DR. YASHASREE POUDWAL

K.J.SOMAIYA HOSPITAL

VISUAL PATHWAY

INTRODUCTION

• THE VISUAL PATHWAY EXTENDS FROM THE 2 RETINAS TO THE VISUAL CORTEX

• THE FIRST ORDER NEURON OF THE VISUAL SYSTEM IS A BIPOLAR CELL ENTIRELY

WITHIN THE RETINA

OPTIC NERVETHE VISUAL NERVE SIGNALS LEAVE THROUGH THE OPTIC NERVES

THE OPTIC NERVES PASS POSTEROMEDIALLY INTO THE CRANIAL CAVITY AND MEET IN THE MIDLINE, FORMING THE OPTIC CHIASMA

THE OPTIC NERVE IS AN OUTGROWTH OF THE BRAIN

ITS FIBRES POSSESS NO NEROLEMMAL CELLS

UNLIKE OTHER PERIPERAL NERVES IT IS SURROUNDED BY MENINGES

Optic nerve

OPTIC CHIASMA• A FLAT MASS OF PARTIALLY DECUSSATING FIBRES WHICH LIES AT

THE JUNCTION OF THE ANTERIOR WALL AND FLOOR OF THE 3RD

VENTRICLE

• ONLY NASAL RETINAL FIBRES CROSS OVER .

• THEY JOIN THE FIBRES FROM THE OPPOSITE TEMPORAL RETINAS

• THESE FIBRES FORM THE OPTIC TRACTS.

1. OPTIC

NERVE

(STUMP)

2. 0PTIC

CHIASMA

3. OPTIC

TRACTS

OPTIC TRACT

• IT IS THAT EXTENT OF THE VISUAL SYSTEM PATHWAY FROM THE

OPTIC CHIASM TO THE LATERAL GENICULATE NUCLEUS OF THE

THALAMUS

• EACH TRACT CONTAINS AXONS FROM GANGLION CELLS FROM BOTH

EYES BUT INFORMSTION FROM ONLY ONE HALF (NASAL OR

TEMPORAL) OF EACH EYE’S VISUAL FIELD

• IN SPLIT –BRAIN PATIENTS WHO HAVE UNDERGONE A CORPUS

CALLOSOMTOMY (USUALLY TO TREAT SEVERE EPILEPSY) THE

INFORMATION FROM ONE OPTIC TRACT DOES NOT GET

TRANSMITTED TO BOTH HEMISPHERES

• SPLIT-BRAINS PATIENTS SHOWN AN IMAGE IN HIS/HER LEFT VISUAL

FIELD WILL BE UNABLE TO VOCALLY NAME WHAT HE/SHE HAS SEEN

OPTIC TRACT(CONTD.)

• BEHIND THE OPTIC CHIASMA, THE OPTIC TRACTS

DIVERGE DORSOLATERALLY, EACH PASSING BETWEEN

THE ANTERIOR PERFORATED SUBSTANCE AND TBER

CINEREUM. THE TRACT CURVES AROUND THE CEREBRAL

PEDUNCLE TO WHICH IT ADHERES.

• OPTIC TRACT FIBRES TERMINATE PRIMARILY IN THE

LATERAL GRNICUALTE NUCLEUS OF THE THALAMUS, BUT

ALSO IN THE SUPERIOR COLLICULUS (CONTROL RPID

DIRECTIONAL EYE MOVEMENT), PRETECTAL AREA (ELICIT

REFLRX MOVEMENTS OF TE YE TO FOCUS ON IMPORTANT

OBJECTS AND ACTIVATE PUPILARRY LIGHT RESPONSE),

SUPRACHIASMATIC NUCLEUS OF THE HYPOTHALAMUS

(CONTROL CIRCARADIAN RYHTHM).

LATERAL GENICULATE BODY• THE OPTIC NERVE FIBRES TERMINATE IN THE LATERAL

GENICULATE BODY, LOCATED AT THE DORSAL END OF HE THALAMUS.

• IT SERVES TWO PRINCIPAL FUNCTIONS.

• 1) IT RELAYS VISUAL INFORMATION FROM THE OPTIC TRACT TO THE VISUAL CORTEX VIA OPTIC RADIATIONS. THIS RELAY IS SO ACCURATE THAT THERE IS EXACT POINT-TO-POINT TRANSMISSION WITH A HIGH DEGREE OF SPATIAL FIDELITY.

