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Chapter 1:  Nervous System

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Functional classification of neurons, anatomical divisions of the nervous system.

• FUNCTIONAL DIVISIONS OF THE NERVOUS SYSTEM
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The neuron , or nerve cell, is the basic functional unit of the nervous system. A neuron includes its cell body and processes (axons and dendrites). Long neuronal processes are frequently referred to as fibers . Neurons are generally classified as either afferent or efferent:

Afferent , or sensory, neurons receive input from peripheral structures and transmit it to the spinal cord and/or brain.

Efferent , or motor, neurons transmit impulses from the brain and/or spinal cord to effectors (skeletal muscle, cardiac muscle, smooth muscle, glands) throughout the body.

The nervous system has two anatomical divisions:

The central nervous system (CNS) includes the brain and spinal cord.

The peripheral nervous system (PNS) consists of spinal nerves, their roots, and branches; cranial nerves (CN) and their branches; and components of the autonomic nervous system (ANS).

Collections of nerve cell bodies in the CNS form nuclei , whereas those in the PNS form ganglia . Ganglia and nuclei contain either motor or sensory neurons. Bundles of axons in the CNS are called tracts . Similar neuronal processes collected in the PNS form nerves . Nerves are categorized based on their CNS origin:

Spinal nerves are attached to the spinal cord. They transmit both motor and sensory impulses and are, thus, considered mixed nerves .

Most CN are attached to the brain. Some CN are either motor or sensory only, while others are mixed.

Spinal Nerves

The spinal cord is composed of segments, as indicated by the 31 pairs of spinal nerves. Each segment has numerous dorsal (posterior) and ventral (anterior) rootlets that arise from the respective surfaces of the spinal cord ( Fig. 1.1 ). Dorsal rootlets contain neuronal processes that conduct afferent impulses to the spinal cord, whereas the ventral rootlets conduct efferent impulses from the spinal cord. Respective rootlets from each segment unite to form dorsal and ventral roots :

The dorsal root contains the central processes of sensory neuronal cell bodies that are located in the dorsal root ganglion (DRG) . The DRG is also called a spinal ganglion. The peripheral processes of these neurons are located in the spinal nerve, its rami, and their branches. These processes end at or form receptors.

The ventral root contains motor fibers. Their neuronal cell bodies are found in the gray matter of the spinal cord: ventral horn if the axons innervate skeletal muscle; lateral horn if the axons supply smooth muscle, cardiac muscle, or glands.

Somatic components of a spinal nerve.

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Chapter 8 Answers: Nervous System

8.2 introduction to the nervous system review questions and answers.

• List the general steps through which the nervous system generates an appropriate response to information from the internal and external environments.  The nervous system extracts information from the internal and external environments using sensory receptors. It then usually sends signals encoding this information to the brain, which processes the information to determine an appropriate response. Finally, the brain sends signals to muscles, organs, or glands to bring about the response.
• What are neurons? Neurons are special nervous system cells that transmit nerve impulses.
• Compare and contrast the central and peripheral nervous systems. The central and peripheral nervous systems are the two main divisions of the nervous system. The central nervous system includes the brain and spinal cord, whereas the peripheral nervous system consists mainly of nerves that connect the central nervous system with the rest of the body.
• Self-marking
• Which major division of the peripheral nervous system allows you to walk to class? Which major division of the peripheral nervous system controls your heart rate?  The somatic nervous system is the major division of the peripheral nervous system that allows you walk to class. The autonomic nervous system controls your heart rate.
• Identify the functions of the three main divisions of the autonomic nervous system.  The function of the sympathetic division of the autonomic nervous system is primarily to control the fight-or-flight response in emergency situations. The function of the parasympathetic division is to control the routine “housekeeping” functions of the body at other times. The function of the enteric division is to provide local control of the digestive system.
• What is an axon, and what is its function? An axon is a long projection of a neuron that transmits nerve impulses to other cells.
• Define nerve impulses. Nerve impulses are the electrical signals sent by the nervous system.
• Explain generally how the brain and spinal cord can interact with and control the rest of the body. Answers may vary. Sample answer:  The brain and spinal cord (i.e. the CNS) can interact and control the rest of the body through the nerves of the PNS.
• How are nerves and neurons related? Nerves are bundles of axons from neurons.
• What type of information from the outside environment do you think is detected by sensory receptors in your ears? Answers may vary. Sample answer:  I think sensory receptors in your ears detect sounds/sound waves.

