8: eye movements

how many muscles are attached to the eye

6 muscles:

4 rectus muscles ( superior, inferior, medial and lateral)

2 oblique ( superior and inferior)

what are the 3 axis of rotation in which the orientation of the eye is defined by

horizontal

vertical

torsional

what is adduction

rotates the eye towards the nose

abduction

rotates the eye away from the nose

elevation

rotates the eye vertically up

depression

rotates the eye down

torsional movements

do not change the line of sight but rotate the eye around it

intorsion: rotates the top oc cornea towards nose

extorsion: rotates the top of the cornea away from the nose

medial rectus adducts the eye the lateral rectus abducts it

superior rectus

adduction: intorsion

abduction: elevation

inferior rectus

adduction: extorsion

abduction: depression

superior oblique

adduction: depression

abduction: intorsion

inferior oblique

adduction: elevation

abduction: extorsion

what 3 cranial nerves control the extraocular muscles

the abducens nerve CN VI

occulomotor nerve CN III

trochlear nerve CN IV

patients with lesions of extraocular musces or their nerves complain about double vision as the image of an object of gaze no longer falls in the same location of both retinae

what does an isolated lesion of the abducens nerve cause

results in a loss of abduction beyond the midline, causing diplopia when patients attempt to look in the direction of the paralysed lateral rectus muscle

what does an isolated region of the occulomotor nerve result in

results in a loss of eye movements medially or upward from the mid position

also leeds to a drooping eye lid (ptosis) and a downward lateral gaze

what does an isolated lesion of trochlear nerve cause

causes deficits in intorsion/extorsion and elevation/depression which in turn leads to a skew deviation and a torsional deficit

what are the 2 reasons we move our eyes

1. to acquire targets ( gaze shifting)

2. to maintain gaze on targets ( gaze holding)

gaze shifting

involves 2 forms of eye movement

saccades- that shift the fovea rapidly to a peripheral target

vergence- moves the eye in opposite directions so that the image is positioned on both fovea

gaze holding

involves 3 forms of eye movements

smooth pursuit- movements keep image of a moving target on fovea

vestibulo-ocular- keep images on retina during bried head movements. driven by vestibular system

optokinetic- hold images during sustained head movements and driven byv siaul stimulation

saccades

very fast

shift the line of sight from one object to anoher

short duration

eg allow us to read text

can control saccades and use them to explore environment

waveform of saccades

saccades have a stereotypical waveform with a single smooth increase and decrease of eye velocity

only the distance of the target from fovea determines the velocity of a saccade

movement of saccades

can voluntarily change the direction and amplitude of saccades

cannot change the velocity

can saccade to visual stimuli, auditory, tactile, memorised locations and verbal commands

what do extraocular motor neurons signal

they signal eye position and velocity

the discharge frequency of extraocularmotor neurons is directly proportional to the position and velocity of the eye

as eye velocityincreases, firing rate of motor neurones increase in a pulse of activity

what happens once the eyes have reached their final position after movement

they are held in place by contraction of extraocular muscles and this leads to a change in the baseline dishcarge rate known as a step in intensity

the saccadic signal of an ocular motor neurone has the form of a step pulse

step vs pulse signal

during a saccade, eye movements control often involves 2 signals

pulse → brief burst of neural activity that moves eye quickly

step → sustained signal that keeps the eye in the new position so the eye doesnt drift back

the saccadic signal

this is the nueral command sent from the brain to extraocular muscles to generate a saccade

height of the step indicates the extent of the saccade

the height of thw pulse indicates the speed of saccade

where do the motor circuits for saccades lie

they lie in the brain stem

the horizontal component of a saccade is organised in the pons and medulla

vertical component organised in mesencephalic reticular formation- in midbrain

in each cirucit lies neurones that are responsible for the step and pulse

what are burst cells

the neurons that give rise to the pulse components

motor circuits for saccades

the burst cells fire before and during ipsilateral saccades

what are medium lead burst cells

cells that make direct connections to the motor neurones

what are long lead burst cells

cells that drive the medium lead burst cells and recieve inputs from highher centres

what are inhibitory burst neurons

they suppress contralateral abducens neurons and are driven by medium lead burst cells

help coordinate the correct muscles during a saccade , ensuring the opposing muscle is switched off

what are omnipause cells

also involved in motor circuit for saccades

fire continously except around the time of a saccade

located in the nucleus of dorsal raphe

theyre GABA-ergic and inhibit the burst neurons

by pausing their firing, it allows burst cells to initiate a saccade

what helps maintain a stable saccade system

contribution of excitation of burst cells and inhibitoion of omnipause cells

tonic activity in neurons in midbrain and nucleus maintain steady signal related to eye positon

lesions in these regions doesnt affect saccades but after saccade the eyes drift back to starting positoin

what are the lateral rectus motor neurons in the pons driven by

driven by medium lead burst neurons which supply the burst and the vestibular and pepositus nuclei neurons , which supply tonic signal

vertical saccades generated in mesencephalic retincular formation

px with brain stem lesoins have characteristc deficits in eye movements

what does lesion in pontine gaze centres result in

results in deficits in horizontal saccades

what do lesions in midbrain gaze centres cause

deficits in vertical saccades

what are saccades controlle dby

controlled by the cerebral cortex

the pontine and mesencephalic burst cells provide motor signals for saccades but their output is controlled by superior colliculus

the SC is a major visuomotor integration region

what is smooth pursuit

keeps the image of a moving target on the fovea by calculating how fast the target is moving and moving the eye accordingly

neurons that signal eye velocity recieves major inputs from MT/MST and from the FEF

