Centres for control of eye movements Flashcards
What is steady fixation?
Maintenance of gaze when head is still
What does the vestibulo-ocular reflex do?
Maintains fixation to compensate for rapid head movements
What does the optokinetic response do?
Slow compensatory movements to maintain fixation on large moving target (slow head movement)
What is smooth pursuit?
Slow conjugate movements for steady tracking of a small moving target
What are saccades?
Rapid conjugate movement to bring the eyes into a new position to enable viewing eccentrically located object
What is vergence?
Generates equal but opposite eye movements to maintain visual target simultaneously on both foveas during change of depth of gaze
Where are the frontal eye fields in the brain?
Premotor cortex in middle frontal gyrus
Contain cells exhibiting pre-saccadic activity for voluntary eye movements and accomodation
Maintains visual attention and reflex eye movements
What does a lesion on one side of the premotor cortex cause optically?
Inability to saccade towards opposite side (eyes deviate to the side of the lesion)
What is the fronto-mesencephalic pathway?
Fibres from frontal eye fields travel within anterior limb of internal capsule, passing through thalamus to midbrain where they decussate and reach superior colliculus
What does the superior colliculus do?
Initiate and control saccades independent of frontal eye fields
Initiates reflex orientating saccades
If frontal eye field damaged, superior colliculi can compensate
What are the paramedian pontine reticular formations?
Neurons in the pons lateral to CN VI nucleus
Horizontal gaze centre
Final common pathway horizontal movements; quick face of nystagmus, saccadic movements, smooth pursuit
How does horizontal gaze occur?
Fibres from the PPRF liase with ipsilateral CN VI nucleus.
CN VI controls ipsilateral lateral rectus but also joins CN III via MSF to control contralateral medial rectus
Vestibular nucleus also contributes fibres
Memorise diagram
Medial longitudinal fasciculus (MSF)?
Fibre tract running from thalamus to anterior horn cells of the spinal cord
Well developed between vestibular nuclei and CN III nucleus
Provides co-ordination of horizontal and vertical gaze
Commonly affected by demylination and ischaemia
Describe the pathway involved in voluntary horizontal gaze to the left
Right frontal eye field Superior colliculus Left PPRF Left CN VI-> left LR Right CN III-> right MR
How is smooth pursuit controlled>
Temporal eye field
Dorsolateral pontine nuclei determine direction and velocity of eye movements necessary to track target
What is the most important structure in vertical gaze?
Medial longitudinal fasciculus in midbrain
How does the cerebellum control eye movements?
Modulating activity of oculomotor system and vestibulocular reflexes
Spatiotemporal translator to tune amplitude of eye movements in response to stimulus
Dysfunction leads to impaired smooth pursuit, impaired fixation and nystagmus
Where are the vestibular nuclei and what do they do?
Lower pons and upper medulla under 4th ventricle
Afference-from vestibular ganglion and spinocerebellar tracts
Efference- cerebellar connection, PPRF, vertical gaze centres and oculomotor nuclei
How does the vestibulo-ocular reflex work?
Semicircular canals -> vestibular ganglion -> vestibular nucleus -> oculomotor nuclei
How does the near response work?
Frontal eye fields-> pretectal convergence centre-> oculomotor nucleus (including Edinger Westphal nucleus)
Causes bilateral accomodation, miosis and convergence
Pathways passes ventrally
EWN effects pupil via ciliary muscle and sphincter pupillae
How does convergence occur?
Activation of both medial rectus subnuclei in CN III nucleus of midbrain
Why can one aspect of the near reflex be lost without affecting the others?
The 3 elements of the near reflex are not tied to each other
What is a major difference between the near reflex and pupil reflexes?
The accomodation pathway reaches the EWN without passing through pretectal nucleus
What is the oculocardiac reflex?
Pressure on eye/stretching of muscles causes nausea and bradycardia
Occurs via trigeminal nerve. Trigeminal spinal nucleus is in the area of visceral moto nuclei in vagus nerve. Vagus nerve is activated
Can occur under GA in strabismus surgery
Which nerve is responsible for reflex lacrimation?
Sensory branches of trigeminal nerve
Synpase in pterygopalatine ganglion
Reflex-parasympathetic but basal tear secretion is sympathetic
What can cause the blink reflex?
Tactile, visual or auditory stimuli
Which nerves are involved in the blink reflex?
Facial nerve to orbicularis muscle closes the eye
What is Bell’s reflex?
Upward movement of globe which occurs with forcible eyelid closure
Absent in 10% of individuals
