Neuroscience: Exploring the Brain PDF

 PART ONE Foundations CHAPTER 1 Neuroscience: Past, Present, and Future 3 CHAPTER 2 Neurons and Glia 23 CHAPTER 3 The Neuronal Membrane at Rest 55...

 PART ONE Foundations CHAPTER 1 Neuroscience: Past, Present, and Future 3 CHAPTER 2 Neurons and Glia 23 CHAPTER 3 The Neuronal Membrane at Rest 55 CHAPTER 4 The Action Potential 81 CHAPTER 5 Synaptic Transmission 109 CHAPTER 6 Neurotransmitter Systems 143 CHAPTER 7 The Structure of the Nervous System 179 Appendix: An Illustrated Guide to Human Neuroanatomy 219 001-022_Bear_01_revised_final.indd 1 12/20/14 2:38 AM  CHAPTER ONE Neuroscience: Past, Present, and Future INTRODUCTION THE ORIGINS OF NEUROSCIENCE Views of the Brain in Ancient Greece Views of the Brain During the Roman Empire Views of the Brain from the Renaissance to the Nineteenth Century Nineteenth-Century Views of the Brain Nerves as Wires Localization of Specific Functions to Different Parts of the Brain The Evolution of Nervous Systems The Neuron: The Basic Functional Unit of the Brain NEUROSCIENCE TODAY Levels of Analysis Molecular Neuroscience Cellular Neuroscience Systems Neuroscience Behavioral Neuroscience Cognitive Neuroscience Neuroscientists The Scientific Process Observation Replication Interpretation Verification The Use of Animals in Neuroscience Research The Animals Animal Welfare Animal Rights The Cost of Ignorance: Nervous System Disorders CONCLUDING REMARKS 3 001-022_Bear_01_revised_final.indd 3 12/20/14 2:38 AM  CHAPTER 1 NEUROSCIENCE: PAST, PRESENT, AND FUTURE 13 cell” was actually the basic unit of brain function. Nerve cells usually have a number of thin projections, or processes, that extend from a cen- tral cell body (Figure 1.15). Initially, scientists could not decide whether the processes from different cells fuse together as do blood vessels in the circulatory system. If this were true, then the “nerve net” of connected nerve cells would represent the elementary unit of brain function. Chapter 2 presents a brief history of how this issue was resolved. Suffice it to say that by 1900, the individual nerve cell, now called the neuron, was recognized to be the basic functional unit of the nervous system. NEUROSCIENCE NEUR ROSCIIENCE TODAY The history of modern neuroscience is still being written, and the accomplishments to date form the basis for this textbook. We will discuss the most recent developments in the coming chapters. Before we do, let’s take a look at how brain research is conducted today and why it is so important to society. Levels of Analysis History has clearly shown that understanding how the brain works is a ▲ FIGURE 1.15 big challenge. To reduce the complexity of the problem, neuroscientists An early depiction of a nerve cell. Published in 1865, this drawing by break it into smaller pieces for systematic experimental analysis. This German anatomist Otto Deiters shows is called the reductionist approach. The size of the unit of study defines a nerve cell, or neuron, and its many what is often called the level of analysis. In ascending order of complexity, projections, called neurites. For a time these levels are molecular, cellular, systems, behavioral, and cognitive. it was thought that the neurites from different neurons might fuse together Molecular Neuroscience. The brain has been called the most complex like the blood vessels of the circulatory system. We now know that neurons are piece of matter in the universe. Brain matter consists of a fantastic variety distinct entities that communicate using of molecules, many of which are unique to the nervous system. These differ- chemical and electrical signals. (Source: ent molecules play many different roles that are crucial for brain function: Clarke and O’Malley, 1968, Fig. 16.) messengers that allow neurons to communicate with one another, sentries that control what materials can enter or leave neurons, conductors that orchestrate neuron growth, archivists of past experiences. The study of the brain at this most elementary level is called molecular neuroscience. Cellular Neuroscience. The next level of analysis is cellular neuroscience, which focuses on studying how all those molecules work together to give neurons their special properties. Among the questions asked at this level are: How many different types of neurons are there, and how do they differ in function? How do neurons influence other neurons? How do neu- rons become “wired together” during fetal development? How do neurons perform computations? Systems Neuroscience. Constellations of neurons form complex circuits that perform a common function, such as vision or voluntary movement. Thus, we can speak of the “visual system” and the “motor system,” each of which has its own distinct circuitry within the brain. At this level of analysis, called systems neuroscience, neuroscientists study how different neural circuits analyze sensory information, form perceptions of the ex- ternal world, make decisions, and execute movements. Behavioral Neuroscience. How do neural systems work