MCI/ FMGE AFMG Physiology notes free PDF download
MCI/ FMGE AFMG Physiology notes free PDF download b. Receptor cells are rods and conesRods and cones are not present on the optic disk; the result is a blind spot.
c. Bipolar cells. The receptor cells (i.e., rods and cones) synapse on bipolar cells, which
synapse on the ganglion cells.
(1) Few cones synapse on a single bipolar cell, which synapses on a single ganglion cell.
This arrangement is the basis for the high acuity and low sensitivity of the cones. In
the fovea, where acuity is highest, the ratio of cones to bipolar cells is 1:1.
(2) Many rods synapse on a single bipolar cell. As a result, there is less acuity in the rods
than in the cones. There is also greater sensitivity in the rods because light striking
any one of the rods will activate the bipolar cell.
d. Horizontal and amacrine cells form local circuits with the bipolar cells.
e. Ganglion cells are the output cells of the retina.
■ Axons of ganglion cells form the optic nerve.
3. Optic pathways and lesions (Figure 2.4)
■ Axons of the ganglion cells form the optic nerve and optic tract, ending in the lateral geniculate body of the thalamus.
■ The fibers from each nasal hemiretina cross at the optic chiasm, whereas the fibers from each temporal hemiretina remain ipsilateral. Therefore, fibers from the left nasal hemiretina and fibers from the right temporal hemiretina form the right optic tract and synapse on the right lateral geniculate body.
■ Fibers from the lateral geniculate body form the geniculocalcarine tract and pass to the occipital lobe of the cortex.
a. Cutting the optic nerve causes blindness in the ipsilateral eye.
b. Cutting the optic chiasm causes heteronymous bitemporal hemianopia.
c. Cutting the optic tract causes homonymous contralateral hemianopia.
d. Cutting the geniculocalcarine tract causes homonymous hemianopia with macular sparing.
4. Steps in photoreception in the rods (Figure 2.5)
■ The photosensitive element is rhodopsin, which is composed of opsin (a protein) belonging to the superfamily of G-protein–coupled receptors and retinal (an aldehydeof vitamin A).
a. Light on the retina converts 11-cis retinal to all-trans retinal, a process called
photoisomerization. A series of intermediates is then formed, one of which is metarhodopsin II.
■ Vitamin A is necessary for the regeneration of 11-cis rhodopsin. Deficiency of vitamin A causes night blindness.
b. Metarhodopsin II activates a G protein called transducin (Gt), which in turn activates a phosphodiesterase.
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