Chapter 13: THE NERVOUS SYSTEM

13 the nervous system Chapter Outline neurons: THE Communication Specialists Neurons are the communication line of the nervous system Properties of a neuron’s plasma membrane allow it to carry signals Nerve Impulses = Action potenTials Action potentials travel away from their starting point A neuron can’t “fire” again until ion pumps restore its resting potential Action potentials are “all-or-nothing” How Neurons Communicate Neurotransmitters can excite or inhibit a receiving cell Competing signals are “summed up” Neurotransmitter molecules must be removed from the synapse information pathways Nerves are long-distance lines Reflexes are the simplest nerve pathways In the brain and spinal cord, neurons interact in circuits OVERVIEW OF the nervous system major expressways: peripheral nerves and the spinal cord The peripheral nervous system consists of somatic and autonomic nerves Autonomic nerves are divided into parasympathetic and sympathetic groups The spinal cord links the PNS and the brain The Brain: Command central The brain’s three main functional areas are the hindbrain, midbrain, and forebrain Cerebrospinal fluid fills spaces in the brain and spinal cord a closer look at the Cerebrum The cerebral cortex is the seat of consciousness The limbic system governs emotions consciousness memory disorders of the nervous system Physical injury is a common cause of nervous system damage In some disorders, brain neurons break down Infections and cancer inflame or destroy brain tissue Headaches only seem like brain “disorders” Various neural disorders affect development, behavior, and mood FOCUS ON HEALTH: the brain on “mind-altering” DRUGS Connections: The nervous system in Homeostasis SUMMARY Review questions self-quiz critical thinking explore on your own YOUR FUTURE Objectives Describe the visible structure of neurons, neuroglia, nerves, and ganglia, both separately and together as a system. Know the functional zones on a neuron. Explain the function of neuroglia. Define membrane potential. Explain why action potentials move in only one direction. Describe the distribution of the invisible array of proteins, ions, and other molecules in a neuron, both at rest and as a neuron experiences a change in potential. Understand how a nerve impulse is received by a neuron, conducted along a neuron, and transmitted across a synapse to a neighboring neuron, muscle, or gland. Explain the importance of myelination of nerve axons. Outline some of the ways by which information flow is regulated and integrated in the human body. Understand the distinction between the central nervous system and the peripheral nervous system as well as the somatic, autonomic, sympathetic, and parasympathetic nervous systems. Summarize the major parts of the human brain, list the major lobes of the cerebral cortex, and describe the principal functions of each. Define meninges, cerebrospinal fluid, white and gray matter. Know the function of the spinal cord. Characterize the major groups of drugs, emphasizing their effects on the nervous system. Key Terms nervous system sensory neurons interneurons motor neurons dendrites axon resting membrane potential threshold action potential sodium-potassium pumps neurotransmitters chemical synapse neuromodulators synaptic integration nerve nerve tract myelin sheath Schwann cells reflex central nervous system (CNS) peripheral nervous system (PNS) ganglia somatic nerves autonomic nerves parasympathetic nerves sympathetic nerves spinal cord white matter gray matter brain medulla oblongata pons cerebellum brain stem cerebrum cerebral hemispheres thalamus hypothalamus meninges cerebrospinal fluid (CSF) blood-brain barrier cerebral cortex motor areas sensory areas association areas limbic system reticular formation memory amnesia concussion paralysis seizure disorders Parkinson’s disease (PD) Alzheimer’s disease meningitis encephalitis Creutzfeldt-Jakob disease glial cancers multiple sclerosis (MS) Guillan-Barre syndrome headache attention deficit hyperactivity disorder (ADHD) mood disorders autism schizophrenia Lecture Outline Ecstasy is a drug that can make you feel really good, at least for a time. The active ingredient is MDMA, an amphetamine-like drug that interferes with the function of serotonin in the brain. Excess serotonin can relieve anxiety, sharpen the senses, and make you feel socially accepted; it can also kill. How we function as individuals depends on whether we nurture or abuse our nervous system. Neurons: The Communication Specialists Neurons are the communication lines of the nervous system. Neurons form the basis of the system’s communication network. There are three types of neurons: Sensory neurons are receptors for specific sensory stimuli (signals). Interneurons in the brain and spinal cord integrate input and output signals. Motor neurons send information from integrator to muscle