BReceptor cells are rods and cones Table

- Rods 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.

Pigment cell layer

Photoreceptor layer External limiting membrane -Outer nuclear layer

Outer plexiform layer Inner nuclear layer

Inner plexiform layer

Ganglion cell layer

Optic nerve layer Internal limiting membrane--

Pigment cell layer

Photoreceptor layer External limiting membrane -Outer nuclear layer

Outer plexiform layer Inner nuclear layer

Inner plexiform layer

Ganglion cell layer

Optic nerve layer Internal limiting membrane--

H = horizontal cell 8 = bipolar cell A = amacrine cell

G = ganglion cell

Figure 2-3. Cellular layers of the retina. (Reprinted with permission from Bullock J, Boyle J III, Wang MB: Physiology, 3rd ed. Baltimore, Williams & Wilkins, 1995, p 76.)

H = horizontal cell 8 = bipolar cell A = amacrine cell

G = ganglion cell

Figure 2-3. Cellular layers of the retina. (Reprinted with permission from Bullock J, Boyle J III, Wang MB: Physiology, 3rd ed. Baltimore, Williams & Wilkins, 1995, p 76.)

Table 2-6. Functions of Rods and Cones

Function

Sensitivity to light

Acuity

Dark adaptation Color vision

Rods

Sensitive to low-intensity light; night vision

Lower visual acuity Not present in fovea

Rods adapt later

Cones

Sensitive to high-intensity light; day vision

Higher visual acuity Present in fovea

Cones adapt first

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 hemia-nopia.

c. Cutting the optic tract causes homonymous contralateral hemia-nopia.

LEFT EYE

RIGHT EYE

Ganglion cell

LEFT EYE

RIGHT EYE

Ganglion cell

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, 16th ed. Norwalk, CT, Appleton & Lange, 1993, p 135.)

Left

Right

Left

Right

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, 16th ed. Norwalk, CT, Appleton & Lange, 1993, p 135.)

d. Cutting the geniculocalcarine tract causes homonymous hemia-nopia with macular sparing.

4. Steps in photoreception in the rods (Figure 2-5)

- The photosensitive element is rhodopsin, which is composed of scotop-sin (a protein) and retinal (an aldehyde of 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 retinal. Deficiency of vitamin A causes night blindness.

b. Metarhodopsin II activates a G protein called transducin, which in turn 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 and, as a result, hyperpolarization of the receptor cell membrane. Increasing light intensity increases the degree of hyperpolarization.

11 - eis retinal Light

All -trans retinal i *

Metarhodopsin II

Activation of G protein (transducin)

Activation of phosphodiesterase

I cGMP

Closure of Na+ channels

Hyperpolarization

Figure 2-5. Steps in photoreception. cGMP = cyclic guanosine monophosphate.

e. When the receptor cell is hyperpolarized, there is decreased release of either an excitatory neurotransmitter or an inhibitory neurotransmitter.

(1) If the neurotransmitter is excitatory, then the response of the bipolar or horizontal cell to light is hyperpolarization.

(2) If the neurotransmitter is inhibitory, then the response of the bipolar or horizontal cell to light is depolarization. In other words, inhibition of inhibition is excitation.

5. Receptive visual fields a. Receptive fields of the ganglion cells and lateral geniculate cells

(1) 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 center of its receptor 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 releases an excitatory or inhibitory neurotransmitter.)

(2) On-center, off-surround is one pattern of a ganglion cell receptive field. Light 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.

(3) Lateral geniculate cells of the thalamus retain the on-center, off-surround or off-center, 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.

(3) 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 (dB), a log scale.

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

- 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.

- Sound waves cause the tympanic membrane to vibrate. In turn, the ossicles vibrate, pushing the stapes into the oval window and displacing fluid in the inner ear (see II D 2 c).

- Sound energy is amplified by the lever action of the ossicles and the concentration of sound waves from the large tympanic membrane onto the smaller oval window.

- is fluid-filled.

- consists of a bony labyrinth (semicircular canals, cochlea, and vestibule) and a series of ducts called the membranous labyrinth. The fluid outside the ducts is perilymph; the fluid inside the ducts is endolymph.

(1) Structure of the cochlea: three tubular canals

(a) The scala vestibuli and scala tympani contain perilymph, which has a high [Na+].

(b) The scala media contains endolymph, which has a high [K+].

-The scala media is bordered by the basilar membrane, which is the site of the organ of Corti.

(2) Location and structure of the organ of Corti

- is located on the basilar membrane.

- contains the receptor cells (inner and outer hair cells) for auditory stimuli. Cilia protrude from the hair cells and are embedded in the tectorial membrane.

- Inner hair cells are arranged in single rows and are few in number.

- Outer hair cells are arranged in parallel rows and are greater in number than the inner hair cells.

- The spiral ganglion contains the cell bodies of the auditory nerve [cranial nerve (CN) VIII], which synapse on the hair cells.

Figure 2-6. Organ of Corti and auditory transduction.

3. Steps in auditory transduction by the organ of Corti (see Figure

- The cell bodies of hair cells contact the basilar membrane. The cilia of hair cells are embedded in the tectorial membrane.

a. Sound waves cause vibration of the organ of Corti. Because the basilar membrane is more elastic than the tectorial membrane, vibration of the basilar membrane causes the hair cells to bend by a shearing force as they push against the tectorial membrane.

b. Bending of the cilia causes changes in K+ conductance of the hair cell membrane. Bending in one direction causes depolarization; bending in the other direction causes hyperpolarization. The oscillating potential that results is the cochlear microphonic potential.

c. The oscillating potential of the hair cells causes intermittent firing of the cochlear nerves.

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