Physiology chapters
Spinal reflexes, muscle spindle and Golgi tendon organ
The muscle spindle and Golgi tendon organ, how the stretch reflex and its reciprocal inhibition work, the role of gamma motor neurons, and the withdrawal and crossed extensor reflexes.
- The muscle spindle senses muscle length and its rate of change; it lies in parallel with muscle fibres and drives the stretch reflex.
- The stretch reflex is monosynaptic (Ia afferent → alpha motor neuron), and reciprocal inhibition relaxes the opposing muscle.
- The Golgi tendon organ lies in series in the tendon, senses tension and inhibits its own muscle via Ib fibres.
On this page
Overview
A reflex is an automatic response to a stimulus, using a fixed pathway: receptor → afferent nerve → centre → efferent nerve → effector.
Spinal reflexes keep posture, protect the body from injury and smooth out movement. They rely on two muscle receptors: the muscle spindle and the Golgi tendon organ.
The muscle spindle
Muscle spindles are small stretch receptors buried among ordinary muscle fibres.
- Each contains 3 to 12 thin intrafusal fibres, inside a capsule.
- The spindle lies in parallel with the ordinary (extrafusal) fibres, so it is stretched when the muscle is stretched.
- Only the ends of intrafusal fibres can contract; the middle (receptor) part cannot.
Intrafusal fibres
- Nuclear bag fibres: nuclei bunched in the middle; mainly signal rate of change of length.
- Nuclear chain fibres: nuclei in a row; signal steady length.
Sensory endings
| Ending | Fibre | Signals |
|---|---|---|
| Primary (annulospiral) | Ia (large, fastest) | Length and velocity of stretch (dynamic and static) |
| Secondary (flower-spray) | II | Length only (static) |
The spindle signals muscle length and how fast it changes. Ia fibres from the primary ending respond strongly to sudden stretch.
Gamma motor neurons
Gamma motor neurons supply the contractile ends of intrafusal fibres.
When they fire, the ends contract and stretch the middle of the spindle. This makes the spindle more sensitive and keeps it active even while the whole muscle shortens.
During voluntary movement, alpha and gamma motor neurons fire together (alpha-gamma coactivation), so the spindle never goes slack.
Gamma motor neurons do not add to the force of the muscle. They only set the sensitivity of the spindle.
The stretch (myotatic) reflex
When a muscle is suddenly stretched, it contracts.
- A tap on the tendon stretches the muscle and its spindles.
- Ia fibres fire and enter the cord through the dorsal root.
- They synapse directly on alpha motor neurons of the same muscle.
- The muscle contracts.
This is the only monosynaptic reflex. Tendon jerks (knee, ankle, biceps, triceps) test it.
Reciprocal inhibition
Branches of the Ia fibre also excite an inhibitory interneuron that inhibits the motor neurons of the antagonist muscle. So when quadriceps contracts in the knee jerk, the hamstrings relax.
Reciprocal inhibition uses one inhibitory interneuron, so it is disynaptic, while the stretch reflex itself is monosynaptic.
Dynamic and static stretch reflexes
- Dynamic: a strong, quick response to sudden stretch (tendon jerk).
- Static: a weaker, continuing response while the muscle stays stretched. It helps maintain muscle tone and posture.
The stretch reflex also damps movement, preventing jerky, oscillating contractions.
The Golgi tendon organ
Golgi tendon organs lie at the junction of muscle and tendon, in series with the muscle fibres.
- They sense tension (force), whether from stretch or from active contraction.
- Their signals travel in Ib fibres.
- In the cord they excite an inhibitory interneuron that inhibits the alpha motor neurons of the same muscle.
This is the inverse stretch reflex (autogenic inhibition).
When tension becomes very high, the muscle suddenly relaxes. This is the lengthening reaction, which protects muscle and tendon from tearing. It explains the sudden give in clasp-knife spasticity.
| Feature | Muscle spindle | Golgi tendon organ |
|---|---|---|
| Location | Among muscle fibres | Muscle-tendon junction |
| Arrangement | In parallel | In series |
| Senses | Length and rate of change | Tension |
| Afferent | Ia and II | Ib |
| Effect on its own muscle | Excites (contraction) | Inhibits (relaxation) |
| Efferent supply | Gamma motor neurons | None |
The withdrawal (flexor) reflex
How it works: a painful stimulus to a limb (stepping on a pin) activates pain fibres, which excite flexor motor neurons through several interneurons.
What you see: the limb is pulled away from the stimulus.
- It is polysynaptic.
- It spreads to several muscles and can continue for a short time after the stimulus stops (after-discharge), thanks to reverberating circuits.
Crossed extensor reflex
At the same time, signals cross to the other side of the cord and excite extensors of the opposite limb. The other leg straightens to take the body's weight.
Stepping on a pin: the injured leg flexes (withdrawal) and the other leg extends (crossed extensor).
Clinical relevance
Brisk reflexes and clonus
How it happens: an upper motor neuron lesion removes descending inhibition, so the stretch reflex is overactive and gamma activity rises.
What you see: brisk tendon jerks, spreading reflexes and clonus (rhythmic contractions after a sudden stretch).
Absent reflexes
How it happens: a break anywhere in the reflex arc: sensory nerve (neuropathy, tabes dorsalis), root, anterior horn cell or motor nerve.
What you see: reduced or absent tendon jerks, often with low tone.
Reinforcement (Jendrassik manoeuvre)
Clenching the teeth or pulling the hands apart while the knee is tapped increases gamma motor activity and can bring out a weak reflex.
Common questions: parallel vs series arrangement, what each receptor senses, Ia, II and Ib fibres, the role of gamma motor neurons, why the stretch reflex is monosynaptic, reciprocal inhibition, the inverse stretch reflex and clasp-knife spasticity, and the withdrawal and crossed extensor reflexes.
Quick revision
- Spindle: in parallel; length and rate of change; Ia and II fibres.
- Gamma motor neurons: set spindle sensitivity; alpha-gamma coactivation.
- Stretch reflex: monosynaptic; reciprocal inhibition relaxes the antagonist.
- Tendon organ: in series; tension; Ib; inhibits its own muscle.
- Lengthening reaction: protects the tendon; basis of clasp-knife.
- Withdrawal reflex: polysynaptic, with crossed extension of the other limb.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
What does the muscle spindle detect?
Muscle length and the rate at which length changes.
What does the Golgi tendon organ detect?
Muscle tension. When tension is high, it inhibits the same muscle.
Why is the stretch reflex called monosynaptic?
The Ia afferent fibre synapses directly on the alpha motor neuron, with no interneuron between them.
What is the crossed extensor reflex?
When one limb withdraws from a painful stimulus, the opposite limb extends to support the body.