1Explain how high density of cones in fovea allows detailed vision (3 marks)
High visual acuity
Each cone connected to 1 neurone
Cone sends separate impulses to brain
2Explain how high density of rod cells allows vision at night (3 marks)
High visual sensitivity
Several rods connected to 1 neurone
Enough neurotransmitters to reach threshold / spatial summation to reach threshold
3Describe how the regular contraction of the atria and ventricles is initiated and coordinated by the heart itself (8 marks)
SAN stimulated
Electrical impulse (across atria)
Atria contract
Fibrous material causes delay
AVN stimulated
Electrical impulse travels to Bundle of His
Purkyne fibres spread impulse across ventricles
Ventricles contract from base up
4Explain why it takes time for rod cells to recover their sensitivity to light after moving into darkness (2 marks)
Rhodopsin broken down by light
Time for resynthesis
5Describe how cone cells allow us to see different colours (2 marks)
3 types
Each pigment sensitive to different wavelength
6Explain how nervous control in a human can cause increased cardiac output during exercise (5 marks)
Coordination via medulla
Impulses along sympathetic nerve
To SAN
Release of noradrenaline
More impulses from SAN
7When heart beats, both ventricles contract at same time. Explain how this is coordinated in the heart after initiation of the heartbeat by the SAN (2 marks)
Electrical activity only through Bundle of His / AVN
Wave of electrical activity through both ventricles at the same time
8Describe how a pacinian corpuscle produces a GP when stimulated (3 marks)
Increased pressure changes stretch-mediated Na+ channels
Na+ ions flow in
Depolarisation
9Explain how a rise in BP results in decrease in the rate if heartbeat (6 marks)
Baroreceptors in aorta
Sends impulses to medulla
Impulses via parasympathetic nerves
To SAN
Inhibits SAN
Reduced stimulation of AVN
10Describe how a hearteat is initiated and coordinated (5 marks)
SAN sends wave of electrical activity across atria, causing atrial contraction
Non-conducting tissue prevents impulses reaching the ventricles
AVN delays impulse whilst blood leaves atria
AVN sends wave of electrical activity down Bundle of His
Causing ventricles to contract from base up