Quiz 126 · Segmental Organization of Motor System

Answers to Quiz 125 · Introduction to and Organization of Motor System

1. Which of these is NOT a feature of volitional movements?

   Answer: Has a short latency

   Volitional movements have a long latency and are slow to execute. A short latency belongs to automatic (reflex) movements. Revise movement and organization of the motor system

2. Central feedback in the control of movement is integrated in the:

   Answer: Spinal cord, brainstem and cortex

   Central feedback mixes the afferent signal with supraspinal signals at the cord, brainstem and cortex. Feedback confined to the cord is the local type. Revise movement and organization of the motor system

3. A child touches a hot pan and snatches the hand away at once. Which feedback system mainly controls this movement?

   Answer: Local feedback

   Withdrawal from a noxious stimulus needs speed more than accuracy, so it runs on local feedback, integrated within the spinal cord alone. Revise movement and organization of the motor system

4. A carpenter drives nails accurately with his eyes open but keeps missing with them shut. Which feedback has he lost?

   Answer: Special sensory feedback

   Special sensory feedback uses visual and auditory input and gives the most accurate movements; hitting a nail with the eyes open is the book's example. Revise movement and organization of the motor system

5. Compared with local feedback, central feedback makes a movement:

   Answer: Slower but more precise

   Processing the signal at several levels delays the movement, but the delay buys precision. That is why central feedback is used when accuracy matters more than speed. Revise movement and organization of the motor system

6. Which part of the motor system receives no direct somatosensory input from the spinal cord?

   Answer: Basal ganglia

   The basal ganglia get no direct somatosensory input from the cord and act on the motor cortex through the thalamus. The cerebellum, by contrast, receives input from almost every sensory modality. Revise movement and organization of the motor system

7. The two extrapyramidal tracts from the brainstem that act mainly on the motor neurons controlling posture are the:

   Answer: Reticulospinal and vestibulospinal

   The reticular nuclei give rise to the reticulospinal tract and the vestibular nuclei to the vestibulospinal tract; both act mainly on postural motor neurons. The corticospinal tracts are pyramidal, and the spinothalamic tracts are ascending. Revise movement and organization of the motor system

8. The motor cortex influences the brainstem nuclei that give rise to the extrapyramidal tracts mainly through:

   Answer: Corticobulbar projections

   Corticobulbar projections are the cortex's indirect route, through the brainstem nuclei. The corticospinal tracts are its direct route to the spinal motor neurons. Revise movement and organization of the motor system

9. Destruction of the motor neurons supplying a muscle produces:

   Answer: Complete paralysis with loss of tone

   Muscle tone depends on intact innervation, so losing the motor neurons abolishes both tone and function, and the paralysis is complete. Revise movement and organization of the motor system

10. During a movement, the antagonist muscle normally:

    Answer: Relaxes, so the movement goes more easily

    Antagonists oppose the movement, so the movement is helped when they relax while the agonists contract. Revise movement and organization of the motor system

11. The local segmental circuit of the spinal cord is essential for:

    Answer: All rapid reflex movements

    Muscle afferents act on the motor neurons of the same muscle, and this segmental loop is needed for every rapid reflex movement. Planning and initiation belong to higher centres. Revise movement and organization of the motor system

12. Cerebellar disease disturbs every aspect of movement because the cerebellum:

    Answer: Takes part in planning, initiation, coordination and termination

    Fed by almost every sense and projecting to the brainstem motor nuclei and motor cortex, the cerebellum works at nearly every stage of a movement, from planning to termination. Revise movement and organization of the motor system

14 questions on chapter 126. 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. One α motor neuron supplies how many muscle fibres?

2 to 12, depending on the muscle10 to 1,000, depending on the muscleExactly one fibre in every muscle5,000 to 10,000 in every muscle

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

The extrafusal fibres the neuron suppliesThe intrafusal fibres of nearby spindlesThe motor neuron and its axonThe neuromuscular junction of each branch

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

Small units with low-threshold motor neuronsLarge units with fast-conducting motor neuronsUnits made of fast-twitch fatigable fibresUnits with the largest number of fibres

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

A lower threshold and faster conductionA higher threshold and faster conductionA lower threshold and slower conductionA higher threshold and slower conduction

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

Are supplied mainly by γ motor neuronsContain no muscle spindles at allMust produce brief bursts of high forceGive continuous support to posture

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

30%70%10%50%

7. Which statement about γ motor neurons is correct?

They supply the extrafusal muscle fibresThey receive direct excitation from Ia afferentsDescending pathways, not sensory afferents, drive themThey produce much of the force of a contraction

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:

Intrinsic muscles of the handDiaphragm, through the phrenic nerveAxial muscles of the trunkMuscles of the shoulder girdle

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

Dorsal horn neurons, to relay sensationPreganglionic neurons, to control the visceraMedial motoneurons, to control postureLateral motoneurons, to control skilled movement

10. Most descending motor pathways influence spinal motoneurons:

Through the γ motor neurons aloneIndirectly, through spinal interneuronsThrough the dorsal root gangliaDirectly, on the motoneuron cell bodies

11. Which of these is an upper motor neuron?

A neuron of the facial motor nucleusA neuron of the vestibulospinal tractA γ motor neuron of the cordAn anterior horn cell of the cord

12. Withdrawal reflexes are mediated by:

Ia afferents from muscle spindlesFlexor reflex afferentsType II afferents from spindlesIb afferents from tendon organs

13. By definition, extensor muscles:

Rotate the limb inwardDecrease the angle at the jointMove the limb toward the midlineIncrease the angle at the joint

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

Corticospinal neuronsα motor neuronsγ motor neuronsSpinal interneurons

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