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38BRS Physiology
■Usually, the current is inward, which produces depolarization of the receptor.
■The exception is in the photoreceptor, where light causes decreased inward current and hyperpolarization.
c. The change in membrane potential produced by the stimulus is the receptor potential, or generator potential (Figure 2.2).
■If the receptor potential is depolarizing, it brings the membrane potential closer to threshold. If the receptor potential is large enough, the membrane potential will exceed threshold, and an action potential will fire in the sensory neuron.
■Receptor potentials are graded in size depending on the size of the stimulus.
5. Adaptation of sensory receptors
a. Slowly adapting, or tonic, receptors (muscle spindle; pressure; slow pain)
■respond repetitively to a prolonged stimulus.
■detect a steady stimulus.
b. Rapidly adapting, or phasic, receptors (pacinian corpuscle; light touch)
■showadeclineinactionpotentialfrequencywithtimeinresponsetoaconstantstimulus.
■primarily detect onset and offset of a stimulus.
6. Sensory pathways from the sensory receptor to the cerebral cortex a. Sensory receptors
■are activated by environmental stimuli.
■may be specialized epithelial cells (e.g., photoreceptors, taste receptors, auditory hair cells).
■may be primary afferent neurons (e.g., olfactory chemoreceptors).
■transduce the stimulus into electrical energy (i.e., receptor potential).
b. First-order neurons
■are the primary afferent neurons that receive the transduced signal and send the information to the CNS. Cell bodies of the primary afferent neurons are in dorsal root or spinal cord ganglia.
c. Second-order neurons
■are located in the spinal cord or brain stem.
■receive information from one or more primary afferent neurons in relay nuclei and transmit it to the thalamus.
■Axons of second-order neurons may cross the midline in a relay nucleus in the spinal cord before they ascend to the thalamus. Therefore, sensory information originating on one side of the body ascends to the contralateral thalamus.
d. Third-order neurons
■are located in the relay nuclei of the thalamus. From there, encoded sensory information ascends to the cerebral cortex.
e. Fourth-order neurons
■are located in the appropriate sensory area of the cerebral cortex. The information received results in a conscious perception of the stimulus.
Action potential
Threshold
Receptor potential
Figure 2.2 Receptor (generator) potential and how it may lead to an action potential.
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B.Somatosensory system
■includes the sensations of touch, movement, temperature, and pain.
1. Pathways in the somatosensory system a. Dorsal column system
■processes sensations of fine touch, pressure, two-point discrimination, vibration, and proprioception.
■consists primarily of group II fibers.
■Course: primary afferent neurons have cell bodies in the dorsal root. Their axons ascend ipsilaterally to the nucleus gracilis and nucleus cuneatus of the medulla. From the medulla, the second-order neurons cross the midline and ascend to the contralateral thalamus, where they synapse on third-order neurons. Third-order neurons ascend to the somatosensory cortex, where they synapse on fourth-order neurons.
b. Anterolateral system
■processes sensations of temperature, pain, and light touch.
■consists primarily of group III and IV fibers, which enter the spinal cord and terminate in the dorsal horn.
■Course: second-order neurons cross the midline to the anterolateral quadrant of the spinal cord and ascend to the contralateral thalamus, where they synapse on thirdorder neurons. Third-order neurons ascend to the somatosensory cortex, where they synapse on fourth-order neurons.
2. Mechanoreceptors for touch and pressure (Table 2.6)
3. Thalamus
■Information from different parts of the body is arranged somatotopically.
■Destruction of the thalamic nuclei results in loss of sensation on the contralateral side of the body.
4. Somatosensory cortex—the sensory homunculus
■The major somatosensory areas of the cerebral cortex are SI and SII.
■SI has a somatotopic representation similar to that in the thalamus.
■This “map” of the body is called the sensory homunculus.
■The largest areas represent the face, hands, and fingers, where precise localization is most important.
5. Pain
■is associated with the detection and perception of noxious stimuli (nociception).
■The receptors for pain are free nerve endings in the skin, muscle, and viscera.
■Neurotransmitters for nociceptors include substance P. Inhibition of the release of substance P is the basis of pain relief by opioids.
a. Fibers for fast pain and slow pain
■Fast pain is carried by group III fibers. It has a rapid onset and offset, and is localized.
