Special Sense: Vision

INB 446L exam II lecture

Neglect syndrome

A perceptual disorder involving the inability to recognize or pay attention to the opposite side of the body and its surroundings…A perceptual disorder involving the inability to recognize or pay attention to the opposite side of the body and its surroundings

Eyeball layers

1. Fibrous Layer (outer)

2. Vascular Layer / Uvea (middle)

3. Retina (inner/nervous)

Sclera Histology & composition

The tough, relatively avascular white portion of the eye providing structural protection and extraocular muscle insertion points. It is composed of dense connective tissue containing Type I collagen

Cornea transparency & structure

The transparent anterior 1/6th of the eyeball. It is completely avascular

Cornea epithelium & repair

Connective tissue fibers are always covered by an epithelial layer Damage to the cornea is difficult to repair because it lacks blood vessels The epithelium secretes a modified lubricant so the eye can move smoothly in its socket

Glaucoma Cause

Caused by the blockage of aqueous humor drainage at the corneoscleral junction (CSJ) via the trabeculae, which elevates intraocular pressure

choroid composition

Caused by the blockage of aqueous humor drainage at the corneoscleral junction (CSJ) via the trabeculae, which elevates intraocular pressure

Ciliary muscle action

Suspensory ligaments attach the ciliary muscle to the lens When the ciliary muscle contracts, it changes the lens shape to refract and focus light

Ciliary body epithelium function

Lined by stratified columnar epithelium which actively secretes aqueous humor fluid into the eye

Iris structure pecularities

The iris has no epithelial layer (because it is already fully protected by outer layers) and contains limited to no blood vessels

Iris Muscles & Innervation

• Dilator pupillae m.: Radially oriented; causes pupil dilation when it contracts under sympathetic control

• Sphincter pupillae m.: Circumferentially oriented; causes pupil constriction under parasympathetic control

lens Fibrillin defects

The lens capsule is rich in Type IV collagen, but the interior lens must stay solid using structural fibrillin proteins. A defect in the fibrillin gene causes Marfan syndrome (a connective tissue disorder), preventing the lens from changing shape

Presbyopia vs. Cataracts

• Presbyopia: Age-related loss of lens elasticity

• Cataract: Clouding of the lens. Surgical correction involves replacing the foggy lens with a clear synthetic one

Vitreous Body Composition

A posterior gel-like mass that is 99% water bound tightly to hyaluronate molecules

Retina Sub-Layers

Divided into two layers: an outer pigmented layer (separates the choroid from the retina and blocks stray light reflection) and an inner neural layer

The 3 Neural Retina Layers

Back: Signals flow forward vertically through three cell layers [pdf_0.1.18]:

1. Photoreceptors: Rods (black &white) and cones (color) adjacent to pigment layer

2. Intermediate layer: Composed of bipolar neurons

3. Deepest layer: Composed of ganglion cells whose axons form the optic nerve

Macula and Fovea Centralis Purpose

Photoreceptors are heavily concentrated in the macula The fovea centralis is the specific spot where you focus incoming light to help you read text

Nasal vs. Temporal Retina Pathways

• Lateral (Temporal) Retina: Processes the medial visual field; axons stay on the same side (ipsilateral) through the optic chiasm

• Medial (Nasal) Retina: Processes the lateral visual field; axons cross over (decussate) at the optic chiasm

Complete Ascending Visual Track Route

Optic Nerve —> Optic Chiasm —> Recombine into a single Optic Tract —>Synapse in the Lateral Geniculate Body (diencephalon) —> Terminate in the Visual Cortices of the Occipital Lobes around the calcarine fissure

Extraocular Muscle Count & Wiring

Exactly 6 muscles move the eye 2 muscles have separate nerves: Superior Oblique (CN IV / Trochlear) and Lateral Rectus (CN VI / Abducens)The other 4 muscles are all innervated by CN III (Oculomotor)

Lacrimal Gland Control & Tear Path

Located at the upper lateral part of the eye Tears are secreted under Facial Nerve (CN VII) control They travel through the canaliculi and sac into the nasolacrimal duct, draining directly into the nasal cavity to be recycled in the digestive tract