THE VISUAL PATHWAY

Overview

FLOWCHART: The Visual Pathway

LIGHT → RETINA (rods & cones)
           ↓
    BIPOLAR CELLS (1st neuron synapse)
           ↓
    GANGLION CELLS (axons form optic nerve)
           ↓
    OPTIC NERVE (CN II)
           ↓
    OPTIC CHIASMA
    ┌──────────────────┐
    │ Nasal fibers     │ → DECUSSATE to opposite side
    │ Temporal fibers  │ → Stay IPSILATERAL
    └──────────────────┘
           ↓
    OPTIC TRACT (each carries:
    - Temporal fibers from ipsilateral eye
    - Nasal fibers from contralateral eye)
           ↓
    LATERAL GENICULATE NUCLEUS (LGN) of thalamus
    (2nd order neuron synapse)
           ↓
    OPTIC RADIATIONS (Geniculocalcarine tract)
    ┌─────────────────────────────────────────────┐
    │ Upper fibers   → parietal lobe → upper cortex │
    │ Lower fibers   → Meyer's loop (temporal lobe) │
    │                → lower cortex                 │
    └─────────────────────────────────────────────┘
           ↓
    PRIMARY VISUAL CORTEX
    (Area 17 / Striate cortex, calcarine sulcus,
    Brodmann's Area 17, occipital lobe)

ANATOMY IN DETAIL

1. Retina

• Photoreceptors (rods for dim light, cones for color/acuity) synapse on bipolar cells, which synapse on retinal ganglion cells
• Ganglion cell axons coalesce at the optic disc (blind spot - no photoreceptors)

2. Optic Nerve (CN II)

• Axons acquire myelin from oligodendrocytes (CNS-type)
• Covered by cranial meninges
• Passes through the optic canal in the sphenoid bone

3. Optic Chiasma

• Situated anterior to the infundibular stalk, ~10 mm above the pituitary gland
• In shape, resembles the letter H (not the Greek letter chi)
• 50% of fibers decussate: nasal retinal fibers cross to the opposite optic tract
• Temporal retinal fibers remain ipsilateral
• Note: Nasal retina receives light from the temporal visual field; temporal retina receives from the nasal visual field

4. Optic Tract

• Each optic tract carries:
  • Temporal fibers from the ipsilateral eye
  • Nasal fibers from the contralateral eye
• Together they represent the contralateral visual hemifield
• Courses around the midbrain to reach the LGN

5. Lateral Geniculate Nucleus (LGN)

• Located in the posterior thalamus
• Has 6 laminae (layers 1,4,6 from contralateral eye; layers 2,3,5 from ipsilateral eye)
• 2nd order neuron synapse occurs here

6. Optic Radiations (Geniculocalcarine Tract)

• Upper fibers: pass through parietal lobe → terminate on upper bank of calcarine fissure (represents lower visual field)
• Lower fibers: loop anteriorly through the temporal lobe as Meyer's loop → terminate on lower bank of calcarine fissure (represents upper visual field)

7. Primary Visual Cortex (Area 17)

• Located in the calcarine sulcus, occipital lobe
• Macula (central vision) is represented at the posterior pole (occipital tip)
• Peripheral vision is represented more anteriorly
• Upper bank of calcarine → receives lower visual field signals
• Lower bank of calcarine → receives upper visual field signals

DIAGRAM: Visual Pathway (from textbook)

DIAGRAM: Optic Radiations and Meyer's Loop

EFFECTS OF LESIONS AT VARIOUS LEVELS

Master Diagram: Lesions and Visual Field Defects

TABLE: Effects of Lesions at Various Levels in the Visual Pathway

KEY CONCEPTS TABLE

SPECIAL NOTES FOR EXAM

• Nasal retinal fibers (carrying temporal field information) from both eyes cross at the chiasma
• A central chiasmal lesion (e.g., pituitary tumor pressing from below) destroys these crossing fibers
• Result: both temporal visual fields are lost

• The lower fibers of the optic radiation loop anteriorly into the temporal lobe
• Temporal lobectomy (e.g., for epilepsy) can cause "pie in the sky" superior quadrantanopia

• The macular area (central 5-10° of vision) has large cortical representation at the occipital pole
• The occipital pole may receive dual blood supply (MCA + PCA)
• A PCA infarct causing hemianopia typically spares the macula

