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Lecture 21

Cell and Systems Biology
Course Code
Melanie Woodin

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CSB332H1S L21; Apr. 02, 2012
Sensory Systems
R info falls on left side of retina
Anatomical Pathways in the Visual System
Optic nerve fibers arise from ganglion cells in retina, end in LGN
(lateral geniculate nucleus part of thal)
LGN axons project to the cerebral cortex
Output from each retina divides at optic chiasm to supply LGN on each
R side of each retina projects to R LGN
o R visual cortex receives info exclusively from L half of
visual field
Ex. ppl w damaged R cortex = blind in left visual field
Convergence & Divergence of Connections
Neurons converge & diverge extensively at every stage
o Ex. human eye contains 100mil rods & cones but only 1mil
ganglion cell axons
Divergence from V1 V2
Extensive funneling of info in eye
Each ganglion cell axon supplies many geniculate cells
3 Principal Features of Photoreceptor Structure:
1. Outer segment: light absorbed by visual pigment
o Packed with recs
o Change membrane pot
2. Inner segment: nucleus, ion pumps, transporters, ribosomes,
mitochondria, ER
o Standard cell structures
3. Synaptic terminal: releases glutamate
o highly specialized terminal characterized by “ribbon”
structures (contain transmitter vesicles)
Rods contain rhodopsin embedded in membranes arranged as
disks (separate from outer membrane)
Cones contain pigment molecs on infolded membranes
continuous w surface
Outer segment connected to inner segment by narrow stalk
Synaptic endings continuously release glutamate in dark
Ion channels constantly open in outer segment Na goes in
Responses of Photoreceptors
Dark (rest): inward current into outer segment depolarization
o Straight line recording in dark
Light turns off ongoing inward current
Vertebrate photorecs respond w a hyperpolarization graded
according to intensity of flash
Visual Pigments
Concentrated in membranes of outer segments
Sensitive molecs
Dense packing in layers traversed by light
o enhances probability that a photon will be trapped on its
way thru outer segment
Darkness: Current (mostly Na+) flows into outer segment of both
rods & cones keeps segment depolarized
o Vm ~ 40 mV
o Na flows thru stalk, Na-K pumps re-establish grads
Light: Closes Na channels allowing Vm to move towards EK (~ -
80 mV) hyperpolarizing the photorec decrease
neurotransmitter release
Transduction by Photoreceptors
Dark: High cytoplasmic [cGMP] keeps cation channels in open
[cGMP] inversely related to intensity of ambient light
o light [cGMP] , fraction of open channels
(A) Dark: Na+ in via cation channels on outer segment depolarization
o Current loop completed in neck of rod w outward movement
of K+ thru inner segment membrane
(B) Light: cGMP closing channels rod hyperpolarized
transmitter release
Na+, K+ & Ca2+[ ]’s maintained by pumps & exchangers in inner
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