Quiz 127 · Muscle Spindle and Golgi Tendon Organ

Answers to Quiz 126 · Segmental Organization of Motor System

1. One α motor neuron supplies how many muscle fibres?

   Answer: 10 to 1,000, depending on the muscle

   Depending on the muscle, one α motor neuron supplies 10 to 1,000 extrafusal fibres. Two to twelve is the number of intrafusal fibres inside one muscle spindle. Revise segmental organization of the motor system

2. Which of these is NOT part of a motor unit?

   Answer: The intrafusal fibres of nearby spindles

   A motor unit is one motor neuron, its axon and branches, their neuromuscular junctions and all the extrafusal fibres it supplies. Intrafusal fibres belong to the spindles and are supplied by γ motor neurons. Revise segmental organization of the motor system

3. As a weak voluntary contraction begins, which motor units are recruited first?

   Answer: Small units with low-threshold motor neurons

   By the size principle, small motor neurons have the lowest threshold, so they fire first and give weak, sustained contractions in slow-twitch fibres. Large units join only when high force is needed. Revise segmental organization of the motor system

4. Compared with a large α motor neuron, a small α motor neuron has:

   Answer: A lower threshold and slower conduction

   Small motor neurons have lower thresholds to synaptic input and conduct more slowly. Large ones conduct fast and are used for high-effort force. Revise segmental organization of the motor system

5. Antigravity muscles are made mainly of slow-twitch fibres because they:

   Answer: Give continuous support to posture

   Slow-twitch, fatigue-resistant fibres suit the continuous work of holding posture. Brief high-force contractions are the job of fast-twitch muscles, such as many flexors. Revise segmental organization of the motor system

6. What share of the fibres arising from the anterior horn cells are γ motor fibres?

   Answer: 30%

   γ motor neurons make up 30% of the fibres from the anterior horn cells; the rest are α fibres. They supply intrafusal fibres and add nothing to muscle force. Revise segmental organization of the motor system

7. Which statement about γ motor neurons is correct?

   Answer: Descending pathways, not sensory afferents, drive them

   γ motor neurons get no input from sensory afferents; descending pathways control them, and this is how supraspinal centres set muscle tone. They supply intrafusal fibres and add no force. Revise segmental organization of the motor system

8. The lateral motoneuron pool takes up roughly 70% of the ventral horn in the cervical enlargement (lower cervical to first thoracic segments) because these segments supply the:

   Answer: Intrinsic muscles of the hand

   The lateral pool supplies distal muscles, and it swells in the brachial enlargement because those segments supply the intrinsic hand muscles used in manipulation. Axial and girdle muscles are served by the medial pool. Revise segmental organization of the motor system

9. The ventromedial group of spinal interneurons projects mainly to:

   Answer: Medial motoneurons, to control posture

   Ventromedial interneurons project to the medial motoneuron group and control posture. Dorsolateral interneurons project to the lateral group and regulate skilled voluntary movement. Revise segmental organization of the motor system

10. Most descending motor pathways influence spinal motoneurons:

    Answer: Indirectly, through spinal interneurons

    Only some descending fibres end directly on motoneurons; most act through interneurons, which makes interneurons the main targets of the descending tracts. Revise segmental organization of the motor system

11. Which of these is an upper motor neuron?

    Answer: A neuron of the vestibulospinal tract

    UMNs arise in motor areas of the brain, such as the motor cortex and brainstem nuclei, and form descending tracts like the vestibulospinal tract. Anterior horn cells, γ motor neurons and cranial motor nucleus cells are all LMNs. Revise segmental organization of the motor system

12. Withdrawal reflexes are mediated by:

    Answer: Flexor reflex afferents

    Flexor reflex afferents carry the withdrawal reflexes. Ia afferents are the main afferents for motor activity in general, and type II and Ib afferents also help. Revise segmental organization of the motor system

