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l!1lD'!ltlfj 43 Neurophysiology 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 l...
l!1lD'!ltlfj 43 Neurophysiology 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. Temporal ~ field ~ '' ' \ I \ I \I '' ' \ I \ I ' \I Left 0 Right C)a ([) CD b CD CD c d Occipital cortex FIGURE 2.4. Effects of lesions at various levels of the optic pathway. (Modified with permission from Ganong WF. Review of Medical Physiology. 20th ed. New York: McGraw-Hill, 2001:147.) 44 BRS Physiology 11-cis retinal ! Light All-trans retinal ! ! ! ! ! ! Metarhodopsin II Activation of G protein (transducin) Activation of phosphodiesterase tcGMP Closure of Na+ channels Hyperpolarization ! Decreased glutamate release RGURE 2.5. Steps in photoreception in rods. cGMP = cyclic guanosine monophosphate. c. Cutting lhe optic tract causes homonymous contralateral hemianopia d. Cutting the geniculocalcarine tract causes homonymous hemianopia with macular sparing. 4. Steps in photoreception in lhe rods (Figure 2.5) The photosensitive element is rhodopsin, which is composed of opsin {a protein) belonging to the superfamily ofG-protein--coupled receptors and retinal (an aldehyde of vitamin A). a. Light on the retina converts 11-c:is retinal to all-tnlns retinal, a process called photoisomerization. A series of intermediates is then formed, one ofwhich is metarhodopsin II. Vitamin A is necessary for the regeneration of ll-cis retinal. Deficiency of vitamin A causes night blindness. b. Metarhodopsin II activates a G protein called transducin (G1), which in tum activates a phosphodiesterase. c. Phosphodiesterase catalyzes the conversion of cyclic guanosine monophosphate (cGMP) to 5' -GMP, and cGMP levels decrease. d. Decreased levels of cGMP cause closure of Na+ channels, decreased inward Na+ current, and, as a result, hyperpolarization of the photoreceptor cell membrane. Increasing light intensity increases the degree of hyperpolarization. I!1[Jt!jfj Neurophysiology 45 e. When the photoreceptor is hyperpolarized, there is decreased release of glutamate, an excitatory neurotransmitter. There are two types of glutamate receptors on bipolar and horizontal cells, which determine whether the cell is excited or inhibited. (1t lonotropic glutamate receptors are excitatory. Therefore, decreased release of glutamate from the photoreceptors acting on ionotropic receptors causes hyperpolarization (inhibition) because there is decreased excitation. (2t Matabotropic glutamate receptors are inhibitory. Therefore, decreased release of glutamate from photoreceptors acting on metabotropic receptors causes depolarization (excitation) because there is decreased inhibition. 5. Receptive visual fields a. Receptive fields of the ganglion calls and lateral geniculate calls (1 t Each bipolar cell receives input from many receptor cells. In turn, each ganglion cell receives input from many bipolar cells. The receptor cells connected to a ganglion cell form the earner of its recaptor field. The receptor cells connected to ganglion cells via horizontal cells form the surround of its receptive field. (Remember that the response of bipolar and horizontal cells to light depends on whether that cell has ionotropic or metabotropic receptors.) (2t On-canter, off-surround is one pattern of a ganglion cell receptive field. Ught striking the center of the receptive field depolarizes (excites) the ganglion cell, whereas light striking the surround of the receptive field hyperpolarizes (inhibits) the ganglion cell. Off-center, on-surround is another possible pattern. (3t Lateral geniculate cells of the thalamus retain the on-center, off-surround or offcenter, on-surround pattern that is transmitted from the ganglion cell. b. Receptive fields of the visual cortex Neurons in the visual cortex detect shape and orientation of figures. Three cortical cell types are involved: (1) Simple cells have center-surround, on-off patterns, but are elongated rods rather than concentric circles. They respond best to bars of light that have the correct position and orientation. (2) Complex cells respond best to moving bars or edges of light with the correct orientation. (3t Hypercomplex cells respond best to lines with particular length and to curves and angles. D. Audition 1. Sound waves Frequency is measured in hertz (Hz). Intensity is measured in decibels (dBt, a log scale. p dB=20logPa where: dB=decibel P = sound pressure being measured P0 =reference pressure measured at the threshold frequency 2. Structure of the ear a. Outer ear directs the sound waves into the auditory canal. b. Middle ear is air filled. contains the tympanic membrane and the auditory ossicles (malleus, incus, and stapes). The stapes inserts into the oval window, a membrane between the middle ear and the inner ear. 46 BRS Physiology Sph l ga"