smooth pursuit movements have a max velocity of about 100 degrees/ sec which is slower than saccades

tracking an object

smooth pursuit occurs prior to a saccade and then smooth pursuit eye movements allow one to track the subsequent movement

what are microsaccades

very small rapid eye movements that occur while a person is trying to fixate on a single point

small and involuntary

what is the superior colliculus SC

a visuomotor integration region

main visual input comes from retina

helps the brain orient the eyes, head and attention towards objects in the environment

recieved input from V1, frontal eye fields and the retina

SC- during smooth pursuit

neurons a small distance from the front edge are activated, leading to small eye movements

for saccades, neurons are activated in a region that represents the point to which the saccade will be directed

just prior to a saccade activity builds up in cells at the target location and decreases in other parts of sc

what 2 regions is the SC divided into

superficial layers

intermediate and deep layers

the 3 superficial layers recieve direct input from retina as well as input from primary visual cortex and neurons in these layers respond to visual stimuli

what is the activity of intermediate and deep layers related to

occulomotor actions

SC indiv neurons

indv neurons within the SC fire before saccades of specific amplitdues and directions

while sc recieves most of its input from the retina its output is controlled by cerebral cortex

what is the movement field of neurone

the region of the visual field that contains the targets for the saccades

most movement related cells have large movement fields and so actual eye movements are encoded by a large number of neurones

frontal eye fields

involved in triggering intentional saccades

the supplementary eye fields, are involved in organising groups of saccades into sequences

parietal eye fields

further back in the bran

involved mainly in reflexive saccades, made in response to changes in the view

vergence movements

smooth pursuit and saccadic systems produce conjugate movements of both eyes

vergence movements produce disconjugate movements of the eyes

when looking at near objects, eyes rotate towards each other; converging

when lookinng at objects in the distance- eyes rotate away from each other; diverging

what do the disconjugate movements ensure

make sure that the object of interest is on the same place in both retinas

vergence is a function of the horizontal rectus muscles and is controlled by neurons in te occulomotor nucleus of the midbrain

what is the vestibular system

part of the body responsible for detecting head movement, maintaining balance and stabilising vision through the vestibulo-ocular reflex

done by measuring the linear and angular acceleration of the head through an ensemble of 5 sensoy organs in the inner ear

how many receptor organs does vestibular labyrinth house

5 receptor organs

linear accelerations are detected by the utricle and saccule

angular accelerations measured by semicircle canals

the 2 vestibular systems can measure linear acceleration along any axis and angular accelerations about any axis

vestibular reflexes: VORs

vest system is reposnible for vaerious reflexes that the body yses to compensate for head movements and the perception of motion in space

the VORs keep the eyes fixed on an object when the head moves

vestib apparatus signals how fast the head is rotating and the occulomotor system uses this to compensate for these movements keeping the visual images motionless on retina

VORs

visual processing is much slower and less efficient than vestibular processing for image stabilisation

if you lose these reflexes it would be hard to fixate on something whilst moving

what are the 3 different VORs that arise from 3 vestibular sense organs

1. rotational vestibulo ocular reflex compensates for head rotation

2. translational vestibulo ocular reflex compensates for linear head movements

3. ocular counter-rolling compensates for head tilt in the vertical

rotational vor

causes eyes to slowly rotate in opposite direction to any rotational head mocement detected by vestivular system

sustained rotation doesnt cause the eyes to be driven past the edge of their orbit instead the eyes make a rapid resetting movement back across the centre of gaze

combination of slow and quick eye movements results in a pattern called nystagmus - vestib signals drives the slow phase of nystagmus while brain stem circuits generate the quick phase

nystagmus

where the eyes make repetitive, uncontrolled movements

consist of a slow drift in one direction followed by a fast corrective movement in opposite

translational VOR

to compensate for linear head movements, the translational vor must take into account the distance to the object being viewed

keeps visin stable when moving head linearly

eg when leaning forard to look at something close, the eyes move slightly backwards to keep the object in focus

ocular counter rolling

since gravity exerts a constant acceleration on the head the vestibular system also senses the orientation of the head relative to gravity

vest system used to estimate the tilting of the head in vertical place and initiate counter rolling response of the eyes to compensate

gaze

coordination of hea and eye movements to direct the fovea

because the head has much higher inertia, during small gaze movements the fovea reaches its target before the head begins to move

small gaze movements trabslates into saccade, followed by head movement with vor

larger gaze movement

the eyes and head move simultaneously

vor temporarily suppressed in order for the eyes and head to move at the same time

problems with vor

it habituates- eg nystagmus in dark slows down and eventually stops

the canals do not respond well to very slow head movements

to compensate, the optokinetic system provides central vest system with visual information that is used to stabilise the visual scene in the eyes

optokinetic reflex

uses the fact that as we move our eyes in spaced fixed objects appear to move in the opposite direction

the reflex drives the eyes in the direction of the observed image motion, which if it were perfect it would stabilise the image o the retina

optokinetic reflex properties to VORs

it builds slowly so as to provide a motion signal that can take over as vestibular signal decays

it responds to very slow visual image motion

circular vection

type of optokinetic stimulation

illusion of self rotation, when body is stationary

visual system sees rotating environment and vest system detects no actual rotation so brain combines conflicting signals

after short period the subject percieves that rotation to have slowed down

perception of self motion in opposite direction builds up

integration of optokinetic and vestibular inputs

vest nuclei recives peripheral input when stimuli rotates at constant speed

but continued rotation means vest input fall to 0

meanhwile optokinetic pathway feeds a rising signal of similar time constant into tsame vestibular brain stem nuclei

hence in normal circumstances the summed output of these 2 signals results in a constant response