31 - Control of Eye Movements Flashcards
Two components of controlling gaze
1) Oculomotor system:
* Moves the eyes in the orbit (whilst head is still)
* Involves extraocular muscles and neural pathways that coordinate movement of each eye.
2) Head-movement system:
* moves the eye sockets as a whole (whilst head moves).
* Involves vestibular system as well as oculomotor system
Saccadic eye movements
Shift fovea rapidly to a new visual target
Smooth pursuit eye movements
Keep image of a moving target on the fovea
Vergence eye movements
Moves eyes in opposite directions.
EG: When reading a book, looking at something near face
Vestibular ocular eye movements
Holds image still on the retina during brief head movements
Optokinetic eye movements
Holds the image stationary during sustained head rotation or translation.
EG watching things pass from a moving car/train, etc
Flicking eye movements
Five major types of eye movements
1) Saccidic
2) Smooth
3) Vergence
4) Vestibular ocular
5) Optokinetic
Where does inferior oblique insert?
Behind equator, just under lateral rectus
Eye movements 1 2 3 4 5 6
Up - Elevation Down - Depression Abduction - Away from midline Adduction - To midline Intortion - Rotating eye to nose Extortion - Rotating away from nose
Superior oblique insertion
Inserts behind equator at an oblique angle close to the medial rectus.
Path of superior oblique
Tendon runs through a fibrocartilage pulley or ‘trochlea’
Major role of oblique muscles
Torsion
Angle of superior rectus insertion to visual axis
~23 degrees
*Testing each eye muscle in isolation
EYE MOVEMENTS
Testing movements of obliques in isolation.
Superior is looking down towards nose (depression and adduction).
Inferior is looking up towards nose (elevation and adduction).
Why is CNVI often affected in disease?
Has a very long path.
Can be affected by things like systemic diabetes, etc.
How can both medial rectus muscles be affected?
Internuclear ophthalmoplaegia.
Part of brain that controls movement of eyes (conscious)
Frontal eye fields (just in front of the motor cortex)
Brain areas involved in coordinating eye movements
1
2
1) Medial longitudinal fasciculus (MLF):
* White matter tract that connects the various cranial nerve nuclei
2) Reticular formation
* Pontine paramedian reticular formation (reticular formation in the pons)
* Mesencephalic paramedian reticular formation (reticular formation in the midbrain)
Part of brain involved in coordinating horizontal eye movements
Reticular formation in the pons (coordinate activity of CNVI and CNIII)
Part of brain involved in coordinating vertical eye movements
Reticular formation in the midbrain (CN IV and III)
Neuronal control of horizontal saccades
1
2
1) Within Pontine Paramedian reticular formation are:
* Burst neurons
- Fire at high frequency just before movement.
- Several types:
* Provide excitatory connections with ispilateral abducens.
* Inhibitory burst neurons suppress the activity of the contralateral abducens
* Omnipause neurons
- Fires continuously EXCEPT during a saccade.
- GABAergic
- Project to contralateral abducens nucleus
Control of horizontal saccades
1
2
3
1) Requires simultaneous excitation of burst neurons and inhibition of omnipause neurons
2) Excitatory Burst neurons in abducens nucleus on one side receive input from Cortex.
3) Inhibitory burst neurons from activated abducens nucleus inhibit contralateral abducens nucleus
Communication between nuclei of CNIII and VI
Effect of damage to medial longitudinal fasciculus (MLF), EG with MS
Lose control of coordination of horizontal eye movements.
Where does the upper motor neuron in horizontal eye saccades originate?
Contralateral frontal eye fields.
Synapses with secondary neuron in superior colliculus
Reflex controlling ability to read something while head is moving
Vestibular ocular reflex
Role of semicircular canals
Position of head in space (Static)
Role of otoliths
Acceleration/deceleration of head
Transduction of movement in otolith organs
Cilia arranged in clusters, with tallest cilia on one side, and shortest on opposite side.
Cilia sit within gel, with calcium carbonate on surface.
When head is moved, cilia bend, depolarise cell.
Action potentials sent from semicircular canals to brain on turning head
Fluid in horizontal canal will move in one direction, increase firing rate.
Fluid in horizontal canal on opposite side of head will move in opposite direction, and will decrease firing rate
Path of information from vestibular system
1
2
3
1) Vestibular information carried by CNVIII (vestibulocochlear nerve) to vestibular nuclei in the medulla (#6).
2) To coordinate head and eye movements information from the vestibular nuclei must be coordinated with the CNIII, CNVI nuclei
3) Connect to medial longitudinal fasciculus
*Vestibular ocular reflex
VESTIBULAR OCULAR REFLEX