together to pro- duce integrated behaviors? For example, are different forms of memory accounted for by different systems? Where in the brain do “mind-altering” drugs act, and what is the normal contribution of these systems to the 001-022_Bear_01_revised_final.indd 13 12/20/14 2:38 AM  14 PART ONE FOUNDATIONS regulation of mood and behavior? What neural systems account for gender-specific behaviors? Where are dreams created and what do they reveal? These questions are studied in behavioral neuroscience. Cognitive Neuroscience. Perhaps the greatest challenge of neuroscience is understanding the neural mechanisms responsible for the higher levels of human mental activity, such as self-awareness, imagination, and lan- guage. Research at this level, called cognitive neuroscience, studies how the activity of the brain creates the mind. Neuroscientists “Neuroscientist” sounds impressive, kind of like “rocket scientist.” But we were all students once, just like you. For whatever reason—maybe we wanted to know why our eyesight was poor, or why a family member suffered a loss of speech after a stroke—we came to share a thirst for knowledge of how the brain works. Perhaps you will, too. Being a neuroscientist is rewarding, but it does not come easily. Many years of training are required. One may begin by helping out in a research lab during or after college and then going to graduate school to earn a Ph.D. or an M.D. (or both). Several years of post-doctoral training usually follow, learning new techniques or ways of thinking under the direction of an established neuroscientist. Finally, the “young” neuroscientist is ready to set up shop at a university, institute, or hospital. Broadly speaking, neuroscience research (and neuroscientists) may be divided into three types: clinical, experimental, and theoretical. Clinical research is mainly conducted by physicians (M.D.s). The main medical specialties associated with the human nervous system are neurology, psychiatry, neurosurgery, and neuropathology (Table 1.1). Many who conduct clinical research continue in the tradition of Broca, attempting to deduce from the behavioral effects of brain damage the functions of various parts of the brain. Others conduct studies to assess the benefits and risks of new types of treatment. Despite the obvious value of clinical research, the foundation for all medical treatments of the nervous system continues to be laid by experimental neuroscientists, who may hold either an M.D. or a Ph.D. The experimental approaches to studying the brain are so broad that they in- clude almost every conceivable methodology. Neuroscience is highly inter- disciplinary; however, expertise in a particular methodology may distin- guish one neuroscientist from another. Thus, there are neuroanatomists, who use sophisticated microscopes to trace connections in the brain; neurophysiologists, who use electrodes to measure the brain’s electrical activity; neuropharmacologists, who use drugs to study the chemistry of TABLE 1.1 Medical Specialists Associated with the Nervous System Specialist Spec Sp ecia iali list st Desc De Description scri ript ptio ionn Neurologist An M.D. trained to diagnose and treat diseases of the nervous system Psychiatrist An M.D. trained to diagnose and treat disorders of mood and behavior Neurosurgeon An M.D. trained to perform surgery on the brain and spinal cord Neuropathologist An M.D. or Ph.D. trained to recognize the changes in nervous tissue that result from disease 001-022_Bear_01_revised_final.indd 14 12/20/14 2:38 AM  CHAPTER 1 NEUROSCIENCE: PAST, PRESENT, AND FUTURE 15 TABLE 1.2 Types of Experimental Neuroscientists Type Description Developmental Analyzes the development and maturation of the neurobiologist brain Molecular neurobiologist Uses the genetic material of neurons to understand the structure and function of brain molecules Neuroanatomist Studies the structure of the nervous system Neurochemist Studies the chemistry of the nervous system Neuroethologist Studies the neural basis of species-specific animal behaviors in natural settings Neuropharmacologist Examines the effects of drugs on the nervous system Neurophysiologist Measures the electrical activity of the nervous system Physiological psychologist Studies the biological basis of behavior (biological psychologist, psychobiologist) Psychophysicist Quantitatively measures perceptual abilities brain function; molecular neurobiologists, who probe the genetic material of neurons to find clues about the structure of brain molecules; and so on. Table 1.2 lists some of the types of experimental neuroscientists. Theoretical neuroscience is a relatively young discipline, in which researchers use mathematical and computational tools to understand the brain at all levels of analysis. In the tradition of physics, theoretical neuroscientists attempt to make sense of the vast amounts of data gen- erated by experimentalists, with the goals of helping focus experiments on questions of greatest importance and establishing the mathematical principles