Lab 8: Vision, Eyeball Movement And Balance Systems II Flashcards
Impulses to the Extraocular Motor Nuclei
In addition to impulses from the vestibular system……
- from brainstem saccade centers (PPRF) in the caudal pons for horizontal gaze
- vertical gaze centers in the rostral midbrain
Frontal Eye Field (FEF)
The motor cortex for VOLUNTARY horizontal and vertical movements of the eyeball. The path for horizontal eye movements involves transmission through the PPRF and MLF to reach the extraocular motor nuclei
Posterior (parietal, temporal, occipital) Eye Fields
Paths through the cerebellum and vestibular nuclei to the Extraocular motor nuclei for visual tracking of an object in space and the slow component of the optokinetic reflex; unknown paths for convergence of eyeballs during near vision
Impulse for reflex responses to bright light (pupillary constriction)
Travel from the retina through the optic nerve and optic tract. Bypass LGN via the brachium of the superior colliculus to reach the pretectal area bilaterally. Impulses travel from the pretectal nuclei bilaterally to the Edinger-Westphal nuclei of the oculomotor complex. This nucleus sends axons to the ciliary ganglion, which sends fibers to the sphincter pupillae muscles
Accommodation (pupil constriction and lens bulge)
pathway is similar to that for pupillary Light Reflex, except that it involves integration in the superior colliculus rather than the pretectal area, and the target is the ciliary muscle, which when contracted, causes the lens to bulge
It’s a response to spatial info from parietal cortex and includes: pupillary constriction, ciliary muscle contraction and convergence of eyeballs (contraction of both medial rectus muscles)
Petrous Parts of the left and right Temporal bones
Contain inner ear and middle ear
Internal Acoustic Meatus
Location of passage of the vestibulocochlear (VIII) and facial (VII) nerves from the petrous part of the temporal bone into the posterior cranial fossa
-location of the vestibular (Scarpa’s) Ganglion
Semicircular Canals
Spaces in the petrous temporal bone
- observe horizontal, anterior and posterior semicircular canals
- relations of semicircular canals to the facial nerve, cochlea and mastoid air cells
- the orientation of the horizontal canal, which is parallel to the ground when the head is tilted 30 degrees downward, so that the eyes look at the ground in front of the feet
Vestibular Nerve Fibers
- enter the CNS at the lateral edge of the brainstem tegmentum
- contain central processes of 1st order vestibular neurons
Vestibular Nuclei
Termination of most vestibular nerve fibers
MLF (medial longitudinal fasciculus)
-contain axons from vestibular nuclei en route to extraocular motor nuclei
Inferior Cerebellar Peduncle
-contains fibers from the vestibular nuclei and vestibular nerve en route to the cerebellum; contain fibers from cerebellum to vestibular nuclei
Frontal Eye Field (location)
in the central part of the middle frontal gyrus, area 8 is the motor cortex for horizontal voluntary eyeball movement
Posterior Eye Fields (parietal, temporal, occipital) Location
Areas in parietal, Temporal and occipital association cortices involved in:
- convergence of eyeballs during near vision
- tracking (pursuit) of an object in space
- the slow component of the optokinetic reflex
Abducens Nuclei
- contains lower motor neurons to the ipsilateral lateral rectus muscle
- inteneurons that send axons to the contralateral MLF
Abducens Nerve Fibers
Intra-axial
Paramedian Pontine Reticular Formation or PPRF
General location: dorsomedial reticular formation, lateral to the MLF
This saccade center is involved in voluntary horizontal gaze. It sends axions to abducens neurons and abducens interneurons
MLF
Fibers from the vestibular nuclei and the PPRF (via the abducens interneurons) that transmit signals to extraocular motor nuclei
Vestibular Nuclei
Send fibers into the MLF for vestibulo-ocular reflexes
Trochlear Nucleus
Motor nucleus to the superior oblique muscle and target of the MLF
Oculomotor Nucleus
Motor nucleus to extraocular muscles and target of the MLF, vertical gaze center, and convergence paths
Superior Colliculus
Involved in the generation of saccadic eye movements and eye-head coordination
General Area of the Entrance of the Vestibular Nerve into the brainstem
Lateral edge of the tegmentum (caudal pons?)
