Answers to Quiz 130 · Regulation of Posture and Movement
Spinal shock after a complete cord transection lasts longest in which species, and for how long?
Answer: Human, 1–4 weeks
Duration follows encephalization of motor function: the more the brain has taken over, the longer the cord stays silent. Man 1–4 weeks, monkey 2–4 days, cat 2–4 hours, frog 2–4 minutes. Revise postural reflexes, spinal animal and spinal shock
The best-supported mechanism of spinal shock is:
Answer: Sudden loss of the tonic bombardment reaching motor neurons from descending fibres
Loss of suprasegmental drive fits the evidence: areflexia is deeper in species with more descending control, and a second transection made below the first after recovery causes no fresh areflexia. Revise postural reflexes, spinal animal and spinal shock
In a paraplegic recovering from spinal shock, the first reflex response to return is:
Answer: Slight flexion of the leg to a noxious stimulus
A feeble flexor withdrawal appears first; the knee jerk follows 1–5 weeks later. The mass reflex may take months, and extensor spasm only appears around six months. Revise postural reflexes, spinal animal and spinal shock
Press a finger against the sole of a spinal animal's foot, then withdraw it: the limb extends and follows the finger. This is:
Answer: The positive supporting reaction
Also called the magnet reaction; tactile receptors start it and the limb becomes a rigid pillar that resists gravity. The negative supporting reaction is the active switching off of this response by stretch of the extensors. Revise postural reflexes, spinal animal and spinal shock
A decerebrate preparation is made by sectioning the brainstem:
Answer: Between the superior and inferior colliculi
Decerebration is a midcollicular section. A cut at the upper border of the midbrain gives the midbrain animal, and one below the medulla gives the spinal animal. Revise medullary integration and decerebrate rigidity
Which statement about the brainstem reticular motor areas is correct?
Answer: The facilitatory area lies in the pons and discharges spontaneously
The large facilitatory area is pontine and fires on its own; the small inhibitory area is medullary and has to be driven by cortex, basal ganglia and cerebellum. Revise medullary integration and decerebrate rigidity
Which of these is NOT one of the brain areas that drive the medullary inhibitory reticular area?
Answer: Pontine reticular formation
Cortex, basal ganglia and cerebellum are the three inhibitory brain areas; the pontine reticular formation is the facilitatory area itself. Basal ganglia act only through their projection to the motor cortex. Revise medullary integration and decerebrate rigidity
The tone of the antigravity muscles is controlled by the vestibulospinal tract through:
Answer: α motor neurons
Vestibulospinal fibres end mainly on α motor neurons supplying extrafusal fibres, so dorsal rhizotomy does not abolish their effect. The reticulospinal tract acts through γ motor neurons and spindle sensitivity, and rhizotomy does abolish that. Revise medullary integration and decerebrate rigidity
In a decerebrate cat, extensor rigidity is at its maximum when the animal is placed:
Answer: On its back
This is the tonic labyrinthine reflex, from the otolith organs through the vestibulospinal tracts: rigidity is maximal supine, falls when the animal is turned on either side, and is least prone. Revise medullary integration and decerebrate rigidity
Which righting reflex does NOT have its centre in the midbrain?
Answer: Optical righting reflex
The optical righting reflex is integrated in the visual cortex, so it needs the cortex intact; the others are midbrain reflexes, the limb righting reflex working through the red nucleus. Revise midbrain integration and righting reflexes
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:
Answer: Neck righting reflex
Stretch of neck muscles is the stimulus and the muscle spindle the receptor, and the response rights shoulders, thorax and then pelvis. The tonic neck reflex is a medullary static reflex that only alters the pattern of rigidity. Revise midbrain integration and righting reflexes
Extensor rigidity in a midbrain animal differs from that of a decerebrate animal in that it:
Answer: Is present only at rest and disappears when the animal walks
The midbrain animal has its phasic postural reflexes back, and they override the static extensor drive during movement. Neither preparation shows any stage of shock. Revise midbrain integration and righting reflexes
In the motor homunculus of the precentral gyrus:
Answer: The feet lie uppermost and the face lowest down
The map is inverted, with feet uppermost. Territory follows the skill with which a part is used, not its bulk, which is why hand, digits, lips, jaw and tongue take up so much of it. Revise cortical integration and motor cortex
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:
Answer: Supplementary motor area, the medial part of area 6
The supplementary motor area plans and programmes motor sequences and controls bimanual tasks. The premotor cortex sets posture at the start of a planned movement, and area 8 concerns eye movements. Revise cortical integration and motor cortex
Which feature is NOT seen in a decorticate animal?
Answer: Loss of temperature regulation
The hypothalamus survives decortication, so temperature regulation and visceral homeostasis are intact, which is why such an animal is easy to maintain. All midbrain reflexes remain, and decorticate rigidity is minimal and seen only at rest. Revise cortical integration and motor cortex
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