The Eye & Muscle

• What is the mammalian eye?

  The mammalian eye is a sense organ, but only a small section contains the photoreceptors that are sensitive to light, the rest of the eye contains structures that ensure the receptors in the retia receive focused light rays at the correct intensity to form an image.
• Summarise the description + role of the main parts of the eye:

  => Conjunctiva

  => Sclera

  => Cornea

  => Aqueous humour

  => Iris

  => Pupil

  => Ciliary body

  => Suspensory ligaments

  => Lens

  => Vitreous humour

  => Retina

  => Fovea

  => Choroid

  => Optic nerve

  => Blind spot

  
• What is accommodation + what type of response is it?

  - Another important function of the eye is to have the ability to change the shape of the lens in order to focus on both near and distant objects.

  
  

  - This is an automatic response - a reflex.

  
• What components of the eye does accommodation involve? (3)

  - Ciliary muscles (a ring of muscle surrounding the lens).

  
  

  - Suspensory ligaments (thread like structures between the lens and the ciliary muscles).

  
  

  - The lens (essentially a flexible fluid filled sac).
• How are ligaments different to muscle?

  Ligaments are not like muscles - they do not stretch, but becomes either taut or slack.
• How is a sharp image formed?

  To form a sharp image light rays have to refracted (bent) to a single point on the retina.
• Where does most refraction occur?

  Most refraction happens at the cornea and the lens completes the refraction by bending the rays by an appropriate amount depending on the distance the object being viewed is away from the eye, to ensure the light rays are focused on the retina.
• Explain what happens when focusing on a distant object. (4)

  - The ciliary muscles relax

  
  

  - Tension in the wall of the eye ball is transferred to the suspensory ligaments pulling them taut

  
  

  - The lens is pulled into a thinner shape

  
  

  - The refractive power of the lens is reduced
• Explain what happens when focusing on a nearby object. (4)

  - The ciliary muscles contract

  
  

  - Tension from the eyeball is not transferred to the suspensory ligaments so they slacken

  
  

  - Less pressure is exerted on the lens so it bulges becoming thicker

  
  

  - The refractive power of the lens is increased

  
• Why can the size of the pupil be altered + why? (2)

  - Bright light can damage the light sensitive cells of the retina and the eye can have difficulty forming an image in very low light intensities.

  
  

  - For this reason the size of the pupil can be altered through the action of the muscles in the iris.
• What is the iris composed of?

  The iris is composed of both radial and circular muscle fibres which act antagonistically to control the size of the pupil.
• What happens to the eye when exposed to bright light? (3)

  - The circular muscles contract

  
  

  - The radial muscles relax

  
  

  - The pupil is constricted and the amount of light entering the eye is reduced (thereby preventing any damage to the light sensitive cells of the retina)

  
• What happens to the eye when exposed to dim light? (3)

  - Circular muscles relax

  
  

  - Radial muscles contract

  
  

  - The pupil dilates allowing more light to enter the eye ensuring enough light reaches the light sensitive cells to form an image

  
• What is the retina made up of?

  The retina is made up of a number of cell types including the photoreceptor light sensitive cells (rods and cones) as well as bipolar cells and ganglion cells.
• Give the structure, light sensitive pigment, number of types, sensitivity, visual acuity + notes on vision provided for rod cells + cone cells.

  
• What is the importance of mitochondria when considering the eye?

  The mitochondria is important in producing the ATP needed for the re-synthesis of rhodopsin from retinal and opsin following light stimulation.

  
• What happens to rod cells in low light intensities?

  As rods have high sensitivity, the rhodopsin breaks down readily at low light intensities, requiring only a small amount of energy - but this can lead to dark adaptation.
• What happens to rod cells in bright light?

  In bright light, virtually all the rhodopsin is broken down and it takes time for it to be resynthesized, so if we move from a high light intensity to a low light intensity, our vision is initially poor, but gradually will improve.
• What can you say about cone cells in varying light intensities?

  The same principle applies to cone cells, however, the pigment iodopsin is present in cones, and is less readily broken down, only producing a generator potential in bright light.
• The arrangement of the cells in the retina is represented here.

