Download Answers to Mastering Concepts Questions

Mastering Concepts
26.1
1. How are nervous systems adaptive to animals?
An animal’s nervous system allows it to perceive and quickly respond to its environment
(including food, shelter, mates, and threats).
2. Describe how nervous systems changed as animal bodies became more complex.
The simplest nervous systems were diffuse nerve nets in the body wall. With bilateral
symmetry, nervous systems became more centralized in the animal’s head end. In
flatworms, ganglia on each side of the head connect to two nerve cords that form a nerve
ladder. Segmented animals such as annelids and arthropods have a larger brain and
peripheral nerves emerging from a ventral nerve cord. Vertebrates have a highly
centralized nervous system with a brain, dorsal spinal cord, and peripheral nervous
system.
3. Distinguish between the central and peripheral nervous systems in vertebrates.
The central nervous system consists of the brain and spinal cord. The peripheral nervous
system consists of nerves that lie outside the brain and spinal cord.
26.2
1. Describe the parts of a typical neuron.
Dendrites are branches that convey sensory input to the neuron’s cell body. The cell
body, which contains the nucleus, mitochondria, and ribosomes, carries out the neuron’s
metabolic functions. 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 surrounds 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.
26.3
1. Describe the forces that maintain the distribution of K+ and Na+ across the cell
membrane in a neuron at rest.
At rest, a neuron has a high concentration of K+ inside the cell (relative to the outside)
and a high concentration of Na+ outside the cell (relative to the inside). The sodiumpotassium pump moves Na+ out of the cell as it moves K+ in. In addition, K+ is
simultaneously repelled by the Na+ outside and attracted to the negatively charged
proteins inside the cell.
2. In what way is the term resting potential misleading?
The term “resting potential” is misleading because the neuron is ready to fire and not
really “at rest.” Also it is expending almost 75% of its energy to maintain the distribution
of K+ and Na+ while “at rest.”
3. Differentiate among a graded potential, the threshold potential, and an action potential.
A graded potential is a local flow of electricity that depends on stimulus strength and
weakens with distance. A graded potential may depolarize the cell to its threshold
potential (around -50mV), which is a signal to open the cell’s Na+ channels (the first step
in an action potential). An action potential is a brief depolarization that propagates along
a nerve fiber.
4. How does an axon generate and transmit a neural impulse?
If the “trigger zone” reaches the threshold potential, Na+ channels in the axon will briefly
open, depolarizing the axon. This depolarization then propagates down the axon as Na+ in
one local area diffuses into the next and brings it to threshold.
5. What prevents action potentials from spreading in both directions along an axon?
During the refractory period, a local area of the axon that has depolarized returns to
resting potential and cannot generate action potentials. Action potentials therefore move
only from the trigger zone toward the tip of the axon.
6. How do action potentials indicate stimulus intensity and type?
The rate of action potentials indicates the strength of a stimulus. The type of neuron
stimulated and the part of the brain that receives the signal indicates the type of stimulus.
7. How do myelin and the nodes of Ranvier speed neural impulse transmission along an
axon?
Myelin-covered parts of the axon lack sodium channels; nodes of Ranvier are gaps in the
myelin sheath where Na+ channels allow depolarization. This arrangement speeds neural
impulse transmission because it allows the action potential to jump between nodes.
26.4
1. Describe the structure of a synapse.
The presynaptic cell ends in a synaptic terminal. Calcium channels are embedded in the
membrane of the terminal, and synaptic vesicles filled with neurotransmitters occupy the
synaptic terminal. The postsynaptic cell has ion channels with receptors for the
neurotransmitters. Between the two cells is a small gap called the synaptic cleft.
2. What event stimulates a presynaptic neuron to release neurotransmitters?
When action potentials depolarize the synaptic terminal, calcium channels open and
calcium diffuses in. This influx triggers the release of the neurotransmitters into the
synaptic cleft.
3. What happens to a neurotransmitter after its release?
After a neurotransmitter is released, some of it travels to receptors on the postsynaptic
cell. Some diffuses away, some is enzymatically inactivated, and some is taken back into
vesicles within the presynaptic cell.