of nervous system organization. The Scientific Process Neuroscientists of all stripes endeavor to establish truths about the nervous system. Regardless of the level of analysis they choose, they work according to a scientific process consisting of four essential steps: observation, replication, interpretation, and verification. Observation. Observations are typically made during experiments de- signed to test a particular hypothesis. For example, Bell hypothesized that the ventral roots contain the nerve fibers that control the muscles. To test this idea, he performed an experiment in which he cut these fibers and then observed whether or not muscular paralysis resulted. Other types of observation derive from carefully watching the world around us, or from introspection, or from human clinical cases. For example, Broca’s careful observations led him to correlate left frontal lobe damage with the loss of the ability to speak. Replication. Any observation, whether experimental or clinical, must be replicated. Replication simply means repeating the experiment on dif- ferent subjects or making similar observations in different patients, as many times as necessary to rule out the possibility that the observation occurred by chance. Interpretation. Once the scientist believes the observation is correct, he or she interprets it. Interpretations depend on the state of knowledge (or ignorance) at the time and on the scientist’s preconceived notions 001-022_Bear_01_revised_final.indd 15 12/20/14 2:38 AM  16 PART ONE FOUNDATIONS (or “mind set”). Interpretations therefore do not always withstand the test of time. For example, at the time he made his observations, Flourens was unaware that the cerebrum of a bird is fundamentally different from that of a mammal. Thus, he wrongly concluded from experimental ablations in birds that there was no localization of certain functions in the cerebrum of mammals. Moreover, as mentioned before, his profound distaste for Gall surely also colored his interpretation. The point is that the correct interpretation often is not made until long after the original observations. Indeed, major breakthroughs sometimes occur when old observations are reinterpreted in a new light. Verification. The final step of the scientific process is verification. This step is distinct from the replication the original observer performed. Verification means that the observation is sufficiently robust that any competent scientist who precisely follows the protocols of the original observer can reproduce it. Successful verification generally means that the observation is accepted as fact. However, not all observations can be verified, sometimes because of inaccuracies in the original report or insufficient replication. But failure to verify usually stems from the fact that unrecognized variables, such as temperature or time of day, contrib- uted to the original result. Thus, the process of verification, if affirmative, establishes new scientific fact, or, if negative, suggests new interpreta- tions for the original observation. Occasionally, one reads in the popular press about a case of scien- tific fraud. Researchers face keen competition for limited research funds and feel considerable pressure to “publish or perish.” In the interest of expediency, a few have actually published “observations” they in fact never made. Fortunately, such instances of fraud are rare, thanks to the scientific process. Before long, other scientists find they are unable to verify the fraudulent observations and question how they could have been made in the first place. The fact that we can fill this book with so much knowledge about the nervous system stands as a testament to the value of the scientific process. The Use of Animals in Neuroscience Research Most of what we know about the nervous system has come from experi- ments on animals. In most cases, the animals are killed so their brains can be examined neuroanatomically, neurophysiologically, and/or neuro- chemically. The fact that animals are sacrificed for the pursuit of human knowledge raises questions about the ethics of animal research. The Animals. Let’s begin by putting the issue in perspective. Throughout history, humans have considered animals and animal products as renew- able natural resources that can be used for food, clothing, transporta- tion, recreation, sport, and companionship. The animals used in research, education, and testing have always been a small fraction of those used for other purposes. For example, in the United States, the number of ani- mals used in all types of biomedical research is very small compared to the number killed for food. The number used specifically in neuroscience research is much smaller still. Neuroscience experiments are conducted using many different spe- cies, ranging from snails to monkeys. The choice of species is generally dictated by the question under investigation, the level of analysis, and the extent to which the knowledge gained can be related to humans. As a rule, the more basic