8.3 Neurons and Glial Cells Review Questions and Answers

• Describe the myelin sheath and nodes of Ranvier. How does their arrangement allow nerve impulses to travel very rapidly along axons? The myelin sheath consists of the lipid layers that cover sections of an axon. Nodes of Ranvier are regularly spaced gaps between sections of myelin sheath along the axon. Myelin sheath is a good insulator, so nerve impulses can travel along a myelinated axon by skipping from node to node, allowing nerve impulses to travel along the axon very rapidly.
• Define neurogenesis. What is the potential for neurogenesis in the human brain? Neurogenesis is the creation of new nerve cells by cell division. This occurs prenatally when the brain is still forming and growing. However, once the brain is mature, if neurogenesis occurs, its extent is not likely to be very great in humans.
• Relate neurons to different types of nervous tissues. Gray matter is nervous tissue in the central nervous system that consists mainly of unmyelinated structures such as the cell bodies and dendrites of neurons. White matter is nervous tissue found in the central nervous system and in nerves of the peripheral nervous system that consists mainly of myelinated axons. The axons in each nerve are bundled together like wires in a cable.
• Compare and contrast sensory and motor neurons. Both sensory and motor neurons carry nerve impulses in the peripheral nervous system between the central nervous system and the rest of the body. However, they carry nerve impulses in different directions. Sensory neurons carry nerve impulses away from the body and toward the brain, whereas motor neurons carry nerve impulses away from the brain and toward the body.
• Identify the role of interneurons. The role of interneurons is to carry nerve impulses back and forth mainly between sensory neurons and motor neurons.
• Identify four specific functions of neuroglia. Answers may vary. Sample answer:  Four specific functions of glial cells are to synthesize myelin, hold neurons in place, supply neurons with nutrients, and regulate the repair of neurons.
• What is the relationship between the proportion of neuroglia to neurons and intelligence? In general, the greater the proportion of neuroglia to neurons, the greater the level of intelligence. This may be true between individuals within a species and also between different species.

8.4 Nerve Impulses Review Questions and Answers

• Define nerve impulse. A nerve impulse is an electrical phenomenon that occurs because of a difference in electrical charge across the plasma membrane of a neuron. It is a sudden reversal of the electrical gradient across the membrane.
• What is the resting potential of a neuron, and how is it maintained? The resting potential of a neuron is an electrical gradient across the plasma membrane of a neuron that is not actively transmitting a nerve impulse. The resting potential is maintained by the sodium-potassium pump, which pumps ions across the cell membrane against their concentration gradients using energy in ATP and transport proteins in the plasma membrane.
• Explain how and why an action potential occurs. An action potential occurs when there is a sudden reversal of the electrical gradient across the plasma membrane of a resting neuron. It travels rapidly down the axon as an electrical current. An action potential occurs because the neuron receives a chemical signal from another cell or some other type of stimulus.
• Outline how a signal is transmitted from a presynaptic cell to a postsynaptic cell at a chemical synapse.  At a chemical synapse, neurotransmitter chemicals are released from the presynaptic cell into the synaptic cleft between cells. The chemicals travel across the synaptic cleft to the postsynaptic cell and bind to receptors embedded in its membrane.
• What generally determines the effects of a neurotransmitter on a postsynaptic cell? The effects of a neurotransmitter on a postsynaptic cell are generally determined by the type of receptor they bind to.
• Identify three general types of effects that neurotransmitters may have on postsynaptic cells. Three general types of effects neurotransmitters may have on postsynaptic cells are excitatory effects, inhibitory effects, and effects that change the cell in more complex ways.
• Explain how an electrical signal in a presynaptic neuron causes the transmission of a chemical signal at the synapse. An action potential is a type of electrical signal. When it reaches the axon terminal of the presynaptic cell, it opens channels that allow calcium to enter the terminal. Calcium causes synaptic vesicles to fuse with the membrane, releasing their contents (chemical neurotransmitter molecules) into the synaptic cleft. This chemical signal then travels to the postsynaptic cell. In this way, an electrical signal in a presynaptic cell gets translated into a chemical signal at the synapse.
• The flow of which type of ion into a neuron results in an action potential? How do these ions get into the cell? What does this flow of ions do to the relative charge inside the neuron compared to the outside?  Sodium ions.  They flow through sodium ion channels in the cell membrane that have opened in response to a signal or stimulation. The inside of the neuron becomes more positive compared to the outside.
• Name three neurotransmitters. Answers may vary. Sample answer:  Glutamate, GABA, and serotonin.