4. How does synaptic integration determine whether a neuron transmits action potentials?
Like in a voting system, synaptic integration involves “adding up” the number of
excitatory and inhibitory signals. If excitatory signals predominate then there will be an
action potential.
26.5
1. What is the difference between cranial and spinal nerves?
A cranial nerve exits the central nervous system at the brain, and a spinal nerve exits at
the 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 autonomic, sympathetic, and parasympathetic
nervous systems.
The autonomic nervous system carries signals to muscles and glands that are under
involuntary control. The autonomic system is further divided into the sympathetic
system, which dominates under stress, and the parasympathetic system, which restores
body systems to normal during relaxed times.
4. How do the sympathetic and parasympathetic nervous systems maintain homeostasis?
These systems continually work together, in opposition to each other, to adjust blood
pressure, breathing rate, nutrient availability, and other body functions to meet the body’s
needs.
26.6
1. What are the functions of the spinal cord?
The spinal cord transmits action potentials between the body and the brain; it also
functions in reflexes.
2. 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.
3. Describe the functions of the neurons that form a reflex arc.
In a reflex arc, a sensory neuron receives a stimulus from a sensory receptor. The axon
of this neuron leads into the gray matter of the spinal cord, where it synapses with the cell
body of a motor neuron whose axon leads to an effector muscle.
4. What are the major structures in the hindbrain, midbrain, and forebrain? What are their
functions?
In the hindbrain, the major structures are the pons, medulla oblongata, and cerebellum.
The pons connects higher brain centers with the spinal cord and connects the forebrain to
the cerebellum. The medulla oblongata regulates breathing, blood pressure and heart rate
and controls reflex centers for hiccupping, sneezing, defecating, coughing, and
swallowing. The cerebellum refines motor messages and coordinates muscle movements.
The midbrain is part of the brainstem. Nerve fibers that control voluntary motor function
pass from the forebrain through the midbrain. The midbrain also participates in hearing
and eye reflexes and regulates consciousness.
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 maintains homeostasis, controlling body temperature,
heartbeat, water balance, blood pressure, hunger, thirst, sexual arousal, and emotions. It
also regulates secretions from the pituitary gland. The cerebrum controls the qualities of
the mind, such as personality, intelligence, and perception.
5. What are the main subdivisions of the cerebral cortex?
The cortex is divided into two hemispheres (the left and right cerebrum), each consisting
of four lobes (frontal, parietal, temporal and occipital).
6. Summarize what researchers know and have yet to learn about memory formation and
storage.
Researchers know that there are brain differences in short and long-term memory. For
example, the hippocampus is involved in long-term but not short-term memory. They
suspect that in short term memory, temporary neuron connections are formed in the
temporal and parietal lobes, and that the connection (and memory) will last only as long
as the circuit is being used. They also suspect that long-term memory involves stable
changes to neuron pathways. Both of these aspects of memory formation and storage,
however, need more research.
7. List some structures that protect the central nervous system.
The meninges, blood-brain barrier, cerebrospinal fluid, skull, and vertebral column
protect the central nervous system from damage.
8. What are some examples of diseases that affect the central nervous system?
Trauma and stroke can damage the central nervous system. Viruses, bacteria, prions, and
fungi can case various infectious diseases. Parkinson disease, Alzheimer disease, and
ALS are noninfectious nervous system disorders.
9. To what extent can the nervous system regenerate?
While peripheral nerves can regenerate, mature neurons of the central nervous system
cannot. However, neurons may form new connections that compensate for the loss of
other neurons.
26.7
1. Use figure 26.22 to explain how saxitoxin affects action potential propagation.
Saxitoxin blocks sodium channels in susceptible clams. Without functioning sodium
channels, neurons do not depolarize and action potentials are not propagated.
2. How did Bricelj and her colleagues demonstrate that sodium channel structure explains
toxin resistance in some clam populations?
The researchers first demonstrated that the Bay of Fundy clams, unlike those at
Lawrencetown Estuary, were not susceptible to the saxitoxin of the algal bloom. They
then investigated the DNA sequences for the sodium channels and discovered one amino
acid difference between the two protein channels. Finally they grew cells in culture with
DNA for expression of both sodium channel variants. The cells were then exposed to the
saxitoxin and rates of sodium flow were measured through the channels. The mutated
channels still allowed sodium to flow (indicating tolerance to the toxin), whereas the wild
type channel did not.
