Download Answers to Mastering Concepts Questions

Mastering Concepts
27.1
1. What role do the senses play in maintaining homeostasis?
The senses monitor internal and external stimuli, including blood pH, body temperature,
levels of ions and water in interstitial fluids, and a host of other physical and chemical
conditions. Information about these stimuli is transmitted to the central nervous system
for processing and may trigger hormonal, chemical, or behavioral adjustments that
maintain homeostasis.
2. Distinguish between sensation and perception.
A sensation is the raw input of a receptor as it arrives at the central nervous system.
Perception is the interpretation of the sensation in the central nervous system, and it
reflects an integration of all sensory input along with memory.
3. What are the major types of sensory receptors?
The major types of sensory receptors are mechanoreceptors, thermoreceptors, pain
receptors, proprioceptors, photoreceptors, and chemoreceptors.
4. What is a receptor potential?
A receptor potential is a graded potential that occurs in a sensory receptor. If the receptor
potential is large enough, it will generate an action potential in the sensory receptor.
5. What is sensory adaptation, and how is it beneficial?
Sensory adaptation is a reduced response to a stimulus. It enables us to tune out
sensations that are the equivalent of irrelevant “background noise.”
27.2
1. Which structures provide the senses of touch, temperature, pain, and position?
Mechanoreceptors in the skin provide the sense of touch. Some free nerve endings in the
skin are thermoreceptors, whereas other free nerve endings detect mechanical damage
and produce the sensation of pain. Position is detected by proprioceptors.
2. What is the role of the cerebral cortex in integrating information about the general
senses?
A portion of the cerebral cortex receives information about the general senses. That input
is mapped to specific body locations so that the sensation can be interpreted.
27.3
1. How does the brain distinguish one odor from another?
A portion of the cerebral cortex distinguishes one odor from another based on the specific
combination of receptor proteins that have transmitted the impulse from the olfactory
epithelium.
2. How does a taste bud function?
A taste bud is a cluster of taste receptor cells. When a food molecule binds to a
chemoreceptor in a taste bud, sensory neurons convey the message to a portion of the
cerebral cortex for processing and integration.
27.4
1. Describe three types of invertebrate eyes.
Some invertebrates have cup-shaped eyes made of photoreceptor cells, as in the eyespots
of a flatworm. Others have compound eyes with tightly packed photoreceptors, as in
insects. Finally, some invertebrates have a single lens eye that is fluid filled and houses
photoreceptors at the back, as in cephalopods.
2. What are the parts of the vertebrate eye?
The sclera includes the white of the eye and the cornea. The choroid is internal to the
sclera; it includes the iris, pupil, and lens. The retina, the innermost layer of the eye,
consists of photoreceptor cells. Most of the eye’s volume is filled with the jellylike
vitreous humor; the watery aqueous humor fills the space between the cornea and lens.
3. How do rods, cones, and pigments participate in vision?
Rod cells and cone cells detect light. Rod cells provide black-and-white vision in dim
light, and cone cells provide color vision in bright light. Both cell types contain lightsensitive pigments that absorb photons of light and trigger receptor potentials that are
passed on to other neurons that send action potentials to the brain.
4. Trace the pathway of information flow from the retina to the visual cortex of the brain.
In the retina, light sensitive pigments in rods and cones absorb light energy of different
wavelengths. In the presence of light, the pigment molecule changes shape and triggers a
receptor potential that stimulates the retina’s bipolar neurons. These send the message to
the ganglion cells. If they become depolarized, the ganglion cells send action potentials
through the visual pathway to the optic nerve. The optic nerve exits the eyeball, traveling
from the retina to the brain. Optic nerves first go to the thalamus. Then the visual
information goes to neurons in the primary visual cortex of the brain.
27.5
1. What are the parts of the ear, and how do they transmit sound?
The outer ear funnels sound waves into the auditory canal that ends in the eardrum. In
response to sound waves, the eardrum and bones of the middle ear move; their
movements jiggle the fluid of the inner ear’s cochlea. Vibration of the fluid in the cochlea
causes cilia of hair cells to move relative to the tectorial membrane. This movement, in
turn, causes the hair cells to release a neurotransmitter that triggers action potentials in
the auditory nerve.
2. How does the vestibular apparatus provide the sense of equilibrium?
The vestibular apparatus includes the utricle and saccule, along with the three
semicircular canals. The utricle and saccule detect whether the head is accelerating
horizontally or vertically. Calcium carbonate granules cause hair cells to bend, triggering
action potentials that the brain senses as acceleration. The semicircular canals detect
whether the head is tilting or rotating. These motions shift the fluid in the semicircular
canals; the fluid, in turn, bends the cilia of hair cells that transmit action potentials to the
nearby cranial nerve and from there to the brain.
27.6
1. Explain how acid stimulates action potentials in the pain-sensing neurons of mice but
not those of naked mole rats.
In both mice and naked mole rats, acid stimulates acid-sensing channels, depolarizing the
pain receptor. In mice, depolarization often reaches threshold potential, so action
potentials are sent to the brain. However, because acid inhibits sodium channels in the
axons of a naked mole rat’s pain receptors, depolarization does not reach threshold
potential and no action potentials are initiated.
2. Why is the inability to sense pain from acid adaptive for naked mole rats?
Acid does not harm the naked mole rats, so it would be distracting for their brains to
receive a constant input of pain from the sensory system.
Write It Out
1. A male moth uses his antennae to detect the concentration of a pheromone on each side
of his body, allowing him to fly toward the female that is producing the pheromone.
Explain how sensory receptors, peripheral nerves, and the central nervous system interact
to allow moths to compare pheromone concentrations in the air surrounding each
antenna.