or gland cells (effectors). Neurons form extended cells with several zones: The cell body contains the nucleus and organelles. The cell body has slender extensions called dendrites; the cell body and the dendrites form the input zone for receiving information. Next comes the trigger zone, called the axon hillock in motor neurons and interneurons; the trigger zone leads to the axon, which is the neuron’s conducting zone. The axon’s endings are output zones where messages are sent to other cells. Only 10% of the nervous system consists of neurons; the rest of the 90% is composed of support cells called neuroglia, or glia. Neurons function well in communication because they are excitable (produce electrical signals in response to stimuli). Properties of a neuron’s plasma membrane allow it to carry signals. The plasma membrane prevents charged substances (K+ and Na+ ions) from moving freely across, but both ions can move through channels. Some channel proteins are always open, others are gated. In a resting neuron, gated sodium channels are closed; sodium does not pass through the membrane, but potassium does. According to the gradients that form, sodium diffuses into the cell, potassium diffuses out of the cell. The difference across the membrane that forms because of the K+ and Na+ gradients results in a resting membrane potential of ?70 millivolts (cytoplasmic side of the membrane is negative). Nerve Impulses = Action Potentials Sufficient signals at the input zone of a resting neuron can trigger reversal of the voltage difference across the membrane. The signal opens gated sodium channels, allowing Na+ to rush into the neuron. The internal charge near the membrane becomes less negative, stimulating more channels to open (positive feedback). When the voltage difference crosses a key threshold level of stimulation, an action potential (nerve impulse) occurs. Thresholds can only be reached in areas of the neuron where there are voltage-sensitive sodium channels. Stimuli must be strong enough to trigger the potential. Action potentials travel away from their starting point. The action potential is self-propagating and moves away from the stimulation site. Potentials can self-propagate because the changes to the membrane potential don’t lose strength. A neuron can’t “fire” again until ion pumps restore its resting potential. By diffusion, some potassium ions will always leak out of the cell and some sodium will always leak in. The sodium-potassium pump uses ATP to actively pump potassium ions in and sodium ions out of the neuron to keep the concentration of sodium ions higher outside, ready for another action potential to form. Action potentials are “all-or-nothing.” Action potentials are all-or-nothing events. Once a positive-feedback cycle starts, nothing stops the full “spiking” of a potential. If threshold is not reached, however, the membrane disturbance will subside when the stimulus is removed. When the action potential is terminated, the sodium gates close, potassium gates open, and the sodium-potassium membrane pumps become operational to fully restore the resting potential. How Neurons Communicate Action potentials can stimulate the release of neurotransmitters. Neurotransmitters diffuse across a chemical synapse, the junction between a neuron and an adjacent cell (between neurons and other neurons, or between neurons and muscle or gland cells). The neuron that releases the transmitter is called the presynaptic cell. In response to an action potential, gated calcium channels open and allow calcium ions to enter the neuron from the synapse. Calcium causes the synaptic vesicles to fuse with the membrane and release the transmitter substance into the synapse. The transmitter binds to receptors on the membrane of the postsynaptic cell. Neurotransmitters can excite or inhibit a receiving cell. How a postsynaptic cell responds to a transmitter depends on the type and amount of transmitter, the receptors it has, and the types of channels in its input zone. Excitatory signals drive the membrane toward an action potential. Inhibitory signals prevent an action potential. Neurotransmitters include the following examples. Acetylcholine (ACh) can excite or inhibit target cells in the brain, spinal cord, glands, and muscles. Serotonin acts on brain cells to govern sleeping, sensory perception, temperature regulation, and emotional states. Some neurons secrete nitric oxide (NO), a gas that controls blood vessel dilation, as in penis erection. Neuromodulators can magnify or reduce the effects of a neurotransmitter. One example includes the natural painkillers called endorphins. Release of endorphins prevents sensations of pain from being recognized. Endorphins may also play a role in memory, learning, and sexual behavior. Competing signals are “summed up.” Excitatory and inhibitory signals compete at the input zone. An excitatory postsynaptic