8.5 Central Nervous System Review Questions and Answers

• What is the central nervous system? The central nervous system is the part of the nervous system that includes the brain and spinal cord.
• How is the central nervous system protected? The central nervous system is protected physically by bones, meninges, and cerebrospinal fluid. It is protected chemically by the blood-brain barrier.
• What is the overall function of the brain? The overall function of the brain is to act as the control centre of the entire organism.
• Identify the three main parts of the brain and one function of each part. Answers may vary. Sample answer:  The three main parts of the brain are the brain stem, which controls vital functions such as breathing; cerebellum, which coordinates body movements; and cerebrum, which controls conscious thoughts.
• Describe the hemispheres of the brain. The hemispheres are the right and left halves of the cerebrum, which are connected by a thick bundle of axons called the corpus callosum. The two hemispheres are similar in shape, and most areas of the cerebrum are found in both hemispheres.
• Explain and give examples of lateralization of the brain. Lateralization refers to differences between brain hemispheres in particular functions. For example, in most people, language functions are more concentrated in the left hemisphere, whereas abstract reasoning and visual-spatial abilities are more concentrated in the right hemisphere.
• Identify one function of each of the four lobes of the cerebrum. Answers may vary. Sample answer:  The frontal lobe controls reasoning. The parietal lobe controls touch. The temporal lobe controls hearing. The occipital lobe controls vision.
• Summarize the structure and function of the cerebral cortex. Explain how the hypothalamus controls the endocrine system. The cerebral cortex is a thin layer of gray matter on the outside of the cerebrum and contains many folds that greatly increase its surface area. It is the part of the brain where most information processing takes place.
• Describe the spinal cord. The spinal cord is a long, thin, tubular bundle of nervous tissue that extends from the brainstem and continues down the centre of the back to the pelvis. It is enclosed within the vertebral column. The centre of the spinal cord contains gray matter, and this is surrounded by white matter.
• What is the main function of the spinal cord? The main function of the spinal cord is to pass nerve impulses back and forth between the brain and the body.
• Explain how reflex actions occur. Reflex actions occur when sensory nerves send impulses that go to the spinal cord and from the spinal cord go to motor nerves without traveling all the way to the brain and back.
• Why do severe spinal cord injuries usually cause paralysis? Severe spinal cord injuries usually cause paralysis because they interrupt the transmission of sensory nerve messages to the brain and motor nerve messages from the brain.
• What do you think are some possible consequences of severe damage to the brain stem? How might this compare to the consequences of severe damage to the frontal lobe? Explain your answer. Answers will vary. Sample answer:  I think that severe damage to the brain stem is likely to be life-threatening, because it controls unconscious vital functions of the body, such as heart rate and breathing. Severe damage to the frontal lobe would be less likely to be life-threatening because it is involved in higher level executive functions such as planning and problem solving. It may, however, cause problems with these functions as well as abstract thought, language, attention, self-control and personality because the frontal lobe is involved in all of these functions.
• Information travels very quickly in the nervous system, but generally, the longer the path between areas, the longer it takes. Based on this, explain why you think reflexes often occur at the spinal cord level, and do not require input from the brain. Answers will vary. Sample answer:  Reflexes often occur to protect us from harm. For instance, there is a reflex that causes you to pull your arm back when you touch something that is too hot. This needs to happen very quickly so that we don’t get hurt. If the information had to travel to the brain before the arm could be moved, the response might be too slow and damage could occur. Therefore, spinal reflexes that don’t require input from the brain allow us to respond more quickly to harmful stimuli.

8.6 Peripheral Nervous System Review Questions and Answers

• Describe the general structure of the peripheral nervous system. State its primary function. The peripheral nervous system consists of all the nervous tissue that lies outside of the central nervous system. It consists of ganglia and nerves. The primary function of the peripheral nervous system is to connect the central nervous system to the rest of the organism.
• What are ganglia? Ganglia are groups of cell bodies in the PNS.
• Identify three types of nerves based on the direction in which they carry nerve impulses.  Three types of nerves based on the direction in which they carry nerve impulses are: sensory nerves, which carry nerve impulses from the body to the CNS; motor nerves, which carry impulses from the CNS to the body; and mixed nerves, which contain both sensory and motor neurons.
• Outline all of the divisions of the peripheral nervous system. The two major divisions of the peripheral nervous system are the somatic and autonomic nervous systems. The autonomic system, in turn, is divided into sympathetic, parasympathetic, and enteric divisions.
• Compare and contrast the somatic and autonomic nervous systems. The somatic and autonomic nervous systems are the two main divisions of the peripheral nervous system. The somatic nervous system primarily senses the external environment and controls voluntary activities, generally under control of the cerebral cortex. The autonomic nervous system primarily senses the internal environment and controls involuntary activities, generally under control of the hypothalamus.
• When and how does the sympathetic division of the autonomic nervous system affect the body? The sympathetic division prepares the body to fight or flee when it is faced with danger. For example, it speeds up the heart rate, widens air passages in the lungs, increases blood flow to the skeletal muscles, and temporarily shuts down the digestive system.
• What is the function of the parasympathetic division of the autonomic nervous system? Specifically, how does it affect the body?  The function of the parasympathetic division of the autonomic nervous system is to return the body to normal after a fight-or-flight response and to maintain internal homeostasis of the body at other times. Specifically, the parasympathetic division slows down the heart rate, narrows air passages in the lungs, reduces blood flow to the skeletal muscles, and stimulates the digestive system to start working again.
• Name and describe two peripheral nervous system disorders. Answers may vary. Sample answer:  Two disorders of the peripheral nervous system include Guillain-Barre syndrome and Charcot-Marie-Tooth disease. In Guillain-Barre syndrome, the immune system attacks nerves of the PNS, leading to muscle weakness and paralysis. The exact cause is unknown, but it appears to be linked to an infection. Most people eventually make a full recovery. Charcot-Marie-Tooth disease is an incurable hereditary disorder that affects predominantly the nerves in the feet and legs. It is characterized by loss of muscle tissue and sense of touch.
• Give one example of how the CNS interacts with the PNS to control a function in the body. Answers will vary. Sample answer: The cerebral cortex of the brain, which is in the CNS, commands the somatic nervous system of the PNS to carry out voluntary motor activities.
• Visual information sensory, somatic
• Blood pressure information  sensory, autonomic
• Information that causes muscle contraction in digestive organs after eating motor, autonomic
• Information that causes muscle contraction in skeletal muscles based on the person’s decision to make a movement motor, somatic

