LAB 7 VISION EYEBALL MOVEMENT AND BALANCE SYSTEMS

LAB #7 VISION, EYEBALL MOVEMENT AND BALANCE SYSTEMS II

The vestibular system involves pathways from vestibular receptors in the inner ear that allow motor adjustments for balance of the eyes & body in 3 -D space. These pathways involve impulse transmission from vestibular receptors to vestibular fibers of the VIII cranial nerve (cell bodies in vestibular ganglia at the internal acoustic meatus). Central processes of these fibers travel mainly to the vestibular nuclei. From the vestibular nuclei, impulses travel primarily to: a) extraocular motor nuclei via the medial longitudinal fasciculus (MLF) for eyeball adjustment to changes in head position (vestibulo-ocular reflex); b) spinal lower motor neurons , via the medial & lateral vestibulospinal tracts for balance of body in space; c) the cerebellum. Afferent fibers of the vestibular nerve also travel to the reticular formation, and the cerebral cortex.

In addition to impulses from the vestibular nuclei, the extraocular motor nuclei also receive input from brainstem saccade centers: paramedian pontine recticular formation (PPRF, for horizontal gaze) and vertical gaze centers (in midbrain for vertical gaze). In general, the vestibular nuclei and saccade centers receive direct or indirect input from receptors (visual, vestibular) and/or the following areas of cerebral cortex: a) frontal eye field (motor cortex for voluntary horizontal movements of the eyeball), which is involved in a path through the PPRF and MLF to reach the extraocular nuclei; b) parietal eye fields (not shown ), which is involved in paths through the cerebellum and vestibular nuclei to the extraocular motor nuclei for tracking and unknown paths for convergence of eyeballs during near vision. The intraocular smooth muscles of the eyeball (dilator and constrictor pupillae muscles; ciliary muscle) adjust intraocular structures in response to changes in ambient light intensity, distance of a visual target, and emotions. Impulses for reflex responses to light (pupillary constriction) travel from the retina through the optic nerve and optic tract. They bypass the lateral geniculate nucleus via the brachium of the superior colliculus to reach the pretectal area. Impulses travel from the pretectal nuclei to the Edinger-Westphal nucleus of the oculomotor complex. This nucleus sends axons to the ciliary ganglion, which in turn sends fibers to the sphincter pupillae muscles. When viewing a near object, the pupil constricts and the lens bulges for close-object focus. The pathway for lens accommodation to near vision is similiar to that for pupillary constriction, except that it involves integration in the superior colliculus rather than the pretectal area and the target is the ciliary muscle. The near-vision response includes: pupillary constriction; ciliary muscle contraction, and convergence of eyeballs (contraction of two medial rectus muscles, in response to spatial information from parietal cortex).

Blood Supply to the Opics Central Artery Ophthalmic Artery Internal Carotid A. Anterior Choroidal Middle Cerebral Posterior Cerebral - Calcarine A.

OUTPUT FROM THE VESTIBULAR NUCLEI

Vestibular nerve fibers – contain central Haines 5 -13 processes of primary vestibular neurons Inferior Cerebellar Peduncle Sends fibers to the cerebellum Vestibular Nuclie Medial Longitudinal Fasciculus (MLF) sends 3 paths of vestibular nuclei 1. MLF extraoclar motor nuclei 2. Inferior Cerebellar Peduncle cerebellum 3. vestibulospinal tracts spinal motor neurons Pontomedullary Junction fibers to the extraocular motor nuclei

OCULOMOTOR PATHWAYS

Frontal Eye Field • in center of middle frontal gyrus • = cortex for horozontal voluntary eye movement Parietal Eye Field • convergence of eyeballs during near vision • tracking (pursuit) • first (slow) component of the opticokinetic reflex Haines 2 -11

Abducens Nucleus -Motor neurons to lateral rectus m. -Interneurons send fibers to MLF Haines 5 -17 MLF PPRF Abducens Nerve Fibers Contralateral FEF Rostral Medulla Level

MLF – en route to midbrain extraocular motor neuclei Rostral Pons Level Haines 5 -19

MLF – en route to midbrain extraocular motor neuclei Rostral Pons Level Haines 5 -20

Trochlear Nucleus MLF – en route to midbrain extraocular motor neuclei Caudal Midbrain Level Haines 5 -23

Occulomotor Nucleus Haines 5 -25 Superior Colliculi Commisure Occulomotor Nerve Fibers Rostral Midbrain Level MLF

Superior Colliculi - receives impulses from retina & visual cortex - sends axons to Edinger-Westphal nucleus for lens accommodation Haines 5 -26 MLF Rostral Midbrain Level

VISCERAL VISUOMOTOR (OCULAR) REFLEXES

pretectal area - a synaptic center for light reflexes - receives input from retina -sends axons to Edinger-Westphal nucleus for pupillary light response Haines 5 -26 posterior commissure - dorsal to cerebral aqueduct = decussation of fibers between pretectal nuclei -> bilateral (consensual) light response Rostral Midbrain Level brachium of superior colliculus -optic tract fibers that bypass lateral geniculate nucleus to reach pretectal area & superior colliculus

Haines 5 -25 Edinger-Westphal nucleus - in oculomotor complex (pale part) - a preganglionic parasympathetic nucleus - receives fibers from the pretectal area & superior colliculi - sends axons via III C. N. to ciliary ganglion for pupillary constriction and lens accommodation oculomotor nerve fibers - axons from Edinger-Westphal nucleus to ciliary ganglion Rostral Midbrain Level