PHGY 210 Lecture Notes - Inner Plexiform Layer, Membrane Potential, Critical Role

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31 Jan 2013
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MICROSCOPIC ANATOMY OF THE RETINA
The most direct pathway for visual information to exit the eye is: photoreceptors bipolar
cells ganglion cells. In response to light, ganglion cells fire action potentials which
propagate down the optic nerve to the rest of the brain. Retinal process is influenced by two
additional cell types:
Horizontal cells: receives input from the photoreceptors and project neurites laterally to
influence surrounding bipolar cells and photoreceptors.
Amacrine cells: receive inputs from bipolar cells and project laterally to influence
surrounding ganglion cell, bipolar cells and other amacrine cells.
There are two important points to remember:
o Only light-sensitive cells in the retina are the photoreceptors. All other cells are influence
by light only via direct and indirect synaptic interactions with the photoreceptors.
o The ganglion cells are the only source of output from the retina
The Laminar Organization of the Retina
Laminar organization means that the cells are organized in layers (see Fig. 9.12). Light must
pass from the vitreous humor through the ganglion cells and bipolar cells before it reaches the
photoreceptors. The pigmented epithelium lies below the photoreceptors and plays a critical role
in the maintenance of the photoreceptors and photopigments. It also absorbs any light that
passes entirely through the retina and minimizes the reflection of light within the eye.
Ganglion cell layer: innermost layer and contains the cell bodies of the ganglion cells.
o Inner plexiform layer: between the ganglion cell layer and inner nuclear layer and
contains the synaptic contacts between the bipolar, amacrine cells and ganglion cells.
Inner nuclear layer: below the ganglion cell layer and contains the cell bodies of the
bipolar cells, the horizontal and amacrine cells.
o Outer plexiform layer: between the outer and inner nuclear layers where the
photoreceptors make synaptic contact with the bipolar and horizontal cells.
Outer nuclear layer: contains cell bodies of the photoreceptors
Layer of photoreceptor outer segments: contains light-sensitive elements of the retina
and is embedded in the pigmented epithelium.
Photoreceptor Structure
Every photoreceptor has four regions: an outer segment, an inner segment, a cell body and
synaptic terminal. The outer segment contains a stack of membranous disks which contain light-
sensitive photopigments that absorb light and triggers changes in the photoreceptor membrane
potential.
There are two types of photoreceptors (the retina is duplex):
o Rod photoreceptors: have a long, cylindrical outer segment, containing many disks. It
has a greater number of disks and a higher photopigment concentration. Thus they are
over 1000 times more sensitive to light than cones. Under night-time lighting (i.e. scotopic
conditions) only rods contribute to vision. All rods contain the same photopigment.
o Cone photoreceptors: have shorter, tapering outer segment with fewer membranous
disks. Under day-time lighting (i.e. photopic conditions), cones mostly contribute to vision.
There are three types of cones, each containing a different pigment and make the cones
sensitive to different wavelengths of light.
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