Muscle Response in Reflex Arcs: Contraction vs. Relaxation Mechanisms

In a reflex arc, the motor response is either contraction (agonist activation) or relaxation (antagonist inhibition), depending on the reflex type and functional goal.

Contraction‑Type Reflexes

• Stretch (myotatic) reflex: Muscle spindle Ia afferent → α‑motor neuron → homonymous muscle contraction (e.g., patellar tendon tap → quadriceps contraction). Maintains tone, resists lengthening.
• Withdrawal (flexor) reflex: Nociceptor → interneuron → flexor contraction + extensor inhibition (reciprocal inhibition) → limb withdrawal from pain.
• Crossed extensor reflex: Contralateral extensor contraction for postural support during withdrawal.

Relaxation‑Type Reflexes (Inhibition)

• Inverse stretch (Golgi tendon) reflex: Ib afferent from GTO → interneuron → inhibition of homonymous α‑motor neuron + antagonist facilitation. Protects against excessive tendon tension.
• Reciprocal inhibition: Ia afferent excites agonist while inhibiting antagonist via Ia inhibitory interneuron (enables smooth movement).
• Autogenic inhibition: Sustained stretch → GTO discharge → muscle relaxation (basis of PNF stretching).

Neural Circuitry

All reflexes follow: Receptor → Afferent neuron → Interneuron(s) → Efferent neuron → Effector (muscle). The interneuron determines excitation vs. inhibition. Supraspinal pathways (corticospinal, reticulospinal) modulate reflex gain.

Clinical Relevance

• Hyperreflexia (UMN lesion) → loss of inhibition → spasticity, clonus.
• Hyporeflexia/areflexia (LMN lesion, peripheral neuropathy).
• Spasticity management leverages reciprocal inhibition & autogenic inhibition (stretching, baclofen, botulinum toxin).

Reflexes are the spinal cord’s “autopilot” for protection and postural tone.