Week Nine Lecture - Stretch Reflex and Motor Neurons

Week Nine Lecture Notes on the Stretch Reflex

Overview

  • Week nine's lecture focuses on the stretch reflex, building on previous knowledge about plexuses and peripheral nerves.
  • Emphasis on understanding the stretch reflex is critical due to its implications in clinical diagnosis, particularly in distinguishing lesions in the central versus peripheral nervous system.

Terminology

Motor Neurons

  • Upper Motor Neuron (UMN):

    • Definition: A motor neuron located in the central nervous system (CNS).
    • Example: Neurons in the corticospinal tracts originating from the primary motor cortex. These neurons travel through the corona radiata and internal capsule to the brainstem, and ultimately synapse on lower motor neurons in the spinal cord.
    • Characteristic: Synapse on lower motor neurons.

  • Lower Motor Neuron (LMN):

    • Definition: A motor neuron located in the peripheral nervous system (PNS); originates in the CNS but extends into the PNS.
    • Examples: Neurons composing spinal nerves or cranial nerves that project to skeletal muscles.
    • Characteristic: Synapses with muscles, causing contraction.

  • Types of Motor Neurons:

    • Cavalcade of upper motor neurons includes:
    • Corticospinal Neurons: Motor neurons from the motor cortex to the spinal cord.
    • Corticobulbar Neurons: Motor neurons from the motor cortex to the brainstem.

Stretch Reflex

Definition

  • A stretch reflex is defined as:
    • A fast, involuntary motor response to muscle stretch, critical for maintaining muscle tone and posture.
    • Occurs without cortical involvement; the action happens locally at the level of the spinal cord or brainstem.

General Characteristics

  • Reflexes, including the stretch reflex, are:

    • Fast
    • Involuntary
    • Unplanned responses to sensory inputs.

  • Examples of Other Reflexes:

    • Chewing Reflex: Involves proprioceptors that detect stretch in masticatory muscles, leading to reflex contraction through the trigeminal nerve.
    • Pupillary Light Reflex: Triggered by light exposure, causing constriction of the iris via the oculomotor nerve.

Mechanism of the Stretch Reflex

Reflex Arc Components

  1. Sensory Afferent Arm:
    • Includes sensory neurons that respond to muscle stretch via muscle spindles (proprioceptors).
  2. Motor Efferent Arm:
    • Comprises lower motor neurons that innervate muscles to cause contraction and response to the stretch.

Reflex Arc Diagram

  • Structure includes:
    • Sensory afferents carrying signals from muscle spindles (via spinal nerves) into the dorsal horn of the spinal cord.
    • Direct synaptic connections to ventral horn motor neurons, resulting in the contraction of the same muscle that experienced stretch.

Types of Reflexes

  • Monosynaptic Reflex: Simplest form where the sensory neuron directly synapses with the motor neuron with no interneurons involved (example: patellar reflex).

Deep Tendon Reflexes (DTRs)

  • Examples include:
    • Patellar Reflex (knee jerk)
    • Biceps Reflex
    • Achilles Reflex
    • Clinical significance in neuroexamination to assess stretch reflex integrity.

Muscle Spindles

Definition & Structure

  • Muscle Spindles:
    • Proprioceptors located in striated muscles that detect stretch.
    • Comprised of intrafusal fibers embedded among extrafusal muscle fibers.
    • Circle and annular spiral endings serve as sensory inputs; separated into two types:
    • Type I A fibers (primary endings): Thicker and more myelinated, more sensitive to stretch.
    • Type II fibers (secondary endings): Thinner and less sensitive.

Functionality

  • Muscle spindles provide unconscious proprioception, maintaining muscle tone and posture.
  • Regulated by gamma motor neurons that adjust spindle tension and thus modify the sensitivity of the stretch reflex.

Regulation of the Stretch Reflex

Sensitivity Regulation

  • Gamma Motor Neurons:
    • Innervate intrafusal fibers at muscle spindle ends, causing contraction and influencing spindle sensitivity.
  • Increased sensitivity results in an upregulated stretch reflex leading to hypertonia; decreased sensitivity causes downregulated reflex and hypotonia.

Clinical Significance of Regulation

  • Upper Motor Neuron (UMN) Lesion:

    • Results in loss of inhibition over gamma motor neurons, causing hyperactivity of stretch reflexes (hypertonia) and exaggerated reflex responses (hyperreflexia).
    • Symptoms: Spasticity, weakness, decreased voluntary control.
    • Example Condition: Cerebral Palsy.

  • Lower Motor Neuron (LMN) Lesion:

    • Results in denervation of muscle affected, leading to loss of motor function and reduced stretch reflex (hyporeflexia).
    • Symptoms: Flaccid paralysis, atrophy, and fasciculations due to lack of innervation.

Summary of Divergences between UMN and LMN Lesions

  • Both lesions present with weakness or paralysis.
  • UMN lesions: Increased tone & hyperreflexia (spastic).
  • LMN lesions: Decreased tone & hyporeflexia (flaccid).
  • UMN lesions show no atrophy, whereas LMN lesions show significant muscle atrophy and fasciculations.

Conclusion

  • Understanding the stretch reflex is crucial for clinical evaluations and diagnostics, allowing differentiation between upper and lower motor neuron lesions based on reflex activity and muscle tone assessments.