PDF: Nerve Cells and Nerve Impulses - Kalat Chapter 1

Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party co...

Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Nerve Cells and Nerve Impulses Chapter 1 P eople talk about growing into adulthood and becoming independent, but in fact almost no human life is truly independent. How often do you hunt your own meat and cook it on a fire you made from scratch? Do you grow your Chapter Outline Module 1.1 The Cells of the Nervous System Neurons and Glia The Blood–Brain Barrier own vegetables? Could you build your own house (with tools you made your- Nourishment of Vertebrate Neurons self )? Have you ever made your own clothing (with materials you gathered In Closing: Neurons Module 1.2 in the wild)? Of all the activities necessary for your survival, which ones—if The Nerve Impulse The Resting Potential of the Neuron any—could you do completely on your own, other than breathe? People can The Action Potential do an enormous amount together, but very little by themselves. Propagation of the Action Potential The Myelin Sheath and Saltatory Conduction The cells of your nervous system are like that, too. Together they accom- Local Neurons In Closing: Neurons and Messages plish amazing things, but one cell by itself is helpless. We begin our study of Learning Objectives the nervous system by examining single cells. Later, we examine how they act After studying this chapter, you should together. be able to: 1. Describe neurons and glia, the cells that Advice: Parts of this chapter and the next assume that you understand the constitute the nervous system. 2. Summarize how the blood–brain barrier basics of chemistry. If you have never studied chemistry or if you have forgot- relates to protection and nutrition of neurons. ten what you did study, read Appendix A. 3. Explain how the sodium–potassium pump and the properties of the membrane lead to the resting potential of a neuron. 4. Discuss how the movement of sodium and potassium ions produces the action potential and recovery after it. 5. State the all-or-none law of the action potential. Opposite: An electron micrograph of neurons, magnified tens of thousands of times. The color is added artificially. For objects this small, it is impossible to focus light to obtain an image. It is possible to focus an electron beam, but electrons do not show color. (© Juan Gaertner/Shutterstock.com)   17 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Module 1.1 1.2 The Cells of the Nervous System N o doubt you think of yourself as an individual. You don’t think of your mental experience as being composed of pieces... but it is. Your experiences depend on the activity of Cerebral cortex: 16 billion neurons a huge number of separate but interconnected cells. To under- Rest of the brain: stand the nervous system, the place to begin is to examine the Less than 1 billion cells that compose it. Cerebellum: 69 billion neurons Neurons and Glia The nervous system consists of two kinds of cells, neurons and glia. Neurons receive information and transmit it to other cells. Glia serve many functions that are difficult to summa- Spinal cord: rize, and we shall defer that discussion until later in this mod- 1 billion neurons ule. The adult human brain contains approximately 86 billion neurons, on average (Herculano-Houzel, Catania, Manger, & Kaas, 2015; see Figure 1.1). The exact number varies from per- son to person. We now take it for granted that the brain is composed of individual cells, but the idea was in doubt as recently as the early 1900s. Until then, the best microscopic views revealed little detail about the brain. Observers noted long, thin fi- bers between one cell body and another, but they could not see whether a fiber merged into the next cell or stopped before it. In the late 1800s, Santiago Ramón y Cajal used newly developed staining techniques to show that a small gap separates the tip of a neuron’s fiber from the surface of the next neuron. The brain, like the rest of the body, consists Figure 1.1 Estimated numbers of neurons in humans of individual cells. The numbers differ from one person to another. (Source: Herculano-Houzel et al., 2015) Santiago Ramón y Cajal, a Pioneer of Neuroscience Santiago Ramón y Cajal (1852–1934) Two scientists of the late 1800s and early 1900s are widely How many interesting facts fail to be con- recognized as the main founders of neuroscience—Charles verted into fertile discoveries because their Sherrington, whom we shall discuss in Chapter 2, and the Bettmann/Getty Images first observers regard them as natural and Spanish investigator Santiago Ramón y Cajal (1852–1934). ordinary things!... It is strange to see how Cajal’s early education did not progress smoothly. At one the populace, which nourishes its imagina- point, he was imprisoned in a solitary cell, limited to one tion with tales of witches or saints, mysteri- meal a day, and taken out daily for public floggings—at the ous