Visual Pathway Notes

Visual Pathway

The visual pathway consists of a series of cells and synapses that carry visual information from the environment to the brain for processing.
Components:
- Retina
- Optic nerve
- Optic chiasma
- Optic tract
- Lateral geniculate body
- Geniculostriate tract (Optic radiation)
- Visual sensory areas (occipital lobe areas 17, 18, and 19)

The optic nerve is composed of axons from ganglion cells of the retina.
Fibers from the four quadrants of the retina maintain their relative positions within the optic nerve.
Fibers from the nasal half of each retina cross in the optic chiasm and enter the optic tract of the opposite side.
Fibers from the temporal half enter the optic tract of the same side.
The optic tract carries these fibers to the lateral geniculate body of the corresponding side.
Finally, fibers are relayed to areas 17, 18, and 19 of the occipital cortex.

The lateral geniculate body is part of the metathalamus.
It consists of gray matter arranged in six layers.
Fibers from the same side (ipsilateral) end in laminae 2, 3, and 5.
Fibers from the opposite side (contralateral) end in laminae 1, 4, and 6.
Macular fibers terminate in the central and posterior part of the body, which is a relatively large area.

Fibers arising from the lateral geniculate body form the geniculocalcarine tract, also known as optic radiation.
These fibers pass through the retrolentiform part of the internal capsule.
The radiation ends in the visual areas of the cerebral cortex (areas 17, 18, and 19).

The occipital cortex (areas 17, 18, and 19) receives impulses from the retinal halves of the same side, corresponding to opposite halves of the field of vision.
The cortical area representing the macula is much larger than that for the peripheral areas.

Primary Visual Area (Area 17): Located at the occipital pole; responsible for visual perception.
Visual Association Areas (Areas 18 & 19): Parastriate cortex
- Area 18: Processes linear stimuli.
- Area 19: Processes angular stimuli.
Higher Visual Association Area (Area 39): Angular gyrus of the parietal lobe; involved in the comprehension of signs and symbols of language through vision.

These areas correlate past and present visual experiences and assess distance, speed, and orientation in 3D space.
Lesion: Visual agnosia—the person is unable to identify an object or a person seen in the past.

The diagram illustrates visual field defects resulting from lesions at different points in the visual pathway:

Optic Nerve (1): Total loss of vision in the right eye.
Optic Chiasm (2): Non-homonymous bitemporal hemianopia.
Optic Tract (3): Contralateral (left) homonymous hemianopia.
Temporal Lobe - Meyer's Loop (4): Superior left homonymous quadrantanopia (pie in the sky disorder).
Parietal Lobe (5): Inferior left homonymous quadrantanopia (pie in the floor disorder).
Geniculocalcarine Tract (6): Contralateral (left) homonymous hemianopia.
Inferior Bank of Calcarine Fissure (7): Superior left homonymous quadrantanopia (with macular sparing).
Superior Bank of Calcarine Fissure (8): Inferior left homonymous quadrantanopia (with macular sparing).
Both Banks of Calcarine Fissure (9): Contralateral (left) homonymous hemianopia (with macular sparing).

Fovea: The central fixation point of each eye; the region of the retina with the highest visual acuity.
Macula: An oval region approximately 3-5 mm in diameter that surrounds the fovea; also has high visual acuity.
Optic Disc: The region where axons leaving the retina gather to form the optic nerve.
- Photoreceptors are absent over the optic disc, creating a small blind spot located 15 degrees lateral and inferior to the central fixation point of each eye.

Rods: More numerous than cones (20:1).
- Have poor spatial and temporal resolution of visual stimuli.
- Do not detect colors.
- Responsible for vision in low-level lighting conditions.
Cones: Less numerous, more highly represented in the fovea.
- Have high spatial and temporal resolution.
- Detect colors.

Lesions in different regions of the primary visual cortex (upper and lower banks of the calcarine fissure) result in specific visual field defects.

Light flashes: Often indicative of retinal detachment.
Rainbow-colored halos around objects: Suggestive of acute glaucoma.
Migraine: May present with visual blurring, scotoma with scintillating appearance, or jagged alternating light and dark zigzag lines (fortification scotoma).
Pulsating colored lights/moving geometric shapes: May indicate occipital seizures.

Junctional Scotoma: Lesion at the junction of the optic nerve and chiasm.
Bitemporal Homonymous Hemianopia
Left Sector Sparing Homonymous Hemianopia: Lesion at the Lateral Geniculate Nucleus (LGN).
Right Superior Quadrantanopia: Temporal lobe lesion.
Left Inferior Quadrantanopia: Parietal lobe lesion.
Left Homonymous Hemianopia with Macular Sparing
Left Incongruous Homonymous Hemianopia
Right Congruous Homonymous Hemianopia
Enlarged Blind Spot

Macular sparing is related to the watershed area concerning blood supply.
The 'macular' visual cortex is supplied by terminal branches of the posterior and middle cerebral arteries.
The visual cortex subserving the midperipheral and peripheral field is supplied only by the PCA.
The macular area is supplied by a more proximal, 'not terminal' vessel.

Globules of mucoproteins and mucopolysaccharides that progressively calcify in the optic disc.
Retinitis Pigmentosa is also mentioned in the context of optic disc drusen.