Physiology of the visual system Flashcards
Describe the steps involved in perception of light
1- The retina contains a sheet of light-responsive photoreceptor cells
2- Each cell has a receptive field that generates an electrical response when stimulated by light
3- Signal is transmitted across the retina and terminates in the cortex
What cells do the visual signal pass through on its way to the geniculate nucleus?
Photoreceptors
Bipolar cells
Ganglion cells
Does light perception cause excitation or inhibition of a membrane potential?
It inhibits membrane potential leading to hyperpolarization
What voltage is the membrane potential at rest in the dark?
-40mV
In the dark are sodium channels open or closed?
Open
- This allows influx of sodium and the maintained depolarisation
What is the mechanism to change membrane potential when light is perceived?
- Light stimulation of rhodopsin leads to g-protein activation
- G-protein activates cGMP phosphodiesterase
- PDE hydrolyses cGMP reducing its concentration and closes Na+ channel
Are rods or cones more numerous?
Rods
Where are cones most concentrated?
Fovea
What does convergence mean?
Convergence is the number of rods/cones sending their signal to a ganglion cell
- The lower the convergence the more sensitive the ganglion cell is to change in light
Is there higher convergence in the rod or cone system?
Rod system
- More rods per ganglion than cones
What are cones meant for?
Vision in daylight
- Responsible for colour vision
What are rods meant for?
Vision in dim light
What aspect of cones are responsible for colour vision?
Short-wave, Middle-wave and long-wave cones containing different opsins.
Is blue short or long wavelength?
Short
Is red short or long wavelength?
Long