Exam 3 Neuro III Flashcards
What is the refractory index? What happens if light strikes a perpendicular surface vs angled?
RI = speed of light in air / speed of light in the translucent material.
If light strikes perpendicular surface, only the speed changes. If the surface is angles, then rays bend if there is a difference in refraction of the two media.
What are diopters a measure of? What is the unit? What is the refractive power of the eye and which area of the high specifically and why?
the more a lens can bend lift rays, the greater it’s refractive power, measured in diopters. 1 diopter = 1 meter/focal length.
Eye = 59 diopters, 2/3s comes from the cornea (anterior cornea). This is bc the RI of the cornea differs markedly from air, while the RIs of the aqueous humor, lens, and vitreous humor are not that different from air’s.
What is presbyopia and why does it happen as we age
Difficult viewing near objects. As we age the lens becomes less elastic and loses ability to form sphere (thicken) as needed for focusing on near objects.
Contrast myopia and hyperopia
Myopia aka nearsightedness, image is focused in front of retina, eye might be too “long”.
Corrected w concave lens.
Hyperopia aka farsightedness. Image is focused behind the retina, “shortened” eyeball.
Corrected w convex lens
2° cataract vs traumatic cataract vs congenital vs radiation cataract
Secondary - form after surgery from other eye probs eg glaucoma, or form due to etc health probs eg diabetes, steroid use.
Traumatic - after eye injury.
Congenital - babies or children develop the cataracts very small and may not affect vision. But might need lens removal.
Radiation - develop after exposure to radiation.
Describe outer, middle and inner layers of the eye
Outer- cornea (refraction), conjunctiva, sclera.
Middle - choroid and iris (papillary dilator = sympathetic innervation, sphincter muscle = parasympathetic innervation).
Outer- neural coat & retina, macula contains fovea (focal point depression).
What are the two components of interocular fluid? Describe them/formationfiltration.
Interocular fluid: aqueous humor and vitreous fluid. The aqueous humor lies in front of lens, freely flowing and continuously produced and reabsorbed. The vitreous fluid is gelatinous mass which allows materials to diffuse but does not flow.
Aqueous humor is formed by active secretion of Na+, which pulls Cl- and HCO3-. These solutes generate and osmotic gradient which pulls fluid (water) from the capillaries.
Fluid flows through pupil into ‘antechamber of eye’, makes it way to place between cornea and iris. There the humor is absorbed into the trabeculae meshwork, and taken away via the canal of Schlemm, ultimately back into blood system.
Describe the fovea.
The fovea is area in center of retiina, 1 mm2, which is specialized for detailed vision. Center fovia has high concentration of cones, while rest of retina dominated by rods.
Describe the structure of a photoreceptor (outer segment, inner segment and synaptic terminal).
Structure of the photoreceptor:
outer segment = made up of discs or folds of membrane. Light-sensitive photochemical located here - rhodopsin in rods, or one of three color photocemicals in cones.
inner segment= cytoplasmic organelles, mitochondria, nucleus
synaptic terminal = synaptic contact with subsequent cells.
Describe the four main characteristics of rods.
Rods 1) Extremely sensitive to light: night vision: outer segment is longer. contains more photopigment. can capture more light. rod is activated by 1 photon. 2) Black & white vision: 1 photopigment. rhodopsin (429nm). 3) Distribution: primarily outside of fovea. 4) High Convergence: many rods synapse on 1 bipolar cell. many bipolar cells synapse on 1 ganglion cell (makes for highly sensitive detector of light but poor spatial resolution).
Describe the 4 main characteristics of cones.
Cones 1) Low light sensitivity: day vision. outer segment is shorter. contains less photopigment. can capture less light. cone is activated by 100+ photons. 2) Color vision: 3 photopigments; blue opsin (419 nm), green opsin (531 nm), red opsin (559 nm). 3) Distribution: primarily located in the fovea. 4) Low convergence: 1 cone synapses on 1 bipolar cell. 1 bipolar cell synapses on 1 ganglion cell (makes for weak light detection but high spatial resolution)
Degree of activation of cones by different monochromatic lights.
Loss of red cones: ?
Loss of green cones: ?
Why is this more common in males? How can it occur in females?
A specific cone will respond to varying degrees to different colored light.
Red-Green color Blindness- if you are missing either red or green cones, you will have trouble distinguishing between green-yellow-orange-red, since these are covered by the green and red cones.
Red-green blindness is X-linked, so phenotype is expressed in males, females are carriers.
If a woman is color blind, what are the chances that her son will be color blind? That her daughter will be color blind?
.
Explain the photochemistry of vision.
Rods and cones have chemicals which decompose in light. Rods have rhodopsin, cones have cone pigments. Outer segment of rod projecting into RPE has concentration of 40% rhodopsin, which is a combination of the protein scotopsin (opsin) and retinal , acarotenoid pigment.
Light energy is absorbed by rhodopsin in the rods, changes from cis to trans form. The all-trans form no longer fits well with scotopsin and they begin to separate to make bathorhodopsin, then lumirhodopsin, metarhodopsin I, then metarhodopsin II, and finally into all trans retinal and scotopsin. It is Metarhodopsin II (activated rhodopsin) which induces the electrical potential. All-trans retinal converts to 11-cis retinal, recombines with scotopsin.
Role of vitamin A: all-trans retinol can be converted to all-trans retinal, then converted by isomerase into cis-retina.
