Chapter 45

1. Mechanical stimulation of the stretch receptor dendrites by stretch of the muscle they innervate causes graded depolarization of the stretch receptor neuron cell body—the receptor potential. The receptor potential spreads to the axon hillock, and if it depolarizes the membrane of the axon hillock to threshold, a Na⁺ action potential is fired in the axon.

2. The interpretation of an action potential depends on its target neuron in the brain. If the action potential arrives in a nerve of the optic tract projecting to the visual cortex, it is interpreted as light, but if a similar action potential arrives in an olfactory nerve projecting to the olfactory bulb, it is interpreted as smell.

3. The molecules responsible for the distinctive flavor of peppers are received through chemoreceptors that activate a G-protein signaling cascade that open cation channels in the receptor membrane. Capsaicin, however, directly activates a TRP protein that also serves as a thermosensor, and that protein itself is a cation channel that opens when activated by its specific stimuli.

4. Adaptation is the property of a sensory system to stop responding to a constant level of receptor stimulation. Adaptation is important when it is of value to recognize when stimulation changes but it is not necessary to have continuous information. An example is the activation of skin tactile sensors by your clothing. Adaptation would not be advantageous when the information is critical, such as pain sensations, or when constant levels of information are essential, such as from the sensory neurons that signal postural muscle tone.

1. The odorant receptor neurons (ORNs) that share the same receptors project to the same glomerular cells in the olfactory bulb. Thus the identification of a particular odorant depends on the pattern of connectivity of the ORNs that are activated.

2. An olfactory system signals intensity of a stimulus—whiff versus repulsive odor—by the frequency of action potentials it generates. The discrimination between a skunk and a skunk cabbage depends on the mixture of odorant molecules.

3. For salt, extracellular fluids including saliva have a high concentration of NaCl, therefore a lower sensitivity is appropriate for sensing concentrations above that constitutive level. For sweet, the selective pressure for its evolution was the ability to discriminate between high and low energy foods. A low sensitivity motivates the selection of higher energy foods. Bitter is a characteristic of many protective, hence poisonous, compounds that evolved in plants to discourage predation, so a high sensitivity helps avoid poisoning.

4. With only five classes of taste receptors we can sense a great variety of tastes because (1) some classes have multiple receptor genes; (2) a substance may trigger multiple receptor classes and therefore create a mixed signal; and (3) information from taste receptors is integrated with information from olfactory receptors as well as thermoreceptors (TRP channels) and tactile receptors.

1. Different mechanosensor cell types enable responses to different aspects of touch such as sharpness, texture, pressure, vibration, and itch. Different rates of adaptation of touch receptors make it possible to discriminate between stimuli that are relevant and those that aren’t. Slowly adapting mechanosensors such as those in postural muscles provide continuous information. Rapidly adapting mechanosensors provide information about changing conditions and also improve spatial and temporal sensory ability.

2. The activity of a muscle spindle stretch receptor increases the activity in the motor neuron to that muscle. The functional significance of this property of muscle spindle stretch receptors is that it enables continuous adjustments to changes in load, such as when you are holding a glass that is being filled. The activity in the Golgi tendon organ inhibits the activity in the muscle creating the stretch of that Golgi tendon organ. The functional significance is the prevention of damage to the muscle and tendons through the generation of too much tension, such as when you are attempting to pick up objects that are too heavy.

3. Different frequencies of sound pressure waves cause different frequencies of flexion of the tympanic membrane, and these movements are transmitted and amplified by the ossicles of the middle ear into vibrations of the oval window membrane of the fluid-filled inner ear. Those vibrations create pressure waves in the fluid of the inner ear. That fluid surrounds the basilar membrane in the vestibular and tympanic canals. The membrane grades from thick at the proximal end of the canal to thin at the distal end. Pressure waves of different frequencies in the fluid of the tympanic canal cause the basilar membrane to vibrate in different locations. The hair cells are on the basilar membrane, and therefore different sets of hair cells are activated by pressure waves of different frequencies.

1. Ommatidia are excellent at detecting movement as moving objects in a visual field switch neighboring ommatidia “on” and “off.” However, since each ommatidium is receiving light from only a small but discrete portion of the total visual field, only a pixilated (low-resolution) image can be formed.

2. When photons excite rhodopsin, its conformation changes and that activates a G protein that activates a phosphodiesterase that converts cGMP to GMP. The decrease in cGMP levels causes the cGMP gated Na⁺ channels to close, reducing the dark current. Na⁺ is also pumped out of the proximal end of the photoreceptor cell and its membrane potential hyperpolarizes.

3. Rods and various cones have different spectral sensitivities because they have slightly different opsins. The structure of the opsin determines which wavelengths of light it will absorbed and activate its associated 11-cis-retinal.

4. You cannot discriminate color in the periphery of your visual field because the cone cells are concentrated in the fovea, which receives light only from the center of the visual field. Rods do not distinguish color and are more abundant in the peripheral retina.

5. Two reasons why vision is impaired when you come from a brightly to a dimly lit environment are that bright light bleaches many rhodopsin molecules, and the amacrine cells have to readjust the range of brightness sensitivity of the retina.

1. The expression of TRPA1 in the python is much greater in the TG than in the DRGs. In contrast, the expression of TRPA1 in the rat snake is low and not very different in the TG and the DRGs. There are no remarkable differences in the expression of TRPV1 in the TG versus the DRGs in either the rat snake or the python.

2. Since the expression of TRPA1 is dramatically higher in the TG of the pit snake but not in the TG of the non-pit snake, these data support the conclusion that the TRPA1 channel plays a role in the function of the pit organ.

3. If expression of the TRPA1 channel confers on a gene expression model system a temperature sensitivity in the range characteristic of the pit organ, that result would be strong evidence that the TRPA1 channel is the IR sensor in the pit organ.

1. According to the data, moths with disrupted tymbal organs displayed the same types of evasive maneuvers as moths with intact tymbal organs. There is no statistical difference in the evasive maneuvers made by silenced verses clicking moths. This finding demonstrates that the moths with the disrupted tymbal organs are still capable of “hearing” the echolocation signals coming from predatory bats and of responding with evasive maneuvers. If a significant difference had been seen in Figure A, this could indicate that tymbal organ disruption impeded a moth’s ability to either hear the bat signals or respond with an evasive behavior.

2. According to the data, moths with intact tymbal organs were captured significantly less often than moths with disrupted tymbal organs. This is true for all silenced moths no matter what type of evasive maneuver they used to escape bat predation. If evasive maneuvers alone were responsible for moth escapes, we would not expect the number of captures between clicking and silenced moths to be significantly different. The fact that they are significantly different supports the hypothesis that the clicking of the tymbal organ serves as an echolocation-jamming device, which when paired with evasive maneuvers results in fewer captures.

3. Different frequency sound waves cause flexion of the basilar membrane at different locations because the basilar membrane varies in thickness and stiffness. At the base it is most stiff and thick and is only flexed by high frequency pressure waves in the cochlear canals. At the apical end it is thinner and less stiff and is flexed by low frequency pressure waves in the cochlear canal. The bat can hear higher frequency sounds than humans can and therefore their basilar membranes must be thicker and stiffer at the basal end than are the basilar membranes of humans.