Mastering Concepts
25.1
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, while the role of the endocrine system is slower and longer lasting.
2. What are the roles of neurons and neuroglia in the nervous system?
Neurons are the communication cells in the nervous system, while neuroglia play a support role.
3. 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 neurons began to accumulate in the head as the nervous system became more centralized. In flatworms, the simplest bilaterally symmetrical animal, ganglia of neurons exist in each side of the head with two nerve cords connected by a nerve ladder. With segmentation came a larger brain and peripheral nerves from a ventral nerve cord. Finally vertebrates have a highly centralized nervous system with a brain, spinal cord and peripheral nervous system.
4. 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 lies outside of the brain and spinal cord.
25.2
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.
25.3
1. Describe the forces that maintain the distribution of K+ and Na+ across the cell membrane in a neuron at rest.
The sodium-potassium pump creates a concentration gradient of Na
+
outside the neuron and K
+
inside. The other force is charge interactions: for example, K
+
is repelled by the
Na
+
outside and attracted to the negatively charged proteins inside the axon.
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. What is the difference between 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 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.
4. 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.
5. 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.
6. How do action potentials indicate stimulus intensity and type?

Strength of a stimulus is indicated by action potential rate, and type of stimulus is indicated by which neuron is stimulated and what part of the brain the signal goes to.
7. How do myelin and the nodes of Ranvier speed neural impulse transmission along an axon?
Nodes of Ranvier are gaps in the myelin sheath where there are Na
+ channels that can allow depolarization; the axon has no channels where it is covered with myelin. This allows the action potential to jump between nodes and speed up the signal.
25.4
1. Describe the structure of a synapse.
The presynaptic cell ends in an axon terminal that contains calcium channels and synaptic vesicles with neurotransmitters. 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 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 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.
25.5
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 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.
25.6
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 brain, gray matter covers the surface and white matter is in deeper brain tissues. In the spinal cord, the 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, whereas white matter contains myelinated axons.
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, 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.
The midbrain also participates in hearing and eye reflexes and regulates consciousness.
- 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.
5. What are the main subdivisions of the cerebral cortex?
The cortex is divided into 4 lobes: frontal, parietal, temporal and occipital; and each lobe is divided into 2 hemispheres (of the left and right cerebrum).
6. How do short- and long-term memories differ?
Short-term memory stores information only for moments, after which it is forgotten; while long-term memory forms more permanent connections between neurons in a pathway so that the information is available for much longer periods.
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?
Diseases that affect the central nervous system include meningitis, Creutzfeld-Jacob disease and its variants, fungal infections, Parkinson disease, and ALS.
9. To what extent can the nervous system regenerate?
While peripheral nerves can regenerate, neurons of the central nervous system cannot.
However, all neurons can form new connections that compensate for the loss of other neurons.
25.7
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 saxitoxin 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.
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.
2. 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 nets seen in cnidarians. In these nets the nerve cells touch one another throughout the body wall and allow nerve signals to spread throughout the body 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 still sufficient for the organism to survive and reproduce in their environment. Since these organisms are adapted to their environment nature does not select against them. The vertebrates, with their more complex nervous systems, are more adapted to more complex lifestyles and so experience different selective pressures and are not in competition with the invertebrates.
3. 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 25.4, 25.5, and 25.8 might be helpful.]
4. How do the location and functions of sensory neurons, motor neurons, and interneurons differ?

Sensory neurons are located in the peripheral nervous system, except for their axons, which end in the central nervous system. They bring information toward the central nervous system and respond to light, pressure from sound waves, heat, touch, pain, and chemicals detected as odors or taste. The location of motor neurons is the reverse; their dendrites and cell bodies are in the central nervous system with their axons in the peripheral nervous system. A motor neuron conducts its message from the central nervous system toward effectors such as muscle or gland cells. Interneurons, located completely within the central nervous system, connect one neuron to another.
5. 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.
6. What causes the wave of depolarization and repolarization constituting an action potential?
Once enough sodium enters to depolarize the trigger zone’s membrane to a threshold potential (about -50 mV), additional sodium gates open, triggering an action potential.
7. Make a chart showing whether potassium channels, sodium channels, and the sodium– potassium pump in a neuron are active or inactive during each of the four phases in figure
25.6.
Phase Sodium channel Potassium channel Sodium-potassium pump
Resting Potential closed Leakage channel open
Delayed channel closed
Always active
Depolarization to threshold open Leakage channel open
Delayed channel closed
Both open Peak Depolarization
Repolarization closed closed Leakage channel open
Delayed channel closed
8. Why is an action potential described as an “all-or-none” process?
Always active
Always active
Always active