potential (EPSP) depolarizes the membrane to bring it closer to threshold. An inhibitory postsynaptic potential (IPSP) either drives the membrane away from threshold by a hyperpolarizing effect or maintains the membrane potential at the resting level. In synaptic integration, competing signals that reach the input zone of a neuron at the same time are summed; summation of signals determines whether a signal is suppressed, reinforced, or sent onward to other body cells. Neurotransmitter molecules must be removed from the synapse. Neurotransmitters must be removed from the synaptic cleft to discontinue stimulation. There are three methods of removal: Some neurotransmitter molecules simply diffuse out of the cleft. Enzymes, such as acetylcholinesterase, break down the transmitters. Membrane transport proteins actively pump neurotransmitter molecules back into the presynaptic cells. Information Pathways Nerves are long-distance lines. Signals between the brain or spinal cord and body regions travel via nerves. Axons of sensory neurons, motor neurons, or both, are bundled together in a nerve. Within the brain and spinal cord, bundles of interneuron axons are called nerve tracts. Axons are covered by a myelin sheath derived from Schwann cells. Each section of the sheath is separated from adjacent ones by a region where the axon membrane, along with gated sodium channels, is exposed—this exposed region is a node. Action potentials jump from node to node (saltatory conduction); such jumps are fast and efficient. There are no Schwann cells in the central nervous system; here processes from oligodendrocytes form the sheaths of myelinated axons. Reflexes are the simplest nerve pathways. A reflex is a simple, stereotyped movement in response to a stimulus. In the simplest reflex arcs, sensory neurons synapse directly with motor neurons; an example is the stretch reflex, which contracts a muscle after that muscle has been stretched. In most reflex pathways, the sensory neurons also interact with several interneurons, which excite or inhibit motor neurons as needed for a coordinated response. In the brain and spinal cord, neurons interact in circuits. The overall direction of flow in the nervous system: sensory neurons >>> spinal cord and brain >>> interneurons >>> motor neurons. Interneurons in the spinal cord and brain are grouped into blocks, which in turn form circuits; blocks receive signals, integrate them, and then generate new ones. Divergent circuits fan out from one block into another. Other circuits funnel down to just a few neurons. In reverberating circuits, neurons repeat signals among themselves. Overview of the Nervous System The central nervous system (CNS) is composed of the brain and spinal cord; all of the interneurons are contained in this system. Nerves that carry sensory input to the CNS are called the afferent nerves. Efferent nerves carry signals away from the CNS. The peripheral nervous system (PNS) includes all the nerves that carry signals to and from the brain and spinal cord to the rest of the body. The PNS is further divided into the somatic and autonomic subdivisions. The PNS consists of 31 pairs of spinal nerves and 12 pairs of cranial nerves. At some sites, cell bodies from several neurons cluster together in ganglia. Major Expressways: Peripheral Nerves and the Spinal Cord The peripheral nervous system consists of somatic and autonomic nerves. Somatic nerves carry signals related to movement of the head, trunk, and limbs; signals move to and from skeletal muscles for voluntary control. Autonomic nerves carry signals between internal organs and other structures; signals move to and from smooth muscles, cardiac muscle, and glands (involuntary control). The cell bodies of preganglionic neurons lie within the CNS and extend their axons to ganglia outside the CNS. Postganglionic neurons receive the messages from the axons of the preganglionic cells and pass the impulses on to the effectors. Autonomic nerves are divided into parasympathetic and sympathetic groups. Autonomic nerves are divided into parasympathetic and sympathetic nerves, which normally work antagonistically toward each other. Parasympathetic nerves slow down body activity when the body is not under stress. Sympathetic nerves increase overall body activity during times of stress, excitement, or danger; they also call on the hormone norepinephrine to increase the fight-flight response. When sympathetic activity drops, parasympathetic activity may rise in a rebound effect. The spinal cord links the PNS and the brain. The spinal cord lies within a closed channel formed by the bones of the vertebral column. Signals move up and down the spinal cord in nerve tracts. The myelin sheaths of these tracts are white; thus, they are called white matter. The central, butterfly-shaped area (in cross-section) consists of dendrites, cell bodies, interneurons, and neuroglia