Mastering Concepts
26.1
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 CNS as all sensory input is
integrated and then combined with memory.
3. What are the major types of sensory receptors?
The major types of sensory receptors are chemo-, photo-, mechano-, thermo-, proprio-,
electro- and pain receptors.
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, tuning out sensations that are the
equivalent of irrelevant “background noise.”
26.2
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 somatosensory cortex?
The somatosensory cortex receives input from receptors. That input is mapped to
specific body locations so that the sensation can be interpreted.
26.3
1. How does the brain distinguish one odor from another?
The cerebral cortex distinguishes one odor from another based on the specific membranebound receptor proteins that have transmitted the impulse.
2. What are pheromones?
Pheromones are scent molecules made by one organism that carry information to and
cause a response in another individual of the same species.
3. How does a taste bud function?
A taste bud is a cluster of taste receptor cells that have concentrations of chemoreceptors.
Each sends an action potential to sensory neurons that take the message to the brain for
processing and integration.
4. What are the five taste sensations that impart a food’s flavor?
The five taste sensations that impart a food’s flavor are sweet, bitter, salty, sour, and
umami.
26.4
1. Describe three types of invertebrate eyes.
Some invertebrates have a cup-shaped eye made of photoreceptor cells, as seen in the
eyespot of flatworms. Others have compound eyes with tightly packed photoreceptors, as
seen in insects. Finally, some insects have a single lens eye that is fluid filled and houses
photoreceptors at the back, as seen in cephalopods.
2. What are the parts of the vertebrate eye?
The parts of the vertebrate eye include the sclera, the choroids, and the retina. The sclera
includes the white of the eye and the cornea. The choroid includes blood vessels, lens,
iris, and pupil. The retina is a layer of photoreceptors at the back of the eye. Most of the
eye’s volume is filled with vitreous humor. Aqueous humor fills the space between the
cornea, iris, and lens.
3. What are the roles of rod cells, cone cells, and light-sensitive pigments in human
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 light-
sensitive 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.
26.5
1. What is the role of mechanoreceptors in the senses of hearing and equilibrium?
Mechanoreceptors detect sound waves and changes in body orientation, and they transmit
this information to processing centers in the brain.
2. 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 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.
3. How does the vestibular apparatus provide the sense of equilibrium?
When body position changes, fluid in the three semicircular canals bends cilia of hair
cells that transmit action potentials to the nearby cranial nerve. The vestibule also has
hair cells and is filled with fluid that contains granules of calcium carbonate. As the head
tilts, these granules land on hair cells. When the cilia bend, the hair cells generate action
potentials that the brain interprets as a change in body position.
26.6
1. What is the significance of the discovery that ragworms have both rhabdomeric and
ciliary photoreceptors?
Vertebrate eyes have ciliary photoreceptors and are hypothesized to have evolved from
invertebrates with rhabdomeric photoreceptors. The presence of both types of cells in
ragworms, where the ciliary cells are in the brain and not involved in light detection,
provides evidence of a possible evolutionary route for the takeover of photoreception by
ciliary cells in vertebrates by a cell that was present in our invertebrate ancestors.
2. How might you test the researchers’ hypothesis that an ancestor of P. dumerilii had
one photoreceptor and both types of opsins?
Specific answers will vary, but your experimental design might include comparing
photoreceptors of P. dumerilii with photoreceptors of species in related clades. It also
might be useful to estimate when c-opsins diverged from r-opsins.
Write It Out
1. How does the peripheral nervous system interact with the central nervous system to
produce perceptions of stimuli?
The peripheral nervous system is responsible for detecting stimuli and transmitting them
to the central nervous system where they can be interpreted.
2. Distinguish between a sense organ and a sensory receptor.
Sense organs (skin, eyes, ears, nose) derive their information from sensory receptor cells,
which detect stimuli. Sensory receptors are portals through which nervous systems
experience the world. Some sensory receptors are neurons, and others are epithelial cells
that communicate with sensory neurons. The sensory neurons, in turn, pass the
information to the central nervous system.
3. What is the role of transduction in the sensory system? How does transduction occur
for each of the senses described in this chapter?
Transduction is the process where the energy of external stimuli is converted to the
energy of action potentials, the form of energy the nervous system is able to interpret. In
touch, pressure on the mechanoreceptors generates the action potential. In temperature
free nerve endings in the skin do transduction. Pain receptors respond to mechanical
damage. Proprioceptors are encapsulated nerve endings embedded in muscles and joints
that detect the body’s position. Chemoreceptors bind to molecules dissolved in a watery
solution for transduction to occur in smell and taste. Light activates chemicals in
photoreceptors and mechanoreceptors in hair cells are moved by fluid and calcium
carbonate granules for transduction in hearing and equilibrium.
4. Describe an example of sensory adaptation other than the ones listed in this chapter.
[Answers will vary]
5. Why is the least painful place to receive an injection the buttock or upper arm, rather
than, say, the inside of the hand?
