Control of Eye Movements Flashcards
What is a saccadic eye movement?
where the fovea shifts rapidly from one target to another
What is a smooth pursuit eye movement?
where the eyes follow a moving image to keep it centred on the fovea
What is a vergence eye movement?
moves eyes in opposite directions
What is a vestibular ocular eye movement?
hold image still on retina during brief head movements
What is an optokinetic eye movement?
holds image stationary during sustained head rotation or translation
What are the 6 extraocular muscles?
lateral rectus, medial rectus, superior rectus, inferior rectus, superior oblique, inferior oblique
What muscle is tested with abduction and elevation?
superior rectus
What muscle is tested with abduction and depression?
inferior rectus
What muscle is tested with adduction and elevation?
inferior oblique
What muscle is tested with adduction and depression?
superior oblique
What muscle is tested with abduction?
lateral rectus
What muscle is tested with adduction?
medial rectus
What neurons are involved in eye movement?
lower motor neurons (cranial nerves), reticular formation in the brainstem and frontal eye fields in the cortex
Which cranial nerve innervates contralateral muscles?
trochlear nerve (IV)
What is the medial longitudinal fasciculus?
a white matter tract in the brainstem connecting cranial nerve nuclei
What is the pontine paramedian reticular formation?
the horizontal gaze centre
What is the mesencephalic paramedic reticular formation?
the vertical gaze centre
What are the two types of neurons in the pontine paramedian reticular formation?
burst neurons which fire at high frequency just before an eye movement and omnipause neurons which fire continuously except when there is an eye movement
Describe the neural control of a saccade
Higher cortical centres signal to burst neurons in the PPRF. Excitatory burst neurons excite the ipsilateral abducens nucleus. Inhibitory burst neurons inhibit the ipsilateral abducens nucleus. The abducens nucleus signals via the MLF to the contralateral oculomotor nucleus. This results in simultaneous contraction of the lateral and medial rectus as well as inhibition of the opposing lateral and medial rectus muscles.
What is internuclear opthalmaplegia?
a defect in the MLF (usually in MS) which means that horizontal eye movements can’t be coordinated
Describe the vestibulo-ocular reflex
Movement of the head to the left causes the left semicircular canal to excite the left vestibular nucleus. The left vestibular nucleus will send an inhibitory signal to the left abducens nucleus and an excitatory signal to the right abducens nucleus. The abducens nucleus signals via the MLF to the contralateral oculomotor nucleus. This results in simultaneous contraction of the lateral and medial rectus as well as inhibition of the opposing lateral and medial rectus muscles.