the process under investigation, the more distant can 001-022_Bear_01_revised_final.indd 16 12/20/14 2:38 AM  CHAPTER 1 NEUROSCIENCE: PAST, PRESENT, AND FUTURE 17 be the evolutionary relationship with humans. Thus, experiments aimed at understanding the molecular basis of nerve impulse conduction can be carried out with a distantly related species, such as the squid. On the other hand, understanding the neural basis of movement and perceptual disorders in humans has required experiments with more closely related species, such as the macaque monkey. Today, more than half of the ani- mals used for neuroscience research are rodents—mice and rats—that are bred specifically for this purpose. Animal Welfare. In the developed world today, most educated adults have a concern for animal welfare. Neuroscientists share this concern and work to ensure that animals are well treated. Society has not always placed such value on animal welfare, however, as reflected in some of the scien- tific practices of the past. For example, in his experiments early in the nineteenth century, Magendie used unanesthetized puppies (for which he was later criticized by his scientific rival Bell). Fortunately, heightened awareness of animal welfare has more recently led to significant improve- ments in how animals are treated in biomedical research. Today, neuroscientists accept certain moral responsibilities toward their animal subjects: 1. Animals are used only in worthwhile experiments that promise to advance our knowledge of the nervous system. 2. All necessary steps are taken to minimize pain and distress experienced by the experimental animals (use of anesthetics, analgesics, etc.). 3. All possible alternatives to the use of animals are considered. Adherence to this ethical code is monitored in a number of ways. First, research proposals must pass a review by the Institutional Animal Care and Use Committee (IACUC), as mandated by U.S. federal law. Members of this committee include a veterinarian, scientists in other disciplines, and nonscientist community representatives. After passing the IACUC review, proposals are evaluated for scientific merit by a panel of expert neuroscientists. This step ensures that only the most worthwhile projects are carried out. Then, when neuroscientists submit their observations for publication in the professional journals, the papers are carefully reviewed by other neuroscientists for both scientific merit and animal welfare con- cerns. Reservations about either issue can lead to rejection of the paper, which in turn can lead to a loss of funding for the research. In addition to these monitoring procedures, federal law sets strict standards for the housing and care of laboratory animals. Animal Rights. Most people accept the necessity for animal experimenta- tion to advance knowledge, as long as it is performed humanely and with the proper respect for animals’ welfare. However, a vocal and increasingly violent minority seeks the total abolition of animal use for human pur- poses, including experimentation. These people subscribe to a philosophi- cal position often called animal rights. According to this way of thinking, animals have the same legal and moral rights as humans. If you are an animal lover, you may be sympathetic to this position. But consider the following questions. Are you willing to deprive yourself and your family of medical procedures that were developed using ani- mals? Is the death of a mouse equivalent to the death of a human being? Is keeping a pet the moral equivalent of slavery? Is eating meat the moral equivalent of murder? Is it unethical to take the life of a pig to save the life of a child? Is controlling the rodent population in the sewers or the roach population in your home morally equivalent to the Holocaust? 001-022_Bear_01_revised_final.indd 17 12/20/14 2:38 AM  18 PART ONE FOUNDATIONS If your answer is no to any of these questions, then you do not subscribe to the philosophy of animal rights. Animal welfare—a concern that all responsible people share—must not be confused with animal rights. Animal rights activists have vigorously pursued their agenda against animal research, sometimes with alarming success. They have manipulated public opinion with repeated allegations of cruelty in animal experiments that are grossly distorted or blatantly false. They have vandalized laborato- ries, destroying years of hard-won scientific data and hundreds of thousands of dollars of equipment (that you, the taxpayer, had paid for). With threats of violence they have driven some researchers out of science altogether. Fortunately, the tide is turning. Thanks to the efforts of a number of people, scientists and nonscientists alike, the false claims of the extrem- ists have been exposed, and the benefits to humankind of animal research have been extolled (Figure 1.16). Considering the staggering toll in terms of human suffering that results from disorders of the nervous system, neuroscientists take the position that it is our responsibility to wisely use all the resources nature has provided, including animals, to gain an understanding of how the brain functions in health and in disease. Recently, a surgical technique perfected on animals was used to remove a malignant tumor from a little girl's brain. We lost some lab animals. But look what we saved. ▲ FIGURE 1.16 Our debt to animal research. This poster counters the claims of animal rights activists by raising public awareness of the benefits of animal research. (Source: Foundation for Biomedical Research.) 