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
optic chiasm
About 60% of the axons from one eye cross over to the other hemisphere at the optic chiasm, the remainder 40% of axons stay on the side of the eye.
optic tract
Once the axons pass the optic chiasm, the optic nerve becomes the optic tract. This distinction is important because now the tract contains axons from both eyes.
There are multiple locations that retinal ganglion axons project to:
1) Lateral Geniculate Nucleus in the thalamus
2) From there, thalamic neurons project to the primary visual cortex (also called striate cortex)
3) suprachiasmatic nucleus
4) superior colliculus in the midbrain
superior colliculus responsibility
Finally, some retinal ganglion neurons also synapse onto midbrain neuron in the superior colliculus, where they are responsible for reflexive orientation movements of head and eyes
suprachiasmatic nucleus importance
This region is important for circadian rhythms, i.e. our internal clock that is dictated by the light/dark cycle
what is prosopagnosia?
The inability to recognize faces, sometimes including ones own face, is known as prosopagnosia
what causes prosopagnosia?
- There is an area in the temporal lobe called the fusiform gyrus that is crucial for pattern recognition. Visual information gets processed in the occipital lobe, but more complex pattern recognition (i.e. an object versus the face) occurs in this gyrus
- Damage to this era can leave individuals without the ability to recognize a face as a face—they see the face in its components parts and not as a “whole”
detecting a change in light
- In order to convey light information, there are different circuits of neurons in the retina that are “on center” or “off center”
- Based on which glutamate receptor is expressed on the bipolar cell, glutamate release (or lack of release with light) can have opposing effects
- mGluR binds glutamate and hyperpolarizes whereas AMPAR binds glutamate and depolarizes
- The purpose of this arrangement is so that changes in light information can be detected with great sensitivity—think of it like the nociceptors and their receptive fields
Central visual pathway
Photoreceptors to bipolar cells to Ganglion cells to LGN to primary visual cortex
Central visual pathway: LGN and PVS highly ordered
- The thalamus and cortex are layered, meaning that within each structure, there are distinct neurons forms a banding of layers
- In the LGN (A), retinal ganglion neurons synapse onto specific regions, with monocular ocular dominance columns being formed
- From the LGN, all of the inputs go to a specific layer in the primary visual cortex called layer 4. In this layer, the ocular dominance columns are binocular.
