Unit 1 - Topic 4 - Communication and signalling - Section D - Nerve impulse transmission - Part II - Initiation of a nerve impulse in response to an environmental stimulus: the vertebrate eye 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 rods and cones
Rods function in dim light but do not allow colour perception. Cones are responsible for colour vision and only function in bright light.
In animals what forms the photoreceptors of the eye
The light-sensitive molecule retinal is combined with a membrane protein opsin
In rod cells what is the retinal-ospin complex called
Rhodopsin
How does rhodopsin become photoexcited
Retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin
What amplifies the signal
A cascade of proteins
What does photoexcited rhodopsin activate
A G-protein, called transducin, which activates the enzyme phosphodiesterase (PDE)
A single photoexcited rhodopsin activates how many G-proteins which activate how many PDE
A single photoexcited rhodopsin activates hundreds of molecules of G-protein. Each activated G-protein activates one molecule of PDE.
What does PDE catalyse
PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP)
What do PDE molecules break down
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 closure of ion channels in the membrane of the rod cells trigger
Nerve impulses in neurons in the retina
What does a very high degree of amplification result in
Rod cells being able to respond to low intensities of light
In cone cells what allows different photoreceptor proteins
Different forms of opsin combine with retinal to give different photoreceptor proteins, each with a maximal sensitivity to specific wavelengths: red, green, blue or UV
