Vestibular System & Control of Posture Flashcards

1
Q

what detects rotary acceleration and deceleration of the head

A

bilateral semicircular ducts

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2
Q

what is the vestibular system

A

semi-circular canals and otoliths

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3
Q

what do the semi-circular canals contain

A

crista ampullaris

hair cells provide graded electrical potentials

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4
Q

what causes action potentials in the semicircular canals

A

deflection of hair cilia in one direction causes an increase in action potential rate

cilia deflected in opposite direction decreases frequency

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5
Q

what occurs when the head rotates

A

a change in endolymph flow in semi-circular canals

rotation in counter-clockwise direction causes endolymph to move clockwise

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6
Q

what occurs do the action potential signals when the head moves

A

cilia in left canal results in excitatory responses –> the contralateral (right side) are hyperpolarized

combined signals from left-right ampulla project to the vestibular nuclei

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7
Q

what signals do head tilt and acceleration/deceleration cause

A

utricle and saccules

stationary and upright –> little or no bending of hair cilia

tilting induces otolith layer to move and produces shift in hair cilia

during acceleration, otolith drag in the opposite direction

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8
Q

how do otolith cause unique pattern of signals

A

macula in each utricle has specificity for different horizontal accelerations

opposite directions inhibit hair cells

saccule are orientated vertically (sensitive to vertical acceleration) –> highly sensitive to gravity

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9
Q

where are the afferent connections in the vestibular nuclei

A

in the medulla (brainstem)

the lateral nucleus is concerned with postural reflexes

inferior nucleus is involved with spinal cord reflexes

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10
Q

where are the efferent connections in the vestibular nuclei and how do they control posture

A

lateral nucleus sends lateral vestibulospinal projections down the cervical and lumbar sections of the spinal cord to regulate flexor and extensory muscles

multiple muscles are required to control posture

inferior nucleus sends medial vestibulospinal projections only to cervical section of the spinal cord to regulate (inhibit) axial muscles

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11
Q

what is the vestibuloocular reflex during head movement and what is the functional significance

A

counter-clockwise head rotation excites horizontal semicircular canal

output signals via lateral and medial vestibular signal the oculomotor nucleus (ipsilateral) and abducens nucleus (contralateral) to produce eye movements to the right

during head movement, the leftward eye movement is inhibited

functional significance is to maintain the centre of visual field on the retina

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12
Q

what maintains posture

A

vestibulospinal reflexes

are initiated when the position of head is changed relative to the vertical axis (if tilted forward, the reflex extend the forelegs and flexes the hind-legs)

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13
Q

when are tonic neck reflexes activated

A

when position of head relative to the rest of the body is altered

muscle spindles in the neck leads to stretching and flexing of legs

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14
Q

what are the vestibular reflexes mediated by

A

the vestibulospinal tract (VST) and medial reticular spinal tract that send convergent information to control motor neurons in the neck

classic vestibulospinal reflex has no movement difference between head and the trunk

both head and trunk rotation induces left leg extension and right leg extension

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15
Q

what is the labyrinthine righting reflex

A

in cats

righting reflex restores normal posture in an animal that has fallen or unbalanced

requires otiliths (utricles and saccules) and semicircular (ampulla) and muscle spindles in neck

the head is intitially adjusted relative to the field of gravity

then the reflective extension of the legs to prepare the cat for landing

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16
Q

what is relative equilibrium

A

regulation of posture with respect to gravity

17
Q

what is static equilibrium

A

in which all forces acting on the body are balanced so that the body rests in an intended position

18
Q

what is dynamic equilibrium

A

ability to progress through an intended movement without losing progress

19
Q

what do postural adjustments require

A

anticipatory motor actions

to maintain balance, voluntary movement must be preceded by counterbalancing movement

20
Q

what do anticipatory responses depend on

A

feed foward control

anticipatory action varies according to behavioural demands

right forearm pressure demonstration

maintenance of posture includes feedback (response) and feedforward (predictive)

21
Q

what are postural adjustments

A

are innate

can be modified by learning

anticipatory response to postural disturbance can be learned

a unexpected shift in platform induces a postural sway

22
Q

how is adaptation of posture learned

A

locomotion

23
Q

what is the motor system controlled by

A

forebrain, brain stem and spinal cord

descending projections from the cebebral cortex (premotor cortex)

sensory feedback information via basal ganglia and cerebellum relay information via thalamus

24
Q

what does organization of voluntary movement include

A

cortex/brain stem/spine; postural adjustments include brain stem/spine; reflexes only requires spinal cord circuits

25
Q

what does basal ganglia interact with and what is it important for

A

thalamus to incorperate sensory feedback for motor plans (learning)

basal ganglia is a critical node for the control of voluntary movement and learning

doesn’t make direct connections with somat-motor cortices or spinal cord

critical for feedback loops with thalamic structures and the frontal cortex

26
Q

what do medial brain stem pathways control

A

activation of movement

27
Q

what do lateral brain stem pathways control

A

adjustments of movements

28
Q

what are muscle spindles encapsulated in

A

sensory receptors that are located within the muscle and function to signal change in the length of the muscle

29
Q

what are muscle spindles used for

A

by CNS to sense relative position of the body

sensory fibre endings spiral around central regions of the intrafusal fibres and respond to stretch

gamma motor neurons induce contractile polar regions

30
Q

what are the functions of intrafusal fibres of muscle spindles

A

single Ia sensory fibres innervate and respond to stretch in all 3 fibres

groups of II sensory fibres innervate nuclear chain fibres and static bag

static motor neurons involved in the magnitude in change in muscle length and involved in the maintenance of postures and slower movements

dynamic –> function to signal velocity, rate of change

dynamic motor neuron used for quick changes in muscle length

31
Q

what is the response of Ia sensory fibres is dependent on

A

motor neuron stimulation

type of gamma motor neuron stimulated conveys different information to the sensorys system in regards to muscle activity

simple stretch induces a dynamic response and a small steady state response

stimulation of either static/dynamic motors have characteristic responses in sensory fibres

32
Q

what is the mechanism of the semicircular ducts during rotary acceleration of the head

A
  1. when head accelerates the semicircular duct and its receptor organ rotate with the head
  2. endolymph acceleration lags behind due to inertia
  3. the difference in rate causes crista ampullaris to “crash” into slower moving endolymph
  4. this causes displacement of gelatinous cupulla in direction opposite of the head
  5. changes firing rate of sensory neurons in CNS
33
Q
A