1.4e 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 are the two photoreceptor cells found in the retina?
rods and cones
What is the function of rods?
they function in dim light but do not allow colour perception
What is the function of cones?
They are responsible for colour vision and only function in bright light
How are photoreceptor cells of the eye formed in animals?
the light sensitive molecule retinal is combined with a membrane protein, opsin
In rod cells what is the retinal-opsin complex called?
rhodopsin
What is the function of retinal?
absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin
What amplifies the signal
a cascade of proteins
What is the role of photoexcited rhodopsin?
- Activates a G-proteins called transducin which activates the enzyme phosphodiesterase (PDE)
- One photo excited rhodopsin activates hundreds of molecules of G-protein.
- Each activated G-protein activates one molecule of PDE
What does PDE do?
- It catalyses the hydrolysis of a molecule called cyclic GMP (cGMP)
- Each active PDE molecule breaks 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
What does the reduction in cGMP concentration result in?
- The closure of ion channels in the membrane of rod cells, which triggers nerve impulses in neurone in the retina
What does a very high degree of amplification result in?
rod cells being able to respond to low intensities of light
What happens in cone cells?
- Different forms of opsin combine with retinal to give different photoreceptor proteins
- Each with maximal sensitivity to specific wavelengths: red, green, blue or UV
