Vestibular System & Control of Posture

Question 1

what detects rotary acceleration and deceleration of the head

Answer 1

bilateral semicircular ducts

Question 2

what is the vestibular system

Answer 2

semi-circular canals and otoliths

Question 3

what do the semi-circular canals contain

Answer 3

crista ampullaris

hair cells provide graded electrical potentials

Question 4

what causes action potentials in the semicircular canals

Answer 4

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

cilia deflected in opposite direction decreases frequency

Question 5

what occurs when the head rotates

Answer 5

a change in endolymph flow in semi-circular canals

rotation in counter-clockwise direction causes endolymph to move clockwise

Question 6

what occurs do the action potential signals when the head moves

Answer 6

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

Question 7

what signals do head tilt and acceleration/deceleration cause

Answer 7

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

Question 8

how do otolith cause unique pattern of signals

Answer 8

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

Question 9

where are the afferent connections in the vestibular nuclei

Answer 9

in the medulla (brainstem)

the lateral nucleus is concerned with postural reflexes

inferior nucleus is involved with spinal cord reflexes

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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

Question 12

what maintains posture

Answer 12

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)

Question 13

when are tonic neck reflexes activated

Answer 13

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

Question 14

what are the vestibular reflexes mediated by

Answer 14

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

Question 15

what is the labyrinthine righting reflex

Answer 15

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

Question 16

what is relative equilibrium

Answer 16

regulation of posture with respect to gravity

Question 17

what is static equilibrium

Answer 17

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

Question 18

what is dynamic equilibrium

Answer 18

ability to progress through an intended movement without losing progress

Question 19

what do postural adjustments require

Answer 19

anticipatory motor actions

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

Question 20

what do anticipatory responses depend on

Answer 20

feed foward control

anticipatory action varies according to behavioural demands

right forearm pressure demonstration

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

Question 21

what are postural adjustments

Answer 21

are innate

can be modified by learning

anticipatory response to postural disturbance can be learned

a unexpected shift in platform induces a postural sway

Question 22

how is adaptation of posture learned

Answer 22

locomotion

Question 23

what is the motor system controlled by

Answer 23

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

Question 24

what does organization of voluntary movement include

Answer 24

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

Question 25

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

Answer 25

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

Question 26

what do medial brain stem pathways control

Answer 26

activation of movement

Question 27

what do lateral brain stem pathways control

Answer 27

adjustments of movements

Question 28

what are muscle spindles encapsulated in

Answer 28

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

Question 29

what are muscle spindles used for

Answer 29

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

Question 30

what are the functions of intrafusal fibres of muscle spindles

Answer 30

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

Question 31

what is the response of Ia sensory fibres is dependent on

Answer 31

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

Question 32

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

Answer 32

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