Structure and function of the eye and retina - need to watch ninja nerd

Describe the layers of the retina

1. ganglion cell layer

2. inner plexiform layer

3. inner nuclear layer

4. outer plexiform layer

5. photoreceptor layer

Describe the pupil

hole in the center of the iris

- allows light to enter the eye and reach the retina

- dark due to light-absorbing pigments in the retina

Describe the iris

- surrounds the pupil and is pigmented part of the eye

- sphincter muscles cause constriction and dilation

- controls pupillary light reflex

- post to cornea and anterior to lens

Which liquid surrounds iris

aqueous humour

When does sphincter muscle contract

In bright light

What is the cornea?

the transparent layer forming the front of the eye

- covers pupil and iris

- no blood vessels

- very sensitive

What is the sclera of the eye?

white of the eye

continuous with the cornea

(outermost fibrous layer)

What is the conjunctiva?

a thin, clear membrane that protects your eye. It covers the inside of your eyelid and the white of your eye (the sclera). The conjunctiva creates the mucus layer that forms part of your tears.

What is the optic nerve?

Carries axons from the retina, exits back of eye, passes through optic formamen and reaches base of brain near the pituitary gland where is dessicates at the optic chiasma

- ends at primary visual cortex in the occipital lobe

How does the retina work

- photoreceptors turn light into electrical signals

- These electrical signals travel from the retina through the optic nerve to the brain. Then the brain turns the signals into the images you see.

What is the macula

area of retina with high cone density

- yellow spot on retina

- surrounds the fovea

What is the fovea

the fovea is the region of the retina with highest density of cones

- small indentation at the centre of the macula

What are rod cells

Cells in the retina that are sensitive to low light intensity (dim light)

What are cone cells

Cells found in the retina that are sensitive to high light intensity (bright light) and can detect different colours.

What is the lens

the transparent structure behind the pupil that changes shape to help focus images on the retina

- thin transparent capsule

- refracts incoming light

- cataract is when lens becomes cloudy

What is the choroid

Middle layer of eye

- between retina and sclera

- contains a pigment that absorbs excess light

- prevents blurring of vision

What is the ciliary body

the part of the eye that connects the choroid to the iris.

What is in the ganglion cell layer

cell bodies of ganglion cells

- the ganglion bundle together to form the optic nerve

- where output neurons of the retina are found

What is in the photoreceptor layer

rod and cones

input layer to the retina

makes connections with neurons in the inner nuclear layer

Describe how information is transmitted to the brain

- photoreceptors translate light into a biological signal

- inner nuclear layer extracts visual information

- retinal ganglion cells form the optic nerve and transmit that signal to the brain

Describe retinal vasculature

supplies inner retina

can be disrupted in glaucoma

describe choroidal vasculature

supplies photoreceptors

disrupted by retinal detachment

What is photopigment found in rods

rhodopsin

found in rods which absorbs light and forms a biological signal.

- retinaldehyde absorbs light - from vitamin A

- exists in different isoforms

- all trans and 11cis retinal are important

Describe the different isoforms of retinal

- vitamin A is all trans retinol

- in the dark it is 11-cis retinal

- upon light exposure it becomes all-trans retinal

What are plexiform layers

Layers between cells

Describe photoreceptors

detect light

- At the top of the photoreceptor, there are many discs and invaginations which increase the SA and amount of plasma membrane for that cell - more rhodopsin can be stored and more light can be absorbed.

What is a chromophore?

pigment which asorbs light

Describe opsin protein

GPCR

7 transmembrane

- amplifies isomerisation of retinal into a signal

- determines the wavelengths that retinal absorbs

How is 11cis retinal an inverse agonist

in the dark is keeps opsin in an inactive state and stops signalling

Describe the phototransduction cascade

1. photon absorption

2. Gprotein (transducin) dissociation

3. Conversion of cGMP --> GMP

4. Closes cGMP gated NA+ channels

5. Less Na+ influx into cell

6. Cell is hyperpolarised and turns off

7. Bipolar cells turn on

- each individual step can be amplified

-The transduction process is the same in rods and cones

- Photoreceptors DO NOT generate actional potentials, they generate receptor potentials instead.

When are photoreceptors depolarised and hyperpolarised

They are depolarised in the dark and hyperpolarised in the light

- more neurotransmitter is released in the dark

- Photoreceptors respond to light exposure with graded hyperpolarisation.

- This results in a reduction in glutamate release at their synaptic terminals.

What is the neurotransmitter of photoreceptors

glutamate

- released in the dark

differences in rods and cones

- rods are more sensitive than cones and have larger signal amplification

- rods are bigger

- cones adjust sensitivity (used during daytime)

- fovea is all cones

How many opsin cone genes do we have

3 cone genes

- red = 564 nm

- green = 533 nm

- blue = 433 nm

Summarise photoreceptors

Absorb light using 11-cis retinaldehyde (retinal) bound to opsin protein

Photoisomerisation to all-trans retinaldehyde induces structural change in opsin

Activates G-protein signaling cascade

Hyperpolarise and reduce release of glutamate

Rods and cones differ in distribution across retina, opsins expressed, morphology and sensitivity to light

How is a signal conveyed from cones to retinal ganglion cells

Via bipolar cells

What are the 2 types of retinal ganglion cells

on cells are excited by light and fire APs

off cells are inhibited by light and excited by darkness

Describe on and off bipolar cells

On - depolarised by flash

Off - hyperpolarised by flash

this is due to synapse between bipolar cells and RGCs

How does an on bipolar cell transmit signals

The on cell has a sign inverting synapse

- hyperpolarisation of the photoreceptors depolarises an on bipolar cell

- this causes depolarisation of the ganglion cell

- this causes firing of action potentials

Which receptors are in sign conserving synapses (off cells)

Ionotropic glu receptors

- cation channels opened by glu

Which receptors are in sign inverting synapses (on cells)

Metabotropic glu receptors

- glu activates signalling cascade closing ion channels

What are horizontal cells

- link cones horizontally in a region of the retina

- inputs from local cones are sign conserving and hyperpolarised by light

- outputs which are sign inverting antagonise the light response

What are horizontal cells?

