PNS

Peripheral Nervous System Overview

Overview of Spinal Nerves

  • Spinal Nerve Characteristics
    • There are 31 pairs of spinal nerves (C1 to Co1).
    • Each spinal nerve is formed from the merger of:
    • Anterior (ventral) root:
      • Comprised of many axons of motor neurons.
      • Cell bodies (somas) located in the anterior and lateral horns of the spinal cord.
    • Posterior (dorsal) root:
      • Comprised of many axons of sensory neurons.
      • Cell bodies (somas) located in the posterior root ganglion.

Naming and Exit of Spinal Nerves

  • Each spinal nerve is named according to the part of the spinal cord from which it originates and a number based on its position.
    • Cervical Nerves:
    • Exit through intervertebral foramina superior to the corresponding vertebra (e.g., C2 nerve exits between C2 and C1 vertebrae).
    • Exception: C8 nerve exits below the C7 vertebra.
    • Thoracic, Lumbar, Sacral, and Coccygeal Nerves:
    • Exit inferior to the vertebra of the same number (e.g., T2 nerve exits between T2 and T3 vertebrae).
    • Lumbar, sacral, and coccygeal nerves have long roots that extend inferiorly before exiting the vertebrae, forming the cauda equina.

Distribution of Spinal Nerves

  • Upon exiting the intervertebral foramen, each spinal nerve subsequently divides into:
    • Posterior Ramus:
    • A small branch that innervates the muscles and skin of the back.
    • Anterior Ramus:
    • A larger branch that splits into multiple other branches.
    • Innervates the anterior and lateral trunk, upper limb, and lower limb at different levels.
    • Participates in the formation of plexuses.
    • Rami Communicantes:
    • Small branches of autonomic fibers extending between spinal nerve and the sympathetic trunk ganglion.
    • Ganglia in the sympathetic trunk are interconnected and run parallel to the vertebral column.

Dermatomes

  • Dermatome: A segment of skin that is supplied by a single spinal nerve.
    • Some overlap exists in innervated regions.
    • Example: T10 dermatome represents a horizontal ring of skin around the umbilicus.
    • Useful for localizing damage to one or more spinal nerves.
    • Example: Loss of sensation in the medial arm and forearm indicates potential damage to C8.
    • Important in referred visceral pain.
    • Example: Pain from appendicitis often felt in the T10 dermatome.

Nerve Plexuses

  • Nerve Plexus: A network of interweaving anterior rami of spinal nerves.
    • Four main plexuses occur bilaterally:
    • Cervical Plexus
    • Brachial Plexus
    • Lumbar Plexus
    • Sacral Plexus
    • Most thoracic spinal nerves and S5-Co1 nerves do not form plexuses.
    • Individual rami branch repeatedly, so damage to one nerve or spinal segment does not lead to total loss of innervation for a muscle or skin region.

Clinical View: Shingles

  • Shingles involve the reactivation of a chickenpox infection
    • The virus remains latent within the posterior root ganglia.
    • Upon reactivation, it travels through sensory axons to a dermatome, causing:
    • A rash and blisters along the dermatome.
    • Symptoms include burning and tingling pain.
    • Treatment: Antiviral medication to reduce severity along with a vaccine to prevent or reduce disease severity.

Characteristics of Reflexes

  • Reflexes: Involuntary responses that require a stimulus to initiate.
    • Characteristics include:
    • Rapid Response: Involves a chain of only a few neurons.
    • Preprogrammed: The response is always the same for a given stimulus.
    • Involuntary: No intention or awareness of the reflex prior to the occurrence.
  • Reflexes serve as survival mechanisms, ensuring responses to potentially harmful stimuli happen immediately, with awareness following.

Components of a Reflex Arc

  • Reflex Arc: The neural pathway responsible for generating a reflex response.
    • It consists of:
    • Sensory receptors
    • Sensory neurons
    • Interneurons
    • Motor neurons
    • Effectors (muscles or glands)

Types of Reflexes

  • Monosynaptic Reflexes: Involve a single synapse between a sensory and motor neuron.
  • Polysynaptic Reflexes: Involve one or more interneurons in the reflex pathway.

Spinal Reflexes

  • Stretch Reflex:
    • A reflexive contraction of a muscle following its extension.
    • Triggered by a muscle spindle receptor, which detects stretch in the muscle.
    • A muscle spindle contains intrafusal muscle fibers that are innervated by gamma motor neurons and wrapped by sensory neurons.
    • When muscles are stretched, the sensory axons from the spindle transmit impulses to the spinal cord:
    • Sensory axon stimulates alpha motor neurons for the same muscle, causing contraction (monosynaptic).
    • Simultaneously excites interneurons that inhibit motor neurons of the antagonist muscle (polysynaptic reciprocal inhibition).
    • Classified as: spinal, somatic, ipsilateral, and innate.

