Special Senses Flashcards
This is the ratio of light in a vacuum to the velocity in that substance.
Refractive index
What is the velocity of light in a vacuum?
300,000 km/sec
When light is passing through the various layers of the eye, where would the greatest differences in refractive indices be located?
When light is transitioning from the air to the cornea
What is a diopter? How are they expressed for convex and concave lenses?
- A diopter is 1 meter/focal length
- Diopters are + for convex lenses
- Diopters are - for concave lenses
Lens strength can vary from ___ to ___ diopters.
20-30 D
The ability to increase refractive power increases or decreases with age?
Decreases
Parasympathetic or Sympathetic excitation increases lens strength?
Parasympathetic
Parasympathetic excitation causes contraction of the ciliary muscle allowing relaxation of suspensory ligaments attached radially around the lens. This causes the lens to become more convex and thereby increasing refractive power. What is this process called?
Accommodation
This condition is characterized by the loss of elasticity of the lens with age and the subsequent loss of accommodation.
Presbyopia
This term is used to describe normal vision in which the ciliary muscle is relaxed in distant vision.
Emmetropia
This condition is also known as “farsightedness” in which the focal point is behind the retina.
Hyperopia
This condition is also known as “nearsightedness” in which the focal point is in front of the retina.
Myopia
What type of lens is used to correct hyperopia? Myopia?
Hyperopia: convex lens. Can temporarily be corrected for by contracting the ciliary muscle
Myopia: concave lens
This condition is characterized by an irregularly shaped cornea (more common) or lens (less common)?
Astigmatism
This is commonly used to test the visual acuity of an individual my measuring the ratio of what that person can see compared to an individual with normal vision.
Snellen eye chart
According to the Snellen eye chart, what is considered normal visual acuity? Less? More?
20/20: normal
20/40: less
20/10: more
Which area of the eye has the greatest visual acuity?
Fovea centralis: composed almost entirely of long slender cones
What allows the fovea centralis to have such great acuity?
The blood vessels, ganglion cells, inner nuclear and plexiform layers are displaced laterally allowing light to pass relatively unimpeded to receptors
What are the three cues of depth perception?
- Relative size
- Moving parallax
- Seteropsis: binocular vision
The closer the object, the larger it appear. This is describing what cue of depth perception?
Relative size (only needs one eye)
As the head moves, objects closer move across the visual field at a greater rate. This is describing what cue of depth perception?
Moving parallax (only needs one eye)
A slight difference in position of the visual image on both retinas, closer objects are more laterally placed. This is describing what cue of depth perception?
Stereopsis-binocular vision. Requires both eyes
This substance is secreted by the ciliary body. It flows into the anterior chamber of the eye and drains into the Canal of Schlemm (vein).
Aqueous humor
This condition is characterized by an increase in intraocular pressure leading to the compression of the optic nerve and eventual blindness.
Glaucoma
This structure is a peripheral extension of the CNS and processes visual signals. It contains photoreceptors, bipolar, ganglion, horizontal, and amacrine cells.
Retina
These are the only retinal cells that are capable of generating action potentials.
Ganglion cells
What happens to photoreceptors when light strikes them?
- Light breaks down rhodopsin (rods) and cone pigments (cones)
- Rhodopsin decreases leading to a decrease in Na+ conductance
- Photoreceptors hyperpolarize
- Less neurotransmitter (glutamate) is released
How does the retina respond to darkness?
- Rods/cones depolarize
- Neurotransmitters increase
- Hyperpolarize “on” bipolar cells and depolarize “off” bipolar cells
How does the retina respond to light?
- Rods/cones hyperpolarize
- Neurotransmitters decrease
- Depolarize “on” bipolar cells and hyperpolarize “off” bipolar cells
These are non-spiking inhibitory interneurons that make complex synaptic connections with photoreceptors. They are responsible for later inhibition and turned off in the dark due to dopamine released from interplexiform cells.
Horizontal cells
These cells connect photoreceptors to either ganglion cells or amacrine cells.
Bipolar cells
What are the two types of bipolar cells?
Invaginating: “on”-hyperpolarized by glutamate
Flat bipolars: “off”-depolarized by glutamate
These cells receive input from many rod bipolar cells (convergence) and project to the feet of cone bipolar cells, which connect to ganglion cells.
Amacrine cells
These cells can be of the “on” or “off” variety and are the only retinal cells that are capable of generating an action potential.
Ganglion cells
What are the three types of ganglion cells?
- X(P) cells
- Y(M) cells
- W cells
These ganglion cells receive input from the bipolar cells, have a small receptive field, have a slow conduction velocity, are slow adapting, and have projections to the parvocellular part of the lateral geniculate nucleus of the thalamus.
P(X) ganglion cells (responsible for color vision)
These ganglion cells receive input from amacrine cells, have a large receptive field, a fast conduction velocity, are fast adapting, and have projections to the magnocellular part of the lateral geniculate nucleus of the thalamus.
M(Y) ganglion cells (sensitive to movement and brightness)
These ganglion cells are the smallest, have the slowest conduction velocity, and have broad receptive fields (rods).
W ganglion cells
What is the receptive field size in the fovea?
The ratio can be as low as 1 cone to 1 bipolar cell to 1 ganglion cell
What is the receptive field size in the peripheral retina?
Hundreds of rods can supply a single bipolar cell and many bipolar cells connected to one ganglion cell
What happens during dark adaptation?
- an increase of retinal sensitivity (10000 fold)
- cones adapt within the first 10 min (100 fold)
- pupil dilation
What are the three types of cones?
- Blue sensitive (445 nm)
- Green sensitive (535 nm)
- Red sensitive (570 nm)
This is a sex-linked trait carried on the X chromosome. It occurs almost exclusively in males, but transmitted by females.
Color blindness (red-green color blindness is the most common)
This condition is characterized by a loss of red cones. Individuals have an overall decrease in the visual spectrum.
Protanope
This condition is characterized by a loss of green cones. Individuals have a normal overall visual spectrum.
Deuteranope
The loss of blue cones is rare and may be under-represented. What is it referred to as?
Blue-weakness
What test is used to find problems distinguishing green, yellow, orange, and red.
Ishihara chart
What is the visual pathway?
- Optic nerve to optic chiasm
- Optic chiasm to optic tract
- Optic tract to the lateral geniculate (synapses occur here)
- Lateral geniculate to the primary visual cortex (geniculocalcarine radiation)
When a lesion occurs in the optic nerve, what happens to the visual field?
Blindness in one eye
When a lesion occurs in the optic chiasm, what happens to the visual field?
Bitemporal hemianopia (loss of peripheral vision/tunnel vision)
When a lesion occurs in the optic tract, what happens to the visual field?
Left/right homonymous hemianopia (loss of an entire visual field)
What Brodmann area is the primary visual cortex?
Brodmann area 17
These cells within the primary visual cortex respond to bars of light/dark above and below layer IV.
Simple cells
These cells within the primary visual cortex are motion dependent, but have the same orientation sensitivity as simple cells.
Complex cells
These cells within the primary visual cortex are rich in cytochrome oxidaase in the center of each ocular dominance band. It is the starting point of cortical color processing.
Color blobs
Visual signals are broken down and sent over parallel pathways. What are these pathways?
Parvo-interblob: high resolution static form of perception (black and white)
Blob: color (V4) and acrhomatopsia
Magno: movement and stereoscopic depth