cells; it is called gray matter. The spinal cord and brain are covered with three tough membranes—the meninges. The spinal cord is a pathway for signal travel between the peripheral nervous system and the brain; it also is the center for controlling some reflex actions. Spinal reflexes result from neural connections made within the spinal cord and do not require input from the brain, even though the event is recorded there. Autonomic reflexes, such as bladder emptying, are also the responsibility of the spinal cord. The Brain: Command Central The brain’s three main functional areas are the hindbrain, midbrain, and forebrain. The hindbrain and midbrain form the brain stem, responsible for many simple reflexes. Hindbrain. The medulla oblongata has influence over respiration, heart rate, swallowing, coughing, and sleep/wake responses. The cerebellum acts as a reflex center for maintaining posture and coordinating limbs. The pons (“bridge”) possesses nerve tracts that pass between brain centers. Midbrain. The midbrain coordinates reflex responses to sight and sound. It has a roof of gray matter, the tectum, where visual and sensory input converges before being sent to higher brain centers. Forebrain. The forebrain is the most developed portion of the brain in humans. The cerebrum integrates sensory input and selected motor responses; olfactory bulbs deal with the sense of smell. The thalamus relays and coordinates sensory signals through clusters of neuron cell bodies called nuclei; Parkinson’s disease occurs when the function of basal nuclei in the thalamus is disrupted. The hypothalamus monitors internal organs and influences responses to thirst, hunger, and sex, thus controlling homeostasis. Cerebrospinal fluid fills spaces in the brain and spinal cord. The brain is protected by bone and meninges. The tough outer membrane is the dura mater; it is folded double around the brain and divides the brain into its right and left halves. The thinner middle layer is the arachnoid; the delicate pia mater wraps the brain and spinal cord as the innermost layer. The meninges also enclose fluid-filled spaces that cushion and nourish the brain. The brain and spinal cord are surrounded by the cerebrospinal fluid (CSF), which fills cavities (ventricles) and canals within the brain. A mechanism called the blood-brain barrier controls which substances will pass to the fluid, and subsequently to the neurons. The capillaries of the brain are much less permeable than other capillaries, forcing materials to pass through the cells, not around them. Lipid-soluble substances, such as alcohol, nicotine, and drugs, diffuse quickly through the lipid bilayer of the plasma membrane. A Closer Look at the Cerebrum There are two cerebral hemispheres. The human cerebrum is divided into left and right cerebral hemispheres, which communicate with each other by means of the corpus callosum. Each hemisphere can function separately; the left hemisphere responds to signals from the right side of the body, and vice versa. The left hemisphere deals mainly with speech, analytical skills, and mathematics; nonverbal skills such as music and other creative activities reside in the right. The thin surface (cerebral cortex) is gray matter, divided into lobes by folds and fissures; white matter and basal nuclei (gray matter in the thalamus) underlie the surface. Each hemisphere is divided into frontal, occipital, temporal, and parietal lobes. The cerebral cortex is the seat of consciousness. Motor areas. Motor areas are found in the frontal lobe of each hemisphere. The motor cortex controls the coordinated movements of the skeletal muscles. The premotor cortex is associated with learned pattern or motor skills. Broca’s area is involved in speech. The frontal eye field controls voluntary eye movements. Sensory areas. Several sensory areas are found in the parietal lobe: the primary somatosensory cortex is the main receiving center for sensory input from the skin and joints, while the primary cortical area deals with taste. The primary visual cortex, which receives sensory input from the eyes, is found in the occipital lobe. Sound and odor perception arises in primary cortical areas in each temporal lobe. Association areas. Association areas occupy all parts of the cortex except the primary motor and sensory regions; each area integrates, analyzes, and responds to many inputs. Neural activity is the most complex in the prefrontal cortex, the area of the brain that allows for complex learning, intellect, and personality. The limbic system governs emotions. Our emotions and parts of our memory are governed by the limbic system, which consists of several brain regions. Parts of the thalamus, hypothalamus, amygdala, and the hippocampus form the limbic system and contribute to producing our “gut” reactions. Consciousness States of consciousness include alertness and sleeping. Consciousness