events and extraordinary occurrences, age of 10—for the crime of not paying attention during his disdains the world around it as commonplace, monotonous and Latin class (Cajal, 1901–1917/1937). (And you complained prosaic, without suspecting that at bottom it is all secret, mystery, about your teachers!) and marvel. (Cajal, 1937, pp. 46–47) 18   Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1.1 The Cells of the Nervous System   19 Cajal wanted to become an artist, but his father insisted membrane permit a controlled flow of water, oxygen, sodium, that he study medicine as a safer way to make a living. He potassium, calcium, chloride, and other important chemicals. managed to combine the two fields, becoming an outstanding Except for mammalian red blood cells, all animal cells anatomical researcher and illustrator. His detailed drawings of have a nucleus, the structure that contains the chromosomes. the nervous system are still considered definitive today. A mitochondrion (plural: mitochondria) is the structure that Before the late 1800s, microscopy revealed few details performs metabolic activities, providing the energy that the cell about the nervous system. Then the Italian investigator Camillo uses for all activities. Mitochondria have genes separate from Golgi found a way to stain nerve cells with silver salts. This those in the nucleus of a cell, and mitochondria differ from one method, which completely stains some cells without affecting another genetically. People with overactive mitochondria tend others at all, enabled researchers to examine the structure of to burn their fuel rapidly and overheat, even in a cool environ- a single cell. Cajal used Golgi’s methods but applied them to ment. People whose mitochondria are less active than normal infant brains, in which the cells are smaller and therefore easier are predisposed to depression and pains. Mutated mitochon- to examine on a single slide. Cajal’s research demonstrated that drial genes are a possible cause of autism (Aoki & Cortese, 2016). nerve cells remain separate instead of merging into one another. Ribosomes are the sites within a cell that synthesize new (Oddly, when Cajal and Golgi shared the 1906 Nobel Prize for protein molecules. Proteins provide building materials for Physiology or Medicine, they used their acceptance lectures the cell and facilitate chemical reactions. Some ribosomes to defend contradictory positions. In spite of Cajal’s evidence, float freely within the cell, but others are attached to the which had persuaded almost everyone else, Golgi clung to the endoplasmic reticulum, a network of thin tubes that trans- theory that all nerve cells merge directly into one another.) port newly synthesized proteins to other locations. Philosophically, we see the appeal of the old idea that neurons merge. We describe our experience as undivided, not The Structure of a Neuron the sum of separate parts, so it seemed right that all the cells in the brain might be joined together as one unit. How the The most distinctive feature of neurons is their shape, which separate cells combine their influences is a complex and still varies enormously from one neuron to another (see Figure 1.3). mysterious process. Unlike most other body cells, neurons have long branching ex- tensions. All neurons include a soma (cell body), and most also have dendrites, an axon, and presynaptic terminals. The tini- The Structures of an Animal Cell est neurons lack axons, and some lack well-defined dendrites. Figure 1.2 illustrates a neuron from the cerebellum of a mouse Contrast the motor neuron in Figure 1.4 and the sensory neu- (magnified enormously, of course). Neurons have much in com- ron in Figure 1.5. A motor neuron, with its soma in the spinal mon with the rest of the body’s cells. The surface of a cell is its cord, receives excitation through its dendrites and conducts membrane (or plasma membrane), a structure that separates impulses along its axon to a muscle. A sensory neuron is spe- the inside of the cell from the outside environment. Most chem- cialized at one end to be highly sensitive to a particular type icals cannot cross the membrane, but protein channels in the of stimulation, such as light, sound, or touch. The sensory (nuclear (ribosomes) envelope) (nucleolus) Endoplasmic reticulum (isolation, modification, transport Nucleus of proteins and other substances) (membrane-enclosed region containing DNA; hereditary control) Figure 1.2 An electron micro- graph of parts of a neuron from the cerebellum of a mouse The nucleus, membrane, and other structures are characteristic of most animal cells. The plasma membrane is the border of the neuron. Magnifi- Plasma membrane cation approximately x 20,000. (control of material Mitochondrion (Source: Courtesy of Dr. Dennis M. D. Landis) exchanges, mediation of cell- (aerobic energy environment interactions) metabolism) Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 20 CHAPTER 1 Nerve Cells