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

THANK YOU.