8.7 Human Senses Review Questions and Answers

• Compare and contrast special senses and general senses. Special senses have specialized sense organs and include vision (eyes), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages). General senses are all associated with touch and lack special sense organs. Instead, touch receptors are found throughout the body, particularly in the skin.
• What are sensory receptors? Sensory receptors are specialized nerve cells that respond to stimuli in the internal or external environment and transform them into nerve impulses.
• Describe the range of tactile stimuli detected in the sense of touch.  Tactile stimuli that are detected in the sense of touch include pressure, vibration, temperature, and pain.
• Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses. Light passes first through the cornea, which helps to focus the light by refracting it. Light next enters the interior of the eye through an opening called the pupil. Light then passes through the lens, which refracts the light even more and focuses it on the retina at the back of the eye. The retina contains photoreceptor cells called rods and cones that convert the light that strikes them into nerve impulses.
• Identify two common vision problems,along with their causes and their effects on vision.  Answers may vary. Sample answer:  Two common vision problems are myopia and hyperopia. Myopia occurs when the eyeball is too long or the cornea is too curved, causing distant objects to be out of focus without affecting near vision. Hyperopia occurs when the eyeball is too long or the lens is not curved enough, causing close objects to be out of focus without affecting distant vision.
• Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses. Sound waves enter the ear canal and strike the eardrum, causing it to vibrate. The vibrations are passed through and amplified by the three tiny bones (hammer, anvil, and stirrup) of the middle ear, which passes the amplified vibrations to the fluid-filled cochlea in the inner ear. The vibrations make waves in the fluid inside the cochlea, which bends the tiny hair cells lining it. The bending of the hair cells causes them to generate nerve impulses.
• What role does the ear play in balance? Which structures of the ear are involved in balance? The semicircular canals in the ear contain fluid that moves when the head changes position. Tiny hairs lining the canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses the brain.
• Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses? Two ways the body senses chemicals are with the sense of taste and the sense of smell. Taste buds on the tongue contain chemoreceptors that sense chemicals in food. Olfactory chemoreceptors in the nasal passages sense chemicals in air.
• Explain why your skin can detect different types of stimuli, such as pressure and temperature. Answers may vary. Sample answer:  Human skin can detect different types of stimuli, because it contains several different types of receptors that respond to different kinds of stimuli by generating nerve impulses. For example, skin contains mechanoreceptors which detect mechanical force or touch, nociceptors that detect painful stimuli, and thermoreceptors that detect temperature.
• Is sensory information sent to the central nervous system via efferent or afferent nerves? Afferent
• Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects. Answers may vary. Sample answer:  Hair cells in the ear are mechanoreceptors that detect sound waves by moving back and forth in the cochlea in response to transmission of sound within the ear. There are also mechanoreceptors in the skin that detect the mechanical stimulation of touch stimuli, such as pressure and vibration.
• If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer. Answers may vary. Sample answer:  For a person to perceive visual stimuli, their brain must be able to interpret the information coming from their retina. Therefore, if a person is blind but their retina is functioning properly, there may be a problem later in the pathway to the brain, such as in the path the optic nerve takes, or in the visual cortex itself.
• When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information? Cone photoreceptors. Cones are located in the retina, particularly in the centre of the retina. The occipital lobe.
• The auditory nerve carries sound information .

8.8 Psychoactive Drugs Review Questions and Answers

• What are psychoactive drugs? Psychoactive drugs are substances that change the function of the brain and result in alterations of mood, thinking, perception, and/or behavior.
• Identify six classes of psychoactive drugs, along with an example of a drug in each class. Examples may vary. Sample answer:  Six classes of psychoactive drugs (and an example of each) are: stimulants (caffeine), depressants (ethanol), anxiolytics (diazepam), euphoriants (MDMA), hallucinogens (LSD), and empathogens (amphetamine).
• Compare and contrast psychoactive drugs that are agonists and psychoactive drugs that are antagonists.  Both agonists and antagonists produce their effects by affecting particular neurotransmitters in the brain. Agonists increase the activity of neurotransmitters, whereas antagonists decrease the activity of neurotransmitters.
• Describe two medical uses of psychoactive drugs. Answers may vary. Sample answer:  Two medical uses of psychoactive drugs are controlling pain and stabilizing mood.
• Give an example of a ritual use of a psychoactive drug. An example of a ritual use of a psychoactive drug is the use of the mescaline-containing peyote cactus for religious ceremonies by Native Americans.
• Generally speaking, why do people use psychoactive drugs recreationally? People generally use psychoactive drugs recreationally to alter their state of consciousness and create a feeling of euphoria.
• Define addiction. Addiction is the compulsive use of a drug despite negative consequences that such use may entail.
• Identify possible withdrawal symptoms associated with physical dependence on a psychoactive drug. Possible withdrawal symptoms associated with physical dependence on a psychoactive drug include tremors, pain, seizures, and insomnia.
• Why might a person with a heroin addiction be prescribed the psychoactive drug methadone? A person with a heroin addiction might be prescribed the psychoactive drug methadone to reduce their cravings and withdrawal symptoms.
• Is Prozac an agonist or an antagonist for serotonin? Explain your answer. It is an agonist for serotonin because it causes more serotonin to be present in the synapse, increasing the activation of serotonin receptors on the postsynaptic cell.
• Is Prozac a psychoactive drug? Explain your answer. Yes, Prozac is a psychoactive drug because it can alter a person’s mood.
• Name three classes of psychoactive drugs that include opioids. Depressants, anxiolytics, euphoriants
• True or False: All psychoactive drugs are either illegal or available by prescription only.  False
• True or False: Anxiolytics might be prescribed by a physician.  True
• Name two drugs that activate receptors for the neurotransmitter GABA. Why do you think these drugs generally have a depressant effect?  Answers may vary. Sample answer: Ethanol (alcohol) and barbiturates. GABA normally has an inhibitory effect on neurons, so activation of GABA receptors can cause a depressant effect.

