ocular motor control: physiology of saccades and fixations in peripheral gaze Flashcards
what are the two types of muscle fibres of recti and oblique EOMs and state how much % each muscle fibre accounts for in the EOM
- fibrillenstrucktur - 80%
- felderstrucktur - 20%
what type of eye movements do the fibrillenstrucktur fibres produce
large, active, fast/phasic contraction (twitch)
what type of eye movements do the felderstrucktur fibres produce
smaller, tonic sustained contraction
which muscle fibre has a larger diameter
fibrillenstrucktur (and smaller diameter are felderstrucktur)
what are myofilaments and where are they found
they are contractive elements found in the muscle fibres, called actin and myosin which are immobilised when cells contract
on average, how many saccades and fixations collectively so we make in 1 minute
~300 saccades and fixations
where abouts do we fixate when making large saccades
peripheral fixation
where abouts do we fixate when making small saccades
central fixation
on average, how long does a saccade take, when looking at something
~20-30 sec
on average, how long do we fixate for, when looking at something
~100-200 msec
what type of contraction do saccades require, and which muscle fibres are used for this
fast muscle contraction involving fibrillenstrucktur fibres to rapidly move the eye
which 2 brainstem generator nuclei are saccadic eye movements mediated by
- PPRF: for horizontal saccades
- riMLF: for vertical saccades
what type of contraction does peripheral fixation require, and which muscle fibres are used for this
tonic muscle contraction by felderstrucktur fibres to prevent the eye drifting and so holds the gaze steady for periods of fixation and object inspection
what special nuclei in the brainstem mediate peripheral fixation and name the type for horizontal fixation and vertical fixation
neural integrator nuclei:
- horizontal gaze holding: PeriHypoglossal nucleus (aka nucleus prepositus hypoglossi)
- vertical gaze holding: intersistial nuclei of cajal
where in the brainstem is the PeriHypoglossal, neural integrator nuclei located
in the medulla, below the abducent nucleus
where in the brainstem is the interstitial nuclei of canal, neural integrator nuclei located
in the upper midbrain, above the oculomotor complex
what causes resistance to eye movements
natural forces e.g. viscous dragging of orbital connective tissue create inertia that resists eye movements
how do the oculo motor neurons respond to resistance to eye movements
oculomotor nuerons:
- fire spontaneously, and this sets a resting tone for the 6 EOMs so they can maintain primary gaze (i.e. equally balanced across the 6 EOMs)
- they send a burst or pulse of increased firing to initiate EOM contraction via the fibrillenstrucktur fibres (i.e. must contract for eyes to move away from straight ahead gaze position)
this brakes the resistance, so the eye can move
what is the neural pulse for a saccade generated by
excitatory burst cells (EBCs) in the PPRF or riMLF
at what frequency do the excitatory burst cells which generate neural pulse for saccades fire at, and how long before do these fire before the saccade begins
EBCs fire at a very high frequency of up to 1000Hz for a brief period
and ~10-15 msec before the saccade begins
what does the firing frequency of the excitatory burst cells (500-1000Hz) determine
the speed of the saccade
what is the firing duration of the excitatory burst cells during a saccade, and what does the duration determine
from 20-3- msec (very brief)
determines the amplitude of the saccade
what does the higher the frequency and longer the duration of a excitatory burst cell result in
faster and larger saccades
lower frequency and shorter duration bursts results in slower and smaller saccades
how does increasing firing rate and durations of an excitatory burst cell of the PPRF relate to saccadic horizontal excursions
firing rate and duration of EBC increases with increasing saccadic horizontal excursions e.g. increases from 5 vs 10 vs 20 degrees from central fixation
how do oculo motor neurons respond to, eyes tending to drift back to primary gaze by elastic restoration forces of the orbital connective tissues supporting the eye
to hold the eye in a new peripheral position after a saccade, the oculo motor neurons:
- send a sustained or tonic step of activity to the EOMs
- so that the tonic felderstrucktur fibres contract (to hold into new position)
- to prevent the drift back into primary position and hold the eye steady in the new gaze position
what causes the eye to drift back into primary gaze position
elastic restoration forces of the orbital connective tissue supporting the eye
what is the sustained or tonic step of activity to the EOMs generated by, in order to prevent the eye drifting back to primary gaze
neural integrator cells (NI) in the PeriHypoglossal nucleus and intersistial nuclei of cajal
how do the neural integrator cells which which receive tonic step firing from the PHN and INC as well as direct pulse of firing from the EBCs of the PPRF and riMLF, match a pulse duration
they integrate its firing rate and duration.
