Answers to Open-Ended Questions
Hoefnagels Essentials 2/e
Chapter 24
Answers to Mastering Concepts Questions
24.1
1. How are nervous systems adaptive to animals?
An animal’s nervous system allows it to perceive and quickly respond to its environment.
In addition, together with the endocrine system, the nervous system also coordinates
many of the actions that maintain homeostasis.
2. What are the roles of neurons and neuroglia?
Neurons are the communication cells in the nervous system, whereas neuroglia play
many supportive roles.
3. Distinguish between the central and peripheral nervous systems.
The central nervous system consists of the brain and spinal cord. The peripheral nervous
system lies outside of the brain and spinal cord.
24.2
1. Describe the parts of a typical neuron.
The cell body, which contains the nucleus, mitochondria, and ribosomes, carries out the
neuron’s metabolic functions. Dendrites are branches that convey sensory input to the
neuron’s cell body. The axon is a long fiber extending from the cell body. An axon
branches at its tip and transmits information from the cell body to other neurons, muscles,
or glands.
2. Where is the myelin sheath located?
The myelin sheath coats sections of the axon of some neurons.
3. In what direction does a message move in a neuron?
Typically a message moves from dendrites to cell body to axon.
4. What are the functions of each of the three classes of neurons?
Sensory neurons bring information to the central nervous system. Motor neurons convey
information from the central nervous system to muscles and glands. Interneurons are
central nervous system neurons that connect one neuron to another.
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24.3
1.What is the difference between the resting potential, the threshold potential, and an
action potential?
A resting potential is the voltage difference between the inside and outside of the
membrane when the axon is not transmitting a signal; the inside of the cell is more
negative than the outside. Threshold potential is slightly less negative than resting
potential. When the membrane reaches threshold potential, an action potential begins.
An action potential is a reversal in the membrane potential, during which the inside of the
cell is briefly positive relative to the outside before returning to resting potential.
2. How does an axon generate and transmit a neural impulse?
If a segment of membrane near the beginning of the axon reaches threshold potential,
then nearby Na+ channels will briefly open, allowing Na+ into the cell. This influx of Na+
causes the membrane potential to become positive. Some Na+ diffuses down the axon,
causing additional Na+ channels to open. The action potential propagates toward the end
of the axon.
3. How does myelin speed neural impulse transmission?
Sodium channels occur only in the gaps in the myelin sheath; myelin-covered parts of the
axon lack sodium channels. This arrangement speeds neural impulse transmission
because it allows the action potential to jump from gap to gap.
24.4
1. Describe the structure of a synapse.
A synapse consists of a “sending” neuron, a “receiving” cell, and the synaptic cleft (the
space between these two cells). The sending neuron’s axon ends in a synaptic terminal
containing synaptic vesicles carrying neurotransmitters. The receiving cell has ion
channels with receptors for the neurotransmitters that diffuse across the synaptic cleft
from the sending neuron.
2. What event stimulates a neuron to release neurotransmitters?
The arrival of an action potential at the synaptic terminal triggers the release of
neurotransmitters.
3. What happens to a neurotransmitter after its release?
After a neurotransmitter is released, some of it travels to receptors on the receiving cell.
Some diffuses away, some is destroyed, and some is taken back into the sending cell.
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24.5
1. Which structures make up the peripheral nervous system?
The peripheral nervous system consists of nerve cells outside of the central nervous
system, primarily nerves that branch off the brain and spinal cord.
2. How do the sensory and motor pathways of the peripheral nervous system differ?
The sensory pathways transmit action potentials from the peripheral nervous system to
the central nervous system, and the motor pathways carry information from the central
nervous system to muscles and glands.
3. Describe the relationships among the motor, somatic, autonomic, sympathetic, and
parasympathetic nervous systems.
The motor system carries action potentials to the muscle and glands. It is divided into the
somatic and autonomic systems, which carry signals to voluntary and involuntary
structures, respectively. The autonomic system is further divided into the sympathetic
system (“fight or flight”) and the parasympathetic system (“rest and repose”).
4. Cite an example of the opposing actions of the sympathetic and parasympathetic
nervous systems.
During a stressful event, the sympathetic nervous system increases breathing rate and
pulse. After the event has ended, the parasympathetic nervous system slows respiration
and heart rate.
24.6
1. What is the relationship between gray matter and white matter in the spinal cord?
In the spinal cord, white matter is on the surface and gray matter forms an H shape in the
center of the spinal cord. Gray matter contains cell bodies and synapses that process
information, whereas white matter contains myelinated axons that transmit information to
and from the brain.
