Physiology Lab Exam 1

Outer wall of eye formed of _ distinct layers

Three (Fibrous tunic, Vascular tunic, Sensory Tunic)

Outermost layer of the eye

Fibrous tunic

Fibrous connective tissue, poorly vascularised. Has 2 regions: the sclera and the cornea

Sclera

• The ‘white of the eye’ = the posterior 5/6 of the fibrous tunic. ~0.5mm thick.

• Densely packed collagen fibers with few elastic fibers

• Maintains shape of the eyeball and protects from injury

Cornea

• Anterior 1/6 of the fibrous tunic

• Transparent, ~0.8mm thick

• Small collagen fibers, few elastic fibers and many proteoglycans

• Inner surface = simple squamous epithelium, outer surface = stratified squamous

  • Protects internal structures, contributes to refractive power

Middle layer of eyeball

Vascular tunic (aka uvea)

3 regions: choroid, ciliary body and iris

Choroid

• Vascular tunic layer

• Thin membrane, posterior 5/6 of eye, incomplete where optic nerve exits

• loose connective tissue, blood vessels and melanocytes.

  • Supply nutrients and oxygen to outer retina

Ciliary body

• Vascular tunic layer

• Continuous w choroid, anterior, encircles periphery of lens

• Ciliary processes that secrete aqueous humor into anterior segments

• Ciliary muscles that control shape of lens (smooth muscles)

• Suspensory ligaments extend from ciliary processes to lens, maintain in upright position

Iris

• Vascular tunic layer

• Pigmented sheath hangs from ciliary body, encircling most of lens.

• Loose connective tissue, smooth muscles fibers, melanocytes

• Central opening = pupil

• Muscle allows to vary in size

  • Sphincter pupillae muscles constrict the pupil

  • Dilator pupillae muscles increase size of pupil

Innermost layer of eyeball

Sensory Tunic or Retina

Two layers (not fused and can come apart)

Condition where two layers of retina come apart

Detached retina

Pigmented layer

• One layer of cuboidal epithelium in the retina

• Contains melanin

  • Adjacent to choroid, extends anteriorly to cover ciliary body and posterior iris

Neural layer

• In retina layer

• Anterior to pigmented layer, extends to margin of ciliary body (area called the ora serrata)

• Composed of 3 layers of neurons:

  • Photoreceptors

  • Bipolar cells

  • Ganglion cells

Photoreceptors

• Deepest in the neural layer, adjacent to the pigmented layer.

• Convert light into electrical impulses, pass to bipolar cells

• 2 types = rods and cones

Bipolar cells

Receive electrical impulse from photoreceptors and pass to ganglion cells

Ganglion cells

• Most anterior of neural retina.

• Long axons, make 90º turn at inner face of retina

• All axons ultimately merge at posterior eye to from optic nerve to brain

Device used to observe the fundus (posterior inner eye wall)

Opthalmoscope

Optic disc

Where optic nerve exits eye (blind spot)

Appears as round yellow region in opthalmoscope

Macula lutea

Round reddish region in opthalmoscope

Contains fovea centralis

Eye examination

Subject sits and looks at distant object over examiner’s shoulder. Observe right eye with your right eye and vice versa. Opthalmoscope turned on and held at arms lenght, directed so light is shining into subjects pupil and observe.

Yellow is optic disc, red is macula lutea, pink is everywhere else. Arteries brighter red than veins

Why do we not see our own optic veins/arteries?

Our brain suppresses the images in normal line of vision

Purkinje Tree exams

Close eyes, hold light next to eyelid at 45º angle and move it from side to side. The branching image is the Purkinje tree

The 6 extrinsic eye muscles originate on the ___ _____ and insert on the ______

bony orbit

sclera

Retinal disparity

Difference in images perceived by either retina. Helps in 3-dimensional vision.

What maintains eyes in midline position

Antagonistic ocular muscles

Nystagmus

When tone of one eye muscle is weak: Eye slowly drifts in one direction then quickly corrects itself. Controlled by vestibulo-ocular reflex

Nystagmus test

Hold pencil 1 meter from bridge of subject nose. Move 30cm to each side then up/down. Hold in each position ~5 sec. Observe movements noting presence/absence of nystagmus (oscillations of eye, slow one way, fast in opposite)

Myopia

• Nearsightedness

• When distant object brought to focus in front of retina

• Usually due to elongated eyeball

• Corrected with concave lens

Hyperopia

• Farsightedness

• Image brought to focus behind retina

• Usually due to shortened eyeball

• Convex lens used

Emmetropia

Normal vision acuity

Snellen eye chart

• Visual acuity test chart. stand 20 feet from eye chart, cover one eye and attempt to read each line

• Person with normal vision can read 20/20 line at 20 feet.

• Person with 20/40 visual acuity must stand 20 feet away from a line that a normal person can read at 40 feet

Astigmatism

• Visual defect due to abnormal curvature of cornea or lens.

• Refraction of light in horizontal plane is different than in vertical plane.