Write It Out
1. Describe some invertebrate nervous systems. Why do animals with simple nervous
systems still exist, even after the more complex vertebrate nervous system evolved?
One invertebrate nervous system is the nerve net typical of cnidarians. In these nets, the
nerve cells touch one another and allow nerve signals to spread throughout the body wall
so that the animal can move its tentacles or swim. Arthropods have a brain and ventral
nerve cord and well developed sensory organs. These animals can exhibit complex
behaviors. Simple nervous systems still exist because they are sufficient for the
organisms to survive and reproduce in their environment.
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 vast, complex network that
integrates input and signals for output (movement, in this case). Motor neurons carry the
output signals to the appropriate muscles to trigger movements.
3. Describe the distribution of charges in the membrane of a resting neuron.
At rest, a neuron’s membrane is polarized. The inside of the neuron carries a slightly
negative electrical charge relative to the outside. This separation of charges creates an
electrical potential that measures around -70 mV.
4. What causes the wave of depolarization and repolarization constituting an action
potential? How does the membrane restore its resting potential?
Once enough sodium enters to depolarize the trigger zone’s membrane to the threshold
potential (about -50 mV), additional sodium channels open, triggering an action potential.
Repolarization occurs when sodium channels close and potassium channels open,
allowing K+ to pour out of the cell. The sodium-potassium pump eventually restores the
resting potential.
5. Write a nonbiological analogy for resting potential and for depolarization, other than
those mentioned in the chapter.
At resting potential, the neuron stores energy across its membrane in the form of a charge
gradient. This potential energy is released as the membrane depolarizes during an action
potential. The resting potential is like a boulder sitting on the edge of a cliff. A small shift
in the boulder’s equilibrium could cause it to fall, releasing energy as it descends.
6. What is the function of each of the four membrane proteins shown in figure 26.6?
When is each membrane protein channel open?
The K+ leakage channel is always open and allows K+ ions to move in and out of the cell.
The Na+ channel opens when the membrane is depolarized to threshold potential,
allowing Na+ ions to enter the cell and causing further depolarization. The delayed K+
channel opens when the depolarization reaches its peak; K+ ions subsequently leave the
axon. The sodium-potassium pump, which functions continuously, helps restore and
maintain membrane potential.
7. How does myelin alter conduction of a neural impulse along a nerve fiber? What
would happen to neural impulse transmission in a myelinated axon without nodes of
Ranvier? Explain.
Myelinated axons conduct impulses faster than those without a fatty sheath. Nodes
between the myelin segments contain high concentrations of sodium channels. Action
potentials “leap” from node to node, bypassing the myelinated portions. If a myelinated
axon lacks nodes of Ranvier, then the axon has no gaps where ions can cross the
membrane; an action potential could not propagate along the length of the axon.
8. Sketch a synapse; label the axon and synaptic terminal of the presynaptic cell, the
postsynaptic cell, the synaptic cleft, the synaptic vesicles, and the receptor proteins.
[Answers will be visual. Figure 26.9 might be helpful.]
9. Speculate about why synapses and neurotransmitters are beneficial adaptations to
animal nervous systems, compared to direct connections between neurons.
Direct connections between neurons, as in the nerve nets of cnidarians (see section 26.1),
mean that a single nervous impulse spreads in all directions throughout the body. At
synapses, neurons release neurotransmitters that excite or inhibit nearby cells in one
direction only, allowing a specialized response to a stimulus.
10. How does a neuron use information from other neurons to determine whether or not
to generate a neural impulse?
Neurotransmitters are released from terminals on axons in response to an action potential.
These chemical messengers diffuse across space between neurons and bind to receptors
on the receiving cell membrane. Neurotransmitter binding alters the permeability of the
membrane in a way that stimulates or blocks depolarization of the receiving cell.
11. Prescription sleep aids like Ambien bind to inhibitory receptors on neurons. Explain
how these drugs slow the production of action potentials in affected parts of the brain.
