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neuroanatomy- visual pathway in brief
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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
THANK YOU