• Astigmatism test chart - stand 3 to 4 feet from astigmatism, cover one eye, stare at center

Retinal image 4 processes

1. Refraction of light rays.

2. Accommodation of the lens.

3. Modification of the pupillary diameter.

4. Convergence of the eyes.

Refraction of light rays

• Bending light rays.

• Image light refracted by cornea, lens, and humors of eye to form inverted image on retina.

• Strength of lens can be changed by making more or less convex. Higher degree convexity, greater strength of lens.

Strength of lens measured in ________

Diopters

Strength = 1/focal length (m)

Accommodation of the lens

• Altering the degree of convexity of the lens to focus image

• Greater curve = more light bends

• When ciliary muscle relaxed, suspensory ligaments tighten and pull lens = convexity decreases (flatter). Distant objects focused (think like pulling a bow and arrow).

• When ciliary muscles contract, suspensory ligament loosens = convexity increases (rounder), near objects (<20 ft) focused

Accommodation (eye)

Ability of the eye to focus on different distances from the lens

Which requires greater degree of convexity, near or far object?

Near object, has more obliquely directed rays

Presbyopia

Elasticity of lens decreases with age, loss of ability to accommodate (literally means “old vision” “Presbus = old man”)

Near point of vision test

Place meter stick under one eye, close opposite. Hold pencil at arm’s length slowly bring pencil toward eye. Record distance at which object out of focus. Repeat for other eye

Accommodation test

• Focus on distant object.

• Place pencil ~ 25cm in front of eyes

• shift focus to pencil

• close eyes for 30-60 sec

• open and note whether pencil or distant is in focus (Should be the “big picture” in focus)

Depth of focus

Greatest distance an object can move and still remain in focus. Main factor is pupil size (small pupillary diameter = depth of focus is greater due to more accurately focused light at centre of pupil)

Sympathetic vs Parasympathetic pupillary movement?

Sympathetic NS controls the dilator pupillae (contraction for fight or flight)

Parasympathetic NS controls sphincter pupillae (dilation)

Pupillary reflex

In response to light, pupil constricts

Same eye = direct light reflex

Other eye = consensual light reflex

Pupillary diameter test

Focus on tip of pencil as slowly brought from 1m in front of you to bridge of nose. Note size of pupils (should constrict)

Do not perform facing window

Consensual Light reflex test

Shine narrow beam of light from right side of subjects eye into right eye. Note changes in both eyes (should constrict)

Do not perform facing window

Extrinsic vs Intrinsic eye muscles

Extrinsic muscles move the eye (originate outside the eye)

Intrinsic muscles control pupillary dilation (contained within the eye entirely)

Convergence

Movement of eyes medially to ensure light rays of near object hit same spot on the retina

Accomplished by actions of extrinsic eye muscles

Convergence test

Focus on tip of pencil as slowly brought from 1m in front of face to bridge of nose. Eyes converge/rotate medially

Approximately how many rods in each eye?

120 million

Rods are found everywhere on the retina except the _____ _________

fovea centralis

Rods three functions

1. Night (scotopic) vision - rods are much more sensitive to light than cones.

2. Black-and-white vision - pigment found in rods = only perceive black white and shades of grey

3. Peripheral vision - rods perceive images outside direct line of vision (outside fovea centralis)

Photochemical reaction when light hits rods

Bleaching:

rhodopsin pigment dissociates to form retinal pigment and opsin protein. Leads to hyperpolarisation, triggering chain of graded potentials. Inactive period follows.

Pigment found in rods

Rhodopsin

Breaks down into retinal and opsin

Vitamin required to restore retinal to rhodopsin in rods

Vitamin A

What causes inactive period after stimulation of rod cell?

Rhodopsin has to be reformed after “bleaching”

Positive afterimage of bright light + cause?

Bright image of light seen

Due to continued firing of rods

Negative afterimage of bright light + cause?

Dark image of bulb

Due to bleaching of visual pigment of rods

Afterimage test (for rods)

• Stand 30cm from white light lamp

• Stare at bulb for 30 sec

• Turn light off and close eyes at same time

• First image you see should be similar

• second image should be different

How many cones in each retina?

7 million

Where are cones found? and where are they MOST concentrated?

Found in the macula lutea highest conc. at the fovea centralis, decreases as you move away

How many cones found in the fovea centralis

~35 thousand within the 1.5mm diameter

Cones three functions?

1. Day (photopic) vision - cones require relatively bright light to function

2. Color vision - various pigments in cones perceive colour

3. Visual acuity

Why are images focused on fovea centralis the sharpest? (4 reasons)

• This is where image focuses when eyes look directly at it

• Many densely packed cones

• Overlying layers of cells (bipolar/ganglion) are displaced to sides of fovea, allowing unimpeded light to cones

• Generally, only one cone synapses with one bipolar cell in this region

Young-Helmholtz theory of color vision

3 systems of cones responding respectively to red, green, and blue light. All other colors seen by brain are interpretations of mixtures of impulses. Color discrimination impaired if one system of cones bleached by continued viewing of object of that color.

Afterimage test (for cones)

• Stare at small red square for 1 minute on large sheet of black paper.