If the threshold stimulus is reached then the action potential will occur and travel all the way down the axon every time. If the threshold stimulus is not reached then there will be no action potential at all.
9. Cyanide is a poison that disables the sodium–potassium pump. Explain how cyanide prevents nerve transmission and causes death.
The sodium-potassium pump is critical to the ability of a neuron to maintain and restore its resting potential. If cyanide disables the sodium-potassium pump, a neuron will not be able to transmit action potentials. All functions of the nervous system will shut down, causing death.
10. In what ways does an action potential resemble “the wave” in a football stadium? In what ways does a graded potential resemble a cheerleader’s attempts to get “the wave” started?
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. A graded potential resembles cheerleaders attempting to get the “wave” started because in a local flow of electrical current, some Na
+
begins to leak into the cell, causing the interior to become less negative. If the stimulus is strong enough, the depolarization will spread to the trigger zone. This chain of events leads up to the action potential.
11. 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 would not propagate the length of the axon.
12. 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 25.8 might be helpful.]
13. A synapse is asymmetrical. What structures do the presynaptic and postsynaptic cells have in common? What structures differ between the two cells?
Both cells have a plasma membrane containing ion channels. The presynaptic cell sends action potentials toward the synapse; the presynaptic cell contains calcium channels and synaptic vesicles storing neurotransmitters. The postsynaptic cell has sodium or potassium channels with receptors for the neurotransmitters released by the presynaptic cell; the postsynaptic cell sends an action potential away from its postsynaptic membrane.

14. Describe how a neuron uses 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 postsynaptic cell.
15. Suppose that a synapse is like a football game’s line of scrimmage and the neurotransmitter is the football. Which structure in the synapse is analogous to the quarterback? The receiver? What event might be analogous to an interception or a fumble?
The presynaptic cell would be the quarterback as it releases the neurotransmitters
(football). The postsynaptic cell would be the receiver. An interception could occur if a drug/chemical were present in the synapse that destroyed the neurotransmitter before it could bind to its receptor. A fumble could occur if a drug/chemical blocked the receptor so that the neurotransmitter were released but was never able to bind to the receptor.
16. Why might an overdose of an SSRI drug result in serotonin toxicity?
An overdose of selective serotonin reuptake inhibitor would cause serotonin to remain in a synaptic cleft for too long. A person might feel excessively sleepy. According to websites on serotonin toxicity, the result might also include anything from nausea, diarrhea, profuse sweating, tremor, and headache to serious symptoms such as tachycardia (elevated heart rate), high blood pressure, hyperthermia, hallucinations, renal failure, coma, and death.
17. 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.
18. 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.
19. List the main subdivisions of the human nervous system, along with their functions.
Central nervous system- integration and process information, reflexes, regulation of 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
20. 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.
21. 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.
Carpal tunnel syndrome is a peripheral nervous system disorder, because the pain and numbness are felt in skeletal muscle areas of the periphery.
22. 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.
23. 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?
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.
24. 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 memory but not short. 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 permanent changes