and Nerve Impulses neuron shown in Figure 1.5 conducts touch information from the skin to the spinal cord. Tiny branches lead directly from the receptors into the axon, and the cell’s soma is located on a little stalk off the main trunk. Dendrites are branching fibers that get narrower near their ends. (The term dendrite comes from a Greek root word meaning “tree.” A dendrite branches like a tree.) The den- drite’s surface is lined with specialized synaptic receptors, at which the dendrite receives information from other neurons. (Chapter 2 concerns synapses.) The greater the surface area of a dendrite, the more information it can receive. Many den- drites contain dendritic spines, short outgrowths that in- crease the surface area available for synapses (see Figure 1.6). The cell body, or soma (Greek for “body”; plural: so- mata), contains the nucleus, ribosomes, and mitochondria. Most of a neuron’s metabolic work occurs here. Cell bodies of neurons range in diameter from 0.005 millimeter (mm) to 0.1 mm in mammals and up to a millimeter in certain inverte- brates. In many neurons, the cell body is like the dendrites— covered with synapses on its surface. The axon is a thin fiber of constant diameter. (The term axon comes from a Greek word meaning “axis.”) The axon con- veys an impulse toward other neurons, an organ, or a muscle. Axons can be more than a meter in length, as in the case of ax- Figure 1.3 Neurons, stained to appear dark ons from your spinal cord to your feet. The length of an axon Note the small fuzzy-looking spines on the dendrites. is enormous in comparison to its width, and in comparison (Source: Photo courtesy of Bob Jacobs, Colorado College) Dendrite Nucleus Myelin sheath Presynaptic Figure 1.4 The components Axon terminals of a vertebrate motor neuron The cell body of a motor neuron Axon hillock is located in the spinal cord. The parts are not drawn to scale. In Muscle reality, an axon is much longer in Dendritic fiber proportion to the soma. Soma spines Cross section of skin Sensory Axon endings Nucleus Skin Soma surface Figure 1.5 A vertebrate sensory neuron Note that the soma is located on a stalk off the main trunk of the axon. As in Figure 1.4, the structures are not drawn to scale. Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1.1 The Cells of the Nervous System   21 B Afferent A (to B) Efferent Shaft (from A) 1mm Spine Figure 1.7 Cell structures and axons It all depends on the point of view. An axon from A to B is an efferent axon from A and an afferent axon to B, just as a train from Washington to New York is exiting Washington and approaching New York. STOP & CHECK 1. What are the widely branching structures of a neuron called? And what is the long, thin structure that carries information Figure 1.6 Dendritic spines to another cell called? Many dendrites are lined with spines, short outgrowths that receive 2. Which animal species would have the longest axons? incoming information. (Source: From K. M. Harris and J. K. Stevens, Society for Neuroscience, “Dendritic 3. Compared to other neurons, would an interneuron’s axon be Spines of CA1 Pyramidal Cells in the Rat Hippocampus: Serial Electron Microscopy relatively long, short, or about the same? with Reference to Their Biophysical Characteristics.” Journal of Neuroscience, 9 (1989), 2982–2997. Copyright © 1989 Society for Neuroscience. Reprinted by permission.) ANSWERS axon is short. ron is contained entirely within one part of the brain, its the feet, nearly 2 meters away. 3. Because an interneu- elephants have axons that extend from the spinal cord to to the length of dendrites. Giorgio Ascoli (2015) offers the occur in the largest animals. For example, giraffes and analogy that if you could expand the dendrite of a reasonably tion to another cell is called an axon. 2. The longest axons typical neuron to the height of a tree, the cell’s axon and its dendrites, and the long thin structure that carries informa- branches would extend for more than 25 city blocks. 1. The widely branching structures of a neuron are called Many vertebrate axons are covered with an insulating material called a myelin sheath with interruptions known as nodes of Ranvier (RAHN-vee-ay). Invertebrate axons do not have myelin sheaths. Although a neuron can have many dendrites, Variations among Neurons it can have only one axon, but the axon may have branches. The Neurons vary enormously in size, shape, and function. The end of each branch has a swelling, called a presynaptic terminal, shape of a neuron determines its connections with other cells also known as an end bulb or bouton (French for “button”). At and thereby determines its function (see Figure 1.8). For ex- that point the axon releases chemicals that cross through the ample, the widely branching dendrites of the Purkinje cell in junction between that neuron and another cell. the cerebellum (see Figure 1.8a) enable it to receive input from Other terms associated with neurons are