Lecture Notes: Muscle Stretch Reflex

Introduction to Reflexes

• Definition: Reflex is a response to a stimulus that occurs without conscious thought.
• Components of Reflexes:
  • Afferent Part: Brings information about a stimulus to the central nervous system (CNS).
    • Involves receptors detecting stimuli.
    • Neurons carry this information into the CNS.
  • Efferent Part: Carries response information from the CNS to the periphery.
    • Neurons carry information away from the CNS to cause a response.

Types of Reflexes

• Reflexes like the muscle stretch reflex occur on the same side, with both afferent and efferent parts on the same side of the body.
• Other reflexes can involve both sides of the body, especially those in the brain stem.

The Muscle Stretch Reflex

• Example: Muscle stretch reflex, particularly the knee-jerk reflex.
• Mechanism:
  • When a skeletal muscle is rapidly stretched, it contracts quickly as a protective response.
  • Real-world Example: The knee-jerk reflex when a doctor taps the tendon below the kneecap.
    • Stretching of the tendon causes the muscle in the thigh to contract involuntarily, resulting in the leg kicking out.

Details of the Knee-Jerk Reflex

• Muscle Spindles:
  • Receptors called muscle spindles detect the stretch.
  • These contain specialized fibers that are stretched along with the muscle.
• Afferent Pathway:
  • Neuron axons wrapped around muscle spindle fibers send stretch information to the CNS.
  • These are somatosensory neurons, forming the afferent part of the reflex.
  • They synapse on neurons in the CNS.

Efferent Pathway and Reflex Response

• Neurons send signals back to the skeletal muscle via lower motor neurons (efferent part).
• Outcome: Causes contraction of the stretched muscle.
• Diminished Reflexes:
  • Can occur due to issues with either afferent or efferent pathways.
  • Problems with somatosensory neurons or lower motor neurons can result in reduced reflex response.

Reflexes and the Central Nervous System

• Reflex actions occur at lower CNS levels (spinal cord or brain stem) without involving higher CNS functions (cognition, emotion, consciousness).

Additional Reflex Mechanisms

• Reciprocal Inhibition:
  • While the stretched muscle contracts, the opposing muscle relaxes.
  • Achieved by inhibitory neurons inhibiting lower motor neurons of the opposing muscle.
• Purpose: Enhances the response by preventing opposition from the antagonist muscle.

Conclusion

• Reflexes serve as protective mechanisms and maintain balance in responses within the nervous system.
• Understanding reflex pathways helps in diagnosing neurological conditions.