Download Nolte Chapter 21 Control of Eye Movements

Nolte Chapter 21 Control of Eye Movements
There are two types of movements of the eyes. The first in conjugate where the two eyes move the
same amount in the same direction. The second is vergence where the two eyes move in opposite
directions (think convergence when looking at a nearby object).
The medial, lateral, superior, and inferior recti form a cross on the eyeball and help move it in the
cardinal directions.
The superior and inferior obliques move the eyes in a more complicated fashion.
horizontally away from the midline is abduction(lateral rectus). towards the nose is adduction(medial
rectus).
vertically is elevation(superior rectus) and depression(inferior rectus)
around the anterior posterior axis is intorsion(superior oblique) and extorsion(inferior oblique).
Contracting one muscle usually involves the relaxation or simulataneous recruitment of others.
Saccades are used to redirect gaze so that a different image falls on the fovea.
smooth pursuit movements are used to track a visual stimulus that is itself moving.
The CNS computes the size of a required movement in advance and then intiates that movement with
no correction during its course (ballistic movement).
Patterns of excitation are provided to motor neurons in the abducens, trochlear, or occulomotor nuclei
by specialized areas of the pontine and mesencephalic reticular formation.
Rapid horizontal movements is controlled by the medial reticular formation/paramedian pontine
reticular formation (PPRF).
Rapid vertical movements are located in the reticular formation just dorsomedial to the red nucleus
(rostral interstitial nucleus of the MLF)/ This proejcts bilaterally to motor neurons for all the elevator
muscles of both eyes and to the motor neurons for the ipsilateral inferior rectus and contralateral
superior oblique.
The frontal eye field, which is in front of the representation of the face in the precentral gyrus (the
posterior portion of the middle frontal gyrus) is involved in the initiation of saccades, projects to the
caudate and PPRF and then to the superior colliculus. Damage here makes one lose the ability to look
voluntarily.
The parietal eye field is involved more in automatic saccades for thinfs that appear in the periphery.
Supplementary eye field may be involved in sequences of multiple saccades.
Movements toward or away from the eyes causes images to fall on increasingly disparate parts of the
two retinas.
The vermis is related to fast movements and vergence while the flocculus and nodulus are related to
slow mvoements.
Vermal areas in the occulomotor vermis regulate the timing of muscle contractions during saccades.
the flocculus changes the gain of the VOR.
Vesitbular Ocular Reflex
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primary afferents have their cell bodies in the vestibular ganglion, interneurons in the vestibular
nuclei, and motor neurons in the abducens, trochlear, and occulomotor nuclei.
the flocculus and the brainstem can cancel our the VOR due to a conscious decision or in the
case of a smooth pursuit.
secondary vestibular fibers project sthrough the MLF and adjacent reticular formation to the
motor neurons of the oculomotor, trochlear, and abducens
a spin to the left would cause the cupula of the left horizontal canal to bulge toward the utricle
and depoalirze the hair cells of this canal and increase CN VIII firing
The left vestibular nuclei would project to the right abducens which would innervate the right
lateral rectus and also project to the occulomotor (by way of the MLF) to contract the left
medial rectus so that the left eye adducts.
the left vestibular will also inhibit the ipsilateral abducens (so that it’s easier for the occulomotor
to adduct via medial rectus contraction). This indirectly causes inhibition of the contralateral
occulomotor