Answers to Quiz 127 · Muscle Spindle and Golgi Tendon Organ
The fibres lying inside the capsule of a muscle spindle are called:
Answer: Intrafusal fibres
Intrafusal, because they sit inside the fusiform capsule. The ordinary contractile fibres outside the capsule are extrafusal; fusimotor is the name for the γ motor supply, not for a muscle fibre. Revise muscle spindle
A muscle spindle measures roughly:
Answer: 100 µm across and 5 to 10 mm long
Diameter about 100 µm, length 5 to 10 mm, with 2 to 12 intrafusal fibres inside the capsule. 100 µm by 1 mm is the Golgi tendon organ. Revise muscle spindle
The primary, annulospiral ending of the muscle spindle is the terminal of a:
Answer: Type Ia afferent fibre
Ia fibres (12 to 20 µm) coil round the centre of the intrafusal fibres as annulospiral endings. Type II fibres form the flower-spray secondary endings, and Ib fibres belong to the tendon organ. Revise muscle spindle
Dynamic γ motor neurons end, through plate endings, on:
Answer: Nuclear bag fibre 1
Dynamic γ neurons supply bag fibre 1 with plate endings; static γ neurons supply bag fibre 2 and the chain fibres with trail endings. Revise muscle spindle
γ motor neurons make up about what share of the fibres in the ventral root?
Answer: 30%
About 30% are γ fibres (3 to 6 µm); the remaining 70% are α motor fibres. Their small size and number make them the small motor nerve system. Revise muscle spindle
Which statement about the γ motor neuron is NOT true?
Answer: It receives direct input from the primary spindle afferents
γ motor neurons take no input from primary sensory afferents; descending pathways set their firing rate. Sensory events change γ discharge only indirectly, by way of those descending pathways. Revise muscle spindle
Stimulating a γ motor neuron makes the muscle contract because:
Answer: the spindle's poles shorten, stretching its centre, so Ia firing rises and α motor neurons are excited
γ fibres reach only the contractile poles of the intrafusal fibres. Their contraction stretches the non-contractile centre, so the primary endings are deformed, afferent firing rises and the α motor neurons are driven. The contraction is indirect, never a direct γ effect on extrafusal fibres. Revise muscle spindle
The largest spindle response is obtained when a γ motor neuron is stimulated in a muscle that is:
Answer: already stretched
Spindle sensitivity varies with γ discharge, and γ stimulation added to an existing stretch gives the maximum response. In a contracting muscle the spindle is unloaded instead. Revise muscle spindle
During a maintained stretch, a raised and steady rate of firing comes from the endings on the:
Answer: nuclear chain fibres
Chain fibre endings give the static response, firing at an increased rate throughout the stretch. Bag fibre endings give the dynamic response: brisk while the muscle is being stretched, less once the stretch is held. Revise muscle spindle
Tendon reflexes come out more easily during Jendrassik's maneuver because:
Answer: proprioceptive input from the hands raises γ efferent discharge through descending pathways
Input from the hooked, pulling fingers excites supraspinal centres, which raise γ discharge and so spindle sensitivity. Anxiety exaggerates tendon reflexes by the same route; γ neurons never take direct input from primary afferents. Revise muscle spindle
Because of α–γ coactivation, the muscle spindle:
Answer: goes on discharging even while the muscle is contracting
The descending pathways that excite γ neurons excite α neurons as well, so the intrafusal poles shorten along with the muscle and the spindle keeps firing, letting it adjust motor neuron discharge right through the contraction. Revise muscle spindle
The Golgi tendon organ is formed by the terminals of:
Answer: group Ib afferent fibres
Ib afferents branch among the collagen bundles of the tendon to form the receptor. Ia and II fibres are the spindle's afferents, and the tendon organ has no motor supply. Revise golgi tendon organ
Compared with the muscle spindle, the Golgi tendon organ lies:
Answer: in series with the muscle fibres, and signals force
The tendon organ takes the whole pull of the muscle, in series with its fibres, and so reports the force developed; the spindle lies in parallel and reports length. Revise golgi tendon organ
A muscle is stretched passively, then made to contract strongly. Its Golgi tendon organs discharge:
Answer: weakly during the passive stretch and maximally during the contraction
A passive stretch is largely absorbed by the elastic muscle fibres, so little force reaches the tendon. Contraction pulls the tendon taut and is the stronger stimulus; a relaxed muscle gives no discharge at all. Revise golgi tendon organ
Stimulating the Ib fibres from a Golgi tendon organ:
Answer: inhibits the α motor neurons, by way of inhibitory interneurons
Ib fibres reach the α motor neuron only through interneurons, and those interneurons are inhibitory. The spindle's Ia afferents are the opposite: a direct, excitatory synapse on the α motor neuron. Revise golgi tendon organ
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