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Summary CCEA GCE Biology - Chapter 5 - The Eye and Muscle Mindmap
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Based on the CCEA A2 textbook, Chapter 5 Mindmap
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NEAR VISIONDISTANT VISION
• Outer part of the cell has many layers of
membrane containing rhodopsin – made of
protein opsin combined with light absorbing
• Rods and cones form a layer in the retina with the compound retinal (from vitamin A)
outer segment containing the pigment closest to the • Rhodopsin breaks down into retinal and
choroid opsin when stimulated by light - Large pupil
• Layer of bipolar sensory cells connect to • Changes the membrane potential of the cell - Small pupil diameter allows as
diameter allows much light as Ciliary muscle contracts
photoreceptors to a layer of ganglion sensory cells and creates a generator potential to form a smaller circle
• Axons of the ganglion cels group together to form the • When a threshold is reached adjacent sufficient light to possible to enter to
enter but protects ensure there is with a smaller diameter
optic nerve that carries impulses to the brain linking neurone (bipolar neurone) is The ciliary muscle relaxes – suspensory ligaments
• Each cone cell synapses with an individual bipolar depolarised and conducts an action potential the light sensitive sufficient light to
to for a larger circle and are not pulled taut so
neurone – giving it discrete link to the brain to form • Mitochondria on the inner segment produce cells from being stimulate the Conjunctiva
pulls the suspensory relax so that there is
an image – giving precise high resolution – enabling ATP needed to resynthesise rhodopsin from damaged photoreceptors. • Thin transparent membrane covering the cornea
ligaments taut – less pressure on the lens
brain to distinguish bet ween t wo points that are very retinal and opsin - Circular muscles - Radial muscles • Protects the cornea from damage
ligaments then pull the and it springs back to its
close together – high visual acuity • Have high sensitivity in low light intensities contract and contract and Sclera
lens thinner so that it normal thick shape,
• A number of rods may synapse with a common bipolar as only requires a small amount of light to radial muscles circular muscles • Tough opaque connective tissue covering the eye – replaced by transparent cornea at front
bends light less. bending the light more
neurone – and a number of these with ganglion cells break down readily – can lead to dark relax, making the relax, making the • Protects against damage: site of attachment of eye muscles
-retinal convergence – allows generator potentials adaption pupil smaller pupil larger. Cornea
from individual rods to combine together (summation) • Bright light virtually all rhodopsin is broken Distant vision Near vision • Front transparent part of sclera
and reach. Threshold required to produce an action down and takes time to be resynthesised High light intensity Low light intensity • Transparent and most refraction (bending) of light occurs here
potential in a bipolar neurone • They provide monochromatic vision – cant – pupil constricted – pupil dilated Aqueous humour
• Transparent watery fluid bet ween cornea and lens
• If the light energy reaching any one rod is insufficient
to stimulate the bipolar neurone but a group of rods
detect the colour of light
- Accommodation – obtaining a focused image. Most light • Maintains the shape of the front pat of the eye
can provide enough generator potential to produce an Rod cells Controlling the light entering the eye is refracted by the cornea – adjustments of the lens Iris
impulse in the bipolar neurone – together with the thickness further bends light rays, irrespective of their • Muscular layer with both circular and radial muscles: contains pigment that absorbs light
ability of rhodopsin to break down more easily than Contains light sensitive cells and the angle that they enter the eye. • Adjusts the size of the pupil to control the amount of light entering the eye
iodopsin is the basis for the sensitivity shown by rod neurones with which they synapse – rod Pupil
cells and cone cells are photoreceptors – light • Gap within the iris
• Limitation of this is the lack of visual acuity or high Arrangement of energy changes the level of polarisation of The retina • The area through which light reaches the lens and enters the centre of the eye
resolution as the rods in each convergence unit only rods and cones in the cell membrane converting the light The Eye and Muscle Specialised effectors that contract Ciliary body
to cause movement, 3 types: • Contains a muscular ring of (ciliary) muscle around the eye: suspensory ligaments extend
provides as much detail as one cone cell the retina energy to a ner vous impulse in the
• Most cones found in the centre of the retina – fovea associated neurones – they are transducers from the ciliary body and hold the lens in place
important in colour vision – no rods in the fovea. • Cardiac – found • Adjusts the shape of the lens to focus light rays
• There are no rods or cones at the blind spot – where Cone cells only in heart tissue Suspensory ligaments
the optic nerve leaves the back of the eye – light • Smooth – found in • Ligaments that connect the ciliary body to the lens
falling on this area will not be detected • Outer segment containing layers of membrane is one shaped and involuntary muscle • Transfers tension in the wall of the eyeball to make the lens thinner: important when
