Muscle Spindle and Golgi Tendon Reflexes — Stretch Reflex vs Inverse Stretch Reflex

A muscle spindle is an encapsulated sensory receptor that tells the nervous system about changes in muscle length. Each spindle contains specialised intrafusal fibres, lying parallel to the ordinary contracting (extrafusal) fibres. There are three types: nuclear bag 1, nuclear bag 2 and nuclear chain fibres, named after the arrangement of their nuclei.

The central (equatorial) part of an intrafusal fibre has almost no contractile filaments, so it senses stretch but does not contract; the polar ends contain contractile elements and are driven by gamma motoneurons. Spindles do not contribute significantly to the force of the muscle. Bag 1 afferents respond mainly to the velocity of stretch (dynamic), while bag 2 and chain afferents respond to the amount of stretch (static).

The stretch reflex is the basis of the deep tendon (muscle stretch) reflex. Tapping a tendon passively stretches its muscle. The stretch lengthens the intrafusal fibres, so Ia afferents fire. They enter through the dorsal root, and monosynaptically excite the alpha motoneuron supplying the same (homonymous) muscle; glutamate is the transmitter at this central synapse. The extrafusal fibres contract and resist the stretch. When the muscle contracts, the spindle's firing falls and the reflex ends.

1. Tendon tap causes passive stretch of the muscle and its spindles.
2. Ia afferent fires; cell body lies in the dorsal root ganglion.
3. Monosynaptic contact (glutamate) on the alpha motoneuron in the anterior horn.
4. Alpha motoneuron fires; extrafusal fibres of the same muscle contract.
5. Branches of the Ia fibre also excite a Ia inhibitory interneuron that inhibits the antagonist muscle (reciprocal inhibition).

In the knee jerk, the quadriceps contracts while the hamstrings are inhibited through the Ia inhibitory interneuron. This second pathway is polysynaptic, so the stretch reflex is called monosynaptic only for the agonist arc: just two neurons (one sensory, one motor) and one synapse.

Spindles do not contribute significantly to the force of the muscle. Gamma motoneurons adjust the tension of the intrafusal fibres — tightening or relaxing them — and so regulate the sensitivity of the spindle and the strength of the reflex response. Acetylcholine mediates this.

• Alpha motoneurons → extrafusal fibres → force and movement.
• Gamma motoneurons → intrafusal fibres → spindle sensitivity, not force.
• Gamma motoneurons make up about 30% of ventral horn motoneurons.
• The Ia afferent system is not a closed loop: descending upper motor neuron pathways influence it.

Golgi tendon organs (GTOs) are encapsulated afferent endings at the junction of muscle and tendon. Each is supplied by a single group Ib sensory axon, slightly smaller than the Ia axons of spindles. Unlike spindles, which lie parallel to the extrafusal fibres, GTOs lie in series with them. When a muscle is passively stretched, most of the lengthening occurs in the muscle fibres, which are more elastic than tendon, so GTOs barely respond. When the muscle contracts, the force acts directly on the tendon and the GTO fires — it is very sensitive to active tension but relatively insensitive to passive stretch.

The Ib axons contact inhibitory interneurons (Ib inhibitory interneurons) in the spinal cord, which synapse on the alpha motoneurons of the same muscle. This circuit is a negative feedback system for muscle tension: it decreases the activation of a muscle when exceptionally large forces are generated, which protects the muscle, and at lower forces it counteracts small changes in tension, for example the loss of force with fatigue. This is the inverse stretch reflex (also called autogenic inhibition).

Both receptors are part of the proprioceptive system. The Ia inhibitory interneuron of the stretch reflex inhibits the antagonist (reciprocal inhibition), whereas the Ib inhibitory interneuron of the GTO reflex inhibits the same muscle. Many exam questions simply ask which interneuron or which afferent belongs to which reflex.

The two reflexes are partners rather than rivals. When an agonist muscle is stretched, its Ia afferents excite the alpha motoneurons of that muscle and activate Ia inhibitory interneurons that suppress the antagonist — reciprocal inhibition. Together this circuit is the stretch reflex, which helps protect against muscle strain and supports posture and bipedal locomotion.

At the other end of the range, a sustained isometric contraction raises tension at the musculotendinous junction and activates Ib afferents, which excite inhibitory interneurons that inhibit the alpha motoneurons of the contracting muscle. Manual therapies such as muscle energy technique use exactly these two circuits: post-isometric relaxation exploits GTO-mediated autogenic inhibition after a contraction held for a few seconds, and reciprocal inhibition exploits the Ia pathway to relax the antagonist.

Deep tendon reflexes (DTRs) are the bedside use of the muscle stretch reflex, first described in 1875 by Erb and Westphal. Reflex hammers of about 80 to 140 g are used and the patient should be relaxed. Always compare with the opposite side.

• Hyperactive reflexes (brisk, with a lighter tap, sometimes spread to synergists or clonus) point to an upper motor neuron lesion, a suprasegmental lesion above the spinal reflex arc.
• Hypoactive or absent reflexes point to a lower motor neuron lesion (nerve root or peripheral nerve) or a break in the afferent limb.
• The Jendrassik manoeuvre (clench the teeth or pull the interlocked fingers apart) reinforces lower-limb reflexes by countering descending inhibition and distracting the patient.
• Some authors separate the tendon reflex (short latency) from the stretch reflex (long latency), but in practice DTR and muscle stretch reflex are used together.

• Which receptor monitors tension? — Golgi tendon organ (Ib), in series at the muscle-tendon junction.
• Which monitors length? — muscle spindle, in parallel with extrafusal fibres.
• Afferent fibre of the knee jerk — Ia; of the GTO — Ib.
• Efferent to intrafusal fibres — gamma motoneuron.
• Monosynaptic reflex — the stretch (myotatic) reflex; the GTO reflex is disynaptic with an inhibitory interneuron.
• Inverse stretch reflex = autogenic inhibition = inhibition and relaxation of the same muscle when tension is high.
• Root values — biceps C5–C6, triceps C7–C8, knee L2–L4, ankle S1–S2.
• Reflex grades and UMN vs LMN reflex changes.