8.9 Case Study Conclusion and Chapter 10 Summary Review Questions and Answers

• Which part of the brain is neuron A located in — the cerebellum, cerebrum, or brain stem? Explain how you know. Neuron A is in the cerebrum, because it is in the cerebral cortex which makes up the outer layer of the cerebrum.
• The cell body of neuron A is located in a lobe of the brain that is involved in abstract thought, problem solving, and planning. Which lobe is this? The frontal lobe
• Part of neuron A travels all the way down to the spinal cord to meet neuron B. Which part of neuron A travels to the spinal cord? The axon
• Neuron A forms a chemical synapse with neuron B in the spinal cord. How is the signal from neuron A transmitted to neuron B? When the action potential in neuron A reaches the axon terminal, it opens channels that allow calcium to enter the terminal. The calcium causes synaptic vesicles containing neurotransmitter to fuse with the membrane of the terminal. This releases neurotransmitter across the synaptic cleft to neurotransmitter receptors on neuron B. This is how the signal is transmitted from neuron A to neuron B.
• Is neuron A in the central nervous system (CNS) or peripheral nervous system (PNS)? CNS
• The axon of neuron B travels in a nerve to a skeletal muscle cell. Is the nerve part of the CNS or PNS? Is this an afferent nerve or an efferent nerve? PNS; efferent nerve
• What part of the PNS is involved in this pathway — the autonomic nervous system or the somatic nervous system? Explain your answer. The somatic nervous system, because this pathway controls a voluntary movement. The somatic nervous system controls voluntary activities and the autonomic nervous system controls involuntary activities.
• What are the differences between a neurotransmitter receptor and a sensory receptor? Answers may vary. Sample answer:  A neurotransmitter receptor is a protein embedded in the membrane of a postsynaptic cell that binds to neurotransmitter from the presynaptic cell. A sensory receptor is a specialized cell that responds to sensory stimuli and transmits the information to the CNS.
• If a person has a stroke and then has trouble using language correctly, which hemisphere of their brain was most likely damaged? Explain your answer. Answers may vary. Sample answer:  The left hemisphere of the brain was most likely damaged because in most people, language is concentrated (lateralized) on the left side. However, it can be different in different people.
• Define an electrical gradient, in the context of a cell. An electrical gradient is a difference in electrical charge across a cell membrane.
• What is responsible for maintaining the electrical gradient that results in the resting potential? The sodium-potassium pump
• Compare and contrast the resting potential and the action potential. Answers may vary. Sample answer:  The resting potential and action potential are both differences in electrical charge across the cell membrane of a neuron, but the resting potential is the electrical potential when the neuron is at rest, and the action potential occurs when a neuron becomes sufficiently stimulated or excited. Also, the action potential is a sudden reversal of the charge difference across the membrane compared to the resting potential. This makes the inside of the membrane more positive than the outside, as opposed to the typical negative resting membrane potential.
• Where along a myelinated axon does the action potential occur? Why does it happen here? Answers may vary. Sample answer:  In a myelinated axon, the action potential occurs at the nodes of Ranvier, which are unmyelinated gaps along the axon. This is because myelin is electrically insulating, preventing ions from flowing across, so the ion flow necessary for the action potential to occur can only happen at the nodes.
• What does it mean that the action potential is “all-or-none?” Answers may vary. Sample answer:  The action potential is said to be “all-or-none” because it either fully happens or it doesn’t happen at all. An action potential always occurs to the same extent, there are not smaller or larger action potentials.
• Compare and contrast Schwann cells and oligodendrocytes.  Schwann cells and oligodendrocytes are both glial cells in the nervous system that produce myelin sheath. However, Schwann cells are in the PNS, while oligodendrocytes are in the CNS.
• For the senses of smell and hearing, name their respective sensory receptor cells, what type of receptor cells they are, and what stimuli they detect. The sensory receptor cells for hearing are called hair cells, they are mechanoreceptors, and they detect sound waves. The sensory receptor cells for smell are called olfactory receptors, they are chemoreceptors, and they detect odor molecules.
• Nicotine is a psychoactive drug that binds to and activates a receptor for the neurotransmitter acetylcholine. Is nicotine an agonist or an antagonist for acetylcholine? Explain your answer. Nicotine is an agonist for acetylcholine because it activates an acetylcholine receptor, thereby mimicking its function.

Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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11.9: Case Study Conclusion: Memory and Chapter Summary

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• Suzanne Wakim & Mandeep Grewal
• Butte College

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Case Study Conclusion: Fading Memory

Figure \(\PageIndex{1}\) illustrates some of the molecular and cellular changes that occur in Alzheimer’s disease (AD), which Rosa was diagnosed with at the beginning of this chapter, after experiencing memory problems and other changes in her cognitive functioning, mood, and personality. These abnormal changes in the brain include the development of amyloid plaques between brain cells and neurofibrillary tangles inside of neurons. These hallmark characteristics of AD are associated with the loss of synapses between neurons, and ultimately the death of neurons.

After reading this chapter, you should have a good appreciation for the importance of keeping neurons alive and communicating with each other at synapses. The nervous system coordinates all of the body’s voluntary and involuntary activities. It interprets information from the outside world through sensory systems and makes appropriate responses through the motor system, through communication between the PNS and CNS. The brain directs the rest of the nervous system and controls everything from basic vital functions such as heart rate and breathing to high-level functions such as problem-solving and abstract thought. The nervous system is able to perform these important functions by generating action potentials in neurons in response to stimulation and sending messages between cells at synapses, typically using chemical neurotransmitter molecules. When neurons are not functioning properly, lose their synapses, or die, they cannot carry out the signaling that is essential for the proper functioning of the nervous system.

AD is a progressive neurodegenerative disease, meaning that the damage to the brain becomes more extensive as time goes on. Figure \(\PageIndex{2}\) illustrates how the damage progresses from before AD is diagnosed (preclinical AD), to mild and moderate AD, and finally to severe AD.

You can see that the damage starts in a relatively small location towards the bottom of the brain. One of the earliest brain areas to be affected by AD is the hippocampus. The hippocampus is important for learning and memory. This explains why many of Rosa’s symptoms of mild AD involve deficits in memory, such as trouble remembering where she placed objects, recent conversations, and appointments.

As AD progresses, more of the brain is affected, including areas involved in emotional regulation, social behavior, planning, language, spatial navigation, and higher-level thought. Rosa is beginning to show signs of problems in these areas, including irritability, lashing out at family members, getting lost in her neighborhood, problems finding the right words, putting objects in unusual locations, and difficulty in managing her finances. You can see that as AD progresses, damage spreads further across the cerebrum, which you now know controls conscious functions such as reasoning, language, and interpretation of sensory stimuli. You can also see how the frontal lobe, which controls executive functions such as planning, self-control, and abstract thought, becomes increasingly damaged.

Increasing damage to the brain causes corresponding deficits in functioning. In moderate AD, patients have increased memory, language, and cognitive deficits compared to mild AD. They may not recognize their own family members, and may wander and get lost, engage in inappropriate behaviors, become easily agitated, and have trouble carrying out daily activities such as dressing. In severe AD, much of the brain is affected. Patients usually cannot recognize family members or communicate and are fully dependent on others for their care. They begin to lose the ability to control their basic functions, such as bladder and bowel control and proper swallowing. Eventually, AD causes death, usually as a result of this loss of basic functions.

For now, Rosa only has mild AD is still able to function relatively well with care from her family. The medication her doctor gave her has helped improve some of her symptoms. It is a cholinesterase inhibitor, which blocks an enzyme that normally degrades the neurotransmitter acetylcholine. With more of the neurotransmitter available, more of it can bind to neurotransmitter receptors on postsynaptic cells. Therefore, this drug acts as an agonist for acetylcholine, which enhances communication between neurons in Rosa’s brain. This increase in neuronal communication can help restore some of the functions lost in early Alzheimer’s disease and may slow the progression of symptoms.

But medication such as this is only a short-term measure and does not halt the progression of the underlying disease. Ideally, the damaged or dead neurons would be replaced by new, functioning neurons. Why does this not happen automatically in the body? As you have learned, neurogenesis is very limited in adult humans, so once neurons in the brain die, they are not normally replaced to any significant extent. However, scientists are studying the ways in which neurogenesis might be able to be increased in cases of disease or injury to the brain. Also, they are investigating the possibility of using stem cell transplants to replace damaged or dead neurons with new neurons. But this research is in very early stages and is not currently a treatment for AD.

One promising area of research is in the development of methods to allow earlier detection and treatment of AD, given that the changes in the brain may actually start 10 to 20 years before the diagnosis of AD. For example, a radiolabeled chemical called Pittsburgh Compound B (PiB) binds to amyloid plaques in the brain and in the future may be used in conjunction with brain imaging techniques to detect early signs of AD. Scientists are also looking for biomarkers in bodily fluids such as blood and cerebrospinal fluid that might indicate the presence of AD before symptoms appear. Finally, researchers are also investigating possible early and subtle symptoms, such as changes in how people move or a loss of smell, to see whether they can be used to identify people who will go on to develop AD. This research is in the early stages, but the hope is that patients can be identified earlier to provide earlier and possibly more effective treatment and to allow families more time to plan.