• 2) IT ‘GATES’ THE TRANSMISSION OF SIGNALS TO THE VISUAL CORTEX. THE NUCLEUS RECIEVES GATING CONTROL SIGNALS FROM THE CORTICOFUGAL FIBRES AND THE RETICULAR AREA OF THE MESENCEPHALON. BOTH OF THESE ARE INHIBITORY AND CAN TURN OFF THE TRANSMISSION THROUGH SELECTED PORTIONS OF THE LATERAL GENICULATE NUCLEUS .

LATERAL GENICULATE BODY(CONTD)

• THE LATERAL GENICULATE NUCLEUS IS DIVIDED INTO 6 LAYERS.

• LAYERS I AND II ARE CALLED MAGNOCELLULAR LAYERS ABECAUSE THEY CONTAIN LARGE NEURONS. THEY RECEIVE INPUT ALMOST ENTIRELY FROM LARGE Y RETINAL GANGLION CELLS. THE MAGNOCELLULAR SYSTEM PROVIDES A RAPIDLY CONDUCTING PATHWAY TO THE VISUAL CORTEX.. THIS SYSTEM IS COLOR BLIND AND ITS POINT-TO-POINT TRANSMISSION IS POOR.

• LAYERS III TO VI ARE CALLED PARVOCELLULAR LAYERS BECAUSE THEY CONTAIN LARGE NUMBER OF SMALL TO MEDIUM SIZED NEURONS. THESE NEURONS RECEIVE THEIR INPUT FROM TYPE X RETINAL GSNGLION CELLS THAT TRANSMIT COLOR AND CONVEY ACCURATE POINT TO POINT INFORMATION BUT ONLY AT A MODERATE VELOCITY OF CONDUCTION.

OPTIC RADIATIONS

• AXONS FROM THIRD ORDER VISUAL NEURONES IN THE LATERAL

GENICUALTE NUCLEUS RUN IN THE RETROLENTICULAR PART OF THE

INTERNAL CAPSULE AND FORM THE OPTIC RADIATION, WHICH

CURVES DORSOMEDIALLY TO THE OCIPITAL CORTEX.

• FIBRES REPRESENTING THE LOWER HALF OF THE VISUAL FIELD

SWEEP SUPERIORLY TO REACH THE VISUAL CORTEX ABOVE THE

CALCARINE SULCUS. THOSE REPRESENTING THE UPPER HALF OF

THE VISUAL FIELD CURVE INFERIORLY INTO THE TEMPORAL LOBE

(MEYER’S LOOP) BEFORE REACHING THE VISUAL CORTEX BELOW

THE CALCARINE SULCUS

PRIMARY VISUAL CORTEX• FROM THE LATERAL GENICULATE NUCLEUS, THE FIBRES PASS BY

THE OPTIC RADIATIONS TO THE PRIMARY VISUAL CORTEX.

• IT IS LOCATED IN THE CALCARINE FISSURE AREA OF THE MEDIAL

OCCIPITAL LOBE. (BRODMANN’S AREA 17 OR V1)

PHYSIOLOGY OF VISION

• AFTER LIGHT PASSES THROUGH THE LENS SYSTEM OF THE EYE AND THEN THROUGH THE VITREOUS HUMOR, IT ENTERS THE RETINA FROM THE INSIDE I.E. IT PASSES FIRST THROUGH THE GANGLION CELLS AND THEN THROUGH THE PLEXIFORM AND NUCLEAR LAYERS BEFORE IT REACHES THE LAYER OF RODS AND CONES LOCATED ALL THE WAY ON THE OUTER EDGE OF THE RETINA.

PHYSIOLOGY OF VISION• BOTH RODS AND CONES CONTAIN CHEMICALS THAT DECOMPOSE ON EXPOSURE

TO LIGHT AND, IN THE PROCESS, EXCITE THE NERVE FIBERS LEADING FROM THE

EYE. THE LIGHT-SENSITIVE CHEMICAL IN THE RODS IS CALLED RHODOPSIN; AND

IN THE CONES, CALLED CONE PIGMENTS OR COLOR PIGMENTS, HAVE

COMPOSITION ONLY SLIGHTLY DIFFERENT FROM THAT OF RHODOPSIN

LESIONS OF THE

VISUAL PATHWAY

• PLOTTING VISUAL FIELD LOSS USUALLY POINTS TO THE APPROXIMATE LOCATION OF THE CAUSATIVE LESION IN THE VISUAL PATHWAY.