is controlled by the reticular formation. The reticular formation determines what fraction of detected stimuli will be perceived by the brain. Sleep has two major stages: slow-wave, “normal,” sleep and REM (rapid eye movement) sleep. Memory Memory is how the brain stores and retrieves information. Learning and adaptive modifications to behavior are possible because of memory, the storage information. Short-term memory lasts from seconds to hours and is limited to a few bits of information. Long-term memory is more permanent and seems to be limitless. Facts are processed separately from skills using separate memory circuits. Facts, such as names or faces, are forgotten or stored in long-term memory, where they can be recalled through association. Skills, such as playing the piano, can only be recalled by doing them. Amnesia is a loss of fact memory; the severity of loss depends on the extent of damage to the brain, but amnesia does not prevent a person from learning new skills. Disorders of the Nervous System Physical injury is a common cause of nervous system damage. A concussion can result from a severe blow to the head, resulting in blurred vision and brief loss of consciousness. Damage to the spinal cord can result in lost sensation, muscle weakness, or paralysis below the site of the injury. Epilepsy is a seizure disorder, often inherited but also caused by brain injury, birth trauma, or other assaults on the brain. In some disorders, brain neurons break down. Parkinson’s disease (PD) is characterized by the death of neurons in the thalamus that normally make dopamine and norepinephrine needed for normal muscle function. Alzheimer’s disease involves the progressive degeneration of brain neurons, while at the same time there is an abnormal buildup of amyloid protein, leading to the loss of memory. Infections and cancer inflame or destroy brain tissue. Meningitis is an often fatal inflammatory disease caused by a virus or bacterial infection of the meninges covering the brain and/or spinal cord. Encephalitis is a very dangerous inflammation of the brain, often caused by a virus. Creutzfeldt-Jakob disease is caused by an infectious protein (prion) that causes holes in brain tissue. While neurons cannot become cancerous, the supportive glial cells can, causing gliomas. Multiple sclerosis (MS) is an autoimmune disease that results in the destruction of the myelin sheath of neurons in the CNS. Headaches only seem like brain “disorders.” Headaches occur when the brain registers tension in muscles or blood vessels of the face, neck, and scalp as pain; migraine headaches are extremely painful and can be triggered by hormonal changes, fluorescent lights, and certain foods, particularly in women. Various neural disorders affect development, behavior, and mood. Individuals with attention deficit hyperactivity disorder have low dopamine and low impulse control. Mood disorders include depression, and generally involve issues with serotonin and dopamine. In autism and Asperger’s syndrome, individuals struggle with thinking, language, and relating to others. Schizophrenia results in paranoid delusions. Focus on Health: The Brain on “Mind-Altering” Drugs Drugs can alter mind and body functions. Psychoactive drugs exert their influence on brain regions that govern states of consciousness and behavior. There are four categories of psychoactive drugs: Stimulants (e.g., caffeine, cocaine, nicotine, amphetamines) increase alertness or activity for a time, and then depress you. Depressants (e.g, alcohol) depress brain activity, limit judgment, and interfere with coordinated movement; blood alcohol concentration (BAC) measures alcohol in the blood to determine the level of intoxication. Analgesics (pain relievers) include morphine and OxyContin, a synthetic derivative; analgesics block pain signals and some may produce euphoria. Hallucinogens (e.g., marijuana) act like depressants at low levels, but may also skew perception and performance of complex tasks. Drug use can lead to addiction. As the body develops tolerance to a drug, larger and more frequent doses are needed to produce the same effect; this reflects physical drug dependence. Psychological drug dependence, or habituation, develops when a user begins to crave the feelings associated with using a particular drug and cannot function without it. Habituation and tolerance are evidence of addiction. Connections: The Nervous System in Homeostasis The nervous system is, along with the endocrine system, responsible for the action and coordination of the other body systems. Suggestions for Presenting the Material One of the most effective comparisons of the nervous system to anything man-made is to the worldwide telephone/computer network. Although the analogy is not perfect, it does convey the truth that billions of individual phone sets can communicate with any other phone, or several phones at one