lecture 4 Flashcards
reasons for making eye movements
to keep the images on the retinas relatively still - clearer
to position the images of interest onto the fovea- most sensitive regions of the retinas
how to keep retinal images steady
keep the head still
what is the VOR vestibular - ocular reflex
reflex involving the vestibular organs
sensory - vestibular organs - part of the inner ear
motor/response- eye/ocular
without the VOR
clear vision would be impossible
if the vestibular organs are lost
balance is quickly recovered
the VOR keeps your eyes
fixated on an area/ object of interest as your head moves
the VOR is mediated by circuits which
do not involve the spinal cord , the BRAIN
not all reflexes are spinal
eye movements bring the image onto the
FOVEA known as SACCADES
saccades
rapid shift of eye position
they can be reflexive, voluntary or spontaneous
an eye moves in two basic ways
very rapid shifts- saccades
much closer smooth movements - VOR
Types of eye movements
conjugate or disconjugate
conjugate
when the eye moves together in the SAME direction through the SAME ANGLE
disconjugate
when the eye move together in OPPOSITE DIRECTIONS
neural mechanisms of reflex saccades
to make a conjugate saccade to the left the
right oculomotor nuclei and left abducens nuclei need to be activated simultaneously
interneurons in the abducens nuclei connect to the
contralateral oculomotor nuclei
to make a left saccade activating signals are sent to the
left abducens nuclei which excites motor neurons and interneurons that carry the signal to the right oculomotor nuclei
but contraction of the two muscles on the left side of the eyes is
NOT SUFFICIENT so the muscles on the right need to RELAX
muscles on the right relax by
shutting off/ inhibiting excitation of these muscles
additional nucleus contained
inhibitory interneurons
what does the inhibitory interneurons do
shuts down the muscle activity on the right as it inhibits the contralateral abducens nucleus
where does the command signals come from
from a region in the pons called the Paramedic Pontine Reticular formation PPRF
what does the PPRF do
contains command signals
generates a burst of excitation that causes the eyes to move rapidly to a new position
another signal is needed that holds
the eyes in their NEW POSITION. the signal is generated by the nucleus prepositus hypoglassi(NPH)
the NPH contains
loops - feedback the output back into the nucleus
the PPRF sends signals to
the motor neurones and interneurons and the NPH
how does the circuit get activated so it produces a saccade
reflex saccades involve pathways from the sensory organs to the superior colliculi and then to the PPRF
The superior colluculi are midbrain nuclei
saccades are also
voluntary -not initiated by stimulus - instead act of will
originates in the frontal lobe of the cerebral cortex
when you make a voluntary saccade you often turn your head aswell but
it was previously sad that when you turn your head it will elicit the VOR response- trying to keep your eyes looking in the same direction
so the VOR needs to be
turned off when you make a voluntary head turn to look in a new direction as it keeps your head fixated
VOR responses depends on
your intent
to look in the SAME direction - VOR response
to look in a new direction = NO VOR response
= NOT REPEATABLE
What would happen if the left abducens nuclei is damaged
it would not be able to transmit signals to the left lateral rectus causing it to contract and pull the eye left
what would happen if the nucleus paragigantocelularis dorsals was damaged (NPD)
Purpose - neurons inhibit neurones in the abducens nuclei - responsible for transmitting signals to the muscles that move the eyes to the right - cause relaxation
so removing inhibition - muscles do NOT relax and will oppose leftward movements