• Fibers leave the optic tract BEFORE reaching LGN to go to the pretectal nucleus
• Therefore: cortical blindness (bilateral occipital lesion) has normal pupils (light reflex intact)
• Lesions at or before the optic tract affect the light reflex

• More POSTERIOR the lesion → more CONGRUOUS the defect
• Optic tract = most incongruous homonymous defect
• Visual cortex = perfectly congruous (identical) defect

SUMMARY FLOWCHART: Lesion Localization

Monocular vision loss
    → Ipsilateral eye, optic nerve, or retina

Bitemporal hemianopia
    → Optic chiasma (central crossing fibers)

Binasal hemianopia (rare)
    → Lateral compression of chiasma (bilateral)

Contralateral homonymous hemianopia (incongruous)
    → Optic tract or LGN

Contralateral homonymous superior quadrantanopia ("pie in sky")
    → Meyer's loop / temporal lobe

Contralateral homonymous inferior quadrantanopia ("pie on floor")
    → Parietal lobe / upper optic radiation

Contralateral homonymous hemianopia (congruous) WITH macular sparing
    → Primary visual cortex (PCA territory)

Bilateral visual loss, normal pupils
    → Cortical blindness (bilateral occipital cortex)

THE VISUAL PATHWAY

(As per Guyton and Hall Textbook of Medical Physiology)

DIAGRAM: Principal Visual Pathways (Guyton Fig. 52.1)

FLOWCHART: The Visual Pathway (Guyton)

LIGHT
  ↓
RETINA
  ├── Rods (dim light / peripheral vision)
  └── Cones (colour / central acuity)
         ↓ (synapse)
    BIPOLAR CELLS
         ↓ (synapse)
    RETINAL GANGLION CELLS
         ↓
    OPTIC NERVE (CN II)
    [Axons of ganglion cells, myelinated, CNS structure]
         ↓
    OPTIC CHIASMA
    ┌─────────────────────────────────────────────────────┐
    │  Nasal retinal fibers (temporal visual field)       │
    │       → CROSS to opposite optic tract               │
    │  Temporal retinal fibers (nasal visual field)       │
    │       → REMAIN IPSILATERAL in optic tract           │
    └─────────────────────────────────────────────────────┘
         ↓
    OPTIC TRACT
    [Each tract = temporal fibers from same eye
                + nasal fibers from opposite eye
     = represents contralateral visual hemifield]
         ↓
    DORSAL LATERAL GENICULATE NUCLEUS (LGN)
    of the thalamus
    [6 layers; "gates" transmission to cortex]
         ↓
    OPTIC RADIATION
    (Geniculocalcarine tract)
    ┌───────────────────────────────────────────────────┐
    │ Upper fibers → Parietal lobe → Upper calcarine    │
    │ Lower fibers → Meyer's loop (temporal lobe)       │
    │              → Lower calcarine                    │
    └───────────────────────────────────────────────────┘
         ↓
    PRIMARY VISUAL CORTEX (Area 17)
    [Calcarine fissure, medial occipital lobe]
         ↓
    SECONDARY VISUAL AREAS (Areas 18, 19)
    [Higher-order processing: form, color, movement]

Collateral Projections from the Optic Tract (Guyton)

THE VISUAL PATHWAY - STEP BY STEP

1. Retina

• Contains rods (120 million; for dim light and peripheral vision) and cones (6 million; for color and fine acuity)
• Signal pathway: Photoreceptors → Bipolar cells → Retinal Ganglion cells
• Ganglion cell axons converge at the optic disc (blind spot - no photoreceptors here, ~15° lateral to central point of vision)

2. Optic Nerve (CN II)

• Formed by axons of retinal ganglion cells
• Covered by cranial meninges; myelin provided by oligodendrocytes
• Physiologically a CNS tract, not a peripheral nerve

3. Optic Chiasma

• Fibers from the nasal half of each retina (receiving light from the temporal visual field) decussate to the contralateral optic tract
• Fibers from the temporal half of each retina remain ipsilateral
• Result: Each optic tract carries information from the contralateral visual hemifield

4. Dorsal Lateral Geniculate Nucleus (LGN)

• Corticofugal fibers from primary visual cortex (backward)
• Reticular areas of the mesencephalon Both are inhibitory and help highlight important visual information.

