In one breath
A strong stretch, or a forceful contraction, makes a muscle relax instead of contract: this is the inverse stretch (inverse myotatic) reflex, also called autogenic inhibition. Its receptor is the Golgi tendon organ, its afferent the Ib fibre, and it acts through an inhibitory interneuron on the muscle’s own α motor neurons, so it is disynaptic. γ motor neurons set how brisk the stretch reflexes are, which is why upper motor neuron lesions bring spasticity, the clasp-knife response and clonus.
Builds on: Golgi tendon organ · Stretch reflex · Leads to: Upper vs lower motor neuron lesions · Medullary integration and decerebrate rigidity
What it is
- Definition: relaxation of a muscle when it is stretched strongly.
- Other names: the inverse myotatic reflex, and autogenic inhibition (“autogenic” because the muscle’s own receptor switches the muscle off).
Reflex arc
Receptors
- Golgi tendon organs (GTO), found in the tendon: each is a net of nerve endings with knob-like tips, spread among the tendon’s fascicles.
- One tendon organ serves 3–25 muscle fibres.
Stimulus
- A stronger stretch than the one that sets off the stretch reflex.
- An active contraction of the muscle sets it off too.
Reflex circuit
| Link | In the inverse stretch reflex |
|---|---|
| Receptor | Golgi tendon organ |
| Afferent | Group Ib fibre |
| Relay | Inhibitory interneuron |
| Target | Homonymous α motor neurons |
| Result | The same (agonist) muscle is inhibited |
- Afferent to interneuron, then interneuron to motor neuron: two synapses, so the reflex is disynaptic.
Draw it: the inverse stretch reflex
Draw the same cord cross-section as for the stretch reflex, with one muscle beside it. Put the tendon organ in the tendon at the end of the muscle, not in its belly, and run its Ib fibre in through the dorsal root. In the grey matter, end the Ib fibre on a black inhibitory interneuron, and from that draw a ”−” synapse on the α motor neuron of the same muscle. Label “2 synapses”, “autogenic inhibition” and “muscle relaxes”.
Disynaptic, not monosynaptic
The stretch reflex has one synapse. The inverse stretch reflex has two, because the Ib fibre reaches the motor neuron only through an inhibitory interneuron. When an MCQ lists “monosynaptic” as a feature of the inverse stretch reflex, that is the false statement.
Functions
- The tendon organ measures the force in the muscle, whether that force comes from a strong stretch or from the muscle’s own contraction.
- When it fires, its reflex connections inhibit the agonist, the very muscle it sits in, and that muscle relaxes.
- A strong enough pull on a muscle therefore switches the muscle off by itself, which is why the reflex is called autogenic inhibition.
Physiological significance
| Muscle spindle | Golgi tendon organ | |
|---|---|---|
| Reflex | Stretch reflex | Inverse stretch reflex |
| Senses | Length | Tension (force) |
| Feedback | Length feedback | Force feedback |
| Afferent | Ia, II | Ib |
| Synapses | One | Two |
| Own muscle | Contracts | Relaxes |
- The two reflexes work as a pair and keep muscle length and tension at the right values for postural adjustments.
- By letting the muscle relax, the inverse stretch reflex protects it from tearing when it is stretched too far.
γ motor neurons, reflexes and tone
- γ motor neurons are the volume knob of the stretch reflexes: they set the spindle’s sensitivity to stretch, and so how strongly the reflex answers.
- The spindle’s own afferents do not synapse on γ motor neurons, so the spindle cannot drive them.
- What drives them is descending input from above the cord: more γ discharge → a more sensitive spindle → livelier stretch reflexes.
- In upper motor neuron paralysis, for example in the recovery phase after spinal transection, the inhibitory input from above is lost, so γ discharge rises:
- the phasic stretch reflexes grow, and the muscle becomes hyper-reactive (brisk jerks);
- the tonic stretch reflexes grow, and the muscle becomes hypertonic.
Muscle tone
Muscle tone is the resistance a muscle offers to being stretched.
| State | Cause |
|---|---|
| Hypertonia (spasticity) | Overactive stretch reflexes: γ discharge ↑ |
| Hypotonia (flaccidity) | Motor neurons damaged, or γ discharge ↓ |
Applied: tone in motor lesions
An upper motor neuron lesion releases the γ motor neurons, so the limb becomes spastic with brisk jerks. Worth knowing, though not in your pages: in a lower motor neuron lesion the motor neurons themselves are lost, so the limb is flaccid and its jerks are lost (see Upper vs lower motor neuron lesions).
Clasp-knife phenomenon
- Stretch a hypertonic muscle, as in upper motor neuron paralysis, and it first resists by contracting. Keep stretching and it relaxes, all at once.
- Example: bending the elbow of such a patient is hard at first, then the resistance suddenly vanishes and the elbow flexes easily.
- The name comes from the feel of shutting a clasp-knife (a folding pocket knife): stiff at first, then a sudden give.
- Physiologists call it the lengthening reaction: it is triggered by lengthening the spastic muscle.
Why it happens (elbow example):
- Tone is high, so the triceps resists flexion strongly at first.
- As flexion goes on, the triceps is stretched further, until the inverse stretch reflex is set off.
- Autogenic inhibition relaxes the triceps, and the resistance to flexion disappears.
Clasp-knife is not rigidity
Worth knowing, though not in your pages: clasp-knife spasticity is an upper motor neuron sign. The lead-pipe or cogwheel rigidity of parkinsonism is felt all through the movement and never suddenly gives way (see Parkinson’s and Huntington’s disease).
Clonus
- Definition: repeated, rhythmic beats of contraction in a muscle that is stretched suddenly and then kept stretched.
- Ankle clonus is the classic example: dorsiflex the foot sharply and hold it there, and the foot beats down into plantar flexion again and again.
- You find it in upper motor neuron paralysis, where overactive spindles drive it.
The cycle:
- The sudden stretch fires the spindle, and the muscle contracts.
- The contraction stops the spindle’s firing, so the muscle relaxes.
- The stretch is still being held, so the spindle fires again, and the cycle repeats.
Exam-answer skeleton: "Inverse stretch reflex" (short note)
- Definition, and the other names: inverse myotatic reflex, autogenic inhibition.
- Receptor (Golgi tendon organ, 3–25 muscle fibres each) and stimulus (strong stretch or active contraction).
- Reflex circuit as a diagram: Ib → inhibitory interneuron → homonymous α motor neuron; disynaptic.
- Function: the tendon organ monitors force, and the agonist relaxes.
- Significance: force feedback alongside the spindle’s length feedback; protection against tearing.
- The spindle vs tendon organ table.
- Applied: the clasp-knife phenomenon (lengthening reaction) in upper motor neuron lesions, and how it differs from clonus.
Asked in exams
- Inverse stretch reflex: Aug 2024, MCQ