Golgi Tendon Reflex

  • Prevents excessive muscle contraction.
    • Involves Golgi tendon organs, proprioceptors located at the muscle-tendon junction.
    • When excessive tension is detected:
    • Their sensory axons excite interneurons in the spinal cord.
      • Some interneurons inhibit the motor neurons of the contracting muscle, leading to muscle relaxation (polysynaptic reflex).
      • Other interneurons may activate motor neurons for the antagonist muscle, leading to its contraction (reciprocal activation).

Withdrawal Reflex

  • Pulls body parts away from painful stimuli.
    • Triggered by nociceptor sensory neurons that transmit signals to the spinal cord where they excite interneurons.
    • Interneurons excite motor neurons of flexor muscles (e.g., hamstrings), resulting in muscle contraction and limb withdrawal.
    • At the same time, other interneurons inhibit motor neurons of extensor muscles (e.g., quadriceps), allowing for rapid withdrawal.

Crossed-Extensor Reflex

  • Occurs alongside the withdrawal reflex.
    • Involves some interneurons that are excited by the nociceptor crossing to the opposite side of the spinal cord to activate motor neurons of extensor muscles on the opposite leg.
    • For instance, when the left leg is withdrawn, the right leg's quadriceps are excited to support body weight as the injured limb withdrawal occurs.

Reflex Testing in a Clinical Setting

  • Reflexes are useful diagnostic tools.
    • Hypoactive Reflex:
    • Diminished or absent reflex response, may indicate:
      • Damage to the spinal cord, muscle disease, or injury at the neuromuscular junction.
    • Hyperactive Reflex:
    • Abnormally strong reflex response, may indicate:
      • Damage to the brain or spinal cord, particularly if accompanied by clonus (rhythmic oscillating movements during reflex testing).

Cranial Nerves

  • Cranial Nerves: Part of the Peripheral Nervous System (PNS) that originate from the brain.
    • Numbered using Roman numerals based on their position, starting with the most anterior nerve.
    • Names often relate to their corresponding functions.
Mnemonics for Cranial Nerve Identification
  • Identification Mnemonic:
    • "Oh, once Olivia takes the anatomy final, very good vacation awaits her."
  • Cranial Nerves:
    1. Olfactory
    2. Optic
    3. Oculomotor
    4. Trochlear
    5. Trigeminal
    6. Abducens
    7. Facial
    8. Vestibulocochlear
    9. Glossopharyngeal
    10. Vagus
    11. Accessory
    12. Hypoglossal
Mnemonic for Function of Cranial Nerves (Sensory, Motor, or Both)
  • Function Mnemonic:
    • "Some say marry money, but my brother says big boobs matter more."
  • Function of Cranial Nerves:
    1. Olfactory - Sensory
    2. Optic - Sensory
    3. Oculomotor - Motor
    4. Trochlear - Motor
    5. Trigeminal - Both
    6. Abducens - Motor
    7. Facial - Both
    8. Vestibulocochlear - Sensory
    9. Glossopharyngeal - Both
    10. Vagus - Both
    11. Accessory - Motor
    12. Hypoglossal - Motor
Individual Cranial Nerves and Functions
  • CN I (Olfactory Nerve): Responsible for the sense of smell.
  • CN II (Optic Nerve): Responsible for the sense of vision.
  • CN III (Oculomotor Nerve): Controls muscles responsible for eye movement, eyelid elevation, and pupil diameter changes.
  • CN IV (Trochlear Nerve): Controls the superior oblique eye muscle.
  • CN V (Trigeminal Nerve): Responsible for somatic sensation from the face and controls chewing movements.
  • CN VI (Abducens Nerve): Controls lateral rectus muscle that abducts the eye.
  • CN VII (Facial Nerve): Controls muscles involved in facial expression and signals taste from the tongue.
  • CN VIII (Vestibulocochlear Nerve): Responsible for senses related to hearing and equilibrium.
  • CN IX (Glossopharyngeal Nerve): Involved in taste and touch sensation from the tongue; also controls a muscle of the pharynx.
  • CN X (Vagus Nerve): Involved in visceral sensation; provides parasympathetic innervation to various body organs.
  • CN XI (Accessory Nerve): Controls muscles associated with the neck and pharynx.
  • CN XII (Hypoglossal Nerve): Controls muscles of the tongue.