Brachium of superior colliculus
- a continuation of optic tract fibers that bypass the LGN
- terminate in the nuclei of the pretectal area and the superior colliculus
Pretectal Area
Lateral to the posterior commissure in the rostral midbrain
- a synaptic center for the pupillary light reflex
- receives input from the retina
- sends axons bilaterally to the Edinger-Westphal nucleus for the pupillary light response
Posterior Commissure
Dorsal to the rostral cerebral aqueduct
-a decussation of fibers between the pretectal nuclei, contributing to the bilateral (consensual) light response
Superior colliculus
Reflex center for visual system
- receives impulses from the retina and the visual cortex
- sends axons to the Edinger-Westphal nucleus for lens accommodation
Edinger-Westphal Nucleus
In oculomotor complex
- a preganglionic parasympathetic (GVE) nucleus
- receives fibers from the pretectal area and superior colliculi
- sends axons via CN III to the ciliary ganglion for supply of the constrictor papillae and the ciliary muscle (lens accommodation)
Oculomotor Nucleus
The GSE group of nuclei that supply extraocular muscles
Oculomotor nerve fibers
Contain axons from:
- GVE Edinger-Westphal nucleus to terminate at the ciliary ganglion for supply of intraocular muscles
- oculomotor nucleus to supply the GSE extraocular muscles
Sympathetic Supply to the Dilator Pupillae
Review the path from the hypothalamus, through the brainstem reticular formation and thoracic spinal cord, sympathetic chain and ganglia, internal carotid plexus, to the dilator pupillae muscle
Pupillary Dilation Response
Involves descending fibers from the hypothalamus that reach the upper thoracic spinal cord and sympathetic neurons that reach the orbit by way of the sympathetic chain, superior cervical Ganglion, and the internal Carotid Plexus. This pathway can be affected by pathology in the brainstem, upper spinal cord, neck and internal Carotid Arterial system
Reticular Formation of the Brainstem
Contains descending fibers from the hypothalamus en route to the spinal cord
Cervical Spinal Cord
Also contains descending fibers en route to the intermediolateral cell column of the upper thoracic spinal cord
Internal Carotid Artery
Is covered by the internal carotid Plexus that sends fibers onto the ophthalmic artery to reach the dilator pupillae muscle
Near-Vision Visceral Reflex Test
Have patient look at near object; look for pupillary constriction of both eyes
Counter-Rolling Test for VOR
Principle
if you have an old-fashioned doll, hold it in front of you. If you turn the dolls head down, the eyes will turn up to remain looking at you in the eye. If you turn the dolls head up, the eyes will rotate down to remain looking at you in the eye. Counterweights behind the eyes cause this counter-rolling action, which essentially keeps the eyes on visual target when the head moves
Counter-Rolling Test
Technique
Hold the patient’s head so the eyes are in primary position. Then, briskly rotate it to the right and left and up and down, and stop it in each rotated position. For example, rotate the head to the right and stop in that position. During the initial rotation to the right, the eyes counter-roll opposite the direction of the head turning (to the left relative to the head), but actually they simply maintain their original position, the eyes quickly turn and align straight ahead of the new head position. Failure of the eyes to counter-roll and remain looking in the direction of the head rotation (think of dolls who have their pupils painted on) indicates failure of the VOR
Counter-Roll Test
Sum up
In an unconscious patient, “looking straight ahead” is common and is maintained during head rotation by counter-rolling if the vestibulo-ocular pathways are intact.
Called “doll’s eyes” response
Caloric Irrigation
Principle
Oculovestibular responses are reflex eye movement in response to irrigation of the external ear canals with cold or warm water
Caloric Irrigation
Example
Ice water: the head is raised to 30 degrees relative to the horizontal plane, and the external ear canals are inspected for cerulean or a perforated tympanic membrane. 50 to 100 ml of ice water is instilled into the canal and the resulting eye movements are noted. Ice water produces a downward convection current in the horizontal semicircular canal and decrease as the tonic firing rate of the vestibular end organ on that side. Consequently, the contralateral vestibular output drives the eyes toward the irrigated side. In an awake patient who is trying to maintain fixation, the slow phase is followed by a fast phase of nystagmus away from the stimulated ear
Caloric Irrigation
Warm water example
Induces the opposite response of the cold water
-COWS (cold opposite, warm same), which refers to the direction of nystagmus in an awake patient. In an unconscious patient, there’s a loss of the fast-phase Nystagmus, and only the tonic deviation of the eyes are seen
Caloric Irrigation Abnormalities
A lack of oculovestibular responses suggests pontine-midbrain dysfunction. Movement of the ipsilateral eye toward the irrigated ear, but no movement of the contralateral eye, suggests an abnormality of the contralateral MLF
