Overview
This lecture covers the anatomy and function of the visual pathway, detailing each component from the eye to the visual cortex, their anatomical relations, and the organization of visual information.
Components of the Visual Pathway
- The visual pathway consists of the retina, optic nerve, optic chiasm, optic tract, lateral geniculate body, optic radiations, and visual cortex.
- Each eye sees both right and left visual fields, divided into nasal (near nose) and temporal (away from nose) halves.
- The retina transmits signals to the optic nerve, which travels to the optic chiasm where fibers cross, then proceeds via the optic tract to the lateral geniculate body and onwards to the cortex.
Representation of Visual Fields
- Each hemisphere of the visual pathway processes the contralateral (opposite) visual field from both eyes.
- The nasal retina receives input from the temporal field, and the temporal retina receives input from the nasal field.
- Only nasal fibers cross at the optic chiasm; temporal fibers remain uncrossed.
- Each optic tract contains uncrossed temporal fibers (from same side) and crossed nasal fibers (from opposite side).
Anatomy of the Optic Chiasm and Related Structures
- The optic chiasm sits above the sella turcica, which contains the pituitary gland.
- Variations in optic chiasm location: central (normal), prefixed (anterior), postfixed (posterior), affecting which nearby structures are impacted by pituitary tumors.
Lateral Geniculate Body and Reflex Pathways
- The lateral geniculate body (LGB) in the thalamus acts as a relay for visual information.
- Some optic tract fibers detour to the superior colliculus (midbrain) for pupillary reflex, where light causes pupil constriction.
Optic Radiations and Visual Cortex
- Optic radiations (geniculo-calcarine pathway) connect the LGB to the visual cortex via two main loops:
- Meyer's loop (inferior fibers, temporal lobe): carries superior field information.
- Baum's loop (superior fibers, parietal lobe): carries inferior field information.
- Primary visual cortex (Brodmann’s area 17) is on the medial occipital lobe, with secondary areas 18 and 19 (now V1-V5).
Visual Streams
- The ventral stream (temporal lobe) processes "what" (object identification and color).
- The dorsal stream (parietal lobe) processes "where" (location and spatial orientation).
Order of Neurons in the Visual Pathway
- Ophthalmology: rods/cones (1st), bipolar cells (2nd), ganglion cells (3rd), LGB (4th).
- Physiology: bipolar cells (1st), ganglion cells (2nd), LGB (3rd).
Key Terms & Definitions
- Retina — Light-sensitive layer of the eye where vision begins.
- Optic Chiasm — Cross-over point for nasal retinal fibers.
- Lateral Geniculate Body (LGB) — Thalamic relay center for visual information.
- Optic Radiations — Nerve fibers from LGB to the visual cortex.
- Meyer's Loop — Inferior optic radiation fibers through the temporal lobe.
- Baum’s Loop — Superior optic radiation fibers through the parietal lobe.
- Primary Visual Cortex (Area 17) — Main cortical area for processing visual signals.
- Ventral Stream — Pathway for object recognition ("what").
- Dorsal Stream — Pathway for spatial awareness ("where").
Action Items / Next Steps
- Review anatomy diagrams of the visual pathway, especially optic chiasm relations.
- Prepare for the next lecture on lesions of the visual pathway.