001-022_Bear_01_revised_final.indd 18 12/20/14 2:38 AM  CHAPTER FIVE Synaptic Transmission INTRODUCTION BOX 5.1 OF SPECIAL INTEREST: Otto Loewi’s Dream TYPES OF SYNAPSES Electrical Synapses Chemical Synapses CNS Chemical Synapses BOX 5.2 PATH OF DISCOVERY: For the Love of Dendritic Spines, by Kristen M. Harris The Neuromuscular Junction PRINCIPLES OF CHEMICAL SYNAPTIC TRANSMISSION Neurotransmitters Neurotransmitter Synthesis and Storage Neurotransmitter Release BOX 5.3 BRAIN FOOD: How to SNARE a Vesicle Neurotransmitter Receptors and Effectors Transmitter-Gated Ion Channels BOX 5.4 BRAIN FOOD: Reversal Potentials G-Protein-Coupled Receptors Autoreceptors Neurotransmitter Recovery and Degradation Neuropharmacology BOX 5.5 OF SPECIAL INTEREST: Bacteria, Spiders, Snakes, and People PRINCIPLES OF SYNAPTIC INTEGRATION The Integration of EPSPS Quantal Analysis of EPSPs EPSP Summation The Contribution of Dendritic Properties to Synaptic Integration Dendritic Cable Properties Excitable Dendrites Inhibition BOX 5.6 OF SPECIAL INTEREST: Startling Mutations and Poisons IPSPs and Shunting Inhibition The Geometry of Excitatory and Inhibitory Synapses Modulation CONCLUDING REMARKS 109 109–142_Bear_05_revised_final.indd 109 12/20/14 3:43 AM  110 PART ONE FOUNDATIONS INTRODUCTION INTRODUC CTION In Chapters 3 and 4, we discussed how mechanical energy, such as a thumbtack entering your foot, can be converted into a neural signal. First, specialized ion channels of the sensory nerve endings allow positive charge to enter the axon. If this depolarization reaches threshold, then action potentials are generated. Because the axonal membrane is excitable and has voltage-gated sodium channels, action potentials can propagate with- out decrement up the long sensory nerves. For this information to be pro- cessed by the rest of the nervous system, however, these neural signals must be passed on to other neurons—for example, the motor neurons that control muscle contraction, as well as neurons in the brain and spinal cord that lead to a coordinated reflex response. By the end of the nineteenth cen- tury, it was recognized that this transfer of information from one neuron to another occurs at specialized sites of contact. In 1897, English physiologist Charles Sherrington gave these sites their name: synapses. The process of information transfer at a synapse is called synaptic transmission. The physical nature of synaptic transmission was debated for almost a century. One attractive hypothesis, which nicely explained the speed of synaptic transmission, was that it was simply electrical current flowing from one neuron to the next. The existence of such electrical synapses was finally proven in the late 1950s by Edwin Furshpan and David Potter, American physiologists who were studying the nervous system of crayfish at University College London, and Akira Watanabe, who was studying the neurons of lobster at the Tokyo Medical and Dental University. We now know that electrical synapses are common in the brains of invertebrates and vertebrates, including mammals. An alternative hypothesis about the nature of synaptic transmission, also dating back to the 1800s, was that chemical neurotransmitters trans- fer information from one neuron to another at the synapse. Solid support for the concept of chemical synapses was provided in 1921 by Otto Loewi, then the head of the Pharmacology Department at the University of Graz in Austria. Loewi showed that electrical stimulation of axons innervating the frog’s heart caused the release of a chemical that could mimic the effects of neuron stimulation on the heartbeat (Box 5.1). Later, Bernard Katz and his colleagues at University College London conclusively demonstrated that fast transmission at the synapse between a motor neuron axon and skeletal muscle was chemically mediated. By 1951, John Eccles of the Australian National University was studying the physiology of synaptic transmission within the mammalian central nervous system (CNS) using a new tool, the glass microelectrode. These experiments indicated that many CNS syn- apses also use a chemical transmitter; in fact, chemical synapses comprise the majority of synapses in the brain. During the last decade, new methods of studying the molecules involved in synaptic transmission have revealed that synapses are far more complex than most neuroscientists anticipated. Synaptic transmission is a large and fascinating topic. The actions of psy- choactive drugs, the causes of mental disorders, the neural bases of learning