time, via a connecting wire or microwave signal. Students ranging in scientific expertise from that of beginning freshman to third-year medical student all agree that the nervous system is one of the most difficult systems to comprehend at any level. Therefore, extra time and thorough explanations are especially needed in this chapter. The function of the neuron membrane in permitting passage of Na+ and K+ ions is at first confusing. Initially, focusing on sodium may be warranted with an expansion to the role of potassium later in the lecture. Although they are no longer used in this text, the words polarized, depolarized, and repolarized are convenient words to express what is happening in an action potential. For example, in Figure 13.4(1) the membrane is polarized, in 13.4(2-3) depolarization is beginning, and in 13.4(4) repolarization is occurring immediately behind the depolarization. Prepare a transparency or slide of an action potential recording (see Figure 13.6). While projecting it on a screen, describe how the different regions of the tracing are related to the flow of sodium and potassium ions. The concept of “all-or-nothing events” and “thresholds” can be illustrated by describing a light switch—which is either ON or OFF. This is especially handy because it is already in the classroom! Emphasize the temporary nature of the acetylcholine bridge across the synapse by comparing it to a pontoon bridge used by the military to cross small streams and rivers. If the students can recite the sequence of structures through which an impulse passes during a reflex arc, such as in Figure 13.9, they have a good grasp of the nerve conduction pathways. Add the process of ion flow across the membranes, and the story is pretty well complete! Distinguish between a neuron and a nerve. Students often have difficulty distinguishing between them. The division of the human nervous system into component parts as presented in Figures 13.10, 13.13, and accompanying text is an arbitrary one so emphasize the “oneness” of the system. The divisions are really ones made by those of us who need to study the interrelated functions. Some terms used to describe the divisions are not parallel. For example, autonomic (which is not a misspelling of the word “automatic” as some students think) is used to designate nerves not under voluntary control, and somatic is used to designate nerves under voluntary control. If this is the first use of the word antagonistic in class, make an effort to remove any negative connotations students have attached to the word during regular conversational use. Tell them there are several instances where body homeostasis is maintained by antagonistic nerves, hormones, and muscles. Many students cannot distinguish the backbone (= vertebral, or spinal, column) from the spinal cord. Give them some assistance by referring to Figure 13.15. Emphasize the continuity of nerve tracts between brain and spinal cord. Stress the primary functions of the spinal cord as a reflex center versus the brain as a sense-interpretation and directed-response center. Students generally find it easier to distinguish between the sympathetic and parasympathetic divisions of the autonomic nervous system if they are told that the sympathetic division is involved in mobilizing “fight-or-flight” reaction while the parasympathetic division produces a general “slowing-down” and “rest-and-digest” response. Classroom and Laboratory Enrichment The concept of thresholds and all-or-nothing events can be demonstrated by using dominoes (or for large classes, several CD cases). Line up about 20 dominoes placed on end and spaced about one inch apart. Ask a student to gently touch one end domino to begin the progressive fall. Emphasize that the student’s touch (threshold stimulus) caused a standing row (polarized) to begin falling (depolarization) at a constant speed (all-or-nothing event). Pose the following question (and demonstrate the answer): Would a greater and faster stimulus cause more rapid falling? To demonstrate repolarization, a second student could begin resetting the dominoes even before the falling is complete. Arrange with a physics student or instructor for a demonstration of an action potential as recorded on an oscilloscope screen. Permit students to demonstrate the knee jerk reflex arc by use of percussion hammers. It is important to ask the subjects to close their eyes to prevent “cheating.” Show a video of an animation of nerve impulse transmission or use the animations in the text. Exhibit models of neurons and neuroglial cells. Provide microscope slides of longitudinal sections and cross-sections of nerves for student viewing. Have microscope slides of neurons available for laboratory demonstration. Many laboratories have preserved brain specimens from humans or other vertebrates, which are valuable aids to comprehending the size and arrangement of brain