visual system

Visual Pathway

The path from the retina to the occipital cortex: Retina → Optic Nerve → Optic Chiasm → Optic Tract → Occipital Cortex.

Retina

A thin, semitransparent, multilayered sheet of neural tissue lining the inner aspect of the eye, originating from the optic vesicle at day 22 of embryonic development.

Phototransduction

The process by which photoreceptors in the retina convert light into an electrical signal.

Rods

Photoreceptors responsible for scotopic (night) vision; they do not detect color, adapt to low-level lighting, and have poor spatial resolution.

Cones

Photoreceptors responsible for photopic (day) vision, color detection, and high spatial resolution; highly represented in the fovea.

Bipolar Cells

Retinal cells that connect photoreceptors to ganglion cells.

Retinal Ganglion Cells (RGCs)

Cells in the retina that receive signals from bipolar cells and project via the optic nerve; each has a receptive field for light excitation or inhibition.

Receptive Field (Ganglion Cell)

The part of the visual field where light excites or inhibits a ganglion cell; smaller near the fovea for finer visual discrimination.

Interneurons (Retina)

Retinal cells including horizontal cells and amacrine cells.

Muller Cells

Supportive retinal cells analogous to astrocytes in the brain, connecting different photoreceptors and ganglion cells.

Retinal Optical Coherence Tomography

A technology responsible for examining the retina's architecture.

Retinal Pigment Epithelium (RPE)

A barrier between the retina and choroid; damage can lead to layer thinning.

Fovea

The central part of the macula containing only cones; it is the site of highest visual acuity, avascular, and receives nutrients by diffusion from the choroid.

Optic Disc

Located approximately 3.5mm (15^{\circ}) nasal to the fovea, it contains axonal projections from retinal ganglion cells forming the optic nerve.

Physiologic Blind Spot

A region temporal to fixation caused by the absence of photoreceptors in the optic disc.

Bartonella Neuroretinitis

A condition characterized by star-shaped macular exudates, often linked to cat scratch disease, involving 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.

Lamina Cribrosa

The structure that separates the intraocular optic nerve from the retrobulbar (myelinated) portion, where RGC axons become myelinated.

Segments of the Optic Nerve

Four segments: Intraocular (1mm), Intraorbital (30mm), Intracanalicular (6mm), and Intracranial (10mm).

Optic Chiasm

The point where the two optic nerves meet and nasal retinal fibers cross; located in the diencephalon, forming part of the floor of the third ventricle, dorsal to the pituitary gland.

Bitemporal Hemianopsia

Visual field defect (loss of vision in the temporal half of both visual fields) typically caused by compressive lesions like pituitary tumors affecting the crossed nasal fibers at the optic chiasm.

Optic Tract

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

Homonymous Lesions

Visual defects occurring after the optic chiasm, affecting the same visual field quadrant in both eyes.

Lateral Geniculate Nucleus (LGN)

The visual relay nucleus of the thalamus, organized into 6 layers, where RGC axons synapse with neurons before projecting to the primary visual cortex.

LGN Layer Organization

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

Optic Radiations

Axons that exit the LGN and fan out to the occipital lobe, including the inferior (Meyer’s Loop) and superior optic radiations.

Meyer’s Loop

The inferior optic radiation, traveling through temporal white matter and carrying information from the inferior retina.

Inferior Quadrantanopsia

Visual field defect sometimes called "pie on the floor," resulting from lesions affecting the superior optic radiations or the superior bank of the primary visual cortex.

Superior Quadrantanopsia

Visual field defect sometimes called "pie in the sky," resulting from lesions affecting the inferior optic radiations (Meyer's loop) or the inferior bank of the primary visual cortex.

Primary Visual Cortex

Located on either side of the calcarine sulcus (Brodmann’s area 17), it receives direct input from the LGN and maintains retinotopic organization.

Retinotopic Organization

Mapping of corresponding parts of the retina to specific areas of the primary visual cortex, with the fovea disproportionately large.

Extrastriate Cortex

Also known as the Visual Association Cortex, it continues visual processing beyond the primary visual cortex, integrating visual information for a unified perceptual experience.

Extrageniculate Visual Pathways

Minor fiber pathways from the optic tract that bypass the LGN, synapsing in the pretectal area and superior colliculus for pupillary responses and visual attention control, and the suprachiasmatic nucleus for circadian rhythms.

Pretectal Area

Part of the extrageniculate visual pathway, receiving fibers from the optic tract, essential for pupillary responses.

Superior Colliculus

Part of the extrageniculate visual pathway, receiving fibers from the optic tract, essential for visual attention control.

Suprachiasmatic Nucleus

Receives signals from the extrageniculate visual pathway related to circadian rhythms and pupillary responses.

