10. Visual System

Sclera

White, protective outer layer

Caudally lined w choroid interior > rentina

Cornea

Cranial mod. of sclera

Transparent, avascular

Unmyelinated nerve endings

Choroid

Vascular pigment layer

Lines posterior 2/3 of sclera

Retina

Layer interior to choroid

Contains photoreceptors

Aqueous humors

2 chambers separated via iris

Clear, water like nutrient supply to cornea & lens

Drains continuously via canal of Schlemm

Iris

Separates cranial and caudal chambers

Dilator and constrictor smooth mm. influence diameter of pupil

Lens

Suspensory ligaments attach lens to cilliary body

Retina

Interrupted by axons of ganglion layer leaving the eye

Ciliary epithelium

Secretes aqueous humor

What condition is caused by a lack of aqueous humor drainage

Glaucoma; can cause blindeness

Opthalmoscope exam

Examination of retina, aqueous humor, cardiovascular system & choroid vessels

Accomodation

Process by which lens changes shape to focus images from various distances onto the retina

Relaxed ciliary muscleq

Increased tension on suspensory ligaments

Pulls lens, flattens

View far images (>20ft)

Contracted ciliary muscle

Decreased tension on ligaments

More spherical lens

View near objects

Presbyopia

Loss of elasticity in the ciliary muscle (lens) due to age

Emetropia

Normal vision

Parallel light rays are in sharp focus

Hyperopia

Farsightedness

Light rays refracted to be in focus on the renita

Myopia

Nearsightedness

Light rays from distant objects focused in from of the retina

Pupil adjustment

Amount of light entering eye through the pupil is proportional the the areaof the pupil

Dilation via radial mm of iris (sympathetic)

Contriction via circular mm (parasympathetic)

Photoreception

Process by which eye detects light energy; achieve bia light-sensitive cells of the retina

Light pathway

Cornea > Aqueous humor > Pupil (lens) > Vitreous humor > RetinaC

Cataracts

Clouding of the lens

Cells of the eye

Ganglion, amacrine, bipolar, horizontal, & photoreceptor cells

Cell density

Variations in density & architecture for functional variations

Interneurons of visual processing

Horizontal & Amacrine cells

Starbust Amacrine cells

Neurons that syn. & release GABA & ACh

Small to large dendritic fields

Retinal Synaptic Light processing

Vertical (protoreceptor-bipolar-ganglion) and horizontal pathways

Horizontal Cells

Mediate lateral interactions among the photoreceptors and bipolar cells

A-type > Axon; B-type >No axon

GABAergic

Inner neuron axons

Unmeylinated

Relatively transparent

Some distortion of light rays

Fovea (macula)

Area of most visual acuity

Fully exposed photoreceptors - 100% cones

Pigments

Behind the retina

Absorb or reflect light dependent on species

Daylight vision

Dark pigment in pigment epithelium absorbs light to prevent blurred imageN

Nocturnal vision

Epithelium contain reflecting pigment - tapetum lucidum

Optimizes light input

Cause of “night shine” reflection

Rods & Cones

Specialized for photoreception

Membranous disc w visual photopigments

Conopsin & Rhodopsin

G-protein coupled light sensitive pigments

Light absorption

Activates opsin > change in conc. of IC second messenger > change in membrane potential

Light sensitive events

The configurational change in the retina > hyperpolarize

light convers cGMP > GMP and Na+ channels close

Cone : Ganglion ratio

Mostly 1-to-1

Humans have 3 different types, other mammals have 2 while felines have 4

Visual fields

Right half of visual field on L half of retina & vice versa

Retino-geniculo-striate Pathway

Retinal axon> Lateral geniculae nucleus (Thalamus) > Ispsilateral primary visual cortex (occipital lobe)

Temporal lobe > optic chiasm > LGN on same side of brain

Nasal retina > Optic chiasm > Contralateral LGN

Hemianopia

Hemianopsia

Visual field loss of either eye on the vertical midline

Typically affect both eyes

Retino-Tectal Pathway

Optic fibers > Colliculi

Turn eye & head towards visual disturbance/sound/stroking

Pupillary Light Reflex (PLR)

Light hits retina > signal travels optic nerve > travels special sense pathway > parasympathetic response via CN III > postganglionic fiber of Ciliary ganglion > circular fibers of iris contract > pupil constricts

When light removed > pupil dilates to resting position

Retino-Hypothalamic Pathway

Eye > RHT > Suprachiasmic nucleus + Melatonin (pineal gland)

Suprachiasmic nucleus > Parareticular nucleus

Pineal gland > Cranial cervical ganglion > Thoracic Intermediolateral nucleus

Parareticular nucleus > Thoracic Intermediolateral nucleus