to neuron pathways. Both of these aspects of memory formation and storage, however, need more research.
25. Brain imaging techniques include CT scans, PET scans, MEG, MRI, and fMRI.
Search the Internet for information about each of these techniques. How does each technique work? What kinds of images does each one produce? What are the advantages and disadvantages of each?
CT- scan— use specialized x-ray machines to create cross sectional images of the body interior that can be viewed and interpreted through a computer. The technique is fast, painless, noninvasive and images bone, soft tissue and vessels simultaneously. However, it does involve the use of x-ray radiation, which is a known carcinogen.
PET scan— uses small amounts of a radioactive material that is introduced to the body and accumulates in the organ or tissues being studied. Images of the body and of body functions (like blood flow) can be produced. PET scans are less invasive and cheaper than exploratory surgery; they offer great detail and their images of body function are a unique advantage. However, there may be slight pain with the injection of the tracer, and there are sometimes mild allergic reactions to it.
MEG— maps brain activity through time using magnetic fields. This is a unique image, but difficulty can arise in accurately interpreting what is happening in the tissue below from the magnetic signals outside the head.
MRI—uses powerful magnetic fields to affect water in the body and produce high contrast images of internal structures. This high contrast is a major advantage in diagnoses. Drawbacks however are the expense and the effects the magnetic fields may have on electromechanical devices implanted in the body. MRI accidents have also been reported. fMRI—a specialized form of MRI that focuses on blood flow changes associated with neural activity in the central nervous system. Advantages and disadvantages similar to above.
26. What is a stroke? Use the Internet to learn the symptoms of stroke. What treatments are available for stroke victims? Why is prompt treatment so important to a patient’s survival and recovery? Why is it common for only one side of the body to be affected?
What are the best ways to prevent stroke?
A stroke involves the death of brain tissue as blood supply is cut off. Symptoms include: severe and sudden headaches that change in intensity with body position; muscle weakness, numbness or tingling on one side of the body; trouble with walking, reading or speaking. Treatments can include medicines to break up and prevent future blood clots and/or surgery to repair ruptured vessels. Prompt treatment is critical to minimize the effects of the stroke on surrounding brain tissue. Strokes generally do not involve both hemispheres of the cerebrum and so only one side (opposite of the stroke side) of the body is affected.
27. How would you test the hypothesis that a nonhuman animal feels pain or thinks?
Which animals would you choose to investigate this question? Why?

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).
28. Albert Einstein’s brain was of normal size, but a part of his parietal lobe was about
15% wider than normal. This area controls mathematical reasoning, imagery, and the ability to visualize objects in space. A particular groove in the area appeared much reduced, leading researchers to speculate that this might have allowed more synaptic connections to form than normal. What additional information would help to determine whether Einstein’s brain distinctions could have accounted for his genius?
To determine whether Albert Einstein’s brain structure accounted for his genius, researchers would have to decide on an objective measure of “genius” (a difficult problem in and of itself) and then acquire brains from people who did and did not meet the standard. Systematic measurements would help reveal whether certain brain structures are associated with genius.
29. 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 glial cells?
Glial cells are mitotically active, so their DNA replicates frequently and is more likely to undergo genetic mutations that lead to cancer.
30. 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.
31. 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?
The research techniques and procedures may invade the patient’s privacy and cause discomfort (or even harm) to the patient, raising ethical questions. Animals may make

good subjects for some types of research, but the results may not apply directly to humans.
Pull It Together
1. How do sensory neurons, motor neurons, and interneurons interact?
Sensory neurons receive input from the environment (like touch or vision) and transmit this information along to interneurons. Interneurons form a vast, complex network that integrates memory with input and signals for output (like speech or movement). Motor neurons carry the output signals to the appropriate muscles to trigger movements.
2. 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.
3. In what direction do action potentials travel within a neuron?
An action potential travels in only one direction across a neuron, from the source of the signal down the axon to the synapses.
4. Add myelin and nodes of Ranvier to this concept map.
“Neurons” leads with “can be wrapped in” to “Myelin”, which leads with “has gaps between cells called” to “Nodes of Ranvier”, which leads with “speeds up” to “Action potentials”.
5. What structures are included in the peripheral nervous system?
The peripheral nervous system (PNS) contains cells in many structures all around the body, such as the retina of the eyes, sensory cells within the ears, olfactory sensors, taste sensors and others. The PNS also includes the motor neurons that control all voluntary and involuntary muscle contractions.
6. What are the names, locations, and functions of the main parts of the human brain?

The cerebrum is the largest part of the human brain. It is the most outward region of the brain, touches the skull, and is divided into two sides (hemispheres). It contains the frontal lobe, temporal lobe, parietal lobe, and occipital lobe. The cerebrum is responsible for sensory integration, voluntary movement, free thought, speech, and learning. Deeper down in the brain is the limbic system, which is mainly associated with emotion. The hippocampus is a part of the limbic system and is associated with memory. The amygdala is a small region associated with simple emotions, and the thalamus and hypothalamus integrate the nervous system with the endocrine system. The last set of structures make up the brain stem, the lowest and most central region on the brain. The cerebellum is associated with balance and coordination. The pons and medulla oblongata connect the brain with the spinal cord and are where involuntary muscle contractions are controlled.
7.
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”.