Nerve Impulse Transmission- Initiation Of A Nerve Impulse In Response To An Environmental Stimulus: The Vertebrate Eye

Question 1

What is the retina?

Answer 1

It is an area within the eye that detects light and contains two types of photoreceptor cells

Question 2

What are the two type of photoreceptor cells within the retina?

Answer 2

Rods and cones

Question 3

How do rods function?

Answer 3

In dim light but do not allow colour perception

Question 4

What are cones responsible for?

Answer 4

Cones are responsible for colour vision and only function in bright light

Question 5

In animals the light-sensitive molecule what is retinal combined with to form the photoreceptors of the eye?

Answer 5

a membrane protein called opsin

Question 6

In the rod cells what is the retinal-opsin complex called?

Answer 6

rhodopsin

Question 6

What is the retinal response to light?

Answer 6

Retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin, which causes the following process involving a cascade of proteins to amplify the signal:
Photoexcited rhodopsin activates a G-protein called transducin. A single photoexcited rhodopsin activates hundreds of molecules of G-protein. Transducin activates the enzyme phosphodieesterase (PDE). Each activated G-protein activates one molecule of PDE. PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP). Each active PDE molecule breakes down thousands of cGMP molecules per second. The reduction in cGMP concentration as a result of its hydrolysis affects the function of ion channels in the membrane of rod cells. There is a closure of ion channels in the membrane of the rod cells causing the inward leakage of positive Na+ ions is halted (so the membrane potential increases) causing hyperpolarisation. Hyperpolarisation triggers nerve impulses in neurons in the retina.

Question 7

What does the high degree of amplification of the retinal response to light result in?

Answer 7

The rod cells being able to respond to low intensities of light

Question 8

In cone cells what do the different forms of opsin combine with?

Answer 8

With retinal to give different photoreceptor proteins, each with a maximal sensitivity to specific wavelengths: red, green, blue or UV