Neurobiology - Lecture 14: The Eye Part 1

Part of Exam 3

LGN

Lateral Geniculate Nucleus; Located in the Thalamus, First Synaptic Relay in the Primary Visual Pathway

Wavelength

Distance between successive peaks or troughs

Frequency

Number of waves per second

Amplitude

Distance between the wave origin and a trough or a peak

Reflection

Bouncing of light rays off a surface

Absorption

Transfer of light energy to a particle or surface

Refraction

Bending of light rays from one media to another due to changes in velocity.

Muscles That Control Pupil Size

Iris Sphincter and Iris Dilator

Conjunctiva

Mucous membrane of the Sclera and Inner Eyelids; Contains immune cells and vasculature

Extraocular Muscle Innervation Pneumonic:

SO4-LR6-EE3

Superior Oblique Muscle: CN-4 (Trochlear)

Lateral Rectus Muscle: CN-6 (Abducens)

Everything Else: CN-3 (Occulomotor); All extrinsic muscles except SO and LR

Lens Framework

Zonule Fibers: Ligaments that suspend the lens

Ciliary Muscle: Responsible for accomodation of the lens

Path of Light Into the Eye

Cornea → Aqueous Humor → Pupil → Lens → Vitreous Humor → Retina

Location of the Optic Disc

Nasal to the fovea

Refractive Power of the Cornea

42 Diopters

Focal Distance

Distance from point of refraction to point where parallel rays converge

Refractive Power

1/ focal distance (m)

Measured in diopters

Accommodation of the Lens

Ciliary muscles contract, zonule fibers relax, lens gets wider

Visual Acuity

Ability to distinguish two nearby points

Visual Angle

Distance across the retina described in degrees

Hyperopia

Farsightedness (Can see Far well); Eye too short, Objects focus behind the retina.

Myopia

Nearsightedness (Can see near well); Eye too long, Objects focus in front of the retina

Presbyopia

Hardening of the lens with age creating a decreased ability to accommodate.

Bifocals developed by Benjamin Franklin

Radial Keratotomy

Small incisions in the peripheral cornea to relax the central cornea.

Corrects myopia

Photorefractive keratectomy (PRK)

A laser reshapes the outer surface of the cornea by vaporizing thin layers

Laser in situ keratomileusis (LASIK)

Thin flap on the outer cornea. A laser reshapes the cornea from the inside.

Strabismus

Loss of depth perception acuity due to misalignment or lack of coordination between eyes.

Esotropic: Eyes cross

Exotropic: Eyes diverge

Cataracts

Clouding of the lens

Glaucoma

Increased intraocular pressure on the optic nerve,- due to failure of aqueous humor to drain via the scleral venous sinus- causes blindness from the periphery to the center.

Detached Retina

A blow to the head or shrinkage of vitreous humor causes the retina to pull away from the wall of the eye. Corrected by Laser surgery.

Retinitis Pigmentosa

Progressive degeneration of photoreceptors causing tunnel vision

Macular Degeneration

Loss of central vision due to a degeneration of the macula.

Anatomy of the Retina (Anterior → Posterior)

Retinal Ganglion Cells→ Amacrine Cells → Bipolar Cells → Horizontal Cells → Photoreceptors

Role of Horizontal and Amacrine Cells

Inhibitory for unnecessary lateral cells.

Pigmented Epithelium

Located behind the photoreceptors of the retina; absorbs scattered light.

Tapetum Lucidum

Reflective layer beneath photoreceptors in many nocturnal animals.

Rods

Photoreceptors with long, cylindrical outer segments containing many disks. Responsible for Black and White vision. 1000x more sensitive to light.

Approximately 92 million

Cones

Photoreceptors with shorter, tapering outer segments with fewer membranous disks. 3 varieties: Red, Green, & Blue; High variability. Responsible for Color vision.

Approximately 5 million