Download - Lecture - Visual Pathway
VISION
Objective
• To discuss the visual pathway, its parts & connections and relationship with the optic reflexes
• Develops from the optic cup, an outgrowth of the diencephalon
• Retina : rods and cones
• Fovea centralis : specialized region in macula for high visual acuity; highest cone density
• Rods- rhodopsin Cones -iodopsin– isomerization leading to hyperpolarization
RETINA
• Contains 5 cell types
1. amacrine cell *
2. bipolar cell
3. receptor cell (rods and cones)
4. ganglion cell
5. horizontal cell *
*inhibitory, uses GABA
Optic nerve
GANGLION CELLS OF THE RETINA
• Axons converge to form the optic disc• Becomes myelinated as the optic nerve• Optic nerve optic chiasm
LGB,
superior optic tracts
colliculus,
pretectal area
LATERAL GENICULATE BODY
• Inputs are arranged in an ORDERLY TOPOGRAPHIC PATTERN
• Receives the contralateral visual field• central visual field represented more
extensively• each layer in the LGB receives inputs
from one eye only (3 layers for ipsilateral, 3 layers for contralateral)
SUPERIOR COLLICULUS
• Receives direct visual input from optic tracts from the visual cortex
• Projects to the pons (tectopontine) and to the spinal cord (tectospinal)
Tectopontine cerebellum
Tectospinal reflex control of head & neck
• Participates in eye movement control by connections with RF
PRE-TECTAL AREA
• Site for mediation of pupillary reflexes
• Receives input from optic tract
• Fibers project to the Edinger-Westphal nucleus
PRIMARY VISUAL CORTEX (Brodmann’s area 17)
• Primary visual receptive area
• Also called “striate area” (contains Gennari’s line)
• Surrounds the calcarine fissure/sulcus
• Cuneus - above the fissure
• Lingual gyrus - below the fissure
TOPOGRAPHIC ARRANGEMENT IN THE CORTEX
• SUPERIOR visual field projects to INFERIOR part of cortex
• LEFT projects to the RIGHT
• CENTRAL projects to the POSTERIOR
• PERIPHERAL projects to the ANTERIOR
BRODMANN AREA 18 & 19
• also called VISUAL ASSOCIATION AREAS
• regions for visual perception or visual sensory processing
• also play a role in visually guided saccades, ocular pursuit movements, accomodation and convergence
OPTIC REFLEXES AND EYE MOVEMENTS
LIGHT REFLEX
• DIRECT LIGHT REFLEX
Pupil constricts promptly when light is flashed into the eye and dilates when removed.
• Follows the usual visual pathway, BUT, instead to the LGB, it goes to the superior colliculus and end in the PRETECTAL AREA
DIRECT LIGHT REFLEX
• From the pretectal area, connects to the E-W nucleus
• E-W nucleus connects with the CILIARY GANGLION ------> CONSTRICTS Iris
muscles
CONSENSUAL LIGHT REFLEX
• Constriction of the contralateral eye
• Accomplished by crossing connections in the light reflex pathway at level of pretectum
CROSSING OVER
INTACT DIRECT & CONSENSUAL
ABSENT DIRECT & CONSENSUAL REFLEX
NO DIRECT/CONSENSUAL REFLEX ON AFFECTED EYE
REFLEXES OF NEAR-POINT REACTION
• When eyes are directed to an object close to the face, 3 reflexes occur :
1. CONVERGENCE
2. ACCOMODATION
3. PUPILLARY CONSTRICTION
CONVERGENCE
• Medial rectus muscles contract to move both eyes to the midline so the image remains focused on the fovea
• IF non-functioning, DIPLOPIA results
ACCOMODATION
• Lenses are thickened by contraction of its ciliary muscles; also maintains a focused image on the fovea
• Ciliary muscles are innervated by parasympathetic neurons in the ciliary ganglion
PUPILLARY CONSTRICTION
• Pupils are narrowed as an AID to regulate the DEPTH of focus
• Separate from the light reflex
CLINICAL CONDITIONS
• ADIE’S TONIC PUPIL
Unilateral dilated pupil (assoc. with absent deep tendon reflexes)
Minimal constriction to light, and pathologically slow re-dilatation
CLINICAL CONDITIONS
• ARGYLL ROBERTSON PUPIL
Prostitute eye (accommodates but does not react)
Damage to the ciliary ganglion or iris is implicated (probably syphilis)
VISUAL FIXATION
• Four visual subsystems act to VISUALIZE AN OBJECT IN THE FOVEA AND KEEP IT THERE AS THE OBJECT OR THE VIEWER MOVES
1. Saccadic
2. Pursuit
3. Vergence
4. Vestibulo-ocular
SACCADIC EYE MOVEMENTS
• Saccades = fast conjugate eye movements to track a MOVING OBJECT
• Voluntary saccades initiated by the parieto-occipital cortex and visual cortex, and by Brodmann’s area 8
• Involuntary saccades initiated by brainstem (e.g. REM, fast phase of nystagmus)
SACCADES
• HORIZONTAL GAZE
The contralateral pontine paramedian reticular formation (PPRF) is an integral part.
Visual inputs (contra)
Cortices----->sup. colliculus--->PPRF---
CN III <------MLF <---------CN VI <-----
(ipsi MR) (contra LR)
SACCADES
• UPWARD GAZE
contra PPRF-----> riMLF ----->interstitial nucleus
CNIII<------post. commisure<----of Cajal
(ipsi)
SR(contra) and IO (ipsi)
SACCADES
• DOWNWARD GAZE
Same with upward gaze, except the IR (CN III) and SO (CN IV) are innervated
PURSUIT EYE MOVEMENTS
• Maintains fixation on slowly MOVING OBJECTS
HORIZONTAL PURSUIT
Visual inputs----->parieto-occipital--
CN VI<------cerebellar vermis<----pons<
& III (ipsi)
VERTICAL PURSUIT - (unknown)
• If pursuit fails, saccades will substitute
VESTIBULO-OCULAR MOVEMENTS
• Prevents image from moving away from the fovea during HEAD MOVEMENTS
• Pathway from medulla to midbrain
• Receives inputs from the semicircular canals to the vestibular nuclei
• Vestibular nuclei reaches CN VI & III (horizontal) and CN IV & III (vertical)
VERGENCE EYE MOVEMENTS
• Allows continual presence of the image of interest on the fovea as the object moves closer or farther away
• Signals come from bilateral parieto-occipital cortex to the midbrain, then to CN III and VI
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