Special Senses: Physiology of the Eye

Special Senses

Physiology of the Eye

Yuni Susanti Pratiwi

Physiology Division Biomedical Sciences Department Faculty of Medicine Universitas Padjadjaran 2020

Eye : the vision

• For vision, the eyes capture the patterns of illumination in the environment as an “optical picture” on a layer of light-sensitive cells, the retina.

• The coded image on the retina is transmitted through the steps of visual processing until it is finally

consciously perceived as a visual likeness of the original image.

Protect the Injury

• Bony socket

• Eyelids and blinking

• Lacrimal gland-tears

• Eyelashes

The eye : fluid-filled sphere enclosed with three specialized tissue layers

• Outermost to innermost :

• Scleral cornea  visible white part

• Consist of transparent cornea

• Choroid/ciliary body/iris

• Highly pigmented choroid : blood vessels to retina

• Retina

• Consist of an outer pigmented layer and an inner nervous tissue layer

• Rod and cones  photoreceptors convert light energy into impulses

Two fluid filled cavities, separated by elliptical lens

Posterior :

Vitreous humor : helps maintain spherical shape of the eyeball

Anterior :

Aqueous humor carries nutrients for cornea and lens

Blood vessels would impede the passage of light of the

photoreceptors

Aqueous humor produce 5 mL/day by a capillary specialized anterior derivative of the choroid layer

Clinical Aspects

• What happened if there is failure of

aqueous humor drainage process?

Controlling amount of light

Iris : ^ringlike

structure within aqueous humor Pupil : round opening in the center of iris

Hey.. What about iris identification

stuffs?

Iris

• Smooth muscle

• Circular : contracts pupil getting smaller ----parasympathetic

• Radial : contracts  pupil getting bigger ---- sympathetic

• Iris muscle controlled by the autonomic nervous system

• Can you know someone is lying by observe their pupil?

The eye refracts entering light to focus the image on retina

• Lights :

electromagnetic radiation

composed individual

packets of energy called photons that travel in wavelike fashion

• Light have variable

wavelengths and intensity

Refraction

Process of refraction

Eye’s refractive structure

• Cornea

• Curvature

• Difference density between air and cornea

• Remain constants

• What happened with astigmatism?

• Lens  adjusted in accomodations

Eye’s refractive structure

Accomodation

• Increase lens strength for near vision

• The strength of the lens depends on its shape 

regulated by ciliary muscle (ciliary body have ciliary muscle and capillary network that produces aqueous humor)

• Ciliary muscle relaxed  flattened lens

• In normal eye  ciliary muscle is relaxed and lens flat for far vision. For near vision  muscle contract so lens become convex and stronger

• Ciliary muscle : sympathetic for relaxation and vice versa

Lacking DNA and protein-

synthesizing machinery, mature lens cells cannot regenerate or repair themselves. Cells in the center of thelens are in double jeopardy.

With advancing age, these non- renewable central cells die and become stiff. With loss of

elasticity, the lens can no longer assume

the spherical shape required to accommodate for near vision.

This age-related reduction in accommodative ability, presby- opia, affects most people by mid- dle age (45 to 50 years

Cataract : elastic fibers in lens become opaque so that the light cannot pass through

Light must pass through several retinal

layers before reaching the photoreceptors

Eye : focus light rays from the environment to the rod and cones (photoreceptor of the retina)  transform the light energy into electrical signal for transmission to the CNS

Self study : explain the embryological development of the retina so retinal layers is facing backwards!

3 layers of excitable layers

• outermost (closest to choroid) containing rod and cones  light sensitive ends face the choroid

• Middle layer : bipolar cells as associated interneurons

• Inner layer : ganglion cells. Axon of the ganglion join to form optic nerves

? What is blind spot?

• Lights must pass through the ganglion and bipolar

layers before reaching the photoreceptors in all areas of retina EXCEPT in the fovea

• Fovea : light strikes the photoreceptors directly

• Fovea : contains only cones (which have greater acuity and discriminative ability compared with rods)

• Fovea is the point of most distinctive vision

• Macula lutea have high concentration of cones and fairly high acuity

Phototransduction by retinal cells

converts light stimuli into neural signals

• Photoreceptors

• Outer segments. It detects the light stimulus

• Contain abundance of light-sensitive photopigments

• Photopigments activated by lights  generate action potentials in ganglion cells  transmit information to the brain

• Photopigment : opsin (integral protein in disc plasma membrane) and retinal (derivative vitamin A). Retinal is light absorbing part

• Inner segments. As metabolic machinery

• Synaptic terminal. Rate of neurotransmitter release

• Phototransduction : converting light stimuli into electrical signal

• Photoreceptors hyperpolarize in the light absorbtion

Photoreceptor Activity in the Light

• Light  cGMP decrease

• Reduction GMP  gated Na close  stops depolarizing Na  hyperpolaritation  closed Ca channel 

reduction of glutamate release

• Photoreceptors are inhibited by their adequate stimulus (hyperpolarized by light) and excited by the absence of stimulation (depolarized by darkness)