the neural integrator cells produce a tonic level of sustained activity that matches the pulse duration (which they then relay to the oculomotor neurons to get them to produce sustained activity)
that is, higher frequency and longer duration pulses associated with faster and larger saccades which elicit larger steps of activity needed for more eccentric fixation
what type of activity does a excitatory burst cell produce
pulse activity
what type of activity does a neural integrator cell produce
step activity
what does neural integration match
the pulse for the saccade to produce step activity appropriate for the shift in gaze
e.g. a smaller pulse (from EBCs) leads to a smaller step (from NICs)
what does the neural integrator cell look at in relation to the pulse, before it produces a step of activity
NIC looks at the frequency and duration of the pulse thats produced by the EBC and under a mathematical process of integration, in order to produce a step of activity to match, the matching process relates to how big the saccade is going to be
e.g. a big saccade = a big movement made and held for a while means the EOMs contract in a sustained way to hold the gaze eccentrically so…a step of activity needs to be a big one in order to hold the fixation with a big eye movement. (a small eccentric fixation does not need much sustained activity to hold that eye movement and saccade)
what does a lesion to the PeriHypoglossal nuclei (PHN) or interstitial nucleus of cajal (INC) result in and what symptoms does this show
failure to hold eccentric gaze
the affected eye drifts back towards the primary position, with a new saccade needed to re-fixate the target of interest
this is manifest by nystagmus, an involuntary see-saw like oscillation of the eye(s), drifting slowly and rapidly jerking back and forth in opposite directions
where are omni pause cells located
in the nucleus raphe interpositus (NRI)
located at the midline near the PPRF and abducens nucleus
what is the job of the omni pause cells
the control of saccades - prevent unwanted saccades:
as they fire continuously to tonically inhibit excitatory burst cells (which they have connections with) in the PPRF and riMLF, except just before and during a saccade, when their firing pauses
when is the only time the continuous firing of omni pause cells actually pause
just before and during a saccade is when their firing pauses i.e. when the burst cells need to fire in order to make a saccade
where in the CNS are EBCs the only cells which don’t fire spontaneously and why
the EBCs which are in the PPRF and riMLF
because their membranes aren’t leaky here
if the EBCs always fired spontaneously in the PPRF and riMLF, then we will make saccades in the horizontal and vertical direction without control.
so pause cells tell burst cells to inhibit their resting discharge by sending an inhibitory neurotransmitter and only allows EBCs to fire action potentials when they want to make a saccade
explain how action potentials of pause cells and action potentials of burst cells are complementary with one another
most of the time, it is the pause cells that fire in a tonic or sustained way, leading to the release of a inhibitory neurotransmitter to the burst cell, which causes it to shut down.
when we want to make a saccade, the pause cell stops firing and stops releasing inhibitory neurotransmitters onto the burst cell
what does a nucleus raphe interpositus (NRI) lesion result in
the dysfunction of pause cells:
opsoclonus = incessant saccades in random directions, with no pauses for fixation, causing the px to essentially be blind
dont have pause cells any longer which means the burst cells fire spontaneously and the person makes unwanted saccades all of the time with no periods of fixation in-between i.e. the eyes are moving all the time
= saccadic suppression, where we cannot see well as the eyes cannot pause long enough to pause on the object to get information on it
what are the pause cells and burst cells in the brainstem saccade generators regulated by
higher supra-nuclear control centres: eye fields
which 2 broad types of saccadic eye movements are the regulation of pause and burst cells by higher supra-nuclear control centres for
- visually cued reflexive saccades: when moving eyes and head quickly to look at a new object and when doing visual scanning and search saccades and need a visual stimulus to make the eyes move
- internally-generated voluntary saccades: can happen in absence of a visual stimulus and is a memory driven eye movement
which 2 higher supra nuclear control centres is responsible for visually cued reflexive saccades
- superior colliculus: midbrain
responsible for saccades when moving eyes and head quickly to look at a new object - parietal eye field: posterior cerebral cortex
one in the left cortex and one in the right, responsible for driving the eye movements when scanning and searching
which type of saccade needs a visual stimulus
visually cued reflexive saccades
which type of saccade does not need a visual stimulus i.e. is memory driven
internally-generated voluntary saccade
which 2 higher supra nuclear control centres is responsible for internally-generated voluntary saccades
- frontal eye field: frontal cerebral cortex
2. supplementary eye field: frontal cerebral cortex
where in the cerebral cortex is the parietal eye field PEF located
posterior cerebral cortex: inferior parietal lobule, just below the intra-parietal sulcus (IPS), within brodmann areas 39 and 40
where in the cerebral cortex is the frontal eye field FEF located
frontal cerebral cortex: posterior end of middle frontal gyrus (MGF), in front of primary motor cortex, within brodmann area 6
where in the cerebral cortex is the supplementary eye field SEF located
frontal cerebral cortex: posterior end of middle frontal gyrus (MFG), above the FEF, within brodmann area 6 and 8