• More rods at the side of the retinal closest to the contains the pigment iodopsin tissue e.g.stomach, focusing on distant objects
centre of the head - facilitate tens peripheral vision • Less readily broken down than rhodopsin and only produces a artery walls Lens
generator potential in bright light • Skeletal – found in • Transparent biconcave structure with refractive properties
• Provide colour vision – 3 different forms each sensitive to a voluntary muscle • Refracts light and focuses light rays on the retina
different wavelength of light tissue e.g. bicep Vitreous humour
• Only contains one type of pigment with absorption peaks • Transparent, jelly-like material bet ween the lens and the back of the eye
corresponding to the colours blue, green and red – TRICHROMATIC • Maintains the shape of the rear part of the eye and supports the lens
THEORY OF COLOUR VISION • Skeletal (voluntary, striated) muscle Retina
• Degree of stimulation of each type of cone determines colour – attached to the skeleton by • Inner layer of the eyeball containing the light sensitive receptor cells (rods and cones)
produced tendons and is under conscious • When stimulated the rods and cones initiate impulses in associated neurones
control Fovea
• Composed of muscle multinucleate • Region in the centre of the retina that is particularly rich in cones and does to contain
fibres, with nuclei arranged just rods
Changes in sarcomere during muscle contraction
below the cell membrane, called the • Part of the eye that gives the clearest daylight colour vision
Muscle contraction Choroid
sarcolemma
Ultrastructure of myofibrils • They contain all the usual organelles • A layer of pigmented cells bet ween the retina and the sclera
and are particularly rich in • Contains blood vessels that supply the retina: prevents reflection of light back through the
• Movement of actin filaments across the myosin filaments causes the • Inside the muscle fibre are highly specialised mitochondria eye
following changes: contractile units called myofibrils, which run • Sarcolemma folds deeply at intervals Optic nerve
- sarcomere shortens, Z lines become closer together, so the I bands become parallel to each other to form transverse or T-tubules • Bundle of sensory nerve fibres that leave the retina
shorter, H zone becomes shorter, however the A band does not change in length • Myofibrils are composed of 2 types of proteins: • Fibres also contain a net work of • Transmits impulses from the retina to the brain
• The strength of the contraction depends on: thick myosin filaments and thin actin filaments tubules called the sarcoplasmic Blind spot
- how long the muscle is stimulated, how many muscle fibres are stimulated and Muscle fibre -> myofibrils -> myofilaments (actin reticulum • Part of the retina where the sensory neurones that unite to form the optic nerve leave
contracting and myosin) the eye
• Myosin filaments lie in the centre of the • Contains no light sensitive cells so is not sensitive to light
1. An action potential arrives at the neuromuscular junction causing the contractile unit – and are stacked parallel to each other and linked by thin discs called the M
release of neurotransmitter substance that crosses the synaptic cleft and line.
depolarises the post synaptic membrane (sarcolemma). The AP travels • Actin filaments slot bet ween edges of the myosin filaments and are linked by a tin disc called
through the extensive T tubule system • When a muscle contrasts the actin filaments are
the Z line (section bet ween 2 Z lines is the basic contractile unit and is called a sacromere)
2. The AP causes calcium ion channels in the sarcoplasmic reticulum to open pulled over the myosin filaments (towards the m- • Two eyes create a single image which provides accurate judgement of distance and the
• A-band – (anisotropic band) contains the areas where actin penetrates bet ween the myosin
3. Ca2+ stored in the sarcoplasmic reticulum diffuse into the sarcoplasm down line), reducing the length of each sarcomere and ability to form three-dimensional images
filaments
their conc gradient therefore the whole muscle – THE SLIDING • In predatory animals, humans and primates eyes are positioned on the front of the head
• I-band – (isotropic band) contains only actin
4. Ca 2+ cause the ancillary protein tropomyosin to move, freeing the actin FILAMENT MECHANISM – the degree of overlap of vision bet ween t wo eyes facilitates excellent judgement of
• H-zone – found in the centre of the sarcomere and contains only myosin
binding sites for the myosin heads to attach, forming actomyosin bridges • It’s possible due to the bulbous heads the extend distance and 3D vision
5. When attached the myosin heads change shape , rotating back to an angle of from the myosin filaments, which are able to • Many prey species have their eye on the site of their heads – provides wider field of
45OC. This pulls the actin filaments over the adjacent myosin filaments attach to complementary receptor sites on the view allowing prey to detect predators more readily
6. ATP attaches to each myosin head & is hydrolysed, releasing energy which actin filaments
causes the myosin head to detach from the stationary actin binding sit and • At rest the actin receptor sites are blocked by an
return to its original position accessory protein – tropomyosin, which
7. The detached myosin heads repeat the process, attachment, rotation, prevents the myosin heads from binding
release about 5 times per second
8. The cycle will continue as long as the muscle receives nervous stimulation and
Ca2+ is present
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