  
• What does light have to pass through before reaching the membrane discs + why is this permitted? (2)

  - As can be seen in the diagram for light to reach the

  membrane discs containing the light sensitive pigments,

  it has to pass through the ganglion cells, bipolar cells

  and the bulk of the rod and cone cell.

  
  

  - This is permitted due to the transparent nature of these cells but it does raise some interesting questions relating to evolutionary biology.

  
• Give an example of a generator potential.


  - When light reaches the photo sensitive pigments some changes occur leading to visual transduction (the

  process of converting light energy into an action potential occurring).

  
  

  - This is called a generator potential.
• What is a generator potential?

  A generator potential is the degree of depolarisation a stimulated receptor can produce.
• In what circumstances would an action potential be produced?

  Only if the generator potential reaches threshold level will it produce an action potential in the neurone.
• What happens when light strikes rhodopsin? (3)

  - When light strikes rhodopsin it causes it to break down into retinal and opsin (a process known as bleaching).

  
  

  - This change results in a change in membrane potential which can be passed on to the neighbouring bipolar cell.

  
  

  - This bipolar cell will then release an amount of neurotransmitter into the synaptic cleft between it and the ganglion cell of the optic nerve.
• What are the two factors that account for the high sensitivity of rod cells?

  The intensity of light needed for this to occur is quite low but another factor that results in the high sensitivity of rods is that many rods can synapse onto 1 bipolar cell - or they converge onto 1 bipolar cell - hence this is known as "retinal convergence".
• Explain the concept of summation. (2)


  - Essentially this means that a small amount of neurotransmitter released from a few rod cells can be added together, in order to open enough ion channels in the bipolar cell to form an action potential.

  
  

  - This is known as summation and also explains why rods provide decreased visual acuity - the image formed in the brain is as a result of a patch of cells being stimulated rather than a single cell.

  
• What happens to rhodopsin after use?

  Before it can be reused, rhodopsin has to be resynthesized after bleaching.
• What does the process of synthesisation of rhodopsin require?

  This process requires energy in the form of ATP (released by the mitochondria found in the inner segment of the rod cell).
• What happens to rhodopsin in light conditions?

  In light conditions the rhodopsin is almost entirely broken down (cones are being used for vision in bright light) - the eye is said to be light adapted.
• Why is the eye said to be dark adapted + what does it explain? (2)

  - Upon entering an area of darkness it takes around 30 minutes for the rhodopsin to reform to allow rods to become sensitive. The eye is said to be dark adapted.

  
  

  - This explains why you can see very little when you initially enter a dark room but as time progresses you begin to see objects more clearly.
• How does the process of visual transduction in cone cells differ from that in rod cells?

  The process of visual transduction in cone cells is very similar, however, these cells do not display retinal convergence and therefore require higher light intensities in order to release sufficient amounts of neurotransmitter.
• What does the lack of retinal convergence in cone cells provide?

  This also explains why they provide high visual acuity - each bipolar cell synapses to 1 cone and therefore the brain can tell exactly which part of the retina.
• What type of vision do cones provide?

  Cones provide colour vision.
• What type of vision do rods provide?

  Rods provide monochromatic vision..
• Explain how cone cells provide colour vision. (4)

  - This is because iodopsin exists in three different forms with each form being sensitive to different wavelengths of light.

  
  

  - The absorption peaks of the 3 types of cone, correspond to blue, green & red - the trichromatic theory of colour vision.

  
  

  - Pure blue light will only break down "blue iodopsin" and only the blue cones will fire impulses to the brain, which is interpreted by the brain as blue.

  
  

  - But as light contains a mixture of blue, green and red light, it is the degree of stimulation of each type of cones that determines the colour that we perceive.
• Give an example of how colour vision works with yellow light.

  Yellow light will break down some of the red iodopsin and some of the green iodopsin so both red + green cones will fire impulses to the brain, and this is interpreted by the brain as yellow.

  
• What must be remembered when referencing reaching a threshold potential?

  Remember it is not the impulses that are added together, it is the additive effect of the generator potentials so enough NT is released to reach threshold for an impulse and AP to be propagated.
• Where are the cones + rods situated in the eye?

  The rods and cones form a layer immediately inside the choroid.
• What does the choroid contain?

  The choroid contains a pigment that prevents internal reflection of light within the eye.