Forty One Flashcards
What are vergence movements and conjugate movements. Explain two examples of each. What is opto-kinetic movement? What is vestibulo-ocular movement? What are their funcitons?
Eye movements are of two types: vergence and conjugate. Vergence movements occur
when eyes shift between distant and near objects. When the shift is from distant to near
objects, the eyes converge; when from near to distant, they diverge. Conjugate
movements occur when the eyes move in the same direction, i.e., to the right, left, up or down.
Two main types of conjugate movements are saccadic and smooth pursuit. Saccadic
movements are voluntary when vision is being moved rapidly from one target to another,
such as searching for something in the horizon or reading a printed page. Saccadic may
also be reflex, as in nystagmus and REM sleep. Smooth pursuits are movements that
keep an image of a moving target fixed on the retinae.
Other types of conjugate movements are optokinetic and vestibulo-ocular. Optokinetic
movements keep targets fixed on the retinae during sustained movements of the head.
Vestibulo-ocular movements keep targets fixed on the retinae during brief movements of
the head.
What six muscles control movement of each eye? What do gaze centers do? Where are they located?
The movement of each eye is controlled by the coordinated action of six muscles: four
recti (superior, medial, lateral, inferior) and two obliques (superior and inferior). The
muscles are innervated by three cranial nerves: the oculomotor, trochlear and
abducens.
The six pairs of external ocular muscles responsible for keeping both eyes focused on the
same object are controlled by gaze centers, highly specialized groups of neurons in the
brainstem and cerebral cortex.
What are the three centers in the brainstem that control eye movement? Where are they located?
There are three centers in the brainstem that control eye movements. The horizontal gaze center is in the pons, and the vertical gaze and vergence centers are in the midbrain.
Where is the horizontal gaze center located? Where do its axons synapse? What movements do they control?
The horizontal gaze center is located in the paramedian pontine reticular formation
(PPRF). The center on each side is responsible for conjugate movements toward that
side; hence, a unilateral lesion results in paralysis of gaze toward the ipsilateral side.
From each center, impulses pass to the ipsilateral abducens nucleus and, via the
contralateral medial longitudinal fasciculus (MLF), to the lower motor neurons in the
oculomotor nucleus innervating the contralateral medial rectus muscle. In this way, the
ipsilateral eye abducts and the contralateral eye adducts.
Where is the vertical gaze center located? How is this gaze center bilateral? Where are upward movements represented? Downward movements?
The vertical gaze center is in the accessory oculomotor nuclei at the rostral end of the
MLF in the midbrain. This gaze center is bilateral and the two centers are interconnected
via the posterior commissure. Upward movements are represented more dorsally,
downward more ventrally.
Where is the vergence center located? What does it do?
The brainstem center controlling convergence and divergence of the eyes, as when directing vision from far to near or near to far objects, is also located in the rostral midbrain near the oculomotor nuclei.
What are the most well known centers associated with eye movements within the cerebral cortex?
Within the cerebral cortex are centers associated with eye movements. The most well
known are the frontal eye field, the parietal and temporal eye fields, and the occipital
eye field.
What is the chief center in the cerebral cortex for voluntary eye movements? Where is it located? What does stimulation of it result in? Where does it project to?
The chief center in the cerebral cortex for voluntary eye movements is located primarily
in the posterior part of the middle frontal gyrus and is commonly called the frontal eye
field. Stimulation of this area results in aversive eye movements in the form of saccades.
The frontal eye field projects to the vertical and horizontal gaze centers, and to the
superior colliculus.
What movements do the posterior parts of the superior parietal lobule affect? What is their input and outputs? Where is the chief cortical center associated with smooth pursuit movements located? What other centers might be involved? Where does it receive input? Where does it send impulses? Where do those impulses then make connnections? What center is associated with optokinetic movements? What are they? What is optokinetic nystagmus?
Areas in the parietal and temporal lobes also influence eye movements. The posterior parts of the superior parietal lobule affects saccadic movements through reciprocal
connections with the frontal eye field and projections to the superior colliculus.
An area in the posterior part of the lateral surface of the temporal lobe appears to be the chief cortical center associated with smooth pursuit movements, although the superior parietal lobule and frontal eye field may also be involved. This area receives input from the visual cortex and sends impulses to dorsolateral pontine nuclei which then make connections with the vestibular nuclei via the vestibulocerebellum.
The temporal eye field is also associated with optokinetic movements. An example of these movements occurs in an individual in a moving vehicle watching an object in the
passing landscape. The eyes will automatically follow the particular object in the landscape until it disappears from view, at which time the eyes move rapidly in the opposite direction and fix on a new object in the landscape. A similar phenomenon occurs when vision is directed at vertical black and white stripes on a slowly rotating drum. The eyes will fix on a particular stripe, follow it until it disappears from view, and then move rapidly in the opposite direction to fix on a new stripe on the drum. These slow drifting and fast return movements are referred to as optokinetic nystagmus.
What makes up the occipital eye field and where is it located? What does it control? What 3 things happen in the near response? What pathways make this happen? Where do the occipital eye field axons synapse for the near response? Where do they go from there? Where do its axons synapse for divergence? What is their pathway?
The primary visual and visual association areas in the occipital cortex form the occipital eye field which controls vergence movements. Convergence occurs when vision is directed from a far to a near target. This phenomenon is called the near response and includes simultaneous contraction of the medial rectus muscles, accommodation of the lenses, and constriction of the pupils. Occipitofugal fibers pass to the vergence centers adjacent to the oculomotor nuclei, which then project to the oculomotor nuclear complex. Somatic oculomotor neurons innervate the medial rectus muscles and visceromotor (parasympathetic) oculomotor neurons influence via postganglionic fibers from the ciliary ganglia the ciliary muscles for the accommodation of the lens and the pupillary constrictor muscles. Divergence occurs via connections to the abducens nuclei which are made through the reticular formation, not the MLF, because divergence is not impaired by MLF lesions.
How is the superior colliculus subdivided? What are its inputs and which division receives which? What are its outputs and pathways? What do it descending outputs control? What is its role in the control of ordinary eye movements?
The superior colliculus consists of alternating gray and white layers that are subdivided
into superficial, intermediate, and deep. The superficial layers receive input directly
from the retina and cortical visual association areas. The intermediate layers receive
input from the frontal eye field. The deep layers receive input from sensory paths
ascending through the brainstem, especially the pain and auditory paths. Output from the
superior colliculus ascends to visual association areas via the pulvinar and descends into
the brainstem and spinal cord. The latter are responsible for reflex turning of the head and
eyes in response to startling pain or auditory stimuli.
The role of the superior colliculus in the control of ordinary eye movements is not
entirely clear. Because of the input it receives from the retina and cortical eye fields and
its output to the brainstem gaze centers, this structure undoubtedly plays a role as a
visuomotor integration center especially concerned with reflex oculomotor movements.
