In one breath
The Golgi tendon organ (GTO) sits in the tendon of a muscle and is built from the branching terminals of a group Ib afferent fibre, about 100 µm across and 1 mm long. It lies in series with the muscle fibres, so tension in the tendon excites it; a strong active contraction is a much better stimulus than a passive stretch, most of which the elastic muscle fibres soak up. Its Ib fibre reaches the α motor neurons only through inhibitory interneurons, so GTO firing damps those neurons down. The tendon organ therefore measures the force a muscle develops, while the spindle measures its length: force feedback beside length feedback.
Builds on: Muscle spindle · Sensory receptors and their classification · Leads to: Inverse stretch reflex and muscle tone · Stretch reflex
Location
- In the tendons of skeletal muscles.
- Worth knowing, though not in your pages: the receptors cluster where the muscle fibres insert on the tendon, at the musculotendinous junction.
- Because the tendon takes the whole pull of the muscle, the receptor sits in series with the muscle fibres — the spindle, by contrast, lies in parallel with them (Muscle spindle).
Structure
- Formed by the terminals of group Ib afferent fibres, which spread out among the collagen bundles of the tendon.
| Feature | Value |
|---|---|
| Afferent fibre | Group Ib |
| Diameter | About 100 µm |
| Length | About 1 mm |
| Arrangement | In series with the muscle fibres |
Your book prints "100 mm"
The chapter gives the GTO diameter as 100 mm, which cannot be right for a receptor 1 mm long. Read it as 100 µm, the same order of size as the muscle spindle’s diameter. Write 100 µm in an exam.
How it is wired into the cord
- The Ib fibre does not end on the α motor neuron itself. It ends on interneurons, and those reach the α motor neuron.
- Those interneurons are inhibitory. So firing the Ib fibres inhibits α motor neuron activity.
- Contrast the spindle: its Ia afferents synapse directly on the α motor neurons and excite them.
Draw it: the Golgi tendon organ
- Draw a few muscle fibres on the left ending on a tendon on the right, so the pull passes straight through the tendon — label this “in series”.
- Inside the tendon draw the collagen bundles, and a nerve fibre entering and branching among them: label it group Ib afferent.
- Mark the receptor 100 µm across and 1 mm long.
- Carry the Ib fibre into the spinal cord through the dorsal root.
- In the cord end it on a small inhibitory interneuron, and take that interneuron to the α motor neuron — draw this second arrow with a minus sign.
- Take the α axon back out to the same muscle, and write “contraction damped down”.
Function
- Being in series, the GTO can be roused either by stretching the muscle or by making it contract.
- The actual stimulus is the force developed in the tendon, however that force arises.
| Muscle state | GTO discharge |
|---|---|
| Relaxed | None |
| Passively stretched | Weak |
| Strongly contracting | Maximum |
- A strong contraction is the stronger stimulus. Contracting muscle pulls hard on the tendon and stretches it a great deal, so the Ib afferents fire briskly.
- Passive stretch is a poor stimulus: the muscle fibres are elastic and take up most of the pull, so little of it reaches the tendon.
Force feedback and length feedback
| Feature | Muscle spindle | Tendon organ |
|---|---|---|
| Site | In the muscle belly | In the tendon |
| Arrangement | Parallel | In series |
| Afferent | Ia and II | Ib |
| Best stimulus | Stretch | Active contraction |
| Route to α neuron | Direct | Via interneuron |
| Effect on α neuron | Excites | Inhibits |
| Signals | Muscle length | Muscle force |
- So the two receptors report different things about the same muscle: the GTO gives force feedback, the spindle gives length feedback. Together they tell the cord both how long a muscle is and how hard it is pulling.
- Worth knowing, though not in your pages: the GTO has no motor supply of its own. The spindle’s sensitivity can be reset by γ motor neurons; the tendon organ has no equivalent, so it reports tendon force plainly.
Applied: protecting the muscle and its tendon
As contraction grows stronger, Ib firing grows with it, and through the inhibitory interneurons it holds back the very muscle that is pulling. Worth knowing, though not in your pages: this is the inverse stretch reflex (autogenic inhibition). Push the load high enough and the inhibition wins, so the muscle suddenly gives way — the clasp-knife response, set out in Inverse stretch reflex and muscle tone.
Three MCQ traps
- Ib is the tendon organ; Ia and II are the spindle. One letter decides the mark.
- In series (tendon organ) versus in parallel (spindle) — this single difference explains why contraction excites the one and unloads the other.
- The spindle’s afferents excite α motor neurons directly; the tendon organ’s inhibit them through an interneuron.
Exam-answer skeleton: "Describe the structure and functions of the Golgi tendon organ" (short note)
- Definition and site: a proprioceptor in the tendon of skeletal muscle, in series with the muscle fibres.
- Structure: terminals of group Ib afferent fibres among the collagen bundles; about 100 µm by 1 mm. Draw the labelled diagram.
- Central connection: Ib fibre → inhibitory interneuron → α motor neuron, so its firing inhibits the motor neuron.
- Stimulus: force developed in the tendon, by contraction or by stretch.
- Why strong contraction beats passive stretch: elastic muscle fibres absorb a passive pull, while contraction pulls the tendon taut.
- Discharge in the relaxed, stretched and contracting muscle.
- Function: force feedback — it signals the force of contraction, and guards muscle and tendon against excessive tension.
- Close by contrasting it with the muscle spindle: force feedback versus length feedback, in series versus parallel, inhibition versus excitation.
Asked in exams
- Golgi tendon organ: Jan/Feb 2022, MCQ · Dec 2023, MCQ