• Suddenly shift gaze to sheet of white paper.

• Positive afterimage is similar

• Negative afterimage is different

  • May see negative afterimage first if fatigue sets in

Relative Positioning of Rods and Cones on Retina test

• Subject sits approx 15 cm from black board at left side. Make white chalk X on board immediately in front of subject’s right eye.

• Subject close left eye and stare at X with right eye throughout test

• Slowly move white paper disc into field of vision from right side, around 2m away

• Mark on board for each disc when first becomes visible to subject. Repeat w red green and blue

  • SHOULD have white further out (before the halfway point) and colours closer as rods are peripheral and cones are central

Trichromancy

No loss of color vision

Color blindness

Impaired color discrimination. Hereditary condition affecting 8% of males and 0.5% of females.

Deuteranomaly

Reduced ability to see green light (most common type, X-linked)

Protanomaly

reduced sensitivity to red light, X-linked

Tritanomaly

Reduced sensitivity to blue light (extremely rare, NOT X-LINKED) Difficult to distinguish blue+yellow, violet+red, blue+green

Monochromacy (Achromotopsia)

Complete loss of color vision, extremely rare, only sees white, greys, and blacks

Color blindess tests

Cambridge color test online

Liquid filling the internal (posterior) cavity of the eye?

Vitreous humor

Passage of light in eye?

• Through pupil

• through lens and vitreous humour

• pass through neural retina, past ganglion cells then bipolar cells

• hits photoreceptor cells = bleaching

Passage of graded potential in neural retina

After bleaching occurs, photoreceptors pass graded potentials anteriorly to a bipolar cell then to a ganglion cell.

Action potential generated by ganglion cell, travels down axon to the brain

Axons of all ganglion cells gather posteriorly to form _____ _____

Optic nerve

Point where optic nerve leaves the eye?

Optic disc (the blind spot)

Blind spot test

Hold the diagram about 20in from face with left eye closed. focus on circle lined up with right eye. Slowly bring page closer to face until cross disappears. Then move until cross reappears. Repeat with left eye

Why don’t we always have a blind spot in our vision?

Due to overlap with our other eye, they cover each others blind spots. When one eye is closed, the brain “fills in the gap” using context of surrounding view.

Where do the two optic nerves meet?

Optic chiasma

Axons from the [medial or lateral] region of each eye cross over at the optic chiasma?

The medial region

Axons from the [medial or lateral] region of each eye do NOT cross over and continue to same side of brain?

Lateral region

Refresh! Tract versus Nerve??

A tract is a collection of axons in the CNS, a nerve is a collection of axons in the PNS

Right optic tract

Formed of fibers from the medial half of left eye and lateral half of right eye.

Left Optic tract

Formed of fibers from the medial half of the right eye and lateral half of left eye.

Lateral geniculate body

Region where most optic tract axons end, found in the thalamus (the relay station between the eye and the visual cortex)

Function of axons that don’t end at the lateral geniculate body?

Function in visual reflexes

Neurons of the lateral geniculate body form fibers of _____ __________ that project to the _______ ______ ______ in the occipital lobe

optic radiations to the primary visual cortex

Visual cortex function?

Integrates retinal messages into single message, translates into mental image and relays to other parts of brain where it is acted upon or ignored.

Binocular vision

Type of vision in humans. Although both eyes look in approx same direction they see slightly different images, but there is overlap.

Provides humans with stereopsis, 3D vision and accurate spatial + depth perception

Depth perception test

Drop pencil into narrow neck bottle with both eyes open. Repeat with one eye open.

Clinical Significance: to determine if the subject has difficulty integrating the overlapping images from the left & right visual fields

Confrontational visual field exam (Donder’s test)

Subject covers one eye stares straight ahead at dark background. Examiner stands out of sight, moves bright object out of visual field laterally. Subject signals when no longer visible in their peripheral view. Repeated for other eye.

Clinical Significance: a visual field test used to determine if the subject suffers from deficits in their peripheral vision

Amsler grid test

Place grid on flat surface. Stand above grid at normal reading distance. Test one eye at a time, focus on centre dot. Note if lines appear straight or they appear broken, distorted, blurred or gaps.

Clinical Significance: to determine if the subject suffers from macular degeneration, glaucoma, and/or a detached retina. Will not be accurate if astigmatism is present but NOT used to test for astigmatism.

Diplopia

“double vision” caused by lack of integration of binocular views from each eye in brain.

Dominant eye test

With both eyes open, hold a pencil in front of you, lined up with object in the distance. Close one eye, open it, close the other. The eye that allows you to keep the pencil in line with the object is your dominant eye.

Two Types of muscle filaments

Actin (thin) and Myosin (thick)

Thick filament

Consists of many myosin molecules whose heads protrude at opposite ends of the filament.

Myosin molecule (5 parts)

Head, tail, flexible hinge region, ATP-binding site, Actin-binding site

Thin filament

Two strands of actin subunits twisted into a helix + two types of regulatory proteins (troponin and tropomyosin)