Quiz 130 · Regulation of Posture and Movement

Answers to Quiz 129 · Descending Pathways

1. About what share of corticospinal fibres cross in the medulla to form the lateral corticospinal tract?

   Answer: About 80%

   About 80% cross just after the pyramid and run in the lateral funiculus as the lateral CST. The other 20% stay uncrossed as the anterior CST. Revise corticospinal (pyramidal) tract

2. The cortex has about 30,000 Betz cells, yet the corticospinal tract has about a million axons. What does this show?

   Answer: Many cortical neurons besides Betz cells send CST axons

   Betz cells of area 4 supply only a small share. Area 6 and the sensory cortex (areas 3, 1, 2 and 5, 7) supply the rest: motor cortex 60%, sensory cortex 40%. Revise corticospinal (pyramidal) tract

3. A 65-year-old hypertensive man bleeds from the lenticulostriate branch of his LEFT middle cerebral artery. Which deficit is most likely?

   Answer: Right hemiplegia of UMN type

   Charcot's artery supplies the posterior limb of the internal capsule, where CST fibres are packed tightly and have not yet crossed. A left capsular lesion therefore paralyses the right side, with UMN features. Revise corticospinal (pyramidal) tract

4. Which of these is a LATERAL system descending pathway?

   Answer: Rubrospinal tract

   The lateral system (lateral CST and rubrospinal) ends on the lateral motor neurons for distal, skilled movement. The other three are medial-system tracts for posture, despite the 'lateral' in one name. Revise corticospinal (pyramidal) tract

5. Spasticity after an upper motor neuron lesion is mainly explained by:

   Answer: Pontine reticulospinal tract freed from cortical inhibition

   Corticoreticular fibres cut along with the CST normally restrain the pontine reticulospinal tract; released, it drives the proximal extensors and tone rises. The medullary tract inhibits extensors, so its overactivity would lower tone. Revise upper vs lower motor neuron lesions

6. Which sign is found in BOTH upper and lower motor neuron paralysis?

   Answer: Loss of superficial reflexes

   Superficial reflexes are long polysynaptic loops that need the descending pathways and an intact lower motor neuron, so either lesion abolishes them. Each of the other signs belongs to only one kind of lesion. Revise upper vs lower motor neuron lesions

7. After poliomyelitis, a child's leg is floppy and wasted, the knee jerk is absent and twitches are seen under the skin. Where is the lesion?

   Answer: Anterior horn cells

   Flaccidity, marked wasting, lost reflexes and fasciculations are LMN signs, and polio destroys anterior horn cells. The other three sites give UMN features. Revise upper vs lower motor neuron lesions

8. Lower motor neuron paralysis does NOT cause:

   Answer: Hypertonia of the affected muscles

   An LMN lesion removes the γ motor neuron drive that keeps up tone, so the muscles are flaccid. Hypertonia (spasticity) is a UMN sign. Revise upper vs lower motor neuron lesions

9. Why are the deep tendon reflexes exaggerated in upper motor neuron paralysis?

   Answer: Higher γ discharge makes the spindles more sensitive

   UMNs normally inhibit lower motor neurons. When that inhibition goes, γ motor neuron discharge rises, the spindle is sensitised and the stretch reflex becomes brisk. Revise upper vs lower motor neuron lesions

10. A lesion confined to the medullary pyramid produces mainly:

    Answer: Distal weakness and a positive Babinski sign

    The pyramid carries mostly lateral CST fibres, and a lesion there spares the corticobulbar fibres that act on the brainstem reticular formation (they leave above the pyramid), so there is no spasticity (tone may even fall). Distal weakness and an extensor plantar remain. Revise upper vs lower motor neuron lesions

11. Diplegia is best described as:

    Answer: Quadriplegia with the legs worse than the arms

    Diplegia is a variety of quadriplegia: all four limbs are involved, the legs more than the arms. Paralysis of one half of the body is hemiplegia, the commonest pattern. Revise upper vs lower motor neuron lesions

12. Which medial-system tract crosses to the opposite side?

    Answer: Tectospinal tract

    The medial descending tracts run on their own side except the tectospinal, which crosses just below the periaqueductal grey. Of the lateral-system tracts, both cross. Revise rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts

13. Extensor rigidity in a decerebrate (midcollicular) animal is mainly due to overactivity of the tract that arises from the:

    Answer: Lateral vestibular nucleus

    Freed from cortical inhibition, the lateral vestibulospinal tract from Deiters' nucleus drives the limb extensors. The red nucleus excites flexors, and the medullary reticulospinal tract from nucleus gigantocellularis inhibits extensors. Revise rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts

14. Which tract is correctly matched with its effect?

    Answer: Medullary reticulospinal: inhibits extensor motor neurons

    The medullary reticulospinal tract is mainly inhibitory to extensors. The pontine reticulospinal and lateral vestibulospinal tracts excite extensors, and the rubrospinal excites flexors. Revise rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts

15. The raphespinal tract is best described as:

    Answer: Serotonergic, damping pain transmission in the dorsal horn

    Fibres from nucleus raphe magnus release serotonin and form part of the endogenous pain-inhibiting system. The noradrenergic tract from the locus ceruleus is the ceruleospinal. Revise rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts

15 questions on chapter 130. 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. Spinal shock after a complete cord transection lasts longest in which species, and for how long?

Human, 1–4 weeksMonkey, 2–4 daysFrog, 2–4 minutesCat, 2–4 hours

2. The best-supported mechanism of spinal shock is:

Sudden loss of the tonic bombardment reaching motor neurons from descending fibresOedema of the cord at the level of the sectionLoss of segmental sensory input through the dorsal rootsDegeneration of the anterior horn cells below the section

3. In a paraplegic recovering from spinal shock, the first reflex response to return is:

Extensor spasm of both lower limbsThe mass reflexSlight flexion of the leg to a noxious stimulusSustained ankle clonus

4. Press a finger against the sole of a spinal animal's foot, then withdraw it: the limb extends and follows the finger. This is:

The vestibular placing reactionThe positive supporting reactionThe negative supporting reactionThe mass reflex

5. A decerebrate preparation is made by sectioning the brainstem:

Immediately below the medullaBetween the pons and the medullaBetween the superior and inferior colliculiAt the superior border of the midbrain

6. Which statement about the brainstem reticular motor areas is correct?

The facilitatory area lies in the pons and discharges spontaneouslyBoth areas discharge only when driven by the cerebellumThe inhibitory area lies in the pons and discharges spontaneouslyThe facilitatory area lies in the medulla and needs cortical drive

7. Which of these is NOT one of the brain areas that drive the medullary inhibitory reticular area?

Basal gangliaPontine reticular formationCerebellumCerebral cortex

8. The tone of the antigravity muscles is controlled by the vestibulospinal tract through:

γ motor neuronsRenshaw cellsα motor neuronsIa inhibitory interneurons

9. In a decerebrate cat, extensor rigidity is at its maximum when the animal is placed:

On its right sideOn its frontHead down and tail upOn its back

10. Which righting reflex does NOT have its centre in the midbrain?

Optical righting reflexBody on head righting reflexNeck righting reflexLabyrinthine righting reflex

11. The head has been righted but the body is still tilted, so the neck muscles are stretched and the thorax and abdomen then come round. This is the:

Tonic neck reflexBody on head righting reflexNeck righting reflexLimb righting reflex

12. Extensor rigidity in a midbrain animal differs from that of a decerebrate animal in that it:

Is confined to the forelimbsIs present only at rest and disappears when the animal walksAppears only after a stage of shock has passed offIs far more severe and involves the flexors as well

13. In the motor homunculus of the precentral gyrus:

Territory is proportional to the bulk of the muscleThe trunk has the largest representation of any partThe feet lie uppermost and the face lowest downThe face lies uppermost and the feet lowest down

14. A patient can move either hand well on its own but cannot get the two hands to work together on a task. The area most likely affected is:

Primary motor cortex, area 4Supplementary motor area, the medial part of area 6Frontal eye field, area 8Premotor cortex, the lateral part of area 6

15. Which feature is NOT seen in a decorticate animal?

Slight rigidity present only at restSeverely impaired hopping and placing reactionsIntact righting reflexesLoss of temperature regulation

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