Sensory receptors on each antenna bind to different numbers of pheromone molecules.
Action potentials are initiated in many of these receptors. The peripheral nerves send
signals from the sensory receptors to the brain, which processes the frequency of action
potentials arriving from each antenna.
2. What is the role of transduction in the sensory system? How does transduction occur
for each of the senses described in this chapter?
In transduction, the energy of external stimuli is converted to the energy of action
potentials that the nervous system can interpret. In touch, pressure on the
mechanoreceptors generates the action potential. Free nerve endings in the skin
transduce information about temperature. Pain receptors respond to mechanical damage.
Proprioceptors embedded in muscles and joints are encapsulated nerve endings that
detect the body’s position. In the senses of smell and taste, chemoreceptors transduce
information about molecules dissolved in a watery solution. Light stimulates pigment
molecules in photoreceptors, and mechanoreceptors in hair cells transduction information
in hearing and equilibrium.
3. Explain why evolution has favored slow sensory adaptation for pain stimuli.
Since pain indicates damage to tissues, a continuous sensation of pain is adaptive because
it induces the animal to protect the injured body part and to avoid further damage.
4. Try as you may, you cannot tickle yourself. Speculate about why it could be adaptive
to respond to surprises but not self-imposed stimuli.
Self-imposed stimuli are generally less threatening than unanticipated stimuli. The brain
therefore often ignores expected stimuli to enhance the perception of sensations with
external causes.
5. How does the nervous system differentiate among odors?
Specialized olfactory receptor neurons are located in a patch of epithelium high in the
nasal cavity. Humans have about 12 million olfactory receptor cells, each with 10 to 20
cilia that increase the surface area for receiving odorant molecules. Odorant molecules
bind to receptors on the surfaces of olfactory receptor cells, and the brain perceives a
smell by evaluating the pattern of olfactory receptor cells that bind odorant molecules.
6. Why might cold medicines that dry the nasal cavity make it harder to smell?
Olfactory receptors (the chemoreceptors in the nose) can only detect odorant molecules
that have dissolved in the watery solution that lines the inside of the nasal cavity. If the
nose is dry, fewer odorant molecules are detected.
7. Explain why some people hold their nose when consuming bad tasting food or
medicine.
The flavor of food comes not only from sensations at the taste buds but also from the
food’s aroma. Holding the nose keeps these airborne molecules from contacting the
olfactory receptors and greatly reduces the flavor of what is being eaten.
8. Suppose you put on glasses belonging to someone who is more farsighted than you.
Draw how light passes through the glasses and into your eye. Why will the glasses blur
your vision?
Refer to figure 27.A. Glasses that correct for farsightedness make light rays converge
over a shorter distance. Therefore, putting on glasses that over-adjust for farsightedness
will cause light to converge in front of the retina. The glasses will blur your vision
because light rays are not converging precisely on the retina.
9. In a disorder called macular degeneration, photoreceptors at the center of the retina die.
How does macular degeneration impair vision? Why is peripheral vision unaffected?
Loss of photoreceptors at the center of the retina will cause a loss of acuity in the center
of the field of view. Peripheral vision should be less affected because photoreceptors at
the periphery of the retina remain intact.
10. In what ways do the cochlea and vestibular apparatus function similarly?
The cochlea and vestibule rely on mechanoreceptors called hair cells. The motion of fluid
causes cilia on the hair cells to bend, triggering action potentials in nerves leading to the
brain.
11. In a rare condition called synesthesia, stimulation of one sense causes stimulation of
another sense. For example, people with synesthesia have reported seeing bursts of color
when stimulated with loud noises. Would you expect synesthesia to be a problem with
sensory receptors, peripheral nerves, or the central nervous system? Explain.
People with synesthesia experience stimuli that are not actually present. Using the
example from the question, loud noises cannot stimulate photoreceptors. Instead,
integration of the sound sensations in the central nervous system stimulates the same
nervous pathways that light ordinarily stimulates. The brain therefore interprets a signal
from the ear as both an auditory and a visual perception, even though the visual stimulus
is absent.
Pull It Together
1. Which sense organs are required for each of the general and special senses?
Sensing touch requires mechanoreceptors in the skin. Sensing temperature requires
thermoreceptors in the skin. Nearly every organ has pain receptors. Sensing body position
requires proprioceptors in muscles and ligaments. The sense of smell requires
chemoreceptors in the nasal cavity, whereas the sense of taste requires chemoreceptors in
the mouth. Vision requires photoreceptors in the eyes. Both hearing and equilibrium
require mechanoreceptors in the ears.
2. Add mechanoreceptors, chemoreceptors, and photoreceptors to the concept map.
“Mechanoreceptors” connects with the phrase “initiate the sensation of” to “Touch,”
“Hearing,” and “Equilibrium.” “Chemoreceptors” connects with the phrase “initiate the
sensation of” to “Smell” and “Taste.” “Photoreceptors” connects with the phrase “initiate
the sensation of” to “Vision.”
3. Describe one way that each sense listed in this concept map can help the body maintain
homeostasis.
[Answers will vary; this sample includes examples.] Touch and pain allow the body to
respond to stimuli that might cause tissue damage. Thermoreceptors allow the body to
maintain the optimal temperature for enzyme function. Knowing the body’s position is
important to resting, conserving energy, and avoiding injury. Smell and taste help
differentiate between nutritious and toxic or spoiled foods. Sight stimulates quick
responses to danger and safety. Hearing informs the body of threats that are not visible.
Equilibrium keeps the body balanced, allowing quick movement away from danger or
toward a food source.