Eye movements two general categories of eye movements



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1. EYE MOVEMENTS

Two general categories of eye movements:

- those designed to get images onto fovea to begin to see them clearly

- those designed to keep images on fovea to continue to see them clearly

- diplopia, double vision, occurs both foveas are not directed at objects of interest


Saccadic eye movements

- as things move around at a given distance from us we need to move both eyes that same amount in same directionconjugate movements, 2 types:

1. Saccades – fast movements to get an image onto the fovea; use them for voluntary eye movements, to look over at periphery, to catch up with something that is moving too fast to track, and for fast phase of nystagmus

- saccades are primarily used in voluntary eye movements; triggered from frontal lobes (frontal eye field – anterior to where head is represented in motor cortex); vertical eye movements represented bilaterally; horizontal saccades  each hemisphere triggers movements to the contralateral side

- saccades can be modified through the vestibular nuclei; if the head is moved during a saccade, vestibuloocular reflex (VOR) compensates for the movement
2. Slow movements used to keep image on fovea
- as things move toward us or away from us we need to either converge or diverge our eyes  vergence movements


Muscle

Cranial nerve

Actions

Lateral rectus

Abducens (VI)

Abduction

Medial rectus

Oculomotor (III)

Adduction

Superior rectus

Oculomotor (III)

Elevation, adduction, intorsion

Inferior rectus

Oculomotor (III)

Depression, adduction, extorsion

Superior oblique

Trochlear (IV)

Abduction, depression, intorsion

Inferior oblique

Oculomotor (III)

Abduction, elevation, extorsion

- groups of subcortical neurons exist that are specialized to generate timing signals; receive inputs from parts of brain that can order eye movements; outputs to motor neurons


- vertical conjugate movements  superior and inferior recti are principal muscles

- timing signals for upward movements located near superior colliculi and posterior commissure (distributed bilaterally)

- timing signals for downward gaze come from deep in midbrain, near dorsomedial edge of red nucleus (bilateral lesions required for function to be disrupted)


- horizontal conjugate movements  coordination of lateral rectus of one eye with contralateral medial rectus; abducens nucleus contains motor neurons for ipsilateral lateral rectus and interneurons whose axons cross the midline at abducens nucleus, ascend in MLF, and end on motor neurons of contralateral medial rectus; timing signals generated in reticular formation near abducens nucleus (paramedian pontine reticular formation, PPRF); reticular formation on one side of the pons sets up signals for saccades to the ipsilateral side
Pursuit (tracking) movements

- track a moving object once its image is on the fovea

- pursuit movements can go 50 degrees/second

- rapidly moving objects require combo of saccades and pursuit movements

- there is a latency for pursuit movements when a target starts to move, so when we start to track something, its image has moved off the fovea (a catch-up saccade is required)
- pursuit movements triggered from visual association cortex in occipital lobes, posterior parts of temporal/parietal lobes (frontal eye fields may be involved)

- vertical movements triggered bilaterally

- unilateral cortical damage impairs horizontal pursuit movements in both directions, but impairment is greater when looking to ipsilateral side

- pathway from cortex to abducens involves flocculus, vermis (via pontine nuclei), and vestibular nuclei; no cerebellum  no pursuit movements


Vergence Movements

- to direct haze to something nearby our eyes do the following (near or accommodation reflex):

1. Pupils constrictimprove optical performance of eye and increase its depth of field (objects at different distance put into acceptable focus); unfortunately, it also reduces the amount of light reaching the retina; brain figures pupil size that provides best tradeoff between acuity and intensity

2. Ciliary muscles contract – lens gets fatter and focuses eyes on nearby object

3. Medial recti of both eyes contract  eyes converge
- preganglionic parasympathetics for pupillary sphincter and ciliary muscle are in oculomotor

nucleus (and medial rectus motor neurons); Edinger-Westphal subdivision; all brainstem components for efferent limb of near reflex are in rostral midbrain

- afferent limb of near reflex is normal visual pathway from eyeball to occipital lobe  visual association cortex of occipital lobe projects to efferent machinery in midbrain


2. Indicate consequences of damage to the oculomotor nerve, abducens nerve, abducens nucleus, and MLF
3. VESTIBULOOCULAR REFLEX

- we can keep our eyes pointed at something even if we’re moving around; head to left compensated by conjugate eye movement to the right  direction of gaze does not change

- vestibular primary afferents  ipsilateral vestibular nucleiMLFoculomotor, trochlear, and abducens nuclei

- semicircular canals work only for the first few seconds of sustained rotation  after that, visual system recognizes that images are slipping on retinavestibuloocular pathway


4.

- continuous rotation  compensatory movements (in opposite direction of rotation) interrupted by fast reset movements in opposite direction (in direction of rotation) called nystagmus (named for direction of fast component); postrotatory nystagmus after rotation has stopped (in opposite direction of original rotation) – cupula deflection

- caloric nystagmus causes one horizontal semicircular canal to cause nystagmus (warm/cool water)
- fast phase of nystagmus dependent upon functioning PPRF

- Optokinetic nystagmus caused by moving a repetitive visual pattern in front of someone; visual inputs to vestibular nuclei cause continued nystagmus (reflex attempt to maintain visual stability)



- spontaneous nystagmus can indicate neural pathology
- flocculus suppresses VOR in times such as when we want to look the left by turning our head; VOR would normally couterrotate the eyes and keep them pointed straight ahead, but flocculus blocks this; flocculus also adjusts the gain of the reflex, so that our eyes move the right amount when vision changes


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