Download Answers to Mastering Concepts Questions

Answers to Mastering Concepts Questions
24.1
1. How is the nervous system’s role in maintaining homeostasis different from that of the
endocrine system?
The role of the nervous system in maintaining homeostasis is nearly instantaneous,
whereas the role of the endocrine system is slower and longer lasting.
2. What are the roles of neurons and neuroglia?
Neurons are the communication cells in the nervous system, whereas neuroglia play a
support role.
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.
Three parts of a neuron are:
- dendrites: branches that receive sensory input and bring it to the neuron’s cell body.
- cell body: contains the nucleus, mitochondria, and ribosomes. The cell body carries
on the normal metabolic cellular functions of the neuron.
- axon: a long fiber extending from the cell body. Axons branch at their terminal ends
and form junctions with other cells, such as other neurons, muscles, or glands. The
role of the axon is to transmit a nerve impulse to another cell.
2. Where is the myelin sheath located?
The myelin sheath is around the axons of neurons.
3. What is the usual direction in which a message moves within a neuron?
Within a neuron, a usual message moves from dendrites to cell body to axon.
4. What are the functions of each of the three classes of neurons?
The three classes of neurons and their functions can be summarized as:
- Sensory neurons bring information to the central nervous system.
- Motor neurons connect the central nervous system to muscles and glands.
- Interneurons are central nervous system neurons that connect sensory neurons with
motor neurons.
24.3
1.What is the difference between the resting potential, the threshold potential, and an
action potential?
A resting potential is when the voltage difference between the inside and outside of the
membrane is not transmitting a signal. The inside of the cell is more negative than the
outside. A threshold potential of around -50mV is the signal to open more Na+ channels.
An action potential is a brief depolarization that propagates along a nerve fiber.
2. How does an axon generate and transmit a neural impulse?
If the “trigger zone” reaches the threshold value then Na+ channels in the axon will
briefly open, depolarizing the axon to its tip. This depolarization then propagates down
the axon as Na+ in one local area diffuses into the next and brings it to threshold.
3. What prevents action potentials from spreading in both directions along an axon?
There is a refractory period as a local area of the axon returns to resting potential.
4. How does myelin speed neural impulse transmission?
The parts of an axon that are coated with a myelin sheath lack sodium channels. Areas
where there are gaps in the sheath, however, do have sodium channels. Action potentials,
therefore, "jump" between the gaps, which speeds the signal.
24.4
1. Describe the structure of a synapse.
The neuron ends in an axon terminal that contains synaptic vesicles with
neurotransmitters. The receiving 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 neuron to release neurotransmitters?
When the axon terminal depolarizes, calcium channels open, and calcium diffuses in.
This triggers the release of the 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 enzymatically inactivated, and some is taken back into
vesicles within the sending cell.
24.5
1. Which structures make up the peripheral nervous system?
The nerves that branch off the spinal cord compose the peripheral nervous system.
2. How do the sensory and motor pathways of the peripheral nervous system differ?
The sensory pathways transmit action potentials to the central nervous system, and the
motor pathways carry them away.
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, which operates under stress, and the parasympathetic system for more relaxed
times.
4. How do the sympathetic and parasympathetic nervous systems maintain homeostasis?
These systems continually work together, in opposition to each other, to maintain balance
in the body.
24.6
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 are the major structures in the hindbrain, midbrain, and forebrain, and what are
their functions?
Brain structures and functions:
- 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.
- Midbrain: The midbrain is part of the brainstem. Nerve fibers that control voluntary
motor function pass from the forebrain through the midbrain portion of the brainstem.
- Forebrain: The major structures in the forebrain are the cerebrum, thalamus, and
hypothalamus. The cerebrum controls the qualities of the mind: notably, personality,
intelligence, and perception. 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.
3. What are the parts and functions of the cerebral cortex?
The cortex is divided into 4 lobes: frontal, parietal, temporal, and occipital. Each lobe is
divided into 2 hemispheres (of the left and right cerebrum). The cerebral cortex is
responsible for interpreting sensory information; control of voluntary movements; and
association, interpretation, and analyzing of information.
4. 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.
5. 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.
6. To what extent can the nervous system heal itself?
Neurons of the central nervous system cannot regenerate. However, all neurons can form
new connections that compensate for the loss of other neurons.
24.7
1. 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.
2. 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.
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.”
24.8
1. Which structures provide the senses of touch, temperature, and pain?
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.
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 that have concentrations of chemoreceptors.
Each sends an action potential to sensory neurons that take the message to the brain for
processing and integration.
3. 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.
24.10
1. 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.
2. What are the roles of photoreceptors and pigments 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.
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 neurons. These 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 lead to neurons in the primary visual cortex of
the brain.
24.11
1. What is the role of mechanoreceptors in the senses 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 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 overlying membrane. This movement, in turn,
causes the hair cells to release a neurotransmitter that triggers action potentials in the
auditory nerve.
