PHYL2002 Study Guide - Final Guide: Transducin, Cell Membrane, Opsin
Cellular Basis of Phototransduction
• Major function of the eye → focus light rays from the environment on
rods and cones
• Rods and cones → photoreceptor cells → transform light energy into
electrical signals for transmission to CNS
• Neural portion of retina is made up of 3 layers
o Outermost layer → rods and cones
o Middle layer → bipolar cells/interneurons
o Inner layer → ganglion cells
• Axons of ganglion cells join to form optic nerve
• Light must pass through ganglion and bipolar layers before reaching the
photoreceptors in all areas except the fovea
• Photoreceptors
o Consist of 3 parts
▪ Outer segment → closest to eye’s exterior → detects light
stimulus
▪ Inner segment → middle → metabolic machinery
▪ Synaptic terminal → closest to eye’s interior → stores and
releases neurotransmitters
o Outer segment contains photopigments → stacked, flattened
membranous discs
o Photopigments → undergo chemical alterations when activated by
light
o Activation of photopigment leads to generation of action potential
in ganglion cells → transmits information to brain for visual
processing
o Photopigment is comprised of
▪ Opsin → integral protein in disc plasma membrane
▪ Retinal → derivative of vitamin A → light absorbing part of
photopigment
o Photoreceptors are inhibited by adequate stimulus →
hyperpolarized by light
o Photoreceptors are excited in absence of stimulation →
depolarized by darkness
• Phototransduction
o Process of converting light stimuli into electrical signals
o Photoreceptors hyperpolarize on light absorption instead of
depolarizing
o Photoreceptor activity in the dark
▪ Photopigment in rods → rhodopsin
▪ Plasma membrane contains chemically gated Na channels
▪ Respond to cGMP → binding of cGMP on Na channels keeps
them open
▪ Absence of light → cGMP concentration is high
▪ Na channels of a photoreceptor are open in absence of
stimulation → unlike other chemically gated channels
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