Unit X (50 & 51 only)
Special Senses
1. Outline the opsin cycle in the retina. Does cis or trans retinal associate with opsins?
 Rodney Bathes Lumber, Minding My Sawmill
 Rhodopsin =light=> Bathorhodopsin => Lumirhodopsin- => Metarhodopsin I =>
Metarhodopsin II => Scotopsin
 Trans-retinol (vit A.) is converted to cis-retinal, which associates with scotopsin to make
rhodopsin, thus closing the cycle
[think: vit. A => oily => -ol]
[also think of a bow being drawn and released for cis=>trans]
2.
How is light transduced into neural input?
 Metarhodopsin II lingers long enough to activate transducin, which activates [cGMP-specific]
phosphodiesterase
 Phosphodiesterase depletes cGMP, which closes cGMP-dependent Na+ channels
 This closure hyperpolarizes the cell, causing a reduction in neurotransmitters elaborated
 First bipolar, then amacrine (in the case of rods) or ganglion cells (cones) receive the input
electrotonically, and ganglion cells finally transmit action potentials to the LGNi
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3.
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Metarhodopsin II=> transducin=> phosphodiesterase => decreased cGMP=> closes Na+ channel=> hyperpolzarization=> decreased neurotransmitters
What is the important molecular mechanism of light adaptation?
Think: you need to reduce the sensitivity of your rods and cones in order to see in bright light
Therefore, your rhodopsin stores should mostly be converted to scotopsin and retinal
Additionally, some [trans] retinal will be reduced to [trans] retinol (vitamin A)
4. Can you order the three color receptors and the rod receptor from widest spectrum sensitivity
breadth to most narrow? What are their peaks in nm? [Test Q]
 Red:
570 nm
 Green:
535 nm
 Rod:
505 nm
 Blue :
445 nm
[sensitivity just so happensto follow descending wavelength order]
5. What is the main type of color blindness? What are the names of its two subtypes? [test Q]
 Red-green; cannot tell red and green apart
 Protanope = no red cones; colors outside of range (e.g. red stoplights) may be dimmed
 Dueteranope = no green cones
6.
What type of ganglion cells are most numerous? What do they transmit?
 X cells, a.k.a. midget cells – transmit fine detail and all color
 => Parvocellular layers (III-VI) of LGN=> primary visual cortex layers IVa and IVcB
i. Least common type is the type that detects change (Y)
7. What part of the brain do we use to change our point of fixation (“voluntary fixation”)? What
part is responsible for locking fixation in place (“involuntary fixation”)?
 Frontal lobe’s premotor region
i. Lesion causes inability to change fixation point
 Occipital lobe’s secondary visual area, specifically its interactions w/the superior colliculus
i. A lesion would make it impossible to fixate; the gaze would jump about
8.
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What is miosis? What is mydriasis?
Miosis = Constriction
[miosis happens ona tiny tiny scale]
Mydriasis = Dilation
[the Mydraal are gigantic]
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In what segment of a rod are the cGMP-gated sodium channels? [test Q]
Outer segment
9.