Inhibitory receptors are embedded in the cell membrane of neurons. When an inhibitory
receptor binds a molecule, such as a neurotransmitter or a drug, the probability that the
neuron will initiate an action potential 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.
12. What is the relationship between a neuron and a nerve?
A neuron is a single communicating cell in the nervous system. A nerve is a bundle of
axons from many neurons, all wrapped in connective tissue.
13. In carpal tunnel syndrome, a nerve in the wrist becomes compressed, causing
numbness or pain in the forearm and hand. Is this a disease of the peripheral or central
nervous system? Explain your answer.
Since carpal tunnel syndrome affects nerves in the hand, it is a peripheral nervous system
disorder.
14. Cerebral palsy is a nervous system disorder that is often caused by a lack of oxygen
during brain development. Impairments in movement, hearing, seeing, and thinking can
result. For each of these symptoms, indicate which region of the cerebrum was most
affected during development.
Movement and thinking disorders reflect damage to the frontal lobe; impaired hearing
reflects damage to the temporal lobe; and impaired sight reflects damage to the occipital
lobe.
15. Traumatic brain injury can occur when a person receives a strong blow to the head or
when an object enters the brain through the skull. Symptoms can include anything from
nausea to loss of sight or hearing to memory loss and personality changes. Why do
symptoms depend strongly on the location and severity of the injury?
The location of the injury will determine which areas of the cortex are involved, and
different cortex areas are involved in different functions (like vision, speech or memory).
Deeper areas of the brain also have functions that differ from the more superficial cortex.
They could be involved in processes like homeostasis regulation or sensory information
transmission to the cortex.
16. What is a stroke? Use the Internet to learn the symptoms of stroke.
A stroke involves the death of brain tissue as the blood supply is cut off. Typical
symptoms include severe and sudden headaches that change in intensity with body
position; muscle weakness, numbness or tingling on one side of the body; and trouble
with walking, reading or speaking.
17. Neuroglia outnumber neurons in the nervous system by about 10 to 1. In addition,
neuroglia retain the ability to divide, unlike most neurons. How do these two
observations relate to the fact that most brain cancers begin in glial cells?
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 are mitotically
active, so their DNA replicates frequently and is more likely to undergo genetic
mutations that lead to cancer.
18. Dentists apply local anesthetics to deaden the pain associated with filling a cavity.
These drugs block sodium channels in neurons surrounding the affected tooth. Major
surgery requires general anesthetics that act on the brain, causing the patient to become
unconscious and unaware of his or her surroundings. Use what you have learned about
the nervous system to explain how local and general anesthetics temporarily eliminate
pain.
Once a local anesthetic blocks sodium channels in the neurons surrounding a tooth, the
neurons can no longer transmit nerve impulses to the brain. The brain remains unaware
of the painful stimulus. A general anesthetic causes a person to become unconscious; he
or she therefore retains no memory of the painful stimulus.
19. Scientists know little about many common illnesses, including migraines and
Alzheimer disease. What ethical considerations make research on these diseases difficult?
What are the limitations of using animals as models to study the nervous system?
Since these disorders originate in the brain, invasive procedures are likely to harm the
patient. In addition, Alzheimer disease (and many other nervous system disorders) arise
gradually and therefore require extensive time commitments for which patients may or
may not be compensated. Animals may make good subjects for some types of research,
but their brains are not identical to ours, so the results may not apply directly to humans.
It may also be difficult to apply animal research to human conditions involving memory,
emotion, and problem-solving.
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.” “Neurons” 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. What are the names, locations, and functions of the main parts of the human brain?
The main parts of the brain are the hindbrain, midbrain, and forebrain. The hindbrain
includes the medulla oblongata, pons, and cerebellum; in general, these structures
regulate essential physiological processes and subconscious movements. The midbrain
transmits information between the forebrain and the spinal cord. The forebrain includes
the thalamus, hypothalamus, and cerebrum. The thalamus relays information to the
cerebrum, the hypothalamus integrates the nervous system with the endocrine system to
regulate homeostasis, and the cerebrum is responsible for sensory integration, voluntary
movement, free thought, speech, and learning.
4. 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.”