Muscle spindle and Golgi tendon organ !
- Mechanism
- Exam priority: medium
Two sensors tell the spinal cord how long a muscle is and how hard it pulls, so movement stays smooth and safe.
Updated 2026-11-01
Also called: Proprioceptors, Gamma motor neurons
See it in 3D: Rectus femoris · Quadriceps tendon · Patellar tendon · Spinal cord · Grey matter of the cord · Dorsal root ganglia
What it is
The muscle spindle lies in parallel with ordinary muscle fibres and senses length and how fast length changes; it drives the stretch reflex. The Golgi tendon organ lies in series at the muscle–tendon junction and senses tension. Gamma motor neurons keep the spindle tight so it stays sensitive while the muscle shortens.
How it works, step by step
- Stretching a muscle stretches the intrafusal fibres inside the spindle capsule.
- Mechanically gated channels on Ia endings open, firing rises in proportion to length and especially to the speed of stretch.
- Ia afferents synapse directly on alpha motor neurons of the same muscle: it contracts and resists the stretch.
- The same Ia fibres excite an inhibitory interneuron that relaxes the antagonist muscle.
- Gamma motor neurons, firing with alpha, shorten the poles of intrafusal fibres so the spindle keeps reporting during contraction.
- As tension rises, the Golgi tendon organ's Ib endings are squeezed between collagen strands and fire.
- Ib afferents excite inhibitory interneurons that reduce alpha drive to that muscle, spreading load and limiting extreme force.
Spindle versus Golgi tendon organ
| Feature | Muscle spindle | Golgi tendon organ |
|---|---|---|
| Arrangement | In parallel with extrafusal fibres | In series, at the muscle–tendon junction |
| Senses | Length and rate of change | Tension (force) |
| Afferent | Ia (primary), II (secondary) | Ib |
| Effect on the same muscle | Excitation (monosynaptic) | Inhibition (disynaptic) |
| Efferent control | Gamma motor neurons | None |
Clinical notes
- Tendon reflexes test the spindle loop: biceps C5–C6, brachioradialis C6, triceps C7, patellar L3–L4, Achilles S1.
- Spasticity after stroke or spinal cord injury: loss of descending inhibition makes the spindle reflex overactive, with clonus and a Babinski sign.
- Lower motor neuron or root lesions abolish the reflex because the arc is broken.
Test yourself
Think of the answer, then tap the question to see it.
Why does a muscle spindle stay sensitive when the muscle shortens during voluntary contraction?
Gamma motor neurons fire with alpha motor neurons and contract the ends of intrafusal fibres, keeping the central sensory region taut (alpha–gamma coactivation).
Tapping the patellar tendon makes the quadriceps contract and the hamstrings relax. What circuit explains both?
Ia afferents excite quadriceps alpha motor neurons monosynaptically and, through an inhibitory interneuron, inhibit hamstring motor neurons (reciprocal inhibition). Segments L3–L4.
Which receptor senses muscle tension and what does it do to its own muscle?
The Golgi tendon organ, in series with the muscle, signals via Ib afferents and inhibits its own muscle through an interneuron, limiting force.
Why does an upper motor neuron lesion cause brisk reflexes while a peripheral nerve lesion abolishes them?
Losing descending inhibition makes the intact spindle arc overactive. A peripheral lesion cuts the afferent or efferent limb, so the reflex cannot occur.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Spinal cord reflexes
- Upper versus lower motor neuron lesions
- Sensory receptors, coding and neuronal circuits
- Types of nerve fibers
- Motor units, recruitment and summation
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 14.3 Motor Responses (OpenStax (Rice University), 2022); 14.1 Sensory Perception (OpenStax (Rice University), 2022); Neuroanatomy, Spinal Cord Morphology (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 55). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.