ocular motor control: physiology of saccades and fixations in peripheral gaze

Question 1

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

Answer 1

• fibrillenstrucktur - 80%

- felderstrucktur - 20%

Question 2

what type of eye movements do the fibrillenstrucktur fibres produce

Answer 2

large, active, fast/phasic contraction (twitch)

Question 3

what type of eye movements do the felderstrucktur fibres produce

Answer 3

smaller, tonic sustained contraction

Question 4

which muscle fibre has a larger diameter

Answer 4

fibrillenstrucktur (and smaller diameter are felderstrucktur)

Question 5

what are myofilaments and where are they found

Answer 5

they are contractive elements found in the muscle fibres, called actin and myosin which are immobilised when cells contract

Question 6

on average, how many saccades and fixations collectively so we make in 1 minute

Answer 6

~300 saccades and fixations

Question 7

where abouts do we fixate when making large saccades

Answer 7

peripheral fixation

Question 8

where abouts do we fixate when making small saccades

Answer 8

central fixation

Question 9

on average, how long does a saccade take, when looking at something

Answer 9

~20-30 sec

Question 10

on average, how long do we fixate for, when looking at something

Answer 10

~100-200 msec

Question 11

what type of contraction do saccades require, and which muscle fibres are used for this

Answer 11

fast muscle contraction involving fibrillenstrucktur fibres to rapidly move the eye

Question 12

which 2 brainstem generator nuclei are saccadic eye movements mediated by

Answer 12

• PPRF: for horizontal saccades

- riMLF: for vertical saccades

Question 13

what type of contraction does peripheral fixation require, and which muscle fibres are used for this

Answer 13

tonic muscle contraction by felderstrucktur fibres to prevent the eye drifting and so holds the gaze steady for periods of fixation and object inspection

Question 14

what special nuclei in the brainstem mediate peripheral fixation and name the type for horizontal fixation and vertical fixation

Answer 14

neural integrator nuclei:
- horizontal gaze holding: PeriHypoglossal nucleus (aka nucleus prepositus hypoglossi)

• vertical gaze holding: intersistial nuclei of cajal

Question 15

where in the brainstem is the PeriHypoglossal, neural integrator nuclei located

Answer 15

in the medulla, below the abducent nucleus

Question 16

where in the brainstem is the interstitial nuclei of canal, neural integrator nuclei located

Answer 16

in the upper midbrain, above the oculomotor complex

Question 17

what causes resistance to eye movements

Answer 17

natural forces e.g. viscous dragging of orbital connective tissue create inertia that resists eye movements

Question 18

how do the oculo motor neurons respond to resistance to eye movements

Answer 18

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

Question 19

what is the neural pulse for a saccade generated by

Answer 19

excitatory burst cells (EBCs) in the PPRF or riMLF

Question 20

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

Answer 20

EBCs fire at a very high frequency of up to 1000Hz for a brief period
and ~10-15 msec before the saccade begins

Question 21

what does the firing frequency of the excitatory burst cells (500-1000Hz) determine

Answer 21

the speed of the saccade

Question 22

what is the firing duration of the excitatory burst cells during a saccade, and what does the duration determine

Answer 22

from 20-3- msec (very brief)

determines the amplitude of the saccade

Question 23

what does the higher the frequency and longer the duration of a excitatory burst cell result in

Answer 23

faster and larger saccades

lower frequency and shorter duration bursts results in slower and smaller saccades

Question 24

how does increasing firing rate and durations of an excitatory burst cell of the PPRF relate to saccadic horizontal excursions

Answer 24

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

Question 25

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

Answer 25

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

Question 26

what causes the eye to drift back into primary gaze position

Answer 26

elastic restoration forces of the orbital connective tissue supporting the eye

Question 27

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

Answer 27

neural integrator cells (NI) in the PeriHypoglossal nucleus and intersistial nuclei of cajal

Question 28

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

Answer 28

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

Question 29

what type of activity does a excitatory burst cell produce

Answer 29

pulse activity

Question 30

what type of activity does a neural integrator cell produce

Answer 30

step activity

Question 31

what does neural integration match

Answer 31

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)

Question 32

what does the neural integrator cell look at in relation to the pulse, before it produces a step of activity

Answer 32

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)

Question 33

what does a lesion to the PeriHypoglossal nuclei (PHN) or interstitial nucleus of cajal (INC) result in and what symptoms does this show

Answer 33

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

Question 34

where are omni pause cells located

Answer 34

in the nucleus raphe interpositus (NRI)

located at the midline near the PPRF and abducens nucleus

Question 35

what is the job of the omni pause cells

Answer 35

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

Question 36

when is the only time the continuous firing of omni pause cells actually pause

Answer 36

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

Question 37

where in the CNS are EBCs the only cells which don’t fire spontaneously and why

Answer 37

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

Question 38

explain how action potentials of pause cells and action potentials of burst cells are complementary with one another

Answer 38

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

Question 39

what does a nucleus raphe interpositus (NRI) lesion result in

Answer 39

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

Question 40

what are the pause cells and burst cells in the brainstem saccade generators regulated by

Answer 40

higher supra-nuclear control centres: eye fields

Question 41

which 2 broad types of saccadic eye movements are the regulation of pause and burst cells by higher supra-nuclear control centres for

Answer 41

1. 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
2. internally-generated voluntary saccades: can happen in absence of a visual stimulus and is a memory driven eye movement

Question 42

which 2 higher supra nuclear control centres is responsible for visually cued reflexive saccades

Answer 42

1. superior colliculus: midbrain
   responsible for saccades when moving eyes and head quickly to look at a new object
2. 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

Question 43

which type of saccade needs a visual stimulus

Answer 43

visually cued reflexive saccades

Question 44

which type of saccade does not need a visual stimulus i.e. is memory driven

Answer 44

internally-generated voluntary saccade

Question 45

which 2 higher supra nuclear control centres is responsible for internally-generated voluntary saccades

Answer 45

1. frontal eye field: frontal cerebral cortex

2. supplementary eye field: frontal cerebral cortex

Question 46

where in the cerebral cortex is the parietal eye field PEF located

Answer 46

posterior cerebral cortex: inferior parietal lobule, just below the intra-parietal sulcus (IPS), within brodmann areas 39 and 40

Question 47

where in the cerebral cortex is the frontal eye field FEF located

Answer 47

frontal cerebral cortex: posterior end of middle frontal gyrus (MGF), in front of primary motor cortex, within brodmann area 6

Question 48

where in the cerebral cortex is the supplementary eye field SEF located

Answer 48

frontal cerebral cortex: posterior end of middle frontal gyrus (MFG), above the FEF, within brodmann area 6 and 8