In one breath
Stretch a muscle whose nerve supply is intact and it contracts: that is the stretch (myotatic) reflex, and it has just one synapse between the spindle afferent and the motor neuron. A sudden stretch, such as a tap on the tendon, gives the brief phasic reflex (the tendon jerk); a sustained stretch gives the tonic reflex, which keeps up muscle tone and posture. The same afferent relaxes the antagonist through an inhibitory interneuron, and that is reciprocal inhibition.
Builds on: Muscle spindle · Segmental organization of the motor system · Leads to: Inverse stretch reflex and muscle tone · Upper vs lower motor neuron lesions
What it is
- Definition: reflex contraction of a muscle in response to its own stretch. The muscle’s nerve supply must be intact.
- It is the main reflex behind posture and movement. The stimulus is stretch of the muscle; the response is contraction of that same muscle.
- The spindle afferents excite the motor neurons directly, so only one synapse separates the afferent limb from the efferent limb: the reflex is monosynaptic.
- It comes in two kinds, phasic and tonic, compared side by side at the end of this page.
Phasic stretch reflex
It is set off through the primary endings of the muscle spindle.
Stimulus
A sudden stretch of the muscle.
Reflex arc
| Part of the arc | In the phasic stretch reflex |
|---|---|
| Receptor | Muscle spindle (primary endings) |
| Afferent | Group Ia fibre (cell body in the dorsal root ganglion) |
| Centre | Spinal cord: one synapse |
| Efferent | α motor neuron |
| Effector | The stretched muscle itself |
Inside the cord, each Ia fibre splits into two branches:
- One branch synapses directly on the homonymous α motor neurons, the ones that supply the stretched muscle (the agonist, or protagonist). The agonist contracts.
- The other branch ends on an inhibitory interneuron, the Golgi bottle neuron, which inhibits the heteronymous motor neurons that supply the antagonist. This path has two synapses (disynaptic), and the antagonist relaxes.
So as the agonist shortens, its antagonist lets go, and the movement goes through unopposed.
- Reciprocal innervation: wiring that excites one set of motor neurons while it inhibits the opposing set.
- Reciprocal inhibition: the inhibition of the antagonist that this wiring produces.
Draw it: the stretch reflex arc
Draw a cross-section of the spinal cord, with the quadriceps beside it and the hamstrings below. Put a spindle in the quadriceps and run its Ia fibre in through the dorsal root (cell body in the dorsal root ganglion) straight to an α motor neuron in the ventral horn. Send that neuron’s axon out through the ventral root back to the quadriceps, and mark the synapse ”+”. From the Ia fibre, draw a second branch to a small black inhibitory interneuron (the Golgi bottle neuron), and from it a ”−” synapse on the α motor neuron of the hamstrings. Label “1 synapse: agonist contracts” and “2 synapses: antagonist relaxes (reciprocal inhibition)”.
Function
- A sudden stretch gives a quick, brief contraction and a rapid movement of the limb.
- Tendon reflexes (jerks) are the everyday examples: a tap on the tendon stretches the muscle suddenly.
- Because it is fast and short-lived, the phasic reflex makes instant corrections to the cord’s motor output, moment by moment, as a movement goes on.
Worth knowing, though not in your pages: the spinal segments of the jerks examined most often.
| Jerk | Segments |
|---|---|
| Biceps | C5, C6 |
| Knee | L2, L3, L4 |
| Ankle | S1, S2 |
Clinical significance
- A tendon jerk shows how excitable the motor neurons are in general.
- Diseases that change this excitability make the jerks exaggerated or depressed, and the pattern tells the clinician what kind of neurological deficit to suspect.
- The γ motor neurons matter most here, because they set the sensitivity of the spindle:
| Jerks | What they point to |
|---|---|
| Exaggerated (brisk) | γ motor neuron discharge ↑ |
| Depressed | γ motor neuron activity ↓ |
Applied: reading the jerks
- Brisk jerks are a sign of an upper motor neuron lesion, in which γ discharge is released from control from above (see Inverse stretch reflex and muscle tone and Upper vs lower motor neuron lesions).
- Worth knowing, though not in your pages: a jerk disappears when any part of its arc is broken (afferent nerve, cord segment, motor neuron or muscle), as in a lower motor neuron lesion. A sluggish jerk can be brought out by reinforcement (Jendrassik’s manoeuvre): the patient hooks the fingers of both hands together and pulls hard just as the tendon is struck, which makes the motor neurons more excitable.
Tonic stretch reflex
Stimulus
A sustained (maintained) stretch of the muscle.
Reflex arc
- Same arc as the phasic reflex, with one difference: the secondary endings join the primary ones. The afferents are therefore Ia and group II fibres.
- Group II fibres, like Ia fibres, synapse directly on α motor neurons, so this reflex too has a single synapse (monosynaptic).
Function
- It contributes to muscle tone, the resistance a muscle offers to being stretched.
- It helps regulate posture. Standing is the classic example:
- Once you are standing, gravity keeps pulling the knees towards flexion, and this stretches the knee extensors, the antigravity muscles.
- The steady stretch drives a steady reflex contraction, so the knees stay straight and you stay up.
- After a long stand, fatigue lets the knees begin to bend. That stretches the quadriceps further, sets off more tonic reflex, strengthens the contraction and straightens the knees again, so you don’t fall.
- A traffic policeman on duty for hours stays upright this way: each small sag at the knee is answered by the tonic stretch reflex of his quadriceps.
Phasic and tonic, side by side
| Phasic | Tonic | |
|---|---|---|
| Stimulus | Sudden stretch | Sustained stretch |
| Spindle endings | Primary | Primary + secondary |
| Afferents | Ia | Ia + II |
| Synapses | One | One |
| Job | Tendon jerk; rapid corrections | Tone; posture as the load changes |
Both are monosynaptic
It is tempting to call the tonic reflex polysynaptic. In your book, group II fibres end directly on α motor neurons, just as Ia fibres do. What separates the two reflexes is the stimulus (sudden vs sustained) and the endings involved (primary alone vs primary + secondary).
Exam-answer skeleton: "Stretch reflex" (short note)
- Definition; monosynaptic; the receptor is the muscle spindle.
- The two types, with the phasic vs tonic table.
- The reflex arc as a labelled diagram: Ia → homonymous α motor neuron, plus the branch through the Golgi bottle neuron to the antagonist.
- Reciprocal innervation and reciprocal inhibition.
- Phasic reflex: tendon jerks and moment-to-moment corrections.
- Tonic reflex: muscle tone and posture (the standing example).
- γ motor neurons set its sensitivity: brisk jerks when γ discharge rises (upper motor neuron lesion), depressed jerks when it falls.
- If it is paired with the inverse stretch reflex, add the comparison table from Inverse stretch reflex and muscle tone.
Asked in exams
- Stretch reflex: Jan 2010, 2 marks · Aug 2016, 4 marks · Jan 2021, 5 marks · Apr 2021, 3 marks · May 2022, MCQ