visual system

The Visual System: Anatomy and Basic Science

Objectives

  • Anatomy of the Visual Pathway: Describe the anatomy of the pathway from the retina to the occipital cortex.

  • Cell Types in Visual Pathway: Define the types of cells involved in the visual pathway.

  • Pupillary Control Pathways: Review the pathways controlling pupillary reactions.

  • Visual Pathway: From the retina to the occipital cortex:

    • Retina → Optic Nerve → Optic Chiasm → Optic Tract → Occipital Cortex.

Visual Pathway Overview

  • Anatomy of the visual pathways as seen from above:

    • Left: Retina → Optic nerve → Optic chiasm → Optic tract → Meyer’s loop → Lateral geniculate nucleus → Optic radiation → Upper bank of the calcarine fissure → Lower bank of the calcarine fissure.

    • Right: Retinal structures (A - H)

Retina

  • Definition: The retina is a thin, semitransparent, multilayered sheet of neural tissue that lines the inner aspect of the eye.

  • Origin: It derives from the optic vesicle, which emerges from the neural fold/tube at day 22 of embryonic development.

Retina Basic Function

  • Light Processing: Light passes through various layers (inverted order) to reach photoreceptors.

  • Phototransduction: Photoreceptors convert light into an electrical signal.

  • Signal Conveyance: The electrical signal is conveyed to the retinal ganglion cells, which project via the optic nerve.

  • Optical Clarity: The retina must be transparent for light to pass through.

  • Types of Connectivity: Bipolar cells connect photoreceptors to ganglion cells, forming the optic nerve.

Retinal Cells and Layers

  • Photoreceptors:

    • Rods:

    • Function: Scotopic (night vision).

    • Characteristics: Do not detect color, adapt to low-level lighting, poor spatial resolution.

    • Cones:

    • Function: Photopic (day vision).

    • Characteristics: Highly represented in the fovea, detect color, high spatial resolution (20 rods for each cone).

  • Bipolar Cells: Connect photoreceptors to ganglion cells.

  • Ganglion Cells (Retinal Ganglion Cells):

    • Each ganglion cell has a receptive field, which is the part of the visual field where light excites or inhibits the cell.

    • Receptive fields are smaller near the fovea, allowing finer visual discrimination.

Other Retinal Cells

  • Interneurons:

    • Include horizontal cells and amacrine cells.

  • Muller Cells: Serve a supportive function analogous to astrocytes in the brain, connecting different photoreceptors and ganglion cells.

Retinal Optical Coherence Tomography

  • Function: Responsible for examining the retina's architecture.

  • Retinal Pigment Epithelium (RPE): Acts as a barrier between the retina and choroid; damage can lead to layer thinning.

Retinal Structures

  • Fovea:

    • Central part of the macula.

    • Contains only cones and is the site of highest visual acuity.

    • Avascular; receives nutrients by diffusion from the choroid.

  • Optic Disc:

    • Size: Approximately 3.5mm (15°) nasal to the fovea.

    • Contains axonal projections from retinal ganglion cells forming the optic nerve.

    • Physiologic Blind Spot: Exists temporal to fixation due to the absence of photoreceptors in the optic disc.

Pathological Examples Related to Retina

  • Bartonella Neuroretinitis: Characterized by star-shaped macular exudates often linked to cat scratch disease; inflammation of the optic nerve and retina.

  • Optic Disc Coloboma: Absence of neural tissue at the optic disc due to developmental failure, resulting in a complete hole.

  • Diabetic Papillopathy: Appearance of a swollen optic nerve as a result of diabetic retinopathy.

Optic Nerve

  • Convergence: RGC axons converge at the optic disc to create the optic nerve.

  • Myelination: Axons become myelinated at the lamina cribrosa; the area behind the eye is where intraocular unmyelinated axons transform into the myelinated optic nerve (retrobulbar).

  • Segments of the Optic Nerve:

    • Intraocular: 1mm.

    • Intraorbital: 30mm.

    • Intracanalicular: 6mm.

    • Intracranial: 10mm.

  • Lamina Cribrosa: Separates the intraocular optic nerve from the retrobulbar portion.

Optic Chiasm

  • Pathway: Where the two optic nerves meet and nasal retinal fibers cross.

  • Location: Part of the diencephalon, forming part of the floor of the third ventricle, dorsal to the pituitary gland.

  • Visual Field Representation:

    • Right optic tract carries information from the left visual field (nasal left eye and temporal right eye).

    • Nasal fibers cross over, while temporal fibers do not.

Pathology Associated with the Optic Chiasm

  • Compressive Lesions: Commonly from pituitary tumors causing lesions that can lead to bitemporal hemianopsia due to crossed nasal fibers affecting the temporal visual field.

Optic Tract

  • Contains fibers from the ipsilateral (temporal) and contralateral (nasal) retinal fields, projecting to the lateral geniculate nucleus (LGN).

  • Lesions: All lesions occurring after the optic chiasm are homonymous, indicating that they affect the same visual field quadrant in both eyes.

Lateral Geniculate Nucleus (LGN)

  • Function: Visual relay nucleus of the thalamus where RGC axons synapse with neurons.

  • Layer Organization:

    • 6 layers in total.

    • Layers 2, 3, 5 receive input from the ipsilateral temporal hemiretina; layers 1, 4, 6 receive from the contralateral nasal hemiretina.

  • Projection: LGN cells send visual information to the primary visual cortex through the geniculocalcarine pathway.

Optic Radiations

  • Axons exit the LGN and fan out to the occipital lobe:

    • Inferior Optic Radiation: Travels through temporal white matter (Meyer’s Loop), carrying information from the inferior retina.

    • Superior Optic Radiation: Travels via the parietal white matter, carrying info from the superior retina.

  • Defects:

    • Inferior quadrantanopsia ("pie on the floor") due to lesions affecting the inferior optic radiations.

    • Superior quadrantanopsia ("pie in the sky") due to lesions affecting the superior optic radiations.

Primary Visual Cortex

  • Location: Situated on either side of the calcarine sulcus; Brodmann’s area 17.

  • Function: Receives direct input from the ipsilateral LGN and maintains retinotopic organization, corresponding parts of the retina to areas of the cortex.

  • Cortical Representation:

    • Fovea represented disproportionately large; peripheral retina represented anteriorly.

  • Lesion Effects:

    • Lesions in the superior bank lead to inferior quadrantanopsia; lesions in the inferior bank lead to superior quadrantanopsia.

Extrastriate Cortex (Visual Association Cortex)

  • Continues visual processing beyond the primary visual cortex, integrating visual information with data from other cortical areas to create a unified perceptual experience.

Extrageniculate Visual Pathways

  • Minor Fiber Pathway: Some fibers from the optic tract bypass the LGN, synapsing directly in the pretectal area and superior colliculus, which are essential for pupillary responses and visual attention control.

  • Suprachiasmatic Nucleus: Also receives signals related to circadian rhythms and pupillary responses.