Scientists are also still trying to fully understand the causes of AD, which affects more than 5 million Americans. Some genetic mutations have been identified that play a role, but environmental factors also appear to be important. With more research into the causes and mechanisms of AD, hopefully, a cure can be found, and people like Rosa can live a longer and better life.

Chapter Summary

In this chapter, you learned about the human nervous system. Specifically, you learned that:

• The nervous system is the organ system that coordinates all of the body’s voluntary and involuntary actions by transmitting signals to and from different parts of the body. It has two major divisions, the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord.
• The PNS consists mainly of nerves that connect the CNS with the rest of the body. It has two major divisions: the somatic nervous system and the autonomic nervous system. The somatic system controls activities that are under voluntary control. The autonomic system controls activities that are involuntary.
• The autonomic nervous system is further divided into the sympathetic division, which controls the fight-or-flight response; the parasympathetic division, which controls most routine involuntary responses; and the enteric division, which provides local control for digestive processes.
• Signals sent by the nervous system are electrical signals called nerve impulses. They are transmitted by special, electrically excitable cells called neurons, which are one of two major types of cells in the nervous system.
• Glial cells are the other major type of nervous system cells. There are many types of glial cells, and they have many specific functions. In general, glial cells function to support, protect, and nourish neurons.
• The main parts of a neuron include the cell body, dendrites, and axon. The cell body contains the nucleus. Dendrites receive nerve impulses from other cells, and the axon transmits nerve impulses to other cells at axon terminals. A synapse is a complex membrane junction at the end of an axon terminal that transmits signals to another cell.
• Axons are often wrapped in an electrically-insulating myelin sheath, which is produced by glial cells. Electrical impulses called action potentials occur at gaps in the myelin sheath, called nodes of Ranvier, which speeds the conduction of nerve impulses down the axon.
• Neurogenesis, or the formation of new neurons by cell division, may occur in a mature human brain but only to a limited extent.
• The nervous tissue in the brain and spinal cord consists of gray matter, which contains mainly the cell bodies of neurons; and white matter, which contains mainly myelinated axons of neurons. Nerves of the peripheral nervous system consist of long bundles of myelinated axons that extend throughout the body.
• There are hundreds of types of neurons in the human nervous system, but many can be classified on the basis of the direction in which they carry nerve impulses. Sensory neurons carry nerve impulses away from the body and toward the central nervous system, motor neurons carry them away from the central nervous system and toward the body, and interneurons often carry them between sensory and motor neurons.
• A nerve impulse is an electrical phenomenon that occurs because of a difference in electrical charge across the plasma membrane of a neuron.
• The sodium-potassium pump maintains an electrical gradient across the plasma membrane of a neuron when it is not actively transmitting a nerve impulse. This gradient is called the resting potential of the neuron.
• An action potential is a sudden reversal of the electrical gradient across the plasma membrane of a resting neuron. It begins when the neuron receives a chemical signal from another cell or some other type of stimulus. The action potential travels rapidly down the neuron’s axon as an electric current.
• A nerve impulse is transmitted to another cell at either an electrical or a chemical synapse. At a chemical synapse, neurotransmitter chemicals are released from the presynaptic cell into the synaptic cleft between cells. The chemicals travel across the cleft to the postsynaptic cell and bind to receptors embedded in its membrane.
• There are many different types of neurotransmitters. Their effects on the postsynaptic cell generally depend on the type of receptor they bind to. The effects may be excitatory, inhibitory, or modulatory in more complex ways. Both physical and mental disorders may occur if there are problems with neurotransmitters or their receptors.
• The CNS includes the brain and spinal cord. It is physically protected by bones, meninges, and cerebrospinal fluid. It is chemically protected by the blood-brain barrier.
• The brain is the control center of the nervous system and of the entire organism. The brain uses a relatively large proportion of the body’s energy, primarily in the form of glucose.
• The brain is divided into three major parts, each with different functions: brain stem, cerebellum, and cerebrum. The cerebrum is further divided into left and right hemispheres. Each hemisphere has four lobes: frontal, parietal, temporal, and occipital. Each lobe is associated with specific senses or other functions.
• The cerebrum has a thin outer layer called the cerebral cortex. Its many folds give it a large surface area. This is where most information processing takes place.
• Inner structures of the brain include the hypothalamus, which controls the endocrine system via the pituitary gland; and the thalamus, which has several involuntary functions.
• The spinal cord is a tubular bundle of nervous tissues that extends from the head down the middle of the back to the pelvis. It functions mainly to connect the brain with the PNS. It also controls certain rapid responses called reflexes without input from the brain.
• A spinal cord injury may lead to paralysis (loss of sensation and movement) of the body below the level of the injury because nerve impulses can no longer travel up and down the spinal cord beyond that point.
• The PNS consists of all the nervous tissue that lies outside of the CNS. Its main function is to connect the CNS to the rest of the organism.
• The tissues that make up the PNS are nerves and ganglia. Ganglia act as relay points for messages that are transmitted through nerves. Nerves are classified as sensory, motor, or a mix of the two.
• The PNS is not as well protected physically or chemically as the CNS, so it is more prone to injury and disease. PNS problems include injury from diabetes, shingles, and heavy metal poisoning. Two disorders of the PNS are Guillain-Barre syndrome and Charcot-Marie-Tooth disease.
• The human body has two major types of senses, special senses, and general senses. Special senses have specialized sense organs and include vision (eyes), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages). General senses are all associated with touch and lack special sense organs. Touch receptors are found throughout the body but particularly in the skin.
• All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include mechanoreceptors (mechanical forces), thermoreceptors (temperature), nociceptors (pain), photoreceptors (light), and chemoreceptors (chemicals).
• Touch includes the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.
• Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and “tells” us what we are seeing.
• Common vision problems include myopia (nearsightedness), hyperopia (farsightedness), and presbyopia (age-related decline in close vision).
• Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and “tells” us what we are hearing.
• The ear is also the organ that is responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses on head position to the brain, which sends messages to skeletal muscle via the peripheral nervous system. The muscles respond by contracting to maintain balance.
• Taste and smell are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food and olfactory receptors in the nasal passages sense chemicals in the air. The sense of smell contributes significantly to the sense of taste.
• Psychoactive drugs are substances that change the function of the brain and result in alterations of mood, thinking, perception, and/or behavior. They include prescription medications such as opioid painkillers, legal substances such as nicotine and alcohol, and illegal drugs such as LSD and heroin.
• Psychoactive drugs are divided into different classes according to their pharmacological effects. They include stimulants, depressants, anxiolytics, euphoriants, hallucinogens, and empathogens. Many psychoactive drugs have multiple effects so they may be placed in more than one class.
• Psychoactive drugs generally produce their effects by affecting brain chemistry. Generally, they act either as agonists, which enhance the activity of particular neurotransmitters; or as antagonists, which decrease the activity of particular neurotransmitters.
• Psychoactive drugs are used for various purposes, including medical, ritual, and recreational purposes.
• Misuse of psychoactive drugs may lead to addiction, which is the compulsive use of a drug despite negative consequences. Sustained use of an addictive drug may produce physical or psychological dependence on the drug. Rehabilitation typically involves psychotherapy and sometimes the temporary use of other psychoactive drugs.

