Oculomotor System III Saccades Flashcards
Describe the neural control of saccades
Their direction is dictated by premotor neurons in two gaze centers in the reticular formation:
- the paramedian pontine reticular formation (PPRF) next to the abducens nucleus is the HORIZONTAL GAZE CENTER
- the rostral interstitial nucleus of the medial longitudinal fasciculus (rostral iMLF) in the midbrain reticular formation near the oculomotor nucleus is the VERTICAL GAZE CENTER.
What is the horizontal vs vertical gaze center
the paramedian pontine reticular formation (PPRF) next to the abducens nucleus is the HORIZONTAL GAZE CENTER
the rostral interstitial nucleus of the medial longitudinal fasciculus (rostral iMLF) in the midbrain reticular formation near the oculomotor nucleus is the VERTICAL GAZE CENTER.
- projects to 3rd and 4th nuclei)
Where is the cell body of internuclear neurons? Where do they project?
in abducens cellls, project to medial rectus on the other side
EBNS
found where?
project to what? What are the effects?
- found in the PPRF
- discharge a high frequency burst (eye velocity)
- project to abducens nucleus (pons)
- also project to neural integrator, meaning the burst signal gets integrated and makes the eye position, which is then sent back into the abducens nucleus
- also project to the IBN on the other side. this means the burst is immediately put onto the IBN, which inhibits all the antagonist motor neurons as well as the EBNs on the other side.
Which neurons have information about eye position/velocity?
Motor neuron, burst neuron, tonic neuron
Motor neurons (MNs) have both
Burst neurons (EBNs and IBNs) have eye velocity
Tonic neuron (neural integrator) has eye position
What are omnipause neurons?
Pause neurons for all directions of saccades.
They have tonic activity that is not related to the eye position at all, not related to the neural integrator either.
On all the time, but a discrete pause every time theres a saccade (every time burst neurons fire)
- OPNs inhibit ALL burst neurons
eg, if youre trying to fixate and read notes and an EBN discharges any action potential, your eye will jiggle a tiny bit which wont allow u to read (instability of vision). OPNs make sure that they eye can stay fixed and be able to read.
What do IBNs project to?
Project to:
- IBNs on the other side
- EBNs on the other side
- abducens nucleus on the other side (which prevents antagonist activity)
They have inhibitory output.
Which part of the brain sends projections to the horizontal (PPRF) and vertical gaze centers (rostral iMLF), providing the MOTOR COMMAND to move the eyes via a saccade to an intended new position for the foveation of a visual stimulus?
The superior colliculus
What is the superior colliculus involved in regarding oculomotion?
Sends projections to the horizontal (PPRF) and vertical gaze centers (rostral iMLF), providing the MOTOR COMMAND to move the eyes via a saccade to an intended new position for the foveation of a visual stimulus
- very strong projections to PPRF and RiMLF
- the command structure thats equivalent of motor cortex (final decision are made here, when you’re going to move your eye and where, etc)
- “master controller” of saccadic eye movements
Describe the organization of the layers of the superior colliculus
Superficial/dorsal layer: visual layer
- retinal ganglion cells go here, as well as the early visual areas
- (like a visual structure, processing visual stimuli)
- just like visual map in V1, fovea is overrepresented
Intermediate layers: motor layers
- signalled by brain areas that decide where were going to look
- motor layers project to the PPRF and iMLF to trigger saccades
These layes are aligned and theres vertical integration so that if you see a visual stimulus, you can make a motor saccade based of this stimulus immediately.
superior colliculus visual map oculomotor system III slides
SCs (superficial - the visual layer) has a visual map
whereas
SCi (intermediate - the motor layer) has a motor map
For motor map:
Usually have a horizontal and vertical component. Some neurons go to both pprf (horizontal gaze center) and riMLF (vertical gaze center), contributing to both horizontal and vertical.
- If youre mostly involved in doing horizontal component, youre going to pprf
- If youre mostly involved in vertical, going to riMLF
- But most neurons are both, very few that just go to one or another
Superior colliculus also contains fixation neurons that project to the omnipause neurons
What are fixation neurons, and how do they function in relation to saccades and omnipause neurons?
Fixation neurons, located at the fovea, exhibit high tonic activity when the eye is not moving. Their activity pauses for most saccades and resumes at a new frequency upon fixation.
Unlike omnipause neurons, their activity does not switch abruptly from 0 to 100 but varies between trials. Their signal is associated with the vigilance of fixation (how strongly one wants to fixate). Fixation neurons provide major input to omnipause neurons, whereas saccade-related neurons primarily project to burst neurons (IBNs and EBNs).
What is a difference between where fixation neurons vs saccade neurons innervate?
Fixation neurons: provide major input to omnipuase neurons
Saccade neurons: major input to burst neurons (IBNs and EBNs)
Which neurons project to PPRF vs riMLF?
Motor layers project directly to PPRF. Medial colliculus triggers a vertical saccade, projecting to riMLF.
Usually have a horizontal and vertical component. Some neurons go to both PPRF and riMLF, contributing to both horizontal and vertical.
- if youre mostly involved in doing horizontal component, youre going to PPRF
- if youre mostly involved in vertical, going to riMLF
- but most neurons are both, very few that just go to one or another
How are horizontal and vertical saccades controlled in the brainstem?
The motor layers of the superior colliculus project to:
- The paramedian pontine reticular formation (PPRF), which controls horizontal saccades.
- The rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), which controls vertical saccades.
Most neurons contribute to both horizontal and vertical movement, but some are more specialized—PPRF is primarily involved in horizontal saccades, while riMLF is primarily involved in vertical saccades.
How do fixation and saccade neurons in the superior colliculus contribute to eye movement control?
The saccade map in the superior colliculus contains both fixation neurons and saccade neurons:
- Fixation neurons are highly active when the eye is not moving, with tonic activity at the fovea. Their activity fluctuates based on the vigilance of fixation (how strongly you want to maintain fixation). They project to omnipause neurons, which help suppress saccades.
- Saccade neurons project mainly to burst neurons (IBNs and EBNs), driving rapid eye movements. During saccades, fixation neuron activity pauses, allowing the shift in gaze.
What is the fastest route of information transmissions to the colliculus?
Retina –> lateral geniculate nucleus (LGN) –> primary visual cortex –> SCs (superficial SC aka visual layer)
NOT retina –> SCs
because of myelination
What are the two parts of the superior colliculus (SC)?
SCs (superficial SC - visual layer)
SCi (intermediate SC - motor layer)
What does overt oriented mean briefly?
Where your visual axis is going
Covert vs overt
Overt is where youre looking and covert is where youre paying attention to.
Covert attention is a lot of the times same as overt. Sometimes its not the same