These are areas of the body with fewer pain receptors. In fact, these areas have fewer
sensations altogether since they are not involved in fine motor control.
6. People with Hansen disease (formerly called leprosy) suffer nerve damage that leaves
them unable to sense pain in their extremities. How might this situation be dangerous?
The sensation of pain can be a life-saving event. If a person does not feel pain, small
wounds may become infected, and bone breaks or burns may remain untreated and cause
tissue damage.
7. 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.
8. Suppose that some male moths lack a functional pheromone receptor protein in their
antennae. Using what you know about natural selection, would you expect this trait to
become more common in the moth population over multiple generations? Explain.
The trait would not become more common. Without a functioning receptor the male
moths would not detect the female pheromone and so would not be attracted for mating.
Without mating the gene coding for the trait would not be passed on.
9. Design an experiment to test the hypothesis that pheromones play a role in human
sexual attraction. Is your experiment both practical and ethical? Explain.
[Answers vary]
10. 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
smell or aroma of food. Holding the nose keeps the airborne molecules from contacting
the olfactory receptors and greatly reduces the flavor of what is being eaten.
11. In what ways are the senses of smell and taste similar? In what ways are they
different?
The senses of smell and taste both depend on chemoreceptors that detect chemicals in the
environment. Both senses require that a stimulus molecule be dissolved in a watery
solution, such as saliva or the moist lining of a nasal passage. In addition, the molecule
must interact with a receptor on a sensory cell’s membrane. However, the sense of taste
is limited to just five stimuli, but smell has many, many stimuli.
12. List the structures of the human eye and their functions.
Iris—regulates the diameter of the pupil
Lens—further bends light and focuses it on the retina
Pupil—the opening that lets light through to the lens
Cornea—the clear outer part of the eye that first bends the light
Retina—contains the photoreceptors
Choroid—the vascular layer that supplies the eye with nutrients and oxygen
Sclera—the outer portion of the eye providing protection and structure
Optic nerve—the cranial nerve that connects the eye to the brain
13. 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.
14. When you enter a darkened room, the rod cells in your eyes boost their production of
rhodopsin, increasing their sensitivity to light over a period of 10 to 30 minutes. How do
you think your eyes adjust when you emerge from a movie theater on a sunny day?
In bright light, nearly all of the rhodopsin becomes nonfunctional, sharply reducing the
sensitivity of the rod cells.
15. In what ways do the cochlea and vestibular apparatus function similarly?
The cochlea and vestibule function all rely on the motion of fluid and the motion of
ciliated hair cells relative to gelatinous membranes. Fluid movement stimulates the
movement of the membranes, causing bending of the cilia of the sensory hair cells, each
of which triggers action potentials. The brain interprets this information.
16. As we age, our senses of sight, hearing, smell, and taste decline. Brainstorm some of
the ways that people compensate for each of these deficits.
[Answers vary]
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 thermo receptors in the skin.
Pain is sensed in every organ.
Sensing body position requires proprioceptors in muscles and ligaments.
Smell requires chemoreceptors in the nasal cavity.
Taste requires chemoreceptors in the mouth and on the tongue.
Vision requires photoreceptors in the eyes.
Hearing requires mechanoreceptors in the ears.
Equilibrium requires mechanoreceptors sensitive to gravity, which are in the ears.
2. Make a chart that lists the types of sensory receptors and the sense organs that use each
type.
(See Table 26.1)
Receptor Type
Mechanoreceptors
Thermoreceptors
Nociceptors, Pain receptors
Proprioceptors, Position receptors
Chemoreceptors
Photoreceptors
Organ(s)
Skin, Ears, Equilibrium
Skin
Everywhere except the brain
Muscles and ligaments
Nasal cavity, mouth and tongue
Eyes
3. What is the role of rods and cones in the sense of vision?
Rods and cones are neurons in the retina that respond to light. Rods respond to light
intensity, or brightness, and cones respond to light wavelength, which we perceive as
color. Ultimately, the brain processes signals from rods and cones into the image we
perceive.
4. Describe one way that each sense listed in this concept map can help the body maintain
homeostasis.
Answers will vary.
Touch allows the body to respond to stimuli that might cause pain.
Thermoreception allows the body to maintain the optimal temperature for enzyme
function.
Pain sensing provides feedback to help the body heal and minimize injury.
Knowing the body’s position is important to resting, conserving energy, and avoiding
injury.
Smell helps to avoid ingesting toxic or spoiled foods.
Taste helps reinforce consumption of nutrients that the body needs.
Sight stimulates quick hormonal responses to danger, and stimulates alternate hormonal
responses when the danger has passed.
Hearing informs the body of threats before they can be seen or heard.
Equilibrium keeps the body balanced, allowing quick movement away from danger or to
a food source.