• The brighter the light is .. The greater the

hyperpolarizing response … the gretare reduction of glutamate response

Further Retinal Processing the Light Input

• Further retinal processing involves different influences of glutamate on two parallel pathways

• Each photoreceptors synapses with two side-by-side bipolar cells, one an on-center bipolar cell and the other an off-center bipolar cell

• These cells, in turn, terminate respectively on on

center-ganglion cells and off-center ganglion cells, whose axons collectively form the optic nerve for

transmission of signals to the brain

Rods

• Retina contains 20 times more rods than cones (120 millions rods compared to 6 million cones per eye)

• Cones are most abundant in the macula lutea. From this point outward the concentration of cones decreases and the concentration of rods increase

• Rods are most abundant in the periphery

Rods and Cones

• Rods have longer outer segment  contain more

photopigment  absorb light more readily

• Rods have higher sensitivity so they can respond to the dim

light of night

• Rods are for night vision, cones are for day vision

Rods and Cones

• Cones have wired to retinal neuron layers

 confer high acuity (sharpness, able to distinguish between two nearby points)

• Each cone has private line connecting it to particular gangluib cells

• Cones provide sharp vision with high resolution for fine detail during the day

• Rods have higher photopigment  more sensitive

• Cones have better detail vision, rods have better sensitivity

Rods and cones

• 4 different photopigment :

• One in the rods

• Three types of cones : red, green and blue

• Each photopigment has the same retinal but different opsin  absorbs different lengths of lights in visible spectrum

• Because the photopigment in the three type of cones each respond selectivity to a different light spectrum  brain can compare response making color vision

• Rhodopsin (one photopigment) only provide visions at night only in shades with different intensities, not

different colour

• The extent to which each of the cone types is

excited is coded and

transmitted in separate parallel pathways to the brain

• Color processing

pathways  primary visual cortex in the occipital lobe of the brain

• Color blindness

Adaptation

• Light Adaptation

• Rids broken down  no longer respond to light

• Dark Adaptation

• Photopigment that previously breakdown in light  gradually generated

? Night blindness

Visual Information is modified and

separated before reaching the visual cortex

• Visual field : the field of view that can be seen without moving the head

• The information that reaches the primary visual cortex in the occipital lobe is not a replica of the visual field

• 1. The image detected on the retina at the onset of

visual processing is upside down and backward because of bending of the light rays. Once it projected to the

brain, the inverted image is interpreted as being in its correct orientation

• 2. The information transmitted from the retina to the brain is not merely a point-to-point record of

photoreceptor activation.

• 3. Various aspects of visual information such as shape, color and motion are separated and projected in parallel pathways to different regions of the cortex. Only when these separate bits of processed information are

integrated by higher visual regions is a reassembled picture of the visual scene perceived

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Thalamus : Dorsal Lateral Geniculate Nucleus/LGN (Lateral Geniculate Body)

Function :

1. relays visual information

• visual information have high accuracy (point to point transmission, spatial fidelity)

• after passing the optic chiasma are derived from one eye and half from the other eye  representing corresponding points from two retinas

• The signals from two eyes are kept apart in the dorsal lateral geniculate nucleus

2. gate the transmission of signals to the visual cortex  control how much of the signal is allowed to pass the cortex

• inhibitory effects : corticofugal fibers and reticular areas of mesencephalon

• when stimulated  turn off transmission through selection portions of LGN

Layers I and II : Magnocellular Layers (large neurons)

• receive input from Y retinal ganglion cells

• rapidly conducting pathway to the visual cortex

• color blind system

• poor point to point transmission

Layers III through VI : Parvocellular layers

• receive input from type X retinal ganglion cells

• moderate velocity of conduction

• transmit color and accurate point to point spatial

transmission

The thalamus and visual cortex elaborate the visual message

• It separates information received from the eyes and relays it via fiber bundles known as optic radiation to different zones in the primary visual cortex located in the occipital lobes

• Each zone process different stimulus : form. Movement, color, depth

• This is no small job, bigger that all afferent fibers bring somatosensory input from all regions

Secondary Visual Areas of the Cortex : Visual Association Areas

 Secondary signals are

transmitted to these areas for analysis of the visual meanings

 Broadmann’s area 18 (V2)

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Vertical

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• visual cortex organizely

into several million columns (@30-50 micrometers, with more 1000 neurons)

• same columnar structure in all cerebral cortex also

other senses, motor and analytical cortical regions

• layer 1,2,3 : short distance layer, pass outward

• layer 4 and 6 : long

distance layer, pass inward

∙ What pathway determine what/who/define colours?

∙ What pathway determine where/when/movement contrast?

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Depth Perception

• Each eye views an object from a slightly different

vantage point  overlapping area seen by both eyes at the same tme  binocular  important for depth

perception

• The brain uses the slight disparity in the information received from two eye to estimate distance, three

dimensional objects in spatial-depth

• ?diplopia

Stereopsis

: ability of the visual system to

interpret disparity between two retinal images leading to perception of distance or depth (depth perception)

good stereopsis requires

good coordination between the eyes and good visual acuity in each eye

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Retinal Disparities

The gap in the left image differs from the gap in the right image.

Thank you