afferent, effer- up to 200,000 other neurons. By contrast, bipolar neurons in ent, and intrinsic. An afferent axon brings information into the retina (see Figure 1.8d) have only short branches, and a structure; an efferent axon carries information away from a some receive input from as few as two other cells. structure. Every sensory neuron is an afferent to the rest of the nervous system, and every motor neuron is an efferent from the nervous system. Within the nervous system, a given neu- Glia ron is an efferent from one structure and an afferent to another Glia (or neuroglia), the other components of the nervous (see Figure 1.7). You can remember that efferent starts with e system, perform many functions (see Figure 1.9). The term as in exit; afferent starts with a as in admit. If a cell’s dendrites glia, derived from a Greek word meaning “glue,” reflects early and axon are entirely contained within a single structure, the investigators’ idea that glia were like glue that held the neu- cell is an interneuron or intrinsic neuron of that structure. rons together. Although that concept is obsolete, the term For example, an intrinsic neuron of the thalamus has its axon remains. Glia outnumber neurons in the cerebral cortex, and all its dendrites within the thalamus. but neurons outnumber glia in several other brain areas, Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 22 CHAPTER 1 Nerve Cells and Nerve Impulses Apical dendrite Dendrites Basilar dendrites Axon (a) Axon (c) 10 mm (b) (d) (e) Figure 1.8 The diverse shapes of neurons (a) Purkinje cell, a cell type found only in the cerebellum; (b) sensory neurons from skin to spinal cord; (c) pyramidal cell of the motor area of the cerebral cortex; (d) bipolar cell of retina of the eye; (e) Kenyon cell, from a honeybee. (Source: Part e courtesy of R. G. Goss) especially the cerebellum (Herculano-Houzel et al., 2015; the message to the next neuron (Ben Achour & Pascual, 2012). Khakh & Sofroniew, 2015). Overall, the numbers are almost This process is a possible contributor to learning and memory equal. (De Pitta, Brunel, & Volterra, 2016). In some brain areas, as- The brain has several types of glia. The star-shaped trocytes also respond to hormones and thereby influence neu- astrocytes wrap around the synapses of functionally related rons (Kim et al., 2014). In short, astrocytes are active partners axons, as shown in Figure 1.10. By surrounding a connection of neurons in many ways. between neurons, an astrocyte shields it from chemicals cir- Tiny cells called microglia act as part of the immune culating in the surround (Nedergaard & Verkhatsky, 2012). system, removing viruses and fungi from the brain. They Also, by taking up the ions and transmitters released by axons proliferate after brain damage, removing dead or damaged and then releasing them back, an astrocyte helps synchronize neurons (Brown & Neher, 2014). They also contribute to closely related neurons, enabling their axons to send mes- learning by removing the weakest synapses (Zhan et al., sages in waves (Martín, Bajo-Grañeras, Moratalla, Perea, & 2014). Oligodendrocytes (OL-i-go-DEN-druh-sites) in Araque, 2015). Astrocytes are therefore important for gener- the brain and spinal cord and Schwann cells in the periph- ating rhythms, such as your rhythm of breathing (Morquette ery of the body build the myelin sheaths that surround and et al., 2015). insulate certain vertebrate axons. They also supply an axon Astrocytes dilate the blood vessels to bring more nutri- with nutrients necessary for proper functioning (Y. Lee et al., ents into brain areas that have heightened activity (Filosa et al., 2012). Radial glia guide the migration of neurons and 2006; Takano et al., 2006). A possible role in information pro- their axons and dendrites during embryonic development. cessing is also likely but less certain. According to a popular When embryological development finishes, most radial hypothesis known as the tripartite synapse, the tip of an axon glia differentiate into neurons, and a smaller number dif- releases chemicals that cause the neighboring astrocyte to ferentiate into astrocytes and oligodendrocytes (Pinto & release chemicals of its own, thus magnifying or modifying Götz, 2007). Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1.1 The Cells of the Nervous System   23 Nancy Kedersha/UCLA/Science Source Axon Schwann cell Astrocyte Capillary (small blood vessel) Astrocyte Schwann cell Radial glia Oligodendrocyte Myelin sheath Nancy Kedersha/UCLA/Science Source Axon Migrating neuron Microglia Microglia Figure 1.9 Shapes of some glia cells Oligodendrocytes produce myelin sheaths that insulate certain vertebrate axons in the central nervous system; Schwann cells have a similar function in the periphery. The oligodendrocyte is shown here forming a segment of myelin sheath for two axons; in fact, each oligodendrocyte forms such segments for 30 to 50 axons. Astrocytes pass chemicals back and forth between neurons and blood and among neighboring neurons. Microglia proliferate in areas of brain damage and remove toxic materials. Radial glia (not shown here) guide the migration of neurons during embryological development. Glia have other functions as well. STOP & CHECK 4. What are the four major structures that compose a neuron? 