Parts of the Eye (Use diagram)
Sclera: Tough, white, connective tissue layer. Protects structures within eye and maintains its shape
Choroid: Richly supplied with blood vessels. Inner part of choroid made up of cells that contain melanin (pigmented epithelium)
Retina: Contains receptor cells, rodes and cones. Axons of cells in retina leave via the optic nerve
Cornea: Thick transparent layer. Focuses light rays onto the retina
Iris: Circular tissue containing pigmented cells. Controls amount of light on the retina
Ciliary body: Contains ciliary muscles which help to control the shape of the lens
Lens: Made up of stacks of long narrow transplarent cells. It is biconvex and bout 4mm thick. Held in position by suspensory ligaments
Humour: Aqueous is "watery", and Vitreous is "glassy". Maintains the shape by exerting outwards pressure.
Focusing and Accommodation
- The most sensitive part of the retina is the fovea
- Light rays are parallel to eachother and are refracted inwards as they pass through the cornea, aqueous humour, lens and vitreous humour
- The function of the lens is to make small adjustments to the refraction of light rays
- Light rays from a distance are parallel whereas light rays which are close are diverging
- The closer the object the more it needs to be refracted (lens is thick and convex)
Focusing and Accommodation continued...
- Changing the shape of the lens is called accommodation- achieved by varying tension on suspensory ligaments. This change in tension is achieved by the ciliary muscles. The ciliary body is joined to the sclera which is constantly pushed out by pressure from the humours
- When ciliary muscles are relaxed the sclera pulls them out wide, thus pulling the suspensory ligaments also. The lens is now pulled outward, thus is thinner and less convex --> distance focusing
- Close objects-->ciliary muscles contract-->sclera pulls inwards-->suspensory ligaments relax and there is less tension, thus lens is thick and convex
- The adjustment of the lens is a reflex action
- Impulses from sensory cells in retina are carried to the brain in the optic nerve. Brain interprets impulses which are then sent to ciliary muscles which contract/relax.
- Vibrations in the ear are transmitted from the air, through the middle ear and through fluid in the cochlea. This causes vibrations in the stereocilia
- Hair cells maintain a resting potential across membranes using Na+/K+ pump, resulting in a -ve potential inside. When they vibrate they are depolarised. This causes them to release a neurotransmitter, causing depolarisation of nerve endings in the cochlea nerve. Generating APs.
- Brain can determine frequency of sound by detecting which neurones are conducting APs. Hair cells nearest oval window are receptive to high frequency & vice versa
- The loudness is determined by frequency of APs. Loud sounds-->greater vibrations-->faster rate of APs
- Our sense of balance depends on receptor cells in the semicircular canals, the utriculus and sacculus. The recpetor cells form synapses with sensory endings in the vestibular nerve. Nerve cells in the vestibular are currently firing APs to the brain. Changes in those impulses provide info. as to head position etc.
- In both the utriculus and sacculus there is a patch of cells called a macula, covered with a gelatinous layer containing tiny crystals of CaCO3 called otoliths.
- Each macula contains hair cells (receptor cells with stiff stereocilia). The ends of the cilia are embedded in the gelatinous layer, and they form synapses with neurones of the vesibular nerve.
- The otoliths are heavy and pulled down by gravity. The otoliths pull the hair cells in any given direction. Causes Na+ channel to open, depolarising the hair cells, causing changes in the pattern of APs.
- Hair cells in the utriculus--> horizontal plane --> when upright
- Hair cells in the sacculus--> vertical plane --> lying down
- Each of 3 semicircular canals are filled with a viscous fluid. Each have a swelling at one end called an ampulla. Inside the ampulla are hair cells with ends embedded in a gelatinous layer called a cupula. Cupulas do not contain CaCO3.
- When the head moves the inertia of the fluid causes it to get momentarily left behind. It collects in the ampulla, exerting a force on the cupula & bending it to one side.
- Movement of cupula pulls on the cilia, causing depolarisation in the hair cells, thus a change in the pattern of impulses carried by nerve cells in the vestibular nerve.
- Two different types of photoreceptor cells, Rods and Cones
- Other cells, special types of neurones, Bipolar cells and Ganglion cells
Structure of Rods and Cones
- Part of cell closest to outside of eye is called the outer segment. Vice versa with the inner segment
- In both there is invaginations in outer segments. In rod cells these invaginations result in stacks which lie freely in the cytoplasm. In a cone cell these invaginations resemble a comb.
- The discs provide a L.S.o.M to contain visual pigments. When light hits, APs are fired to the optic nerve
- Rod cell's pigment molecules contain a protein called opsin to which light absorbing compound retinal is attached. This whole molecule is called rhodopsin
- Cone cells have 3 types of pigment, in 3 types of cone cells. These include Pigments B,G and R
- Inner segment contains the nucleus and the mitochondria
The Retina continued...
Bipolar and Ganglion cells
- Bipolar cells have a cell body with two sets of processes
- They form synapses with either a single cone cell or many rod cells. Whereas the other process is long and synapses with a ganglion cell
How a rod cell responds to light
- When no light is falling on it a rod cell maintains a potential difference of -40mV across its membranes. It uses the sodium-potassium pump to do this, however in rod cells there are open channels that allow Na+ ions to pass through the plasma membrane in the outer segment and K+ channels that allow the K+ ions to pass through in the inner segment. Na+ into the outer, and K+ out of the inner
- However, light falling onto rhodopsin causes retinal to change shape from 11-cis-retinal to all-trans-retinal
- The retinal no longer fits onto opsin, thus the whole rhodopsin molecule changes shape
The Retina continued...
- Na+ and K+ channels close. Thus the potential difference increases to -70mV. The rod cell is hyperpolarised.
- Rhodopsin breaks down until 11-cis-retinal is formed again
- Dark adaptation: when in dim light and opsin and all-trans-retinal recombine
How cone cells respond to light
- Same as rod cells but cone cells require more light before pigment breaks down and hyperpolarisation occurs
- Each of the 3 pigments responds to a different wavelength of light, Pigment B = short (blue), Pigment G = Green, Pigment R = long (red)
- Brains compares intensity of signal from pigments to identify colour
Release of neurotransmitter
- Niether rod nor cone cells generate APs. Most depolarised when no light present thus release transmitter substance here, i.e. glutamate
The Retina continued...
The Roles of Bipolar and Ganglion cells
- Neurotransmitter diffuses across synaptic cleft and reaches a bipolar cell. It either depolarises or hyperpolarises it
- This transmitter then diffuses across a 2nd synaptic cleft and binds with receptors on the ganglion cell
- The change in frequency of the APs fired off from Ganglion cells depends on the amount of transmitter substance
- Visual acuity: the resolution of image perceived by the brain
- info. from rod cells is taken collectively, thus resulting in poor visual acuity as many large dots instead of crisp small dots
- info. from cone cells is seperate, thus resulting in a high resolution and clear image
The Brain and the Perception of images
- As they pass into the brain the nerve impulses pass along 3 different pathways. These convey messages about colour, shapes and movement and spacial relationships
- Interconnections between pathways = parallel processing
- The picture that we see in pour brain is the result of the integration of info. from these 3 different pathways
- Image provides us with the most useful info. about our environment
Effects of ageing on vision
- When the ciliary muscles contract, loosening the tension on the suspensory ligaments, the lens no longer springs back to shape. The lens remains thin, thus light rays are not refracted enough and remain unfocused
- Also the lens can become less transparent. Proteins that the cells in the contain may denature. If the proteins coagulate then the lens can become cloudy white, forming a cataract.
- Sometimes the lens may need to be completely removed