots: C1-C4, located deep to the sternocleidomastoid muscle.
  • Cutaneous Branches:
    1. Lesser occipital (C2-C3) - skin over the back of the head.
    2. Great auricular (C2-C3) - skin over the parotid and mastoid region.
    3. Transverse cervical (C2-C3) - skin across the neck.
    4. Supraclavicular (C3-C4) - skin over the clavicle.
  • Motor Branches:
    • Muscular branches to: sternomastoid, trapezius, infra hyoid muscles (ansa cervicalis).
    • Phrenic nerve (C3-C5) - supplies diaphragm, essential for respiration.

Functional Significance

  • Every peripheral nerve can be classified as either sensory or motor based on its function.
  • Injuries to nerves can result in motor deficits, sensory loss, or both.

Section Two: Stretch Reflex

  • The stretch reflex is a fundamental reflex mechanism.
  • Importance in clinical neurology for understanding motor neuron and sensory input.
  • Stretch reflexes involve sensory neurons that communicate directly with motor neurons.

Clinical Relevance

  • Understanding the stretch reflex is key for interpreting upper and lower motor neuron lesions.
  • Importance of thorough knowledge for practical applications in clinical settings.

Peripheral Nerve Pathologies

Cauda Equina Syndrome

  1. Definition: Impaired function of multiple nerves from the cauda equina, including lumbar and sacral nerves.
  2. Causes: Traumatic injury, intervertebral disc herniation.
  3. Symptoms:
    • Lower back pain.
    • Loss of sensory and motor function in lower limbs.
    • Saddle anesthesia.
    • Bowel and bladder dysfunction due to injury affecting autonomic nerves.
    • Sexual dysfunction.

Summary

  • This comprehensive exploration of peripheral nervous system highlights the importance of understanding spinal nerves, plexuses, sensory and motor neuron classifications, and critical pathologies like cauda equina syndrome.The stretch reflex mechanism serves as a foundational topic further linking both practical and clinical applications in neurology studies.