Anatomy of the Human Eye

60 Pages·2006·11.61 MB·English

Anatomy of the Human Eye
Retina
• Despite its peripheral location is part of the CNS.
• Converts light energy into action potentials that travel out
the optic nerve into the brain.
• Is layered, relatively simple for a CNS structure.
• Surrounded on one side by pigmented epithelium. Contains
melanin that helps reduce backscattering of light. Also
plays a role in maintenance of photoreceptors.
• 5 types of neurons in the retina: photoreceptors, bipolar
cells, ganglion cells, horizontal cells and amacrine cells.
• A direct 3 neuron chain is the basic unit of transmission.
Photoreceptor (rods and cones) to bipolar cell to
ganglion cell (axons form the Optic nerve)
Structural Differences Between Rods and Cones

rods and cones are distinguished by:
• shape
• type of photopigment they contain
• distribution across the retina
• pattern of synaptic connections
• specialized for different aspects of vision
Rod system-low spatial resolution but extremely
sensitive to light
Cone system- high spatial resolution but is relatively
insensitive to light.

Phototransduction
• Photoreceptors do not exhibit action potentials-
light causes a graded change in membrane
potential that changes the rate at which
neurotransmitter is released.
– The NT is thought to be glutamate
• Light absorption leads to hyperpolarization of
the neuron. This leads to less release of
neurotransmitter to post-synaptic cell.
Cones and rods hyperpolarize in response to light
What does light do?
• In the dark, resting potential of the photoreceptor
is - 40 mv.
• Light shining onto outer segment leads to the
hyperpolarization of the photoreceptor and
reduction of neurotransmitter released.
• In the dark the number of Ca++ channels open at
the synaptic terminal is high, and rate of
neurotransmitter release is high.
• In the light number of open channels is reduced
and rate of neurotransmitter release is reduced.
cGMP gated Na+ channels are key
in dark channel open due
to cGMP binding.
Na+ rushes in
cell depolarized
exposure to light causes cGMP
breakdown, closing Na+ permeable
channels, and hyperpolarization