parts. Alternatively, use a dissectible model of the brain to illustrate the location of its parts. Show a film or video on brain function. Use a spinal cord/vertebral column cross-sectional model to illustrate the relation between the two structures. A herniated disc model could further be used to demonstrate why so much pain and functional loss are associated with herniated discs. Using live Hydra and Dugesia (planaria), test for nervous response to touch, vibration, light, mild acid or alkali, and heat. Are there differences between the two species with respect to degree and speed of response? Classroom Discussion Ideas Why are teenagers and young adults so susceptible to taking recreational drugs such as ecstasy? Do you think persons at a “rave” even know what drugs they might be taking? Would an education campaign about the dangers of ecstasy have any deterrent effect on young “ravers”? Why or why not? What would the result of demyelinization of axons be, such as occurs in multiple sclerosis? If neurons operate under the all-or-nothing principle, how are we able to distinguish soft sounds from loud sounds, or a gentle touch from a crushing blow? Upon hearing that salt was not good for him, a freshman college student began a fanatical program to eliminate all sodium chloride from his diet. By cooking his own meals, he was able to eliminate virtually all sodium. What complications could he expect as a result of his brash action? To most amateur musicians, the playing of 16th notes is a challenge, but to trumpet virtuoso Wynton Marsalis, 32nd and 64th notes are a breeze. Describe the action of nerves and tongue muscles that regulate the air flow through the mouthpiece. Why would a physician’s tapping of the knee or elbow reveal the status of the nervous system in general, not just the condition of those two joints? Why does “jumping” conduction afford the best possible conduction speed with the least metabolic effort by the nerve cell? Why would drinking large amounts of coffee or other caffeine-containing beverages tend to make a person “nervous” or “jittery”? Do invertebrates, such as the cockroach, feel pain? The central nervous system and closely associated ganglia house the cell bodies of neurons. As opposed to the peripheral axons and dendrites, the cell bodies in the CNS are not regenerated after traumatic injury. What advantages and disadvantages does this structural arrangement pose for humans? Why does a small speck of food stuck between your teeth feel like a large chunk when rubbed with your tongue? The exact mode of action of the famous, and now banned, insecticide DDT has never been elucidated (after nearly 50 years of research). However, textbooks describe it as a “central nervous system” poison. What does this imply? Explain why elderly people may be unable to remember what they ate for breakfast but can relate the details of a teenage romance. Discuss the characteristics of brain disorders such as Parkinson’s disease and Alzheimer’s disease. Term Paper Topics, Library Activities, and Special Projects The neurons of the human body can communicate one with the other much the same as telephones in a city can intercommunicate. In the telephone system, wires touch wires to pass the impulse, but neurons are not directly “wired.” Investigate the effects on the body of the elimination of synapse function such as would be caused by organophosphate pesticides, which inhibit acetylcholinesterase. One of the most effective antidotes for the organophosphate poisoning referred to above is atropine. Investigate its mechanism of action. Based on what you find, could administration of atropine be harmful if organophosphate poisoning has not occurred? Using the mode of action of organophosphate insecticides as a tool, delve into the similarities and differences between the physiology of insect and human nerve function. Ask a resource person to explain the consequences of central nervous system damage as opposed to peripheral damage. Investigate some of the factors that determine the speed at which an impulse is conducted in a neuron. Why are injuries to the central nervous system, such as gunshot wounds, more permanently debilitating than those to the peripheral system? What is the basis for “healing” accomplished by the practice of chiropractic? What are its strengths and weaknesses? Prepare an argument for the suppression of a presently readily available drug, say alcohol, or prepare an argument for the legalization of marijuana. Explore the research relating dreams to actual events—past and future. What do dreams tell us about ourselves? Discuss the basis for the use of acupuncture for relieving pain or providing anesthesia. Research the “gate theory” of pain transmission. Investigate the use of biofeedback for controlling pain, heart rate, and other autonomic functions. Videos, Animations, and Websites VIDEOS PBS NOVA – How Does