55-101 Lecture Notes - Suprachiasmatic Nucleus, Signal Transduction, Retina Horizontal Cell

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Outline of Lecture 61 and 62 (03-25 C and 03-25 D; Yau)
Photoreceptors and Retina
0. Introduction
- Images from the retina are analyzed for form/movement/color, then later for depth
- Development of retina: omitted from this outline, not covered in lecture
I. Structure of retina
- The retina is composed of three main layers
1) Outer nuclear layer: photoreceptors, no direct blood supply
2) Inner nuclear layer: bipolar, horizontal, amacrine, Mueller glial cell bodies
3) Ganglion cell layer: retinal ganglion cells
- The outer and inner plexiform layers are sites of synapses between layers
II. Photoreceptors
- There are two types of photoreceptors
1) rods: most sensitive in dim light, only one type
2) cones: less sensitive to light, three types mediate color vision
- Distribution of photoreceptors is not uniform: fovea has high density of cones and few rods,
periphery has more rods
- The outer segment of rods and cones have stacks of disks with high density of pigment and
provide a high probability of absorption of an incident photon
- Pigment epithelial cells serve to (1) absorb light not absorbed by the rods/cones using melanin,
(2) phagocytose shedded fragments of rods/cones, (3) regenerate pigment
III. Visual pigments
- Pigment is composed of a chromophore covalently bound to an opsin 7TM protein
- Humans have 4 pigments (1 rod, 3 cones) each maximally sensitive at different wavelengths
- Upon photon absorption, the chromophore undergoes several configuration changes; the
configuration that triggers vision takes several ms to reach; the ultimate result is the release of
chromophore, and binding of fresh chromophore with opsin
IV. Phototransduction
- Rods/cones hyperpolarize in response to light in a graded fashion with respect to intensity, with
cones requiring more light and responding more rapidly
- Phototransduction mechanism is shown on p. 10, a prototypical signal transduction mechanism
- In dark, cGMP keeps a nonselective cation channel open, and cell is depolarized
- Rhodopsin is coupled to G-protein that activates phosphodiesterase and lowers [cGMP]
- Ca influx plays a key negative feedback function and mediates light adaptation
V. Synaptic connections in the retina (see Figure on p. 12)
- Throughput pathway: photoreceptor bipolar retinal ganglion cell optic nerve
- Lateral associations: between photoreceptors, and via horizontal and amacrine cells
- There are distinct synapse morphologies in the retina
- Synaptic triad: photoreceptor with horiztonal-bipolar-horizontal cells, and contains a synaptic
ribbon
- Dyad: proximal bipolar end synapses with two cells
- Primary nxt’s in the retina are Glu (excitatory) and GABA (inhibitory)
VI. Information processing in the retina
- The receptive field of a single photoreceptor is bigger than itself due to connections with
neighboring photoreceptors; for the same reason, the receptive field of horizontal cells is bigger
than the multiple photoreceptors it contacts
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