Excitation by light exposure: light causes the photopigments to become hyperpolarized; decomposing rhodopsin decreases Na+ current in outer segment of rod. K leaks out through non=gated channels. The Na-K ATPase maintains gradients.
Activated rhodopsin stimulates the G-protein trasnducin, which activates a cGMP phosphodiesterase. This in turn breaks down cGMP, thereby closes cGMP-gated Na+ channels. Rhodopsin kinase inactivates the activated rhodopsin.
Compare the polarization of photoreceptor cells in dark vs light conditions.
Photoreceptor cells are hyperpolarized by light.
In darkness, receptor cells are ‘depolarized’ (In depolarized state, Ca2+ channels are OPEN and transmitter release rate is high.) > exposure to light causes graded hyperpolarization based on intensity, (saturates at @-65 to -70mV). Hyperpolarization reduces Ca2+ current, reducing transmitter release.
Cation channels regulated by cGMP. In the dark, high cGMP levels keep channels open/cell depolarized.
In light, cGMP levels fall > closing cation channels and hyperpolarize the cell.
Photoreceptors project to____.
Horizontal cells project to ___, are always ___.
Bipolar cells project to ____, may be ___or ____.
Amacrine cells project to ____.
Ganglion cells make up the ____.
Photoreceptors project to bipolar and horizontal cells.
horizontal cells project to bipolar cells, always inhibitory, provide lateral inhibition.
Bipolar cells project to ganglion and amacrine cells, may be excitatory or inhibitory.
Amacrine cells project to bipolar cells and ganglion cells
Ganglion cells make up the retinal output, make up the optic nerve.
Descriube the W, X, and Y ganglion cells that make up the optic nerve. Speed, info inputs/outputs, vision.
W Ganglion cells- small and slow, receive information from rods, dendrites spread widely, sense directional movement, dark vision.
X ganglion cells- medium size and speed, most numerous, small dendritic field, fine detail vision.
Y ganglion cells- largest and fastest, large dendritic fields, poor localization but rapid detection of change and movement.
Photoreceptors have circular receptive fields.. Subsequent to these are bipolar cells which are ‘on-center’ or ‘off-center’. Explain.
On-center cell is depolarized with light in the center of receptive field, and hyperpolarized within the annulus. Off-center is hyperpolarized with light in the center of receptive field and depolarized within the annulus.
Most of the information in the visual field comes from what? Of what is this largely a function?
Most of the information in the visual field comes from detection among ganglion cells of the differences in level of illumination: luminance contrast. This is largely a function of on and off centers and surrounds.
Explain the sequence of events involved in receptive field vision.
- Light on the photoreceptor hyperpolarizes the cell, reducing neurotransmitter (glutamate) release.
- On-center bipolar cell has mGluR6 receptors (GPCRs). Receptor binding activates intracellular cascade which closes cGMP –gated Na+ channels, has hyperpolarizing effect. Decrease in transmitter from the photoreceptor will depolarize this cell (releases it from hyperpolarizing effect of transmitter).
- Off-center bipolar cell has ionotropic receptors, receptor binding depolarizes cell.
- Bipolar cells release transmitter in response to depolarization (they don’t fire APs, have graded potentials).
- Light in CENTER hyperpolarizes photoreceptor, which DEPOLARIZES on-center bipolar cell, which will release excitatory transmitter on ‘on-center’ ganglion cell.
What is lateral inhibition?
Further contributing to the generation of contrast is lateral inhibition. Horizontal cells connect laterally with photoreceptors. These are inhibitory cells. In the case depicted in the figure, the center photoreceptor is stimulated by a point of light, while the side photoreceptors in this case are in the dark. The direct pathway from photoreceptor to bipolar to ganglion is activated. This would produce contrast- if all three receptors were activated, then they would neutralize each other through the lateral inhibitory circuit.
Why do we experience a continuous visual field?
The visual fields of the two eyes are blended by the minority of fibers crossing at the chiasm- for ex. the left eye’s nasal field is blended with the right eye’s temporal field. The end result is that we experience a continuous visual field.
Note how the topographical arrangement of the retinal cells is preserved throughout the projection including the visual cortex.
Muscle control of eye movements: describe the 3 pairs of muscles. What cranial nerves project to them?
A. Muscle control of eye movements: 3 pairs of muscles; 1) medial and lateral recti- move side to side,
2) superior and inferior recti, move eyes up and down, and
3) superior and inferior obliques, rotate eyes to align visual fields.
Cranial Nerves: 3rd Oculomotor Nerve, 4th Trochlear Nerve, and 6th Abducens Nerve.
Humans fixate on objects via voluntary fixation or involuntary fixation. From where do the voluntary and involuntary signals originate?
What are the three continous types of movements of the eye and what do they achieve when they converge? What are saccades? What is “pursuit movement’?
The voluntary fixation signal originates in premotor areas of cortex (dysfunction of these areas makes it difficult to unlock the eyes from a fixation point). Involuntary signals start in secondary visual areas of occipital cortex.
Three continous types of movements: 1) a continuous tremor, 2) a slow drift, and 3) sudden flicking movements. These movements correct and keep a fixation point on the fovia.
Watching a constantly changing scene (riding in a car) requires jumping from one fixation point to another several times per second- these jumps are known as saccades. Fixating on a moving target = “pursuit movement”. These and orientation movements to visual disturbances appear to require the superior colliculi, mediated via the medial longitudinal fasciculus.