and memory—indeed, all the operations of the nervous system—cannot be understood without knowledge of synaptic transmission. Therefore, we’ve devoted several chapters to this topic, mainly focusing on chemical synapses. In this chapter, we begin by exploring the basic mechanisms of synaptic transmission. What do different types of synapse look like? How are neu- rotransmitters synthesized and stored, and how are they released in response to an action potential in the axon terminal? How do neurotransmitters act on the postsynaptic membrane? How do single neurons integrate the inputs provided by the thousands of synapses that impinge upon them? 109–142_Bear_05_revised_final.indd 110 12/20/14 3:43 AM  CHAPTER 5 SYNAPTIC TRANSMISSION 111 BOX 5.1 OF SPECIAL INTEREST Otto Loewi’s Dream O ne of the more colorful stories in the history of neuro- science comes from Otto Loewi, who, working in Austria in I awoke again, at three o’clock, and I remembered what it was. This time I did not take any risk; I got up imme- the 1920s, showed definitively that synaptic transmission diately, went to the laboratory, made the experiment on between nerves and the heart is chemically mediated. The the frog’s heart, described above, and at five o’clock the heart has two types of innervation; one type speeds the chemical transmission of the nervous impulse was conclu- beating of the heart, and the other slows it. The latter type sively proved... Careful consideration in daytime would of innervation is supplied by the vagus nerve. Loewi isolated undoubtedly have rejected the kind of experiment I per- a frog heart with the vagal innervation still intact, stimulated formed, because it would have seemed most unlikely that the nerve electrically, and observed the expected effect: the if a nervous impulse released a transmitting agent, it would slowing of the heartbeat. The critical demonstration that this do so not just in sufficient quantity to influence the effec- effect was chemically mediated came when he applied the tor organ, in my case the heart, but indeed in such an ex- solution that had bathed this heart to a second isolated frog cess that it could partly escape into the fluid which filled heart and found that the beating of this one also slowed. the heart, and could therefore be detected. Yet the whole The idea for this experiment had actually come to Loewi in nocturnal concept of the experiment was based on this a dream. Below is his own account: eventuality, and the result proved to be positive, contrary In the night of Easter Sunday, 1921, I awoke, turned to expectation. (Loewi, 1953, pp. 33–34.) on the light, and jotted down a few notes on a tiny slip of The active compound, which Loewi called vagusstoff paper. Then, I fell asleep again. It occurred to me at six (literally “vagus substance” in German), turned out to be o’clock in the morning that during the night I had written acetylcholine. As we see in this chapter, acetylcholine is down something most important, but I was unable to deci- also a transmitter at the synapse between nerve and skeletal pher the scrawl. That Sunday was the most desperate day muscle. Unlike at the heart, acetylcholine applied to skeletal in my whole scientific life. During the next night, however, muscle causes excitation and contraction. TYPE ES OF SYNAPSES TYPES We introduced the synapse in Chapter 2. A synapse is the specialized junction where one part of a neuron contacts and communicates with an- other neuron or cell type (such as a muscle or glandular cell). Information generally flows in one direction, from a neuron to its target cell. The first neuron is said to be presynaptic, and the target cell is said to be postsynaptic. Let’s take a closer look at the different types of synapse. Electrical Synapses Electrical synapses are relatively simple in structure and function, and they allow the direct transfer of ionic current from one cell to the next. Electrical synapses occur at specialized sites called gap junctions. Gap junctions occur between cells in nearly every part of the body and interconnect many non-neural cells, including epithelial cells, smooth and cardiac muscle cells, liver cells, some glandular cells, and glia. When gap junctions interconnect neurons, they can function as electri- cal synapses. At a gap junction, the membranes of two cells are separated by only about 3 nm, and this narrow gap is spanned by clusters of special proteins called connexins. There are about 20 different subtypes of con- nexins, about half of which occur in the brain. Six connexin subunits com- bine to form a channel called a connexon, and two connexons (one from each cell) meet and combine to form a gap junction channel (Figure 5.1). The channel allows ions to pass directly from the cytoplasm of one cell 109–142_Bear_05_revised_final.indd 111 12/20/14 3:43 AM  112 PART ONE FOUNDATIONS Cell 1 ▼ FIGURE 5.1 Gap junction A gap junction. (a) Neurites of two cells connected by a gap junction. (b) The enlargement shows gap junction channels, which bridge the cytoplasm of