Flashcards in Eye Movements I-II Deck (12)
4 types of eye movement
1. Smooth Pursuit
3. OKN; VOR
Smooth Pursuit (describe and how fast they can be)
To keep an object on the fovea
Rate of about 50 degrees per second and are completely dependent on visual input
Jumps of eyes
To bring an object onto the fovea
Make 3/sec on average!
Optokinetic nystagmus (when looking at telephone poles passing by in the car)- you track it and then saccade to the next one
Vestibular ocular reflex (keeps your eyes on a target when moving your head)
A combination of pursuit and saccades.
Non-conjugate eye movements
Moving the fovea to an object closer (convergence) or farther away (divergence).
Conjugate vs vergence movements
Conjugate movements- both eyes moving in the same direction. These can be fast, as in saccades, or slow as in tracking movements. Conjugate eye movements are initiated by a variety of sensory inputs, the most important among them being visual and vestibular inputs.
Vergence movements- eyes moving in opposite directions
Example: near reflex (the combination of changes our eyes undergo when we attempt to focus on a near object).
Several things happen at once. Both medial recti contract, pulling the eyes nasally. The pupils constrict to increase the depth of field. The ciliary muscles contract, allowing the lens to become fatter and thus focus on a near object. All together, these actions are called the near or accommodation reflex. All the motor neurons and preganglionics are in the oculomotor nuclei. These are driven by visual input to association areas of the visual cortex.
Control of the VOR for a person sitting on a chair that is rotated to the right
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
As 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). Excitation on the right is transmitted to right vestibular nuclei. Cells in the vestibular nuclei project by way of the medial longitudinal fasciculus (MLF) to excite left lateral rectus motor neurons in the abducens nucleus.
Co-activation of abducens motor neurons and internuclear interneurons (about equal numbers of these in the abducens nucleus) whose axons cross over and ascend in the medial longitudinal fasciculus to excite right medial rectus motor neurons. Any stimuli that excite abducens motor neurons always also excite these internuclear neurons, thus ensuring co-activation of lateral rectus and contralateral medial rectus. The internuclear neurons serve as "pattern generators" for conjugate horizontal gaze. The axons from these interneurons cross the midline at the level of the abducens nucleus.
When VOR is damaged
MLF damage can disconnect the coordination of medial and lateral recti during horizontal gaze movements
How you can decide if the medial rectus motor neurons and/or nerve are intact with internuclear ophthalmoplegia?
Medial rectus may be unable to participate in horizontal gaze saccades or pursuit and yet the eyes work normally in vergence movements where both medial recti contract.
What is the most likely structure to be affected with internuclear ophthalmoplegia?
MLF because of its length
Describe nystagmus and give examples where it occurs
It is characterized by alternating smooth pursuit in one direction and saccadic movement in the other direction.
Clinical conditions where it can occur include:
Damage to the vestibular system or the cerebellum
Lesions in perietal lobe
Cranial nerve 8
Internuclear opthalmoplegia (one eye can abduct, the other eye can’t adduct. The lesion is on the side of the eye that can’t adduct.) The abducted eye will have nystagmus.