5. Which kind of glia cell wraps around the synaptic terminals of axons? ANSWERS terminal. 5. Astrocytes. 4. Dendrites, soma (cell body), axon, and presynaptic The Blood–Brain Barrier Neuron Although the brain, like any other organ, needs to receive Astrocyte nutrients from the blood, many chemicals cannot cross from the blood to the brain (Hagenbuch, Gao, & Meier, 2002). The Synapse enveloped by astrocyte mechanism that excludes most chemicals from the vertebrate brain is known as the blood–brain barrier. Before we exam- Figure 1.10 How an astrocyte synchronizes associated axons ine how it works, let’s consider why we need it. Branches of the astrocyte (in the center) surround the presynaptic ter- minals of related axons. If a few of them are active at once, the astrocyte absorbs some of the chemicals they release. It then temporarily inhibits all Why We Need a Blood–Brain Barrier the axons to which it is connected. When the inhibition ceases, all of the When a virus invades a cell, mechanisms within the cell ex- axons are primed to respond again in synchrony. trude virus particles through the membrane so that the im- (Source: Based on Antanitus, 1998) mune system can find them. When the immune system cells Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 24 CHAPTER 1 Nerve Cells and Nerve Impulses discover a virus, they kill it and the cell that contains it. In ef- Brain tissue fect, a cell exposing a virus through its membrane says, “Look, immune system, I’m infected with this virus. Kill me and save Fat- solu ble the others.” mol ecu This plan works fine if the virus-infected cell is, for exam- le ple, a skin cell or a blood cell, which the body replaces easily. Gluc However, with few exceptions, the vertebrate brain does not ose tran replace damaged neurons. If you had to sacrifice brain cells spor t whenever you had a viral infection, you would not do well! To minimize the risk of irreparable brain damage, the body lines Ami no-a the brain’s blood vessels with tightly packed cells that keep out cid tran most viruses, bacteria, and harmful chemicals. spor t However, certain viruses do cross the blood–brain bar- rier (Kristensson, 2011). What happens then? When the ra- bies virus evades the blood–brain barrier, it infects the brain and leads to death. The spirochete responsible for syphilis also Charged penetrates the blood–brain barrier, producing long-lasting molecules and potentially fatal consequences. The microglia are more – CO2 + effective against several other viruses that enter the brain, Cell wall tight mounting an inflammatory response that fights the virus with- junction out killing the neuron (Ousman & Kubes, 2012). However, CO2 this response may control the virus without eliminating it. When the chicken pox virus enters spinal cord cells, virus Endothelial cell particles remain there long after they have been exterminated O2 from the rest of the body. The virus may emerge from the spi- Large O2 nal cord decades later, causing a painful condition called shin- molecule gles. Similarly, the virus responsible for genital herpes hides in the nervous system, producing little harm there but periodi- cally emerging to cause new genital infections. Blood vessel Brain tissue How the Blood–Brain Barrier Works Figure 1.11 The blood–brain barrier The blood–brain barrier (see Figure 1.11) depends on Most large molecules and electrically charged molecules cannot cross from the blood to the brain. A few small, uncharged molecules such as O2 the endothelial cells that form the walls of the capillaries and CO2 cross easily, as can certain fat-soluble molecules. Active transport (Bundgaard, 1986; Rapoport & Robinson, 1986). Outside the systems pump glucose and amino acids across the membrane. brain, such cells are separated by small gaps, but in the brain, they are joined so tightly that they block viruses, bacteria, and other harmful chemicals from passage. “If the blood–brain barrier is such a good defense,” you Ottersen, 2003). For certain other chemicals, the brain uses might ask, “why don’t we have similar walls around all our active transport, a protein-mediated process that expends other organs?” The answer is that the barrier keeps out useful energy to pump chemicals from the blood into the brain. chemicals as well as harmful ones. Those useful chemicals in- Chemicals that are actively transported into the brain include clude all fuels and amino acids, the building blocks for proteins. glucose (the brain’s main fuel), amino acids (the building For these chemicals to cross the blood–brain barrier, the brain blocks of proteins), purines, choline, a few vitamins, and needs