the two cells. Ions and small molecules can pass in both directions through these channels. (c) Six connexin subunits comprise one connexon, two connexons comprise one gap junction channel, and many gap junction channels comprise one gap junction. Cell 2 (a) Cell 1 Gap junction channels cytoplasm Gap junction 3.5 nm 20 nm Connexon Connexin Cell 2 cytoplasm Ions and Channel formed by pores (b) small molecules in each membrane (c) to the cytoplasm of the other. The pore of most gap junction channels is relatively large. Its diameter is about 1–2 nm, big enough for all the major cellular ions and many small organic molecules to pass through. Most gap junctions allow ionic current to pass equally well in both directions; therefore, unlike the vast majority of chemical synapses, elec- trical synapses are bidirectional. Because electrical current (in the form of ions) can pass through these channels, cells connected by gap junctions are said to be electrically coupled. Transmission at electrical synapses is very fast and, if the synapse is large, nearly fail-safe. Thus, an action potential in the presynaptic neuron can produce, with very little delay, an action potential in the postsynaptic neuron. In invertebrate species, such as the crayfish, electrical synapses are sometimes found between sensory and motor neurons in neural pathways mediating escape reflexes. This mechanism enables an animal to beat a hasty retreat when faced with a dangerous situation. Studies in recent years have revealed that electrical synapses are common in every part of the mammalian CNS (Figure 5.2a). When two neurons are electrically coupled, an action potential in the presynaptic neuron causes a small amount of ionic current to flow across the gap junc- tion channels into the other neuron. This current causes an electrically mediated postsynaptic potential (PSP) in the second neuron (Figure 5.2b). Note that, because most electrical synapses are bidirectional, when that second neuron generates an action potential, it will in turn induce a PSP in the first neuron. The PSP generated by a single electrical syn- apse in the mammalian brain is usually small—about 1 mV or less at its peak—and may not, by itself, be large enough to trigger an action 109–142_Bear_05_revised_final.indd 112 12/20/14 3:43 AM  CHAPTER 5 SYNAPTIC TRANSMISSION 113 Cell 1 Action Vm of cell 1 0 potential Dendrite Record Vm of cell 1 – 65 Record Vm 0 1 2 3 of cell 2 Time (msec) Gap – 63 junction Vm of cell 2 – 64 Electrical PSP Dendrite – 65 0 1 2 3 Time (msec) Cell 2 (a) (b) ▲ FIGURE 5.2 Electrical synapses. (a) A gap junction interconnecting the dendrites of two neu- rons constitutes an electrical synapse. (b) An action potential generated in one neuron causes a small amount of ionic current to flow through gap junction chan- nels into a second neuron, inducing an electrical PSP. (Source: Part a from Sloper and Powell, 1978.) potential in the postsynaptic cell. One neuron usually makes electrical synapses with many other neurons, however, so several PSPs occurring simultaneously may strongly excite a neuron. This is an example of syn- aptic integration, which is discussed later in the chapter. The precise roles of electrical synapses vary from one brain region to another. They are often found where normal function requires that the activity of neighboring neurons be highly synchronized. For example, neu- rons in a brain stem nucleus called the inferior olive can generate both small oscillations of membrane voltage and, more occasionally, action po- tentials. These cells send axons to the cerebellum and are important in motor control. They also make gap junctions with one another. Current that flows through gap junctions during membrane oscillations and ac- tion potentials serves to coordinate and synchronize the activity of inferior olivary neurons (Figure 5.3a), and this in turn may help to control the fine timing of motor control. Michael Long and Barry Connors, working at Brown University, found that genetic deletion of a critical gap junction protein called connexin36 (Cx36) did not alter the neurons’ ability to gen- erate oscillations and action potentials but did abolish the synchrony of these events because of the loss of functional gap junctions (Figure 5.3b). Gap junctions between neurons and other cells are particularly com- mon early in development. Evidence suggests that during prenatal and postnatal brain development, gap junctions allow neighboring cells to share both electrical and chemical signals that may help coordinate their growth and maturation. Chemical Synapses Most synaptic transmission in the mature human nervous system is chemical, so the remainder of this chapter and the next will now focus exclusively on chemical synapses. Before we discuss the different types of 109–142_Bear_05_revised_final.indd 113 12/20/14 3:43 AM  114 PART ONE FOUNDATIONS (a) With gap junctions: Action potential Vm of cell 1 –0 Record Vm of cell 1 Oscillations 1 – 65 Gap junction Vm of cell 2 –0 2 – 65 Record Vm of cell 2 (b) Without