24.12
1. What is the evidence that the presence of algal toxins is a selective force on softshell
clam populations?
Comparisons between two populations of clams living in two different environments with
two different selective pressures, revealed that in the presence of regular algal blooms,
clams exhibit resistant sodium channels.
2. How did Bricelj and her colleagues demonstrate that sodium channel structure explains
toxin resistance in some clam populations?
Using a laboratory set up, the researchers first demonstrated that the Bay of Fundy clams,
unlike those at Lawrencetown Estuary, were not susceptible to the toxin of the algal
bloom. They then investigated the DNA sequences for the sodium channels and
discovered coding for 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 measured
through the channels. The mutated channels still allowed sodium to flow when the wild
type channel did not.
Answers to Write It Out Questions
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.
2. Sketch two neurons, with one synapsing on the other. In your sketch, label the
dendrites, axons, cell bodies, myelin sheath, and synapse.
[Answers will be visual. Figures 24.2, 24.3, and 24.6 might be helpful.]
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.
4. What causes the switch in the distribution of charges when an axon propagates an
action potential?
Once enough sodium enters to depolarize the trigger zone’s membrane to a threshold
potential, additional sodium gates open, triggering an action potential.
5. 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."
6. How does the myelin sheath increase the speed at which an axon conducts a neural
impulse?
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.
7. How do neurons use neurotransmitters to communicate with other cells.
Releasing the neurotransmitters to diffuse across a synaptic cleft causes ion channels to
open and either excite or inhibit the receiving cell.
8. A scientist discovers a way to stop production of a protein required for recycling of
synaptic vesicles. What will happen to the amount of neurotransmitter in the synaptic
cleft?
If synaptic vesicles could not be recycled then new batches of neurotransmitter could not
be packaged and released, thus the quantity of neurotransmitter in the cleft would
decrease.
9. List the main subdivisions of the human nervous system, along with their functions.
 Central nervous system: integrates and processes information, reflexes, regulates the
body
 Peripheral nervous system: (a) the sensory portion gathers stimuli information and
sends to the central nervous system; and (b) the motor portion carries motor signals to
muscle and glands
 Somatic system: a subdivision of the motor PNS that carries signals to voluntary
skeletal muscle
 Autonomic system: a subdivision of the motor PNS that carries signals to involuntary
cardiac and smooth muscle and glands
10. Why can the loss of reflexes be a possible indication of damage to the central nervous
system?
Reflex arcs have a pathway through the spinal cord, so that is one possible place for the
damage to be located.
11. How would you test the hypothesis that a nonhuman animal feels pain or thinks?
Which animals would you choose to investigate this question?
One way would be to measure electrical activity in regions of the brain associated with
pain or problem solving. Using a range of animals with different levels of intelligence,
from invertebrates to fishes to mammals, would yield interesting information on pain
perception and thought (although the question of pain would certainly raise ethical
questions).
12. Consider the suggestion that humans use only 10% of their brains. Given the brain’s
energy demands, does this claim make sense? Do studies of brain-damaged patients
support or refute this statement?
Nervous tissue requires much energy to maintain resting potential; it would be
disadvantageous for an organism to spend energy maintaining resting potential in parts of
the brain that are unused. In addition, the plasticity of brains after trauma might suggest
that many brain cells actively participate in neural pathways. Humans use much more
than 10% of their brains.
13. Neuroglia outnumber neurons 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?
Cancers are cells that have lost the ability to regulate cell division. Cells have to divide
in order to form tumors. Neurons don’t divide, so it would be less likely that they would
lose the ability to regulate cell division.
14. 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.
15. 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 generate the action potential. In temperature, free nerve endings in the skin do transduction. Pain receptors respond to mechanical
damage. Chemoreceptors bind to molecules dissolved in a watery solution for
transduction to occur in smell and taste. Light activates chemicals in photoreceptors, and
vibrations in fluid move mechanoreceptors in hearing.
16. 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.
17. 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
18. What is the role of rods and cones in the sense of sight?
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.
19. Describe one way that each sense listed in this chapter 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.
Answers to Pull It Together Questions
1. What are the main parts of a neuron?
A neuron has a central cell body; short extensions (called dendrites) reaching in many
directions from the cell body; typically only one long extension (called the axon or nerve
fiber); and meeting points with other neurons called synapses that form at the end of
axons.
2. Describe the functions of the central nervous and peripheral nervous systems.
The central nervous system is responsible for the integration of sensory information and
the direction of motor information. The peripheral nervous system receives the
information and then performs the action directed by the central nervous system.
3. Add the somatic, autonomic, sympathetic, and parasympathetic nervous systems to this
concept map.
“Peripheral nervous system” leads with “is divided into” to “Somatic nervous system”
and “Autonomic nervous system”. “Autonomic nervous system” leads with “is divided
into” to “Sympathetic nervous system” and “Parasympathetic nervous system”.
4. Make a chart that lists the types of sensory receptors and the sense organs that use each
type.
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