Visual Pathway

The path from the retina to the occipital cortex: Retina

→ Optic Nerve

→ Optic Chiasm

→ Optic Tract

→ Occipital Cortex.

Retina

A thin, semitransparent, multilayered sheet of neural tissue lining the inner aspect of the eye, originating from the optic vesicle at day 22 of embryonic development.

Phototransduction

The process by which photoreceptors in the retina convert light into an electrical signal.

Rods

Photoreceptors responsible for scotopic (night) vision; they do not detect color, adapt to low-level lighting, and have poor spatial resolution.

Cones

Photoreceptors responsible for photopic (day) vision, color detection, and high spatial resolution; highly represented in the fovea.

Bipolar Cells

Retinal cells that connect photoreceptors to ganglion cells.

Retinal Ganglion Cells (RGCs)

Cells in the retina that receive signals from bipolar cells and project via the optic nerve; each has a receptive field for light excitation or inhibition.

Receptive Field (Ganglion Cell)

The part of the visual field where light excites or inhibits a ganglion cell; smaller near the fovea for finer visual discrimination.

Interneurons (Retina)

Retinal cells including horizontal cells and amacrine cells.

Muller Cells

Supportive retinal cells analogous to astrocytes in the brain, connecting different photoreceptors and ganglion cells.

Retinal Optical Coherence Tomography

A technology responsible for examining the retina's architecture.

Retinal Pigment Epithelium (RPE)

A barrier between the retina and choroid; damage can lead to layer thinning.

Fovea

The central part of the macula containing only cones; it is the site of highest visual acuity, avascular, and receives nutrients by diffusion from the choroid.

Optic Disc

A pale, circular area located approximately 3.5mm (15^{\circ}) nasal to the fovea. It marks where axonal projections from retinal ganglion cells gather to form the optic nerve and is noteworthy for the absence of photoreceptors, which causes the physiologic blind spot.

Physiologic Blind Spot

A region temporal to fixation caused by the absence of photoreceptors in the optic disc.

Bartonella Neuroretinitis

A condition characterized by star-shaped macular exudates, often linked to cat scratch disease, involving 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.

Lamina Cribrosa

The structure that separates the intraocular optic nerve from the retrobulbar (myelinated) portion, where RGC axons become myelinated.

Segments of the Optic Nerve

Four segments: Intraocular (1mm), Intraorbital (30mm), Intracanalicular (6mm), and Intracranial (10mm).

Optic Chiasm

The point where the two optic nerves meet and nasal retinal fibers cross; located in the diencephalon, forming part of the floor of the third ventricle, dorsal to the pituitary gland.

Bitemporal Hemianopsia

Visual field defect (loss of vision in the temporal half of both visual fields) typically caused by compressive lesions like pituitary tumors affecting the crossed nasal fibers at the optic chiasm.

Optic Tract

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

Homonymous Lesions

Visual defects occurring after the optic chiasm, affecting the same visual field quadrant in both eyes.

Lateral Geniculate Nucleus (LGN)

The visual relay nucleus of the thalamus, organized into 6 layers, where RGC axons synapse with neurons before projecting to the primary visual cortex.

LGN Layer Organization

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

Optic Radiations

Axons that exit the LGN and fan out to the occipital lobe, including the inferior (Meyer’s Loop) and superior optic radiations.

Meyer’s Loop

The inferior optic radiation, traveling through temporal white matter and carrying information from the inferior retina.

Inferior Quadrantanopsia

Visual field defect sometimes called "pie on the floor," resulting from lesions affecting the superior optic radiations or the superior bank of the primary visual cortex.

Superior Quadrantanopsia

Visual field defect sometimes called "pie in the sky," resulting from lesions affecting the inferior optic radiations (Meyer's loop) or the inferior bank of the primary visual cortex.

Primary Visual Cortex

Located on either side of the calcarine sulcus (Brodmann’s area 17), it receives direct input from the LGN and maintains retinotopic organization.

Retinotopic Organization

Mapping of corresponding parts of the retina to specific areas of the primary visual cortex, with the fovea disproportionately large.

Extrastriate Cortex

Also known as the Visual Association Cortex, it continues visual processing beyond the primary visual cortex, integrating visual information for a unified perceptual experience.

Extrageniculate Visual Pathways

Minor fiber pathways from the optic tract that bypass the LGN, synapsing in the pretectal area and superior colliculus for pupillary responses and visual attention control, and the suprachiasmatic nucleus for circadian rhythms.

Pretectal Area

Part of the extrageniculate visual pathway, receiving fibers from the optic tract, essential for pupillary responses.

Superior Colliculus

Part of the extrageniculate visual pathway, receiving fibers from the optic tract, essential for visual attention control.

Suprachiasmatic Nucleus

Receives signals from the extrageniculate visual pathway related to circadian rhythms and pupillary responses.