■Slow pain is carried by C fibers. It is characterized as aching, burning, or throbbing that is poorly localized.
t a b l e 2.6 Types of Mechanoreceptors
Type of Mechanoreceptor |
Description |
Sensation Encoded |
Adaptation |
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Pacinian corpuscle |
Onion-like structures in the |
Vibration; tapping |
Rapidly adapting |
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subcutaneous skin (surrounding |
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unmyelinated nerve endings) |
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Meissner corpuscle |
Present in nonhairy skin |
Velocity |
Rapidly adapting |
Ruffini corpuscle |
Encapsulated |
Pressure |
Slowly adapting |
Merkel disk |
Transducer is on epithelial cells |
Location |
Slowly adapting |
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b. Referred pain
■Pain of visceral origin is referred to sites on the skin and follows the dermatome rule. These sites are innervated by nerves that arise from the same segment of the spinal cord.
■For example, ischemic heart pain is referred to the chest and shoulder.
C.Vision
1. Optics
a. Refractive power of a lens
■is measured in diopters.
■equals the reciprocal of the focal distance in meters.
■Example: 10 diopters = 1/10 m = 10 cm
b. Refractive errors
(1) Emmetropia—normal. Light focuses on the retina.
(2) Hypertropia—farsighted. Light focuses behind the retina and is corrected with a convex lens.
(3) Myopia—nearsighted. Light focuses in front of the retina and is corrected with a biconcave lens.
(4) Astigmatism. Curvature of the lens is not uniform and is corrected with a cylindric lens.
(5) Presbyopia is a result of loss of the accommodation power of the lens that occurs
with aging. The near point (closest point on which one can focus by accommodation of the lens) moves farther from the eye and is corrected with a convex lens.
2. Layers of the retina (Figure 2.3) a. Pigment epithelial cells
■absorb stray light and prevent scatter of light.
■convert 11-cis retinal to all-trans retinal.
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
Horizontal cell
Amacrine cell
Bipolar cell
Ganglion cell
Direction of light
Figure 2.3 Cellular layers of the retina. (Reprinted with permission from Bullock J, Boyle J III, Wang MB. Physiology. 4th ed. Baltimore: Lippincott Williams & Wilkins, 2001:77.)
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t a b l e |
2.7 |
Functions of Rods and Cones |
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Function |
Rods |
Cones |
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Sensitivity to light |
Sensitive to low-intensity |
Sensitive to high-intensity |
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light; night vision |
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light; day vision |
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Acuity |
Lower visual acuity |
Higher visual acuity |
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Not present in fovea |
Present in fovea |
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Dark adaptation |
Rods adapt later |
Cones adapt first |
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Color vision |
No |
Yes |
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b. Receptor cells are rods and cones (Table 2.7).
■ 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.
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 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 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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Temporal |
Nasal |
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field |
field |
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Left |
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Right |
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eye |
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eye |
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Ganglion cell |
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Left |
Right |
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b |
Optic nerve |
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a |
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Optic chiasm |
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c |
Pretectal |
Optic tract |
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region |
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Lateral geniculate body
Geniculocalcarine tract
d
Occipital cortex
a
b
c
d
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.)
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.
a. 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.
(1) Ionotropic glutamate receptors are excitatory. If decreased release of glutamate from the photoreceptors interacts with ionotropic receptors, there will be hyperpolar
ization (inhibition) because there is decreased excitation.
(2) Metabotropic glutamate receptors are inhibitory. If decreased release of glutamate from photoreceptors interacts with metabotropic receptors, there will be depolarization (excitation) because there is decreased inhibition.
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 has ionotropic or metabotropic receptors.)
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11-cis retinal
Light
All-trans retinal
Metarhodopsin II
Activation of G protein (transducin)
Activation of phosphodiesterase
cGMP
Closure of Na+ channels
Hyperpolarization
Figure 2.5 Steps in photoreception in rods. cGMP = cyclic guanosine |
Decreased glutamate release |
monophosphate. |
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(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 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.
(3) Hypercomplex cells respond best to lines with particular length and to curves and angles.
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D.Audition
1. Sound waves
■Frequency is measured in hertz (Hz).
■Intensity is measured in decibels (dB), a log scale.
dB = 20 log P P0
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.
■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 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.
c. Inner ear (Figure 2.6)
■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.
Scala vestibuli
Scala media
Tectorial membrane
Spiral ganglia |
Basilar membrane |
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Scala tympani |
Figure 2.6 Organ of Corti and auditory transduction.