special mechanisms not found in the rest of the body. iron (Abbott, Rönnback, & Hansson, 2006; Jones & Shusta, No special mechanism is required for small, uncharged 2007). Insulin and probably certain other hormones also molecules such as oxygen and carbon dioxide that cross cross the blood–brain barrier, at least in small amounts, through cell walls freely. Also, molecules that dissolve in the although the mechanism is not yet known (Gray, Meijer, & fats of the membrane cross easily. Examples include vitamins Barrett, 2014; McNay, 2014). A and D and all the drugs that affect the brain—from antide- The blood–brain barrier is essential to health. In people pressants and other psychiatric drugs to illegal drugs such as with Alzheimer’s disease or similar conditions, the endothe- heroin. How fast a drug takes effect depends largely on how lial cells lining the brain’s blood vessels shrink, and harmful readily it dissolves in fats and therefore crosses the blood– chemicals enter the brain (Zipser et al., 2007). However, the brain barrier. barrier poses a difficulty for treating brain cancers, because Water crosses through special protein channels in nearly all the drugs used for chemotherapy fail to cross the the wall of the endothelial cells (Amiry-Moghaddam & blood–brain barrier. Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1.1 The Cells of the Nervous System   25 STOP & CHECK sperm also rely overwhelmingly on glucose.) Because metab- olizing glucose requires oxygen, neurons need a steady supply 6. Identify one major advantage and one disadvantage of having of oxygen. Although the human brain constitutes only about a blood–brain barrier. 2 percent of the body’s weight, it uses about 20 percent of its 7. Which chemicals cross the blood–brain barrier passively? oxygen and 25 percent of its glucose (Bélanger, Allaman, & 8. Which chemicals cross the blood–brain barrier by active Magistretti, 2011). transport? Why do neurons depend so heavily on glucose? They can and sometimes do use ketones (a kind of fat) and lactate for ANSWERS fuel. However, glucose is the only nutrient that crosses the certain vitamins, and iron. membrane. 8. Glucose, amino acids, purines, choline, passively. So do chemicals that dissolve in the fats of the blood–brain barrier in large quantities. carbon dioxide, and water cross the blood–brain barrier Although neurons require glucose, glucose shortage is tage). 7. Small, uncharged molecules such as oxygen, rarely a problem, except during starvation. The liver makes tage) and also keeps out most nutrients (a disadvan- glucose from many kinds of carbohydrates and amino acids, as well as from glycerol, a breakdown product from fats. A more 6. The blood–brain barrier keeps out viruses (an advan- likely problem is an inability to use glucose. To use glucose, the body needs vitamin B1, thiamine. Prolonged thiamine deficiency, common in chronic alcoholism, leads to death of Nourishment of Vertebrate Neurons neurons and a condition called Korsakoff ’s syndrome, marked Most cells use a variety of carbohydrates and fats for nu- by severe memory impairments. trition, but vertebrate neurons depend almost entirely on glucose, a sugar. (Cancer cells and the testis cells that make Module 1.1 In Closing Neurons What does the study of individual neurons tell us about behav- or axons that cannot find each other. In short, the locations, ior? Everything the brain does depends on the detailed anat- structures, and activities of your neurons are the basis for omy of its neurons and glia. In a later chapter we consider the everything you experience, learn, or do. physiology of learning, where one slogan is that “cells that fire However, nothing in your experience or behavior follows together, wire together.” That is, neurons active at the same from the properties of any one neuron. The nervous system is time become connected. However, that is true only if the more than the sum of its individual cells, just as water is more neurons active at the same time are also in approximately the than the sum of oxygen and hydrogen. Our behavior emerges same place (Ascoli, 2015). The brain cannot connect dendrites from the communication among neurons. Summary 1. Neurons receive information and convey it to other cells. 5. Because of the blood–brain barrier, many molecules The nervous system also contains glia, cells that enhance cannot enter the brain. The barrier protects the nervous and modify the activity of neurons in many ways. 18 system from viruses and many dangerous chemicals. 23 2. In the late 1800s, Santiago Ramón y Cajal used newly 6. The blood–brain barrier consists of an unbroken wall of discovered staining techniques to establish that the ner- cells that surround the blood vessels of the brain and spi- vous system is composed of separate cells, now known as nal cord. A few small, uncharged molecules such as water, neurons. 