the Brain Work? Video on the human brain. _http://www.pbs.org/wgbh/nova/body/how-does-the-brain-work.html_ Films for the Humanities and Sciences Brain and Nervous System: Your Information Superhighway _ffh.films.com/id/10135/Brain_and_Nervous_System_Your_Information_Superhighway.htm_ ANIMATIONS MedTropolis – The Virtual Body Interactive animations of the human brain. _http://medtropolis.com/virtual-body/_ PBS NOVA – Diagnosing Brain Damage Interactive animation of the investigating brain damage. _http://www.pbs.org/wgbh/nova/body/diagnose-brain-damage.html_ WEBSITES The National Institutes of Health Overview of MDMA addiction and treatment. _http://www.drugabuse.gov/publications/infofacts/mdma-ecstasy_ Harvard University The whole brain atlas. _http://www.med.harvard.edu/AANLIB/home.html_ Spinal Cord Injury Resource Center Overview of the spinal cord, spinal cord injuries, and treatments. _http://www.spinalinjury.net/_ Possible Responses to Review Questions Sensory neurons collect and relay information to the spinal cord and brain. Interneurons receive input and signal other neurons. Motor neurons effect change in response to signals. Functional zones are zones of specialized function within a motor neuron. The zones include: input zone, trigger zone, conducting zone, and output zone. An action potential is a “nerve impulse.” It occurs when the threshold of the resting membrane potential is reached, causing the resting potential to reverse and send a signal along the membrane. A chemical synapse is a gap between a neuron’s output zone and the input zone of either another neuron or a muscle cell or gland cell. Excitatory synapses receive signals that drive the membrane toward an action potential; inhibitory synapses have the opposite effect. During the process of synaptic integration, incoming competing signals to an input zone are added up or summed and integrated. Depending on how the signals tally, they are suppressed, reinforced, or sent onward to other cells. A reflex is a simple movement that is always the same in response to a given stimulus. The stretch reflex describes how a muscle behaves under gravity or after a load has stretched it; if additional load is added to a load you already carry, stretching of the muscles signals the body to maintain contraction in position, preventing you from dropping your load. Neurons are individual cells of the nervous system, whereas nerves are the long axons of neurons enclosed by connective tissue. The somatic system describes voluntary responses and the autonomic system describes involuntary responses. Parasympathetic nerves perform “housekeeping” activities, while sympathetic nerves perform tasks related to the fight-or-flight response. Possible Responses to Critical Thinking Questions The reason for analyzing the cerebrospinal fluid to determine if cells are infected rather than checking the blood is based on the blood-brain barrier concept. Cells can be in the fluid that is bathing the brain and spinal cord and yet not be in the blood from which this cerebrospinal fluid was derived. All too often we think of babies as miniature adults; they are not. The undeveloped blood-brain barrier is one good example. The adult body can process perfectly normal hormones, amino acids, ions, etc., even if their concentrations vary somewhat to the high side. Even the “baddies” such as alcohol, caffeine, and nicotine can be detoxified. But to the susceptible fetus and newborn, any of these substances could enter the brain in concentrations capable of causing damage. So a conscientious mother would limit her indulgences for the health of her unborn baby. And both parents would be careful in the foods they provide to their newborn. Cocaine would dramatically affect mental functioning. The neurons of the brain involved with reasoning and intellectual activities are underactive, suggesting cocaine use could lead to making very bad decisions. Possible Responses to Explore on Your Own Questions The reflex arc for the knee will be similar to the reflex arc shown for the arm in Figure 13.9 of the text book. The knuckle (or rubber hammer) is the “stimulus” that promotes the stretch reflex; the tap stimulates sensory receptors in the tendon, forming an action potential that propagates away from the knee back up the sensory neuron to the interneurons in the spinal cord. Synapses will allow communication between the sensory neuron and the interneuron. A signal will then be routed back from the spinal column to the knee (to motor neurons). The potential will move from the end of axons of the motor neurons to the muscles, causing contraction and the “jump” of your lower leg associated with the tap. 136 Chapter Thirteen The Nervous System 157 156 Chapter Thirteen The Nervous System 157 156 Chapter Thirteen The Nervous System 157 156 Chapter Thirteen The Nervous System 157 156 Chapter Thirteen The Nervous System 157