Vestibular System & Control of Posture Flashcards
what detects rotary acceleration and deceleration of the head
bilateral semicircular ducts
what is the vestibular system
semi-circular canals and otoliths
what do the semi-circular canals contain
crista ampullaris
hair cells provide graded electrical potentials
what causes action potentials in the semicircular canals
deflection of hair cilia in one direction causes an increase in action potential rate
cilia deflected in opposite direction decreases frequency
what occurs when the head rotates
a change in endolymph flow in semi-circular canals
rotation in counter-clockwise direction causes endolymph to move clockwise
what occurs do the action potential signals when the head moves
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
what signals do head tilt and acceleration/deceleration cause
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
how do otolith cause unique pattern of signals
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
where are the afferent connections in the vestibular nuclei
in the medulla (brainstem)
the lateral nucleus is concerned with postural reflexes
inferior nucleus is involved with spinal cord reflexes
where are the efferent connections in the vestibular nuclei and how do they control posture
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
what is the vestibuloocular reflex during head movement and what is the functional significance
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
what maintains posture
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)
when are tonic neck reflexes activated
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
what are the vestibular reflexes mediated by
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
what is the labyrinthine righting reflex
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
what is relative equilibrium
regulation of posture with respect to gravity
what is static equilibrium
in which all forces acting on the body are balanced so that the body rests in an intended position
what is dynamic equilibrium
ability to progress through an intended movement without losing progress
what do postural adjustments require
anticipatory motor actions
to maintain balance, voluntary movement must be preceded by counterbalancing movement
what do anticipatory responses depend on
feed foward control
anticipatory action varies according to behavioural demands
right forearm pressure demonstration
maintenance of posture includes feedback (response) and feedforward (predictive)
what are postural adjustments
are innate
can be modified by learning
anticipatory response to postural disturbance can be learned
a unexpected shift in platform induces a postural sway
how is adaptation of posture learned
locomotion
what is the motor system controlled by
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
what does organization of voluntary movement include
cortex/brain stem/spine; postural adjustments include brain stem/spine; reflexes only requires spinal cord circuits
what does basal ganglia interact with and what is it important for
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
what do medial brain stem pathways control
activation of movement
what do lateral brain stem pathways control
adjustments of movements
what are muscle spindles encapsulated in
sensory receptors that are located within the muscle and function to signal change in the length of the muscle
what are muscle spindles used for
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
what are the functions of intrafusal fibres of muscle spindles
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
what is the response of Ia sensory fibres is dependent on
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
