Chapter 7 Vision Flashcards
What are the functions of the following eye structures?
- Retina
- Cornea
- Lens
- Ciliary Muscles
- Optic Nerve
- Extra-Ocular Muscles
Retina: the layer of neurons at the back of the eye
Cornea and lens: focuses and refracts light to form an image on the retina
Ciliary muscles: adjust and focus light by changing the shape of the lens.
Optic Nerve: sends visual information to the brain
Extra-Ocular Muscles: three pairs of muscles that extend from the outside of the eyeball to the bony eye structure.
What is accommodation in the visual system?
Accommodation is the process by which ciliary muscles adjust the lens to bring nearby objects into focus.
What are two common vision problems?
Farsightedness: unable to focus clearly on near items due to an in-elasticity of the lens.
Myopia: unable to focus clearly on distant items because the eyeball is too long so the light is focused before it reaches the retina.
What cells make up the retina?
Photoreceptors: sensory neurons that detect light
Bipolar cells: receive information from photoreceptors and pass them on to ganglion cells
Ganglion cells: their axons form the optic nerve which carries visual information to the brain.
Horizontal cells: specialized cells that connect to both photoreceptors and bipolar cells. They facilitate communication between cells in the same layer.
Amacrine Cells: specialized ells that connect both ganglion and bipolar cells. Important for inhibitory interactions with the retina.
What are the two types of photoreceptors and what systems do they correspond to?
Rod Cells: correspond with the scotopic system. They are very light sensitive, they operate well in dim light but are insensitive to colour. There is a great deal of convergence in the scotopic system. Multiple Rod cells converge into one ganglion cell.
Cone cells: correspond with the photopic system. They are not as light sensitive, they require more light but are colour sensitive. We have multiple types of cones. There is less convergence in the photopic system.
What is unique about the potentials in the visual receptor cells in comparison to other neurons?
Cells in the retina only operate on graded potentials, not action potentials.
How to photoreceptors respond to light?
Photoreceptors respond to light by releasing less neurotransmitters.
Photoreceptors cells have special photo-pigment receptors that capture light particles (photons). When photons hit this receptor it HYPERPOLARIZES the cell, decreasing the release of neurotransmitters. In the dark, photoreceptors steadily release neurotransmitters. The size of the Hyperpolarization determines the amount of neurotransmitter released.
What are some ways that the visual system responds to changing light intensities?
The Pupil contracts or relaxes in response to increasing or decreasing light conditions respectively. The Parasympathetic system contracts the pupil and the sympathetic system dilates the pupil.
It has a range fractionation where cells are divided in their sensitivity to light.
Photoreceptors are also adaptive: they change their individual sensitivity to match the average level of ambient illumination.
With all this in mind, we can see how the visual system actually responds to changes in light rather than absolute illumination.
What is visual Acuity?
It is the level of detail we can see. Visual acuity is sharpest at the fovea, the center of our visual field.
What are the characteristics of the Fovea?
It is the centre region of the retina. Has the highest density of tightly packed cone cells and it receives direct light input that does not pass through other cells or blood vessels (like the rest of the retina). Rods are absent from the fovea.
What is the distribution pattern of rods and cones across the retina?
Cones are most concentrated at the fovea. Rods are absent from the fovea and more concentrated at the periphery of the retina.
What is the optic disc?
The optic disc is on the nasal side of the fovea. It is where blood vessels and ganglion axons leave the eye. There are no photoreceptors here and this creates a blind spot.
Describe the visual pathway from the eye to the extrastriate cortex.
In the eye, the retinal image is inverted and reversed in comparison to the visual field.
The axons of the ganglion cells form the optic nerve. which reaches the occipital cortex.
From the occipital cortex, information travels to the optic chiasm and the axons from the temporal half of each retina continue into the optic tract on the same side. Axons from the nasal haves cross over to the optic tract on the opposite side.
Some axons in the optic tract terminate in the superior colliculus in the midbrain but most axons terminate in the lateral geniculate nucleus (LGN) of the thalamus.
The axons project back and forth between the primary visual cortex and the LGN of the thalamus in a pathway called optic radiations.
Information then projects into the extrastriate cortex.
What do photoreceptors release?
Photoreceptors release glutamate at rest. When they are hyperpolarized, they release less glutamate.
What effect does glutamate have on the post-synaptic bipolar cells?
It depends on the type of receptor in the bipolar cells.
- If it is an on-centre bipolar cell than glutamate is inhibitory and light is excititory (the cell fires with light stimulation because there is less glutamate.)
If it is an off centre bipolar cell than glutamate is excitory and light is inhibitory (the cell is less likely to fire with light because there is less glutamate from the photoreceptor