18 oxygen, and carbon dioxide cross the barrier freely. So do 3. Neurons contain the same internal structures as other molecules that dissolve in fats. Active transport proteins animal cells. 19 pump glucose, amino acids, and a few other chemicals 4. Neurons have these major parts: a cell body (or soma), into the brain and spinal cord. Certain hormones, includ- dendrites, an axon with branches, and presynaptic ing insulin, also cross the blood–brain barrier. 24 terminals. Neurons’ shapes vary greatly depending 7. Neurons rely heavily on glucose, the only nutrient that on their functions and their connections with other crosses the blood–brain barrier in large quantities. They cells. 19 need thiamine (vitamin B1) to use glucose. 25 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 26 CHAPTER 1 Nerve Cells and Nerve Impulses Key Terms Terms are defined in the module on the page number on page 589. Interactive flash cards, audio reviews, and cross- indicated. They are also presented in alphabetical order with word puzzles are among the online resources available to help definitions in the book’s Subject Index/Glossary, which begins you learn these terms and the concepts they represent. active transport 24 glia 21 nodes of Ranvier 21 afferent axon 21 glucose 25 nucleus 19 astrocytes 22 interneuron 21 oligodendrocytes 22 axon 20 intrinsic neuron 21 presynaptic terminal 21 blood–brain barrier 23 membrane 19 radial glia 22 cell body (soma) 20 microglia 22 ribosomes 19 dendrites 20 mitochondrion 19 Schwann cells 22 dendritic spines 20 motor neuron 19 sensory neuron 19 efferent axon 21 myelin sheath 21 thiamine 25 endoplasmic reticulum 19 neurons 18 Thought Question Although heroin and morphine are similar in many ways, heroin exerts faster effects on the brain. What can we infer about those drugs with regard to the blood–brain barrier? Module 1.1 End of Module Quiz 1. Santiago Ramón y Cajal was responsible for which of these discoveries? A. The human cerebral cortex has many specializations C. The nervous system is composed of separate cells. to produce language. D. Neurons communicate at specialized junctions called B. The brain’s left and right hemispheres control differ- synapses. ent functions. 2. Which part of a neuron has its own genes, separate from those of the nucleus? A. The ribosomes C. The axon B. The mitochondria D. The dendrites 3. What is most distinctive about neurons, compared to other cells? A. Their temperature C. Their internal components, such as ribosomes and B. Their shape mitochondria D. Their color 4. Which of these do dendritic spines do? A. They synthesize proteins. C. They hold the neuron in position. B. They increase the surface area available for synapses. D. They metabolize fuels to provide energy for the rest of the neuron. 5. What does an efferent axon do? A. It controls involuntary behavior. C. It carries output from a structure. B. It controls voluntary behavior. D. It brings information into a structure. 6. Which of the following is a function of astrocytes? A. Astrocytes conduct impulses over long distances. C. Astrocytes create the blood–brain barrier. B. Astrocytes build myelin sheaths that surround and D. Astrocytes synchronize activity for a group insulate axons. of neurons. Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1.1 The Cells of the Nervous System   27 7. Which of the following is a function of microglia? A. Microglia remove dead cells and weak synapses. C. Microglia dilate blood vessels to increase blood supply B. Microglia build myelin sheaths that surround and to active brain areas. insulate axons. D. Microglia synchronize activity for a group of neurons. 8. Which of these can easily cross the blood–brain barrier? A. Fat-soluble molecules C. Proteins B. Chemotherapy drugs D. Viruses 9. Which of these chemicals cross the blood–brain barrier by active transport? A. Oxygen, water, and fat-soluble molecules C. Proteins B. Glucose and amino acids D. Viruses 10. What is the brain’s main source of fuel? A. Glucose C. Chocolate B. Glutamate D. Proteins 11. For the brain to use its main source of fuel, what does it also need? A. Steroid hormones C. Thiamine B. Vitamin C D. Acetylsalicylic acid Answers: 1C, 2B, 3B, 4B, 5C, 6D, 7A, 8A, 9B, 10A, 11C. Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Module 1.2 The Nerve Impulse T hink about the axons that convey information from the touch receptors in your hands or feet toward your spinal cord and brain. If the axons used electrical conduction, they transmit impulses slightly faster than those closer to the brain (Stanford, 1987)! In short, the properties of impulse conduction in an axon could transfer information at a velocity approaching the speed are amazingly well adapted to your needs for information of light. However, given that your body is made of water and transfer. Let’s examine the mechanics of impulse transmission. carbon compounds instead of copper wire, the strength of an impulse would decay rapidly as it traveled. A touch on your shoulder would feel stronger than a touch