Flashcards in eye movements I-III Deck (19)
What are the four types of eye movements?
1. Smooth pursuit: tracking (to keep an object on the fovea). 2. Saccades: rapid, ballistic (to bring an object onto the fovea. 3. Vestibular ocular reflex (VOR) and optokinetic nystagmus (OKN): a combination of pursuit and saccades. 4. Vergence: moving the fovea to an object closer (convergence) or farther away (divergence).
What is the difference between conjugate and vergence eye movements?
Conjugate: eyes move same amount in same direction (as in the vestibulo-ocular reflex). Can be fast (saccades) or slow (tracking). Initiated by sensory inputs. Vergence: eyes moving in opposite directions, e.g., when both eyes turn nasally. Both medial recti contract, pupils constrict, ciliary muscles contract (lens becomes fatter to focus near). This is seen in the near/accomodation reflex
What is a saccade and how is it generated?
This is a rapid movement that brings the eyes to a predetermined target or position at a rate of up to 700 degrees per second. A very high frequency burst (as high as 1000 Hz) is neededin extraocular muscles for the initial acceleration, and then a carefully calculated steady rate is required to maintain the new eye position
What are smooth pursuit eye movements, how fast can they be, and why?
Once the object of interest is on or near the fovea, we use slower movements, pursuit, to track a moving object. Tracking movements that are initiated by a moving stimulus involve analysis of the visual scene by cells in visual association cortex
what is an optokinetic nystagmus
rhythmic pattern of saccades and tracking movements. Similar in appearance to vestibular nystagmus. Elicited by a moving visual stimulus. During slow phase (tracking), eye moves in same direction as object. During fast phase (saccade), eye moves rapidly back to starting position
what is nystagmus
sawtooth movement of eyes: slow ramp (tracking) followed by fast ramp (saccade). direction of nystagmus defined by the direction of the rapid saccade, i.e., "right-beating nystagmus." inappropriate nystagmus can be indication of damage to vestibular system or cerebellum
How is coordinated contraction of both eyes accomplished
interneuronal pattern generators in oculomotor and abducens nuclei
What is the maximum speed of tracking/smooth pursuit
Tracking movements or "smooth pursuit" can only maintain foveation at a rate of about 50 degrees per second and are completely dependent on visual input. For stimuli moving faster than that such asin normal visual analysis of the world, a combination of saccades and smooth pursuit is used
Where are the pattern generators for vertical and horizontal saccades?
vertical saccades:near the oculomotor nuclei. horizontal saccades: in the reticular formation near the abducens nucleus (the paramedian pontine reticular formation, abbreviated PPRF).
Where are the control centers for saccades
Cortex and superior colliculus. The frontal eye field (just anterior to head representation in motor cortex) can activate the saccades directly to reticular formation or via the superior colliculus to the reticular formation
describe input to the suprior colliculus
There is not only a retinotopic map (direct input from the retina) but also an auditory spatial map and a somatotopic map, all superimposed on a motor map for the movement resulting from the saccade
What are the effects of damage to the frontal eye field, superior colliculus and both
If the frontal eye field is damaged, there is a temporary loss of the ability to generate saccades. If the superior colliculus is damaged, saccades are less accurate and occur less often but still happen. If both the frontal eye field and the superior colliculus are damaged, there is permanent loss of the ability to make saccades
Are horizontal saccades ipsilateral or contralateral
contralaterl-a saccade to the left is driven by activity in the right frontal eye field.
Describe the control of the VOR for a person sitting on a chair that is rotated to the right (clockwise rotation if you look down on them from above).
Rightward head rotation is signaled by horizontal canals, leftward eye rotation is produced by excitation of motor neurons to the left lateral rectus in left abducens nucleus, and excitation of right medial rectus by motor neurons in right oculomotor nucleus
describe what happens in the semi circular canals when the head rotates to the right
fluid in the horizontal semi-circular canal lags behind, resulting in deflection of the cupula in both horizontal canals. In the right horizontal canal the deflection results in depolarization of the hair cells, in the left horizontal canal the hair cells are hyperpolarized (opposite effects always occur in two paired canals).
Pathway of vestibulocular reflex when head turns to right
depolarized hair cells on right side only > vestibular nerve > synapse at vestibular nuclei > cross midline > > medial longitudinal fasciculus > excitation of left lateral rectus motor neurons in abducens nucleus AND abducens internuclear interneurons > interneuron fibers cross midline again and ascend as MLF to excite right medial rectus motor neurons > co-activation of left lateral rectus and right medial rectus
Which cells serve as pattern generators for conjugate horizontal gaze
Internuclear interneurons in the vestibulo-ocular reflex
What is internuclear ophthalmoplegia? Who is affected
Medial longitudinal fasciculus damage- disconnects the coordination of medial and lateral recti during horizontal gaze movements. Common in MS.The medial rectus muscle and motor neurons may be normal, but the muscle doesn't function properly during horizontal gaze