-cells that extend across the outer portion of the retina at the level of the synapses between photoreceptors and bipolar cells

-can facilitate or inhibit communication between photoreceptors and ganglion cells, thereby altering the sensitivity of the retina

- horizontal cells are much more responsive when there is light in the middle

What are amacrine cells

- Where bipolar cells synapse with ganglion cells

- inhibitory link causes response modulation at the inner plexiform layer (bipolar to ganglion cell synapse)

What causes modulation at outer plexiform layer

Horizontal cells

modulates info from photoreceptor to bipolar cell synapse

Overview of flow of information in the retina

How does the optic nerve send signals to the brain

sends multiple simplified versions of the same scene

Where are photoreceptors located

neural layer of the retina (at the back)

name the layers of the retina from posterior to anterior

1. Pigment epithelium

2. Photoreceptors (rods and cones)

3. Outer limiting membrane

4. Outer nuclear layer

5. Outer plexiform layer

6. Inner nuclear layer

7. Inner plexiform layer

8. Ganglion cell layer

9. Nerve fibre layer

10. Inner limiting membrane

Which layer of the retina absorbs the light

Pigmented layer

Function of outer limiting membrane

Provides mechanical support to retina - helps to maintain structure

What does outer nuclear layer consist of

cell bodies and nuclei of photoreceptor cells

What does outer plexiform layer consist of

Axons of photoreceptors synapse with bipolar cells

What is the inner nuclear layer composed of

Cell bodies of inter neurons which connect outer and inner plexiform layer

(bipolar cells)

What happens in inner plexiform layer

Axons of interneurons synapse with dendrites of ganglion cells

What happens in ganglion cell layer

Ganglion cell bodies reside

What is in the nerve fibre layer

Axons of ganglion cells are located

- axons travel to posterior centre of retina

- exit eye through optic disc

- form optic nerve (CN II)

- carries visual info to the brain

What is the purpose of internal limiting membrane

Separates retina from vitreous humour of eyeball

What are rods and cones

photoreceptors

- modified neurons

- hyperpolarise upon light stimulus to deliver impulse to brain

Structure of photoreceptors

3 segments

- Synaptic terminal: connects to interneurons such as bipolar cells

- Inner segment: cell body

- Outer segment: Detects light

Which vision does rhodopsin allow

black and light

What is the area of the eye with highest visual acuity

fovea

- high density of cones

Which vision does photopsin allow

- colour

- bright light vision

- photopsin is very sensitive to light

What are the different types of cones

S cones - blue

M cones - green

L cones - red

Why are rods considered low acuity

Multiple rods converge onto one ganglion cell

- makes the image fuzzy and low res

Why are cones high acuity

each cone connects to its own ganglion cell

- high acuity

Which cells have rhodopsin and which have photopsin

Rhodopsin = rods

Photopsin = cones

Both proteins are

- G-protein coupled receptors

Which layer of retina contains rod cell bodies

Outer nuclear layer

Which layer of retina contains bipolar cells

inner nuclear layer

Red (L) cones are sensitive to which wavelengths

Sensitive to long-wave light

Green (M) cones are sensitive to which wavelengths

medium-wave light

Blue (S) cones are sensitive to which wavelengths

Short wave light

Function of pigment epithelium

adjacent to the choroid, absorbs light to reduce back reflection of light onto the retina

What is in the outer plexiform layer

synapses between axons of photoreceptors and dendrites of intermediate neurons

What is in inner nuclear layer

Cell bodies of bipolar, horizontal, and amacrine cells and muller cells

What are muller cells

Müller cells

- glial cell in retina

- homeostatic and metabolic support of retinal neurons

What is in ganglion cell layer

cell bodies of ganglions

What is in optic nerve fibre layer

axons of ganglion cells

Describe fibrous layer of eye

cornea and sclera

avascular

Describe uvea/middle/vascular layer

iris, ciliary body, choroid

- drained by sup/inf ophthalmic veins into cav sinus

2 layers of retina

- pigmented layer (outer)

- neural layer

Visual pathway steps

SOOLOP

Second cranial nerve

Optic chiasm

Optic tract

LGN (thalamus)

Optic radiation

Primary visual cortex

Which nuclei controls eye reflexes

superior colliculi in brainstem

Is contraction sympathetic or parasymoathetic

- Contraction upon exposure to light - parasympathetic

- Dilation in dark - sympathetic

Describe pupillary light reflex

- fibres synapse in superior colliculi of brainstem

- fibres travel from nasal and temporal sides

- reach pretectal nucelus of superior colliculus and decussate

- travel to EDW nucleus

- PNS fibres arise from EDW nucleus to ciliary ganglion

- short ciliary nerve arises and causes constriction via CN3

What is functional model of disability

caused by deficits and limits ability to do functional activity

Social model

disability is caused by barriers of social environment

Medical model disability

Disability caused by health conditon