gap junctions: Vm of cell 3 Record Vm –0 of cell 3 3 No gap junction – 65 4 Vm of cell 4 –0 – 65 Record Vm 0 1 2 3 4 5 of cell 4 Time (sec) ▲ FIGURE 5.3 Electrical synapses can help neurons to synchronize their activity. Certain brain stem neurons generate small, regular oscillations of Vm and occasional action potentials. (a) When two neurons are connected by gap junctions (cells 1 and 2), their oscillations and action potentials are well synchronized. (b) Similar neurons with no gap junctions (cells 3 and 4) generate oscillations and action potentials that are entirely uncoordinated. (Source: Adapted from Long et al., 2002, p. 10903.) chemical synapses, let’s take a look at some of their universal character- istics (Figure 5.4). The presynaptic and postsynaptic membranes at chemical synapses are separated by a synaptic cleft that is 20–50 nm wide, 10 times the width of the separation at gap junctions. The cleft is filled with a matrix of fibrous extracellular protein. One function of this matrix is to serve as a “glue” that binds the pre- and postsynaptic membranes together. The presynaptic side of the synapse, also called the presynaptic element, is usually an axon terminal. The terminal typically contains dozens of small membrane-enclosed spheres, each about 50 nm in diameter, called synaptic vesicles (Figure 5.5a). These vesicles store neurotransmitter, the chemical used to communicate with the postsynaptic neuron. Many axon terminals also contain larger vesicles, each about 100 nm in diameter, called secretory granules. Secretory granules contain soluble protein that appears dark in the electron microscope, so they are sometimes called large, dense-core vesicles (Figure 5.5b). 109–142_Bear_05_revised_final.indd 114 12/20/14 3:43 AM  CHAPTER 5 SYNAPTIC TRANSMISSION 115 Axon terminal (presynaptic element) Secretory granules Mitochondria Synaptic Active zone Membrane cleft Postsynaptic differentiations Synaptic density vesicles Receptors Postsynaptic dendrite ▲ FIGURE 5.4 The components of a chemical synapse. Dense accumulations of protein adjacent to and within the mem- branes on either side of the synaptic cleft are collectively called mem- brane differentiations. On the presynaptic side, proteins jutting into the cytoplasm of the terminal along the intracellular face of the mem- brane sometimes look like a field of tiny pyramids. The pyramids, and the membrane associated with them, are the actual sites of neurotrans- mitter release, called active zones. Synaptic vesicles are clustered in the cytoplasm adjacent to the active zones (see Figure 5.4). The protein thickly accumulated in and just under the postsynaptic membrane is called the postsynaptic density. The postsynaptic density contains the neurotransmitter receptors, which convert the intercellular chemical signal (i.e., neurotransmitter) into an intracellular signal (i.e., a change in membrane potential or a chemical change) in the postsynaptic cell. As we shall see, the nature of this postsynaptic response can be quite varied, depending on the type of protein receptor that is activated by the neurotransmitter. CNS Chemical Synapses. In the CNS, different types of synapse may be distinguished by which part of the neuron is postsynaptic to the axon terminal. If the postsynaptic membrane is on a dendrite, the synapse is said to be axodendritic. If the postsynaptic membrane is on the cell body, the synapse is said to be axosomatic. In some cases, the postsynaptic membrane is on another axon, and these synapses are called axoaxonic (Figure 5.6). When a presynaptic axon contacts a postsynaptic dendritic spine, it is called axospinous (Figure 5.7a). In certain specialized neu- rons, dendrites actually form synapses with one another; these are called 109–142_Bear_05_revised_final.indd 115 12/20/14 3:43 AM  116 PART ONE FOUNDATIONS Mitochondria Presynaptic terminal Postsynaptic cell Active zone (a) Synaptic vesicles Dense-core vesicles (b) ▲ FIGURE 5.5 Chemical synapses, as seen with the electron microscope. (a) A fast excitatory synapse in the CNS. (b) A synapse in the PNS, with numerous dense-core vesicles. (Source: Part a adapted from Heuser and Reese, 1977, p. 262; part b adapted from Heuser and Reese, 1977, p. 278.) Soma Synapse (a) (b) (c) Dendrite Axon ▲ FIGURE 5.6 Synaptic arrangements in the CNS. (a) An axodendritic synapse. (b) An axoso- matic synapse. (c) An axoaxonic synapse. 109–142_Bear_05_revised_final.indd 116 12/20/14 3:43 AM  CHAPTER 5 SYNAPTIC TRANSMISSION 117 (a) Axons (b) Presynaptic terminals Postsynaptic dendritic spine Axon Axon Postsynaptic (c) elements (d) Presynaptic terminals (a) Active zones Axon ▲ FIGURE 5.7 Various shapes and sizes of CNS synapses. (a) Axospinous synapse: A small presynaptic axon terminal contacts a postsynaptic dendritic spine. Notice that presynaptic terminals can be recognized by their many vesicles, and postsynaptic elements have postsynaptic densities. (b) An axon branches to form two presyn- aptic terminals, one larger than the other, and both contact a postsynaptic soma