In addition to the nervous system, there is another system of the body that is important for coordinating and regulating many different functions – the endocrine system. You will learn about the endocrine system in the next chapter.

Chapter Summary Review

• Which part of the brain is neuron A located in — the cerebellum, cerebrum, or brain stem? Explain how you know.
• The cell body of neuron A is located in a lobe of the brain that is involved in abstract thought, problem-solving and planning. Which lobe is this?
• Part of neuron A travels all the way down to the spinal cord to meet neuron B. Which part of neuron A travels to the spinal cord?
• Neuron A forms a chemical synapse with neuron B in the spinal cord. How is the signal from neuron A transmitted to neuron B?
• Is neuron A in the central nervous system (CNS) or peripheral nervous system (PNS)?
• The axon of neuron B travels in a nerve to a skeletal muscle cell. Is the nerve part of the CNS or PNS? Is this an afferent nerve or an efferent nerve?
• What part of the PNS is involved in this pathway — the autonomic nervous system or the somatic nervous system? Explain your answer.
• What are the differences between a neurotransmitter receptor and a sensory receptor?
• The axon terminal
• The nodes of Ranvier
• The dendrites
• The cell body
• True or False. Glial cells produce action potentials.
• True or False. The spinal cord consists of white matter only.
• True or False. Axons may be more than a meter long in adult humans.
• If a person has a stroke and as a result has trouble using language correctly, which hemisphere of their brain was most likely damaged? Explain your answer.
• head region
• trunk and leg regions
• Define what an electrical gradient is, in the context of a cell.
• What is responsible for maintaining the electrical gradient that results in the resting potential?
• Compare and contrast the resting potential and the action potential.
• Where along a myelinated axon does the action potential occur? Why does it happen here?
• What does it mean that the action potential is “all-or-none?”
• The neurotransmitter itself
• The specific receptor for the neurotransmitter on the postsynaptic cell
• The number of synaptic vesicles in the axon terminal
• Whether it is in a sensory neuron or a motor neuron
• Compare and contrast Schwann cells and oligodendrocytes.
• True or False. The cerebellum makes up most of the brain and is divided into four lobes.
• True or False. The hypothalamus is part of the brain.
• Mechanoreceptors
• Nociceptors
• Photoreceptors
• Chemoreceptors
• For the senses of smell and hearing, name their respective sensory receptor cells, what type of receptor cells they are, and what stimuli they detect.
• True or False. Sensory information such as smell, taste, and sound, are carried to the CNS by cranial nerves.
• True or False. The parasympathetic nervous system is a division of the central nervous system.

Attributions

• Characteristics of AD by National Institute on Aging, National Institutes of Health; public domain via Wikimedia Commons
• Alzheimer’s Disease, Spreads through the Brain by National Institute on Aging, National Institutes of Health; public domain via Flickr.com
• Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0