Physiology: The Retina Flashcards
What is the duplex theory of vision?
Suggests that greater convergence in the rod system results in increased sensitivity (for light) but low visual acuity
Name two cells types which influence signal processing through lateral connections
Horizontal cells, amacrine cells
Describe the organisation of the cells in the retina
The cells in the retina are organized inside-out: the first cells to receive stimulation are the innermost photoreceptors, and the cells that exit the eye to carry information to the brain are the outermost cells
Describe the direct (vertical) pathway for signal transmission
Photoreceptors (rods and cones, first order neurones) → bipolar cells (second order neurons) → ganglion cells (third order neurons) → axons of ganglion cells merge to form the optic nerve
Name the two forms of photoreceptor
Rods and cones
How do amacrine cells influence signal processing?
Receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells
Describe the molecular mechanism of the ‘dark current’
Light activates the retinal of the visual pigment molecules (rhodopsin) which activates transducin and through a molecular cascade this decreases cGMP
cGMP-gated Na+ channels will close, so there will be greater efflux of K+ compared to influx of Na+ → positive charged ions leaving cell, which is hyperpolarising
What is the function of photoreceptors?
Converts electromagnetic radiation to neural signals (transduction)
Where are cones found?
Central retina (fovea)
Describe the structure of a photoreceptor
Outer segment → inner segment → cell bodies → synaptic terminals
What is the function of rods?
High light sensitivity so see in dim light, but have low visual acuity so are achromatic
What is the function of cones?
See in normal daylight as have low light sensitivity, but have high visual acuity so see colour
Describe the structure of phodopsin
Composed of retinal (Vitamin A derivative) + opsin (GPCR)
How do photoreceptors convert electromagnetic radiation to neural signals?
Photoreceptors are depolarised at rest and hyperpolarise with light exposure due to the ‘dark current’
What is the ‘dark current’?
cGMP-gated Na+ channel that is open in the dark and closes in the light
Which region of cones is responsible for the perception of colour?
Cones have different opsins (GPCR) for discrete wavelengths which allow perception of colour
How do horizontal cells influence signal processing?
Receive input from photoreceptors and project to other photoreceptors and bipolar cells
