Chapter 10 - Vestibulospinal Flashcards
What is a “fictive” motor pattern?
The word “fictive” means “created by imagination”, so a fictive motor pattern implies that a motor response is generated but not actuated, as in the case of an experimental prep with an immobilized animal, or pharmacologically ablated muscular response. Ressponses can be monitored in motor axons.
What is reafference?
Sensory signals that are a result of an animals own actions. The notion of a distinction between a sensory response from an action we do ourselves versus something that is done to us is the basis for the theory of efference copy. Tickling is an example in that we cannot tickle ourselves, in theory because of reafference. We send an “efferent copy” of a motor command copy that allows us to distinguish the sensory input caused by that motor command from an outside input.
How does computerized dynamic posturography assess components of a multisensory strategy for posture?
By sequentially neutralizing them. Ankle inputs can be elimated by tilting the platform to match sway (sway referencing), optic flow patterns can be neutralized by sway referencing the visual surround or by blind-folding.
Can removal of vision alone (eg blindfolding) distinguish those with a defective labyrinth from normal controls?
No, you must also remove proprioceptive inputs to distinguish the two groups by posture assessments.
Center of mass (CoM) must be aligned with center of pressure (CoP) for postural equilibrium. What are three strategies people select to right themselves when off-balance?
ankle, hip, and stepping strategies.
Ankle strategy - extensor torque about the ankles that movies CoP anteriorly beyond the perturbed CoM.
Hip strategy - flex hips, extend legs (bend over) and move CoM posteriorly.
Stepping strategy - stepping forward to avoid an imminent fall.
What strategy do those with labyrinthine dysfunction prefer, even if another strategy would be more appropriate?
They tend to use an ankle strategy even when a hip strategy is more appropriate, potentially leading to postural instability (Horak et al. 1990)
What is a postural control strategy?
A pattern of motor activity, sometimes involving activation of muscles throughout much of the body, that restores stable equilibrium or produces a controlled, intentional disequilibrium. These strategies can anticipate a motor act and are not therefore, strictly reflexes.
Can postural control strategies be learned?
Yes! For example, gymnasts or high-wire artists can develop automatic skills that compensate at latencies approaching 100 ms, longer than reflexes (10-50 ms) but shorter than reaction times (200 ms or longer).
What is the primary source of vestibulospinal input to the upper cervical spinal cord?
The medial vestibulospinal tract (MVST) . . . with a small contribution from the lateral vestibulospinal tract (LVST) and secondary contributions from reticulospinal tract (RST).
What is the main vestibulospinal input to the limb segments of the spinal cord?
The lateral vestibulospinal tract (LVST) with secondary contributions from the reticulospinal tract (RST).
Does the medial vestibulospinal tract project to the limb segments of the spinal cord?
Not as far as we know (only the MVST and reticulospinal tract (RST)).
What is the apparent function of the angular vestibulo-collic reflex?
In experiments by Flourens (1824) and Ewald (1892), stimulation of a single canal results in a rotation of the head in the plane of that canal. The same is true for electrical stimulation, a right horizontal canal electrical input can cause a rotation of the head away from the stimulated side (ie to the left).
Is the angular vestibulo-colic reflex (VCR) subserved primarily by short-latency disynatpic pathways?
No, it appears disynaptic pathways serve a minor function in the reflex and that it is primarily driven by more complex multisynaptic pathways, as evidence by lesion studies, large phase lags relative to afferent vestibular discharge
