In one breath
The muscle spindle is a stretch receptor that lies parallel to the ordinary muscle fibres and signals muscle length and the speed at which it changes. Inside its capsule are 2 to 12 intrafusal fibres (nuclear bag 1, bag 2 and chain fibres), whose non-contractile centres carry the sensory endings (Ia primary, type II secondary) and whose contractile ends are made to shorten by γ motor neurons. Stretching the muscle, or firing γ motor neurons, stretches the spindle’s centre and raises Ia firing, which excites the α motor neurons of the same muscle. So the spindle is the muscle’s length-feedback device, and γ discharge, set by descending pathways, adjusts its sensitivity and with it muscle tone.
Builds on: Sensory receptors and their classification · Segmental organization of the motor system · Leads to: Golgi tendon organ · Stretch reflex · Inverse stretch reflex and muscle tone
What a spindle is
- A specialised sense organ found in every skeletal muscle of mammals.
- Plentiful in muscles that make fine movements, and common too in postural muscles, especially those rich in slow-twitch fibres.
- Named for its long fusiform (spindle) shape.
- Its own fibres are intrafusal, because they sit inside the fusiform capsule. The ordinary contractile fibres of the muscle, outside the capsule, are extrafusal.
- It responds to a change in muscle length and to the velocity of lengthening.
Structure
| Feature | Value |
|---|---|
| Intrafusal fibres | 2 to 12 |
| Diameter | About 100 µm |
| Length | 5 to 10 mm |
| Usual make-up | 2 bag fibres, 4 or more chain fibres |
- A connective tissue capsule encloses each spindle.
- The spindle lies parallel to the extrafusal fibres, and its ends are fixed to the tendon or to the sides of extrafusal fibres. This side-by-side arrangement is what lets a stretch of the muscle reach the spindle.
Nuclear bag fibres
- Longer than chain fibres, and swollen in the middle into a “bag” that holds many nuclei.
- Usually two per spindle:
- bag 1: myosin ATPase activity low; at its best in the dynamic phase, while length is still changing;
- bag 2: myosin ATPase activity high; at its best in the static phase, once a stretch is being held.
Nuclear chain fibres
- Thinner and shorter, lying alongside the bag fibres, with no true bag.
- Their nuclei lie in a single row, like the links of a chain.
- Usually four or more per spindle.
| Feature | Bag fibres | Chain fibres |
|---|---|---|
| Size | Longer, thicker | Shorter, thinner |
| Nuclei | Grouped in a central bag | In a row |
| Number | Usually 2 (bag 1, bag 2) | 4 or more |
| Best phase | Bag 1 dynamic, bag 2 static | Static |
Where the receptors are
- The central part of each intrafusal fibre is non-contractile and carries the sensory endings.
- The ends (poles) hold the contractile elements.
- So anything that stretches the centre excites the spindle.
Innervation
The spindle has both a sensory (afferent) and a motor (efferent) nerve supply.
Afferent (sensory) fibres
| Feature | Primary ending | Secondary ending |
|---|---|---|
| Fibre | Ia | II |
| Diameter | 12–20 µm | 6–12 µm |
| Conduction | Faster | Slower |
| Shape | Annulospiral | Flower-spray |
| Site | Centre of the fibres | Paracentral |
| Lies on | Bag 1, bag 2 and chain | Mainly chain fibres |
- Primary (annulospiral) endings are the Ia terminals, wound in a spiral round the centre of the intrafusal fibres. The Ia fibre divides in two: one branch goes to bag 1, the other to bag 2 and the chain fibres.
- Secondary (flower-spray) endings are the type II terminals, named for their flower-like look. They lie just to either side of the centre (paracentral), usually on chain fibres.
Efferent (motor) fibres
- The spindle has its own motor neurons, the γ motor neurons, also called fusimotor fibres.
- They are small fibres from the anterior horn cells, 3–6 µm across, and make up only about 30% of the fibres in the ventral root (the other 70% are α).
- Small and relatively few, they form the small motor nerve system.
- They supply the contractile poles of the intrafusal fibres:
- on bag 1 their terminals are plate endings, ending on motor end plates;
- on bag 2 and the chain fibres they are trail endings.
There are two kinds of γ motor neuron:
| Feature | Dynamic γ | Static γ |
|---|---|---|
| Supplies | Bag 1 | Bag 2 and chain |
| Ending | Plate | Trail |
| Boosts firing in | Ia fibres | Type II fibres |
| Only during | Change in length | Held stretch |
- Between them they set how sensitive the spindle is to length (static γ) and to the speed of change in length (dynamic γ). Your book puts this as γ motor neurons “monitoring” length and its rate of change.