• LESIONS CENTRAL TO THE RETINA ARE USUALLY ATTRIBUTABLE TO THE OPTIC NERVE LESIONS

• LESIONS OF THE OPTIC CHIASMA, INVOLVING CROSSING NERVE FIBRES, PRODUCE BILATERAL FIELD DEFECTS EG. IN CASE OF A PITUTARY ADENOMA THERE IS BITEMPORAL HEMIANOPIA

• LESIONS OF THE OPTIC TRACT ARE RARE BUT DISTINCTIVE. THE TRACT CONTAINS CONTRALATERAL NASAL AND IPSILATERAL TEMPORAL RETINAL FIBERS AND DAMAGE WILL CAUSE HOMONYMOUS HEMIPANOPIA WITH SUBSTANTIAL INCONGRUITY. (DISSIMILAR DEFECTS IN BOTH FIELDS)

• LESIONS OF THE LATERAL GENICULATE BODY CAUSES QUADRANTIC VISUAL FIELD DEFECTS (PIE IN THE SKY DEFECT)

• LESIONS OF THE OCCIPIATAL LOBE ALSO SHOW QUADRANTIC FIEL DEFECTS DUE TO THE ANATOMIC ARRANGEMENT HOWEVER IN MOST CASES MACUALR SPARING.

• BILATERAL DESTRUCTIONOF THE OCCIPITAL LOBE CAUSES SUBJECTIVE BLINDNESS

LESIONS OF THE VISUAL PATHWAY

VISUAL DEFECTS

AND PSYCHIATRY

ANTON-BABINSKI SYNDROME• ALSO CALLED CORTICAL BLINDNESS

• MOST STRIKING FORM OF ANOSOGNOSIA.

• PATIENTS WITH THIS SYNDROME BEHAVE AS IF THEY CAN SEE DESPITE THEIR LACK OF SIGHT..

• CORICAL BLINDNESS IS DUE TO BILATERAL DAMAGE OF THE OCCIPITAL LOBES DUE TO HYPOXIA, VASOSPASM OR CARDIAC EMBOLISM.

• OFTEN ASSOCIATED WITH PARIETAL LOBE DYSFUNCTON AS WELL. LEADS TO DISCONNECTION SYNDROME- THE AREA OF THE PARIETAL LOBE THAT INTEGRATES VISUAL WITH OTHER SENSORY INFORMATON IS SEPARATED.

• THERE ARE MANY THEORIES WHY PATIENTS WITH ANTON SYNDROME DENY THEIR BLINDNESS. ONE HYPOTHESIS IS THAT DAMAGE TO THE VISUAL CORTEX RESULTS IN INABILITY TO COMMUNICATE WITH SPEECH-LANGUAGE AREAS OF THE BRAIN. VISUAL IMAGERY S RECEIVED BUT CAN NOT BE INTERPRETED. THE SPEECH CENTERS OF THE BRAIN CONFABULATE A RESPONSE

Computed

tomography

of the brain

revealed

infarction of

both occipital

lobes

GABRIEL

ANTON-

WROTE ON

SELF

AWARENESS

OF FOCAL

BRAIN

DISEASES

FUNCTIONAL VISUAL LOSS• HYSTERIC AND MALINGERING TYPES OF VISUAL LOSS ARE SIMILAR; ONLY THE

UNDERLYING PSYCHOLOCIAL REASONS FOR THE LOSS VARY.

• IN EVERY PATIENT IN WHOM FUNCTIONAL VISUAL LOSS IS SUSPECTED, ORGANIC

DISEASE MUST BE RULED OUT AND THE PATIENT RE-EXAMINED AT REGULAR

INTERVALS

• LOSS OF CENTRAL VISION- IS A COMMON FUNCTIONAL COMPLAINT. IF THE LOSS

IS RELATED TO EMOTIONAL GAIN RATHER THAN FINANCIAL, IT IS USUALLY

BILATERAL. BILATERAL LOSS OF VISION ALLOWS A PERSON TO COMPLETELY

RETREAT FROM WHATEVER IT IS THEY FEEL THEY CAN NO LONGER COPE WITH,\.

MONOCULAR FUNCTIONAL LOSS DOES OCCOUR BUT USUALLY FINANCIAL GAIN IS

INVOLVED (INSURANCE LITIGATION)

• PERIPHERAL VISION LOSS- THE TRADIONAL FIELD DEFECT IN HYSTERICS IS THE

SPIRAL FIELD DEFECT.

THE END

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