2. What are the major structures in the hindbrain, midbrain, and forebrain, and what are
their functions?
In the hindbrain, the major structures are the medulla oblongata, pons, and cerebellum.
The medulla oblongata regulates essential physiological processes like breathing. The
pons is the “bridge” that connects the forebrain to the medulla and the cerebellum. The
cerebellum refines motor messages and coordinates subconscious muscle movements.
The midbrain is part of the brainstem (along with the pons and medulla oblongata). It
participates in hearing and eye reflexes and regulates consciousness; nerve fibers that
control voluntary motor function pass from the forebrain through the midbrain.
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The forebrain consists of the thalamus, hypothalamus, and cerebrum. The thalamus is a
relay station that receives sensory information and sends it to the correct portion of the
cerebrum. The hypothalamus links the nervous and endocrine systems, sending and
receiving neural and hormonal signals that help it maintain homeostasis in temperature,
blood pressure, and many other conditions. The cerebrum controls the qualities of the
mind, such as personality, intelligence, and perception; the cerebrum includes the
cerebral cortex and parts of the limbic system (which also includes the hypothalamus and
thalamus).
3. How do short- and long-term memories differ?
Short-term memory stores information only for moments, after which it is forgotten.
Long-term memory forms more permanent connections between neurons in a pathway so
that the information is available for much longer periods.
4. What are some examples of disorders that affect the central nervous system?
Trauma and stroke can damage the central nervous system, as can various infectious and
noninfectious diseases (such as Parkinson disease).
5. To what extent can the nervous system heal itself?
Mature neurons of the central nervous system cannot regenerate. However, neurons may
form new connections that compensate for the loss of other neurons.
24.7
1. Distinguish between sensation and perception.
A sensation is the raw sensory input that 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.
2. What role do the senses play in maintaining homeostasis?
The senses monitor internal and external stimuli, including blood pH, hormone
concentrations, 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
adjustments that maintain homeostasis.
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?
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A receptor potential is a change in membrane potential that occurs in a sensory receptor.
If the receptor potential is large enough, it will generate an action potential in the sensory
neuron.
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.”
24.8
1. Which structures provide the senses of touch, temperature, and pain?
Mechanoreceptors in the skin provide the sense of touch. Some receptors in the skin are
thermoreceptors that sense temperature, whereas others detect mechanical damage and
produce the sensation of pain.
2. How does the brain participate in the general senses?
The brain receives input from receptors. That input is mapped to specific body locations
so that the sensation can be interpreted.
24.9
1. How does the brain detect and identify odors?
Olfactory receptor cells in the nose send signals to the brain's olfactory bulb. The cerebral
cortex identifies the odor based on the specific membrane-bound receptor proteins that
have transmitted the impulse.
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.
3. What are pheromones?
Pheromones are chemical signals that cause a response in another individual of the same
species.
24.10
1. 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,
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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.
2. What are the roles of photoreceptors and pigments in vision?
Rod cells and cone cells are photoreceptors, which detect light. Rod cells provide blackand-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.
3. 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 various neurons. These send action
potentials to the optic nerve, which sends the information to the visual cortex for
processing and interpretation.
24.11
1. What is the role of mechanoreceptors in the sense of hearing?
Mechanoreceptors in the cochlea detect sound waves that vibrate the fluid of the inner
ear. 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 at 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 overlying membrane. This movement, in
turn, causes the hair cells to release a neurotransmitter that triggers action potentials in
the auditory nerve.
Write It Out
1. How do the nervous and endocrine systems differ in how they communicate?
One difference is the speed at which the two communication systems act. The nervous
system’s electrical impulses travel so rapidly that their effects are essentially
instantaneous. The endocrine system acts more slowly. Neurons secrete neurotransmitters
that affect adjacent cells; in contrast, endocrine glands secrete chemical messages called
hormones that circulate throughout the bloodstream and take hours or longer to exert
their effects.
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2. Explain how sensory neurons, interneurons, and motor neurons work together as an
insect moves toward a chemical stimulus.
Sensory neurons receive input from the environment (the chemical stimulus) and transmit
this information along to interneurons. Interneurons form a complex network that
integrates the input and generates the appropriate response signals (movement, in this
case). Motor neurons carry the response signals to the appropriate muscles to trigger
movements.
3. Describe the distribution of charges in the membrane of a resting neuron.
At rest, the inside of a neuron’s membrane carries a slightly negative electrical charge
relative to the outside.