Signals from the two eyes are kept separate in LGN and remain so all the way to the visual cortex.

5. Optic Radiation (Geniculocalcarine Tract)

• Upper fibers: Pass through parietal lobe → terminate on upper bank of calcarine fissure
• Lower fibers: Loop anteriorly through temporal lobe (Meyer's loop) → terminate on lower bank of calcarine fissure

6. Primary Visual Cortex (Area 17 / Striate Cortex)

• Located in the calcarine fissure, medial occipital lobe
• Macula (central 5-10° of vision) has disproportionately large representation at the posterior occipital pole
• Peripheral fields project anteriorly along the calcarine fissure
• Has 6 layers with columns detecting orientation, movement, and color

PERIMETRY AND THE VISUAL FIELD

• Perimetry maps the visual field by moving a small light or object through all areas while the subject fixes on a central point
• The blind spot is ~15° lateral to the central point of vision in each eye

EFFECTS OF LESIONS AT VARIOUS LEVELS IN THE VISUAL PATHWAY

Master Table (Guyton-based)

VISUAL FIELD DEFECT DIAGRAM

VISUAL FIELDS          LESION SITE
L Eye    R Eye
[●   ]  [   ●]   A. Optic nerve right → Right monocular blindness
[■   ]  [   ■]   B. Optic chiasm (central) → Bitemporal hemianopia
[   ■]  [   ■]   C. Right optic tract → Left homonymous hemianopia
[  ▲ ]  [  ▲ ]   D. Right Meyer's loop → Left superior quadrantanopia
[  ▼ ]  [  ▼ ]   E. Right parietal optic rad → Left inferior quadrantanopia
[ ■○ ]  [ ■○ ]   F. Right visual cortex → Left homonymous hemianopia + macular sparing (○)

(■ = field lost; ○ = macula spared; ▲ = upper quadrant lost; ▼ = lower quadrant lost)

KEY CONCEPTS TABLE

SUMMARY FLOWCHART: Lesion Localization

MONOCULAR visual loss (one eye only)
    → Retina / Optic nerve (prechiasmal)

BITEMPORAL hemianopia (both temporal fields lost)
    → Optic chiasma (central - crossing nasal fibers)
    → Classic cause: Pituitary adenoma

BINASAL hemianopia (both nasal fields lost)
    → Lateral pressure on both sides of chiasm (rare)

CONTRALATERAL HOMONYMOUS HEMIANOPIA
    → Optic tract / LGN / Optic radiation / Visual cortex

    + INCONGRUOUS → Optic tract or LGN (anterior)
    + "PIE IN SKY" (superior quadrant) → Meyer's loop (temporal lobe)
    + "PIE ON FLOOR" (inferior quadrant) → Parietal lobe (upper radiation)
    + MACULAR SPARING → Primary visual cortex (PCA territory)
    + CONGRUOUS, no macular sparing → Optic radiation (entire)

BILATERAL LOSS + NORMAL PUPILS
    → Cortical blindness (bilateral occipital cortex)

IMPORTANT PHYSIOLOGICAL POINTS (Guyton)

1. The "New" vs. "Old" visual system: Guyton distinguishes a new system (LGN → visual cortex, for conscious perception of form/color) and an older system (to superior colliculus, pretectal nuclei, hypothalamus) for reflex responses and circadian rhythms.
2. LGN gating function: The LGN does not merely relay - it gates signals via corticofugal feedback and mesencephalic reticular inputs. This "highlights" important visual information.
3. Magno vs. Parvo channels: Magnocellular (M, layers I-II) = fast, color-blind, poor spatial resolution. Parvocellular (P, layers III-VI) = slow, color, fine spatial detail. Both remain segregated through LGN to cortex.
4. Removing primary visual cortex → Loss of conscious vision but NOT loss of pupillary reflex (pupillary light reflex is mediated via pretectal nuclei, which receive fibers from the optic tract before the LGN).
5. Macular representation: The macula (central 5-10°) has a disproportionately large representation at the posterior occipital pole and is supplied by both MCA and PCA - hence macular sparing in PCA territory infarcts.