1u- Initiation of a nerve impulse in response to an environmental stimulus Flashcards
What is the retina?
The area within the eye that detects light and contains two types of photoreceptor cells: rods and cones
What can be said about rod cells?
They function in dim light but do not allow colour perception
What can be said about cone cells?
They are responsible for colour vision and only function in bright light
In animals what two things are combined to form the photoreceptors of the eye?
- the light-sensitive molecule retinal
- a membrane protein, opsin
In rod cells what is the retinal-opsin complex called?
Rhodopsin
What happens when retinal absorbs a photon of light?
Rhodopsin changes conformation to photoexcited rhodopsin
What does photoexcited rhodopsin do?
Activates a G-protein, called transducin, which activates the enzyme phosphodiesterase (PDE)
How many molecules of G-protein does a single photoexcited rhodopsin activate?
Hundreds
What does PDE catalyse the hydrolysis of?
A molecule called cyclic GMP (cGMP)
How many cGMP molecules does each active PDE molecule break down?
Thousands per second
What does the reduction in cGMP concentration as a result of its hydrolysis affect the function of?
Ion channels in the membrane of rod cells
What does the hydrolysis of cGMP result in?
The closure of ion channels in the membrane of rod cells, which triggers nerve impulses in neurons in the retina
What results in rod cells being able to respond to low intensities of light?
A very high degree of amplification
In cone cells, different forms of opsin combine with retinal to give what?
Different photoreceptor proteins each with a maximal sensitivity to specific wavelengths: red, green, blue or UV