4. In what ways does an action potential resemble a crowd doing “the wave” in a football
stadium?
An action potential resembles “the wave” in a football game because it creates a series of
electrochemical changes that propagate like a wave along the nerve fiber. Successive
groups of ion channels open, then close, similar to how successive groups of people
stand, then sit, as they do "the wave."
5. How does myelin alter conduction of a neural impulse along a nerve fiber? What
would happen to neural impulse transmission in an axon without gaps in the myelin
sheath? Explain.
Myelinated axons conduct impulses faster than those without a fatty sheath. Gaps
between the myelin segments contain high concentrations of sodium channels. Action
potentials “leap” from gap to gap, bypassing the myelinated portions. An axon with a
continuous myelin sheath could not propagate action potentials because sodium ions
could not enter or exit the axon.
6. Speculate about why synapses and neurotransmitters are beneficial adaptations to
animal nervous systems, compared to direct connections between neurons.
Direct connections between neurons could mean that a single nervous impulse might
spread in all directions. Responses to stimuli might therefore be very generalized. At
synapses, neurons release neurotransmitters that excite or inhibit nearby cells in one
direction only, allowing a one-way flow of information and more specialized responses to
stimuli.
7. Prescription sleep aids like Ambien activate membrane receptors that allow negatively
charged ions to enter neurons. Explain how these drugs slow the production of action
potentials in affected parts of the brain.
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When the drug activates the membrane receptors, negatively charged ions enter the
neurons. As a result, the probability that affected neurons will initiate action potentials
decreases. Ambien therefore slows the production of action potentials in the brain areas
that the drug targets. In this case, decreased action potential production causes sleep.
8. Consider the suggestion that humans use only 10% of their brains. Given the brain’s
energy demands, does this claim make sense?
This claim does not make sense. Nervous tissue requires a lot of energy to maintain
resting potential; it would be wasteful for an organism to spend energy to maintain such a
large brain and then not use most (or all) of it.
9. Neuroglia outnumber neurons in the nervous system by about 10 to 1. In addition,
neuroglia retain the ability to divide, unlike neurons. How do these two observations
relate to the fact that most brain cancers begin in neuroglia?
Neuroglia greatly outnumber neurons, so if any brain cell becomes cancerous, there is a
good chance it will arise among the neuroglia. In addition, neuroglia can divide, so their
DNA replicates frequently and is therefore more likely to undergo genetic mutations that
lead to cancer.
10. 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. Thermoreceptors in the skin transduce
information about temperature. Pain receptors respond to mechanical damage. 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 transduce vibrations in the ears.
11. 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 24.B. 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.
12. What are the roles of rods and cones in the sense of sight?
Rods and cones are photoreceptor cells in the retina that respond to light. Rods provide
black-and-white vision in dim light, and cones respond to light wavelength (which we
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perceive as color) in bright light. Ultimately, the brain processes signals from rods and
cones into the image we perceive.
13. In a rare condition called synesthesia, stimulation of one sense triggers the perception
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 directly 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. Add axons and myelin to the concept map.
“Action potentials” leads with the phrase “propagate along” to “Axons.” “Axons” leads
with the phrase “release” to “Neurotransmitters.” “Axons” leads with the phrase
“communicate with other cells across” to “Synapses.” “Axons” leads with the phrase
“can be wrapped in” to “Myelin,” which leads with the phrase “speeds the conduction of”
to “Action potentials.”
2. What structures are included in the peripheral nervous system?
The peripheral nervous system includes sensory neurons that transmit information to the
central nervous system and motor neurons that control all voluntary and involuntary
muscle contractions.
3. Add the somatic, autonomic, sympathetic, and parasympathetic nervous systems to this
concept map.
“Peripheral nervous system” leads with the phrase “is divided into” to “Somatic nervous
system” and “Autonomic nervous system.” “Autonomic nervous system” leads with the
phrase “is divided into” to “Sympathetic nervous system” and “Parasympathetic nervous
system.”
4. Add mechanoreceptors, chemoreceptors, and photoreceptors to the concept map.
“Sensory receptor cells” connects with “that respond to touch, pressure, or vibration are
called” to “Mechanoreceptors.” “Mechanoreceptors” connects with “occur in the” to
“Ears.” “Sensory receptor cells” connects with “that respond to chemical stimuli are
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McGraw-Hill Education.
called” to “Chemoreceptors.” “Sensory receptor cells” connects with “that respond to
light are called” to “Photoreceptors.” “Photoreceptors” connects with “are found in the”
to “Eyes.”
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McGraw-Hill Education.