13. By definition, extensor muscles:

    Answer: Increase the angle at the joint

    Extensors increase the joint angle and flexors decrease it. The lower-limb extensors work against gravity, so they are the main antigravity muscles. Revise segmental organization of the motor system

14. The final common path, through which the CNS acts on skeletal muscle, is formed by the:

    Answer: α motor neurons

    The α motor neurons are the final common path: the route by which all central activity reaches the force-producing extrafusal fibres. γ motor neurons only adjust spindle sensitivity. Revise segmental organization of the motor system

15 questions on chapter 127. Pick one answer for each, then save. Your answers stay in this browser, and the right ones are shown at the top of the next quiz.

1. The fibres lying inside the capsule of a muscle spindle are called:

Fusimotor fibresExtrafusal fibresAnnulospiral fibresIntrafusal fibres

2. A muscle spindle measures roughly:

10 µm across and 1 to 2 mm long100 µm across and about 1 mm long1 mm across and 50 to 100 mm long100 µm across and 5 to 10 mm long

3. The primary, annulospiral ending of the muscle spindle is the terminal of a:

Type Ia afferent fibreDynamic γ efferent fibreType Ib afferent fibreType II afferent fibre

4. Dynamic γ motor neurons end, through plate endings, on:

Nuclear bag fibre 2The nuclear chain fibresNuclear bag fibre 1Bag fibre 2 and the chain fibres

5. γ motor neurons make up about what share of the fibres in the ventral root?

50%10%70%30%

6. Which statement about the γ motor neuron is NOT true?

It is driven mainly by descending pathwaysIt raises the spindle's sensitivity to stretchIt receives direct input from the primary spindle afferentsIt supplies the contractile ends of the intrafusal fibres

7. Stimulating a γ motor neuron makes the muscle contract because:

the receptor region of the spindle shortens, which silences the Ib afferentsthe α motor neurons of the antagonist are inhibited through interneuronsthe γ axon ends on the extrafusal fibres and shortens them directlythe spindle's poles shorten, stretching its centre, so Ia firing rises and α motor neurons are excited

8. The largest spindle response is obtained when a γ motor neuron is stimulated in a muscle that is:

fully relaxeddenervated of its α supplyalready contracting maximallyalready stretched

9. During a maintained stretch, a raised and steady rate of firing comes from the endings on the:

motor end plates of bag fibre 1nuclear chain fibrestendon collagen bundlesnuclear bag fibres

10. Tendon reflexes come out more easily during Jendrassik's maneuver because:

proprioceptive input from the hands raises γ efferent discharge through descending pathwaysthe Golgi tendon organs are silenced by the pull on the handsα motor neuron discharge falls, so the muscle is more easily stretchedthe primary spindle afferents excite the γ motor neurons directly

11. Because of α–γ coactivation, the muscle spindle:

is excited only by passive lengthening of the muscleloses its sensitivity to the rate of stretchfalls silent as soon as the muscle begins to shortengoes on discharging even while the muscle is contracting

12. The Golgi tendon organ is formed by the terminals of:

group Ia afferent fibresstatic γ efferent fibresgroup II afferent fibresgroup Ib afferent fibres

13. Compared with the muscle spindle, the Golgi tendon organ lies:

in parallel with the muscle fibres, and signals forcein series with the muscle fibres, and signals forcein parallel with the muscle fibres, and signals lengthin series with the muscle fibres, and signals length

14. A muscle is stretched passively, then made to contract strongly. Its Golgi tendon organs discharge:

in neither, because they answer only to a change in muscle lengthmaximally during the passive stretch and weakly during the contractionweakly during the passive stretch and maximally during the contractionequally in both, since the tendon force is the same

15. Stimulating the Ib fibres from a Golgi tendon organ:

inhibits the α motor neurons, by way of inhibitory interneuronsexcites the α motor neurons directly, with no interneuron betweenhas no effect until the muscle is stretched passivelyexcites the γ motor neurons and so raises spindle sensitivity

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