10/12 Eye Movements Flashcards
• Be able to define and explain the concept of foveation and its importance to eye movements
• Fovea is most sensitive part of retina. Foveation is directing our eyes to a target in such a way it is projected on the fovea. Requires visual feedback as source of info on eye movement accuracy
• Be able to define and explain the three different classes of horizontal eye movements
- Saccades: quick, darting conjugate movements which rapidly direct eyes to a new target (conjugate=both eyes in same direction)
- Smooth pursuit: slow conjugate movement with allows tracking a moving object or stationary object while moving.
- Con or di vergence. Convergent is disconjugate movement of both eyes towards midline
• What are the two types of saccades and what controls them
- Frontal lobes BA 8 control voluntary saccades
- Superior colliculi control reflexive saccades to simple stimuli. Accomplished in association with tectospinal tract which moves head and neck
• Describe the pathway of voluntary horizontal saccades
• Begins in BA 8 frontal eye fields. Projects to brainstem crossing in dorsal midbrain. Collaterals synapse in superior colliculus. Synapses in paramedian pontine reticular formation then CN VI n. then in MLF synapse in CN III because six fired medial rectus subnucleus causes saccade to contralateral space
• Describe the trochlear nerve function
• The trochlear nerve innervates trochlear muscle acts as a pully to change direction of muscle. Trochlea is slightly in front of eye which switches direction of the muscle superior oblique. Pulls eye down and in
• Identify the voluntary smooth pursuit pathway
• Origin BA 39 (angular gyrus) or TOPJ. Compute speed and direction of moving object
• Describe what the reflexive smooth pursuit (vestibulo-ocular reflex) is and how it works
• VOR allows maintenance of foveation. Coordinated by reflex arc and does not require cortical input. The semicircular canals are stimulated by ipsi head movement. The signal comes from vestibular nerve to vestibular nucleus to contralateral CN VI n. Abducens nerve and MLF to ipsilateral CNIII medial rectus subnucleus. A head turn to left will stimulate left vestibular n. and eyes right. With excitation from ipsilateral head movement (CNIII), there is inhibition of opposive III and VI.
• Explain the near triad
• To bring a near object into focus involves convergence, accommodation (lens curvature) and pupillary constriction. These are the near triad. Precise pathways are not known. Accommodation of lens occurs via parasympathetic pathways that control pupillary constriction (CN III ciliary muscle). Bilateral medial rectus contraction controls inward eye movement (CN III)
• List and define nerve and gaze palsy’s associated with eye movements
- Nerve palsy: cannot activate muscle. Infranuclear (distal to nucleus) lesion. Unilateral and ipsi. Can be mimicked by muscle lesion. CN III, IV, or VI.
- Gaze palsy: cannot look in a direction (neither eye can look left). Usually lesion in FEFs, PPRF, or CN VI n. FEFs—cannot look contralaterally, eyes towards lesion. PPRF or CN VI n.—cannot look ipsilaterally with either eye. Supranuclear= FEF, PPRF. Nuclear= CN VI only.
- Mixture of gaze and nerve: MLF lesion or MLF + nuclear lesion.
• 1 ½ syndrome: combo ipsi gaze palsy and INO. Eyes fixed straight ahead at rest. No ipsi gaze in either eye. No contra gaze in ipsi eye. Contra medial rectus cannot be excited from ipsi MLF excitation.
• List and define eye movement nerve palsies
- Third nerve palsy: down and out. EOMs that work are lateral rectus and superior oblique. Ptosis and mydriasis
- Fourth nerve palsy: cannot depress when adducted. Cannot intort. Head tilt down to contralateral side.
- Sixth nerve palsy. Eyes cannot abduct.
- Sixth n. lesion: gaze palsy to ipsi side cannot be overcome by VOR
Internuclear ophthalmoplegia. Lesion of MLF. Eyes can move normally. No resting abnormality. Eye ipsi cannot adduct. Contra eye exhibits nystagmus (involuntary movement). Bilateral MLF lesion stops both eyes moving medially
• Explain how to differentiate CN VI n. lesion from PPRF.
• Test VOR because vestibular n. input is intact. If right PPRF is damaged, the left vestibular n. can drive right abducens n. the crossed MLF to CN III n is also intact to contract right medial rectus
• Explain how to differentiate between cortical and midbrain lesions
• FEF are close to BA4 so lesions affecting BA4 or CST also affect FEF or projection to pons. FEF lesions result in contralateral saccade deficit and ipsi gaze preference (right way eyes—away from weak side). Pontine lesions affecting CST and PPRF cause contra paralysis and contra gaze preference (wrong way eyes).
• Be able to recognize pupillary reflex deficits and their most likely anatomical etiology
- Left occulo-motor nerve lesion: left eye permanently dilated
- Left horners syndrome. Right eye dilated in dark room. Dilation lag going from light to dark.
- Left afferent pupillary defect: normal except during swinging flashlight test.
• Be able to describe and define the optokinetic reflex and its anatomy
• Allows tracking of sequential moving targets when one leaves visual field but there is another and your head stays still. Involves parieto-occipito-temporal junction (smooth pursuit area) which excites PPRF. Ipsi. Saccade generated comes from FEF ipsi allowing contra saccade. Optokinetic strip