on your abdomen. The Resting Potential of the Neuron Short people would feel their toes more strongly than tall peo- Messages in a neuron develop from disturbances of the resting ple could—if either could feel their toes at all. potential. Let’s begin by understanding the resting potential. The way your axons actually function avoids these prob- All parts of a neuron are covered by a membrane about lems. Instead of conducting an electrical impulse, the axon 8 nanometers (nm) thick. That is about one ten-thousandth regenerates an impulse at each point. Imagine a long line of the width of an average human hair. The membrane is com- people holding hands. The first person squeezes the second posed of two layers (free to float relative to each other) of person’s hand, who then squeezes the third person’s hand, and phospholipid molecules (containing chains of fatty acids and so forth. The impulse travels along the line without weakening a phosphate group). Embedded among the phospholipids are because each person generates it anew. cylindrical protein molecules through which certain chemi- Although the axon’s method of transmitting an impulse cals can pass (see Figure 1.12). prevents a touch on your shoulder from feeling stronger than When at rest, the membrane maintains an electrical one on your toes, it introduces a different problem: Because gradient, also known as polarization—a difference in axons transmit information at only moderate speeds (varying from less than 1 meter/second to about 100 m/s), a touch on your shoulder reaches your brain sooner than will a touch on your toes, although you will not ordinarily notice the differ- Phospholipid Protein ence. Your brain is not set up to register small differences in molecules molecules the time of arrival of touch messages. After all, why should it be? You almost never need to know whether a touch on one part of your body occurred slightly before or after a touch somewhere else. In vision, however, your brain does need to know whether one stimulus began slightly before or after another one. If two adjacent spots on your retina—let’s call them A and B—send impulses at almost the same time, an extremely small differ- ence between them in timing tells your brain whether light moved from A to B or from B to A. To detect movement as accurately as possible, your visual system compensates for the fact that some parts of the retina are slightly closer to your brain than other parts are. Without some sort of compensa- tion, simultaneous flashes arriving at two spots on your ret- ina would reach your brain at different times, and you might Figure 1.12 The membrane of a neuron perceive movement inaccurately. What prevents this illusion Embedded in the membrane are protein channels that permit certain ions is the fact that axons from more distant parts of your retina to cross through the membrane at a controlled rate. 28   Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1.2 The Nerve Impulse   29 Axons from Computer other neurons Amplifier Intracellular microelectrode Soma Reference Figure 1.13 Methods for microelectrode recording activity of a neuron Diagram of the apparatus and a Axon sample recording. (Source: Fritz Goro) electrical charge between the inside and outside of the cell. Ion pores K+ The electrical potential inside the membrane is slightly nega- tive with respect to the outside, mainly because of negatively charged proteins inside the cell. This difference in voltage is called the resting potential. Membrane of neuron Researchers measure the resting potential by inserting a very thin microelectrode into the cell body, as in Figure 1.13. K+ The diameter of the electrode must be small enough to enter Na+ without damaging the cell. The most common electrode is a fine glass tube filled with a salt solution, tapering to a tip di- Ion ameter of 0.0005 mm or less. A reference electrode outside pathways K+ the cell completes the circuit. Connecting the electrodes to a voltmeter, we find that the neuron’s interior has a negative potential relative to its exterior. The magnitude varies, but a Na+ typical level is –70 millivolts (mV). Forces Acting on Sodium Figure 1.14 Ion channels in the membrane of a neuron When a channel opens, it permits some type of ion to cross the mem- and Potassium Ions brane. When it closes, it prevents passage of that ion. If charged ions could flow freely across the membrane, the mem- brane would depolarize, eliminating the negative potential in- side. However, the membrane has selective permeability. That of the sodium–potassium pump, sodium ions are more is, some chemicals pass through it more freely than others do. than 10 times more concentrated outside the membrane Oxygen, carbon dioxide, urea, and water cross freely through than inside, and potassium ions are more concentrated in- channels that are always open. Several biologically important side than outside. ions, including sodium, potassium, calcium, and chloride, cross