128. Spinal Reflexes – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Spinal reflexes are rapid, automatic, and stereotyped motor responses to specific sensory stimuli. They maintain muscle tone, posture, coordinated movement, and protect the body from injury. Higher motor centers regulate reflexes by modifying the sensitivity of spinal reflex pathways. A reflex arc consists of a receptor, sensory neuron, interneuron, motor neuron, and effector muscle.
• The major spinal reflexes are stretch reflex, inverse stretch reflex, and withdrawal reflex.
• Stretch (myotatic) reflex is the contraction of a muscle in response to its own stretch.
  • Receptor: Muscle spindle.
  • Afferent fibers: Group Ia (and Group II for tonic reflex).
  • Efferent neuron: Alpha motor neuron.
  • It is a monosynaptic reflex.
  • Phasic stretch reflex is produced by sudden stretch and causes rapid contraction (e.g., patellar tendon reflex).
  • Tonic stretch reflex is produced by sustained stretch and maintains muscle tone, posture, and antigravity function.
  • Reciprocal inhibition relaxes the antagonist muscle while the agonist contracts, ensuring smooth movement.
• Clinical importance of tendon reflexes:
  • They assess the integrity of the spinal cord and descending motor pathways.
  • Hyperreflexia suggests an upper motor neuron lesion with increased gamma motor neuron activity.
  • Hyporeflexia or areflexia indicates damage to the reflex arc or lower motor neurons.
• Inverse stretch (inverse myotatic) reflex is relaxation of a muscle in response to excessive tension.
  • Receptor: Golgi tendon organ.
  • Afferent fiber: Group Ib.
  • It is a disynaptic reflex mediated by inhibitory interneurons.
  • It produces autogenic inhibition, protects muscles and tendons from excessive force, and provides force feedback, whereas the muscle spindle provides length feedback.
• Gamma motor neurons regulate muscle spindle sensitivity. Increased gamma discharge enhances both phasic and tonic stretch reflexes, maintaining muscle tone and posture. Loss of descending inhibition causes hyperreflexia, spasticity, and hypertonia. Damage to lower motor neurons causes hypotonia (flaccidity).
• Clasp-knife phenomenon is a feature of spasticity in upper motor neuron lesions. Initial resistance to passive movement is followed by sudden relaxation due to activation of the Golgi tendon organ-mediated inverse stretch reflex. Clonus consists of rhythmic involuntary contractions produced by sustained stretch and is another sign of an upper motor neuron lesion.
• Withdrawal (flexor) reflex is a polysynaptic protective reflex that withdraws a body part from painful stimuli.
  • Receptor: Nociceptors.
  • Afferent fibers: Mainly Group III and IV.
  • Ipsilateral flexors contract while extensors relax (reciprocal inhibition).
  • The crossed extensor reflex extends the opposite limb to maintain balance.
  • Important features include irradiation, divergence, after-discharge, prepotency, and local sign.
• Properties of spinal reflexes include adequate stimulus, convergence, divergence, facilitation, inhibition, spatial and temporal summation, occlusion, habituation, and sensitization. Alpha motor neurons form the final common pathway through which all motor commands reach skeletal muscles.