The Human Eye

Stimulus

• a chemical or physical change in the internal or external environment which is detected by receptors eliciting a response

Transduction

• when stimulus energy (kinetic, force, heat) is converted into chemical energy in the form of nerve impulses

Biological Transducers

receptor cells 

1. cornea

2. iris

3. lens

4. sclera

5. retina

6. choroid

7. fauvea

8. optic disc

9. optic nerve

10. vitreous body 

11. suspensory ligaments

12. ciliary body

13. anterior chamber filed with aqueous humor

14. pupil

Orbits

• eye socket which holds the eye

• has a tiny hole at the end

What is the spherical shape of the eye maintained by

• sclera

• aqueous and vitreous humors → create hydrostatic pressure to support the eye shape

list the outermost to innermost tissue layers of the eye 

• sclera and cornea

• choroid, ciliary body, lens, iris

• retina 

Ancillary Structures

• external structures but necessary for eye functioning

• eyebrows and eyelids

• conjunctiva - on the sclera and eyelid

  • transparent mucous membrane

  • lubricates eye surface

  • secretes lysozyme to destroy bacteria

Tears 

• produced by the lacrimal gland 

• consist of

  • water/salts

  • mucilage

  • oils

  • antibodies and lysozyme

• drain into lacrimal canaliculi and lacrimal sac

• surplus tears drain into nasal cavity

Sclera 

• outermost layer of the eye

• opaque whitish-yellow appearence due to collagen fibres

• protects, supports the eye 

Cornea

• transparent dome over iris

• refracts light

• aqueous humor behind it increases the refractive index

(angle light rays to retina for clearest image)

Iris

• circular muscular diaphragm

• controls the PUPIL

• pigment over iris muscles determines eye color

• separates the anterior and posterior aqueous humors.

Pupil 

• black due to absorption of light

• opens/closes to let light in

• opening in the iris 

red pupil effect

• when a bright light enters the eye and the light reflects off the choroid blood vessels inside

The Pupil Light Reflex

• iris muscels are smooth muscles controlled by the autonmic nervous system 

• when the circular muscles contract 

  • pupil contracts - allowing less light in

• radial muscles contract 

  • pupil dilates - allowing more light in 

• this is a reflex 

Choroid 

• blood rich to supply retina

• pigment cells absorb light - no reflection 

• WHY INSIDE THE EYE APPEARS DARK 

Accommodation Structures

• ciliary body - marks where sclera joins cornea

• muscles + blood vessels

• secretes aqueous humour

• suspensory ligaments join ciliary body to lens

• ciliary muscles

  • circular and longitudinal bring about lens accomodation

Lens accommodation 

• where the lens shifts its forward or backward so an image always forms on the retina 

3 words for the lens + describe how lens focuses light

• plastic, biconvex and transparent

• focuses light via refraction

  • when object is far away the lens thins due to relaxation of ciliary muscles

  • when object is close lens becomes more rounded due to contraction of ciliary muscles

The Retina

• composed of photoreceptor cells

• Rods - which work best in dim light

• Cones - which work best in high light intensity

Optic Disc 

• blind spot, neurons meet at the optic nerve to take information to the brain

Fovea 

• most sensitive spot due to a large number of cone cells 

• highest image clarity and sharpest image 

Aqueous Humour

• salt solution located behind the cornea which refracts light

• secreted by ciliary body

• drains into blood vessles via canal of schlemm

  • blockage of this canal causes glaucoma

Vitreous Humour 

• clear semi-solid gel

• refracts light even more

• supports eyeball via hydrostatic pressure 

Layers of the retina

• photoreceptor layer

  • rods and cones

  • embedded in the choroid, prevents light bouncing back

• intermediate layer

  • bipolar neurons which connect the photoreceptor and internal surface layer

• internal surface layer

  • ganglion cells

  • axon and dendrites of optic nerve

4 main parts of the rods and cones 

1. Outer segment

   1. photosensitive part - flattened membranous vesicles containing photosensitive pigments

   2. transform light into a generator potential (graded pot - which results in an action p)

2. Constriction

   1. cytoplasm constricts due to pinch in outer membrane

3. Inner Segment

   1. nucleus, mitochondria needed for polysomes for protein sysntehsis (pigments and membranous vesicles) and energy

   2. metabolically active part of the cell

4. Synaptic Region 

   1. connect bipolar neurons

Main difference between Rods and Cones

• Rods have multiple connections to bipolar neurons allowing for synaptic convergence 

• Cones only have one connection to each bipolar neuron - high visual acuity.

What cells help with image processing 

• Horizontal Cells and Amacrine cells help with image processing

Distinguish between rods and cones

rods	cones
convergence	visual acuity
work best in low light (scotopic vision) absent in fovea periphery of retina monchromatic vision	used for vision in bright light (photopic vision) concentrated in fovea center of retina trichromatic (Color) vision

convergence

• multiple rods are connected to a single bipolar neuron

• this allows for summation of a generator potential

• this makes the rods more sensitive to dim light - as summation will result in an action potential being generated

• decreases visual acuity as the brain cannot tell which rod the signa came from producing a fuzzy image (in dim light shapes appear fuzzy)

Visual Acuity

• related to the sharpness of vision

• visual acuity is the ability to distinguish two or more stimuli of identical intensity as two distinct stimuli

---

• cones have individual connections to bipolar neurons

• the brain can tell exactly from which con the stimulus is coming from - each cone triggered is a separate stimulus

• stimulus must be strong enough to generate a GP → AP

• ONLY POSSIBLE IN BRIGHT LIGHT

• THUS OBJECTS LOOK THE SHARPEST WHEN IN BRIGHT LIGHT AND DIRECTLY LOOKED AT (fovea)

Sensory Transduction

• in the dark an inhibitory neurotransmitter called glutamate which prevents bipolar neurons from exciting the ganglion cells 

• then in the light glutamate is inhibited, hence the cell is no longer hyperpolarized

• ganglion cells are excited and generate impulses leading to the brain 

Rhodopsin

• photosensitive pigment found in rods

• made up of 11-cis retinal + opsin

• in the light 11-cis retinal becomes all-trans retinal

• As a result opsin bleaches (no longer fit)

• Once released (bleached) opsin triggers a cascade in the cells causing Na+ channels to be inhibited

• this causes a hyperpolarization

• prevents the release of glutamate

How is a hyperpolarization achieved 

• in the dark there is a dark current

  • where na+ flows from the inner segment into the outer segment

• in the presence of light → opsin is released

• opsin causes cGMP(cyclic guanosine monophosphate) to change into GMP

• cGMP keeps the Na+ channels open, while GMP closes them

• the hyperpolarized membrane now no longer releases glutamate

• bipolar neurons depolarize and generate an AP

• generates AP in ganglion cells

• sends impulse to brain

Cones in Humans 

• 3 cones with different opsin molecules 

• S- cones → blue 

• M Cones → Green

• L - Cones → Red 

• TRICHROMATIC VISION

• most animals have dichromatic vision

• sea turtles have extra cones so can see UV light aswell 

Color Blindness

• sex linked on the X-chromosome

• deficiency of one or more cones causes this

• most common is red-green

Nocturnal Vision - adapations

• only rods and no cones

• wide eyes - allow more light in

  • reduced eye movement (eyes are too big for their skull)

  • hence they have incredible neck rotation ability

• larger pupils - let more light in

• tapetum lucidium which reflects light back into the retina

• spherical lens + wide cornea to increase refraction