- Worth knowing, though not in your pages: static γ fibres also raise the steady firing of the Ia endings during a held stretch, not only that of the type II endings.
- The spindle also receives β efferents, both dynamic and static.
- Worth knowing, though not in your pages: β fibres are skeletofusimotor: one axon supplies both extrafusal and intrafusal fibres.
Afferent discharge patterns
- Stimulating the spindle’s motor supply gives two patterns of afferent discharge, dynamic and static; γ and β fibres can each produce both.
- Dynamic fusimotor fibres give the dynamic response, and static fusimotor fibres the static response.
- Your book ties the dynamic response to the bag fibres and the static response to the chain fibres.
Bag 2 is the odd one out
In one place your book calls the bag fibres dynamic, yet it also says bag 2 responds best to a held stretch and is supplied by static γ. Learn this version: bag 1 = dynamic; bag 2 and chain = static. Hence dynamic γ goes to bag 1, and static γ to bag 2 and the chain fibres.
Draw it: the muscle spindle
- Draw two long extrafusal fibres, and between them a spindle-shaped capsule lying parallel to them, its ends fixed to the tendon.
- Inside the capsule draw the intrafusal fibres: bag 1 and bag 2 (long, with a swollen centre full of nuclei) and four or more chain fibres (shorter and thinner, with nuclei in a row).
- Shade both ends of every fibre and label them contractile poles. Leave the middle plain and label it non-contractile receptor region.
- Ia fibre (12–20 µm): coil it round the centre of the fibres as the annulospiral (primary) ending, with one branch to bag 1 and one to bag 2 plus the chain fibres.
- Type II fibre (6–12 µm): draw flower-spray (secondary) endings just beside the centre, mainly on the chain fibres.
- γ efferents (3–6 µm) to the poles: dynamic γ with plate endings on bag 1, static γ with trail endings on bag 2 and the chain fibres.
- Label the Ia and II fibres “to dorsal root” and the γ fibres “from anterior horn cells, via ventral root”.
Functions
Response to stretch: length feedback
- The spindle is stretch-sensitive: stretch deforms its sensory endings and sets off action potentials in the afferents, and the firing rate rises with the degree of stretch.
- Because the spindle lies in parallel with the extrafusal fibres, stretching the muscle stretches the spindle.
- The spindle afferents run straight to the α motor neuron cell bodies in the cord, with no interneuron between. Their firing excites those neurons, and the muscle contracts.
- When the muscle contracts, the spindle is no longer stretched, so its firing falls and the excitation of the α motor neurons fades.
- So stretch leads to contraction through the spindle, and contraction damps the spindle down again. The spindle and its reflex link to the motor neurons are a feedback device that controls muscle length (Stretch reflex).
The sequence: muscle stretched → spindle stretched → afferent firing ↑ → α motor neurons excited → muscle contracts → spindle unloaded → afferent firing ↓.
Effects of γ stimulation
- Firing the γ motor neurons also makes the muscle contract, but not directly.
- γ discharge makes the contractile poles of the intrafusal fibres shorten.
- This stretches the central part, where the primary endings are. The endings are deformed and the afferent discharge rises.
- The afferents excite the α motor neurons, and the muscle contracts.
- So γ motor neurons increase the spindle’s sensitivity to stretch.
- A muscle can be made to contract directly, through its α motor neurons, or indirectly, through γ motor neurons and the spindle.
- The spindle’s sensitivity varies with γ discharge, and firing γ motor neurons in a muscle that is already stretched gives the largest spindle response.
- Worth knowing, though not in your pages: this γ → spindle → Ia → α route is widely called the γ loop. Your chapter describes it without using the name.
Draw it: spindle firing under stretch and γ stimulation
Draw a sparse row of Ia spikes for the resting spindle, as a reference, then four rows below it:
- Muscle stretched: more spikes.
- Muscle contracting (α fired): few or no spikes, because the spindle is unloaded.
- γ fired: more spikes, because the poles stretch the centre.
- γ fired while the muscle is stretched: the densest row, the maximum response.
Draw it: the γ loop
- Draw the spinal cord with two anterior horn cells, α and γ, and a descending pathway with arrows to both (α–γ coactivation).
- Take the γ axon to the poles of an intrafusal fibre, and write “poles contract”.
- Show the centre stretched and the Ia ending firing faster.
- Bring the Ia fibre back into the cord and end it directly on the α motor neuron.
- Take the α axon to the extrafusal fibres, and write “contraction”.
- Label the γ motor neuron “no sensory input: driven from above”.
Dynamic and static responses
Stretch excites the primary endings on both bag and chain fibres, but the two give different patterns.
| Feature | Dynamic response | Static response |
|---|---|---|
| Endings on | Bag fibres (bag 1) | Chain fibres (and bag 2) |
| Firing | High while stretching, lower once held | Raised all through a held stretch |
| Signals | Rate of stretch | Amount of stretch |
- The primary ending shows both responses, so one ending reports two things at once: how much the muscle has lengthened, and how fast it is lengthening.
- Put simply: the static response answers “how far has the muscle been stretched?” and the dynamic response “how fast is it being stretched?”
- Worth knowing, though not in your pages: the secondary (type II) endings give an almost purely static response, so they report length alone.
Draw it: dynamic and static responses
- Top line, muscle length: flat, then a ramp upward (stretching), then a flat plateau (stretch held).
- Below it, the bag fibre ending: spikes crowded together on the ramp, thinning out on the plateau.
- Below that, the chain fibre ending: spikes at a raised, even rate right along the plateau.
- Label the ramp “dynamic phase” and the plateau “static phase”.
Control of γ discharge
- γ motor neurons get no input from primary sensory afferents. They are driven mainly by descending pathways, which tune the spindle’s sensitivity by setting the γ firing rate.
- Sensory input can still change γ firing, but only indirectly, by stirring up those descending influences.
- Jendrassik’s maneuver: the subject hooks the bent fingers of the two hands together and tries to pull them apart, and tendon reflexes then come out more easily. Proprioceptive input from the hands excites supraspinal centres, and these raise γ discharge through the descending pathways.
- Anxiety also raises γ discharge, so tendon reflexes are exaggerated in anxious people.
Applied: Jendrassik's maneuver
When a tendon reflex such as the knee jerk is hard to get, the patient locks the fingers of both hands and pulls hard as the tendon is tapped. The extra γ discharge makes the spindles more sensitive, and the reflex appears (Stretch reflex).
α–γ coactivation
- The descending pathways that excite γ motor neurons excite the α motor neurons as well, so γ discharge rises together with α discharge. This is α–γ colinkage (α–γ coactivation).
- Because of it, the spindle keeps firing even while the muscle is contracted, and so it can go on adjusting motor neuron discharge for the whole contraction.
- Worth knowing, though not in your pages: the intrafusal poles shorten along with the extrafusal fibres, which keeps the spindle’s centre taut instead of letting it go slack.
Four MCQ traps
- The spindle’s afferents are Ia and II. Ib fibres come from the Golgi tendon organ.
- Flower-spray (secondary) endings sit mainly on chain fibres; annulospiral (primary) endings coil round the centre of all the fibres.
- γ stimulation makes the poles contract and so stretches the centre; it does not shorten the spindle’s receptor region.
- The spindle gives length feedback, and the tendon organ gives force feedback (Golgi tendon organ).
Exam-answer skeleton: "Describe the structure and functions of the muscle spindle, with a labelled diagram" (long essay)
- Definition and site: a stretch receptor in all skeletal muscles, numerous in fine-movement and postural muscles; parallel to the extrafusal fibres; senses length and the velocity of lengthening.
- Structure: capsule; 2 to 12 intrafusal fibres; about 100 µm by 5–10 mm; bag 1, bag 2 and chain fibres compared; non-contractile centre, contractile poles. Draw the labelled diagram.
- Afferent supply: Ia annulospiral primary endings (12–20 µm) on all the fibres; type II flower-spray secondary endings (6–12 µm), paracentral, mainly on chain fibres.
- Efferent supply: γ fusimotor fibres (3–6 µm, 30% of the ventral root) to the poles; dynamic γ with plate endings on bag 1, static γ with trail endings on bag 2 and chain; β efferents.
- Response to stretch: firing rises with stretch; Ia ends directly on α motor neurons; contraction unloads the spindle; the spindle as a length-feedback device.
- Effects of γ stimulation: poles contract, centre stretched, Ia firing rises, α excited; γ sets spindle sensitivity and so muscle tone; maximum response when γ fires in a stretched muscle. Draw the γ loop and the firing traces.
- Dynamic and static responses: which fibres show each, what each signals; the primary ending shows both.
- Control of γ discharge (descending pathways, Jendrassik’s maneuver, anxiety) and α–γ coactivation; end with a spindle-versus-tendon-organ comparison (length versus force feedback).
Asked in exams
- Muscle spindle and gamma motor neurons: Nov 2013, 10 marks · Jan/Feb 2022, 15 marks · Jan 2023, MCQ · Apr 2023, MCQ · Oct 2024, MCQ