Study Notes on Spinal Cord Motor Function and Reflexes

Peripheral Motor System and Spinal Cord Function

Overview

  • Discussion focus: Spinal cord motor function,
    • Importance of coordination of movement at the spinal cord level.
    • Examination of spinal reflexes.
    • Signs of lower motor neuron lesions.

Reflexes

  • Definition: Reflexes are involuntary motor responses to an external stimulus.
    • Clinical examination of reflexes is crucial for understanding peripheral and central nervous system functionality.
    • Spinal reflexes can operate independently of brain input but brain does modulate stretch reflexes through spinal cord neural activity.

Basic Stretch Reflex

  • Definition: A muscle contraction that occurs in response to a quick stretch (Monosynaptic reflex).
  • Example: Quad tendon tapped with a reflex hammer.
    • Mechanism: Quick stretch activates muscle spindles within fibers.
    • Sensory pathway: Type 1A fibers carry information to the spinal cord.
    • Alpha motor neurons (one of two types of lower motor neurons) get depolarized, leading to muscle fiber contraction.

Reciprocal Inhibition

  • Definition: Simultaneous action during a muscle stretch reflex, where an antagonistic muscle is inhibited from contracting.
  • Example: In the same quad tendon tapping scenario,
    • 1A fibers project to the spinal cord and inhibit alpha motor neurons for the hamstring, preventing contraction of the hamstring muscle.

Golgi Tendon Organ and Autogenic Inhibition

  • Function: The Golgi tendon organ senses tension in muscles.
  • Reflex responsibility: Modulates muscle contraction during movement.
    • Known as autogenic inhibition.
  • Example: Quad tendon tapped, stretch sensed by muscle spindle and tension sensed by Golgi tendon organ.
    • 1B sensory axon fibers relay information to inhibit alpha motor neurons and prevent quad muscle contraction.

Other Spinal Reflexes

Flexor Withdrawal Reflex
  • Occurrence: Activated by noxious stimulus applied to a limb.
  • Mechanism: Sensory information travels to the spinal cord,
    • Signals alpha motor neurons to stimulate flexor muscles, causing limb to move away from the noxious stimulus.
  • Example: If the right lower limb contracts to withdraw from a stimulus,
    • The opposite leg (left limb) remains extended due to stimulation of alpha motor neurons on the other side of the spinal cord, ensuring balance.
Cross-Extension Reflex
  • Function: Occurs simultaneously with the flexor withdrawal reflex to maintain balance during withdrawal.

Lower Motor Neuron Lesions

  • Causes: Trauma, demyelinating diseases, infections, chronic neuropathy.
  • Signs:
    1. Hyporeflexia: Decrease or loss of reflexes.
    2. Paresis: Weakness of muscles; may lead to paralysis if severe.
    3. Muscle Atrophy: Loss of muscle bulk due to disuse or nervous system damage.
    4. Abnormal Muscle Tone:
    • Hypotonia: Decreased tone.
    • Flaccidity: Complete lack of tone in muscles.

Key Takeaways

  • Spinal reflexes can occur without brain input.
  • Deep tendon reflex (muscle stretch reflex) is a monosynaptic reflex initiated by a quick muscle stretch.
  • Reciprocal inhibition prevents antagonist muscle contraction, whereas Golgi tendon reflex (autogenic inhibition) checks muscle tension.
  • Flexor withdrawal reflex reacts to harmful stimuli, while cross-extension reflex aids in balance.
  • Recognizing signs of lower motor neuron lesions: hyporeflexia, paresis, paralysis, muscle atrophy, hypotonia, and flaccidity is essential in patient assessments.