Exam 3: VISION Flashcards
adequate stimulus for vision
light
what is the first neuron for vision?
- the retina
- contains the neurons that will turn the light stimulus into a neural signal
- The neurons that detect this light and turn it into an action potential send their axons via the optic nerve to the brain
- The spot at which the optic nerve leaves the eye ball has no retina, meaning that there is a blind spot in this region. Your vision therefore has a blind spot that you normally do not notice
Photoreceptors
- Photoreceptors are located in the inner most layer of the retina, but are the neuron that directly respond to light and depolarize in response
- There are two types: rods and cones. Rods are for low light stimuli and cones are bright light stimuli
- Photoreceptors project their axons to bipolar cells
Bipolar cells
depolarize or hyperpolarize in response to photoreceptors and communicate with ganglion cells
Ganglion Cells
activate in response to bipolar cells and send their axons to the brain via the optic nerve.
Horizontal Cells and Amacrine Cells
supports sensitivity to light contrast, over a wide range of light intensities
Cones vs. Rods
- The convergence for the rod system makes it poor for visual acuity whereas the cone system is excellent for that
- There are more cones in the fovea, which is the region of the retina this best for visual acuity
- Cones contain three different types of opsins, which allow for detection of short, medium and long wavelengths of light. Rods only have one photopigment.
key properties of photoreceptors:
- Light changes the membrane potential in a graded fashion, meaning you get graded potentials and not action potentials in photoreceptors. While action potentials are not generated, the graded potentials do cause neurotransmitter release from photoreceptors
- Photoreceptors are tonically active, meaning in the dark they release neurotransmitter (glutamate). When light shines on the photoreceptors, they hyperpolarize and decrease neurotransmitter release.
Phototransduction: Cone sequence
- cone
- bipolar cell
- ganglion cell
Phototransduction: cGMP
- The graded potentials are created by cyclic GMP (cGMP), a second messenger. In the dark there is a lot of cGMP, the function of which is to open ion channels that let in sodium and calcium. This is how the cell becomes depolarized
- When light strikes the photoreceptor, it decreases levels of cGMP, meaning the cation channels close and the cell does not depolarize
Phototransduction: potassium leak channels
There are potassium “leak” channels always open (in both light and dark). In the dark, the influx of calcium and sodium cations overpowers the potassium efflux. In the light, when the cation ion channel closes, potassium efflux now causes a hyperpolarization
Phototransduction: opsins
- mechanism by which light causes a reduction in cGMP levels
- There is a family of proteins on the membrane of photoreceptors called opsins.
- The opsin molecule responds to light by causing transducin to activate, which in turn cleaves cGMP. This prevents ion channels from opening and depolarizing the receptor.
Phototransduction: Rod sequence
- rod
- bipolar cell
- amacrine cell
- ganglion cell
what causes color blindness?
- Most so-called color-blind humans (actually color-deficient humans) have dichromatic vision and can distinguish short-wavelength stimuli (blue) from long-wavelength stimuli (not blue).
- Most color-blindness in humans is due to the absence of cones sensitive to medium-wavelength light (M cones).
- Because women have two X chromosomes, a defective gene encoding for the medium-wavelength pigment on the X chromosome is compensated for by the other normal gene X. This is why men are much more likely to be dichromats than women are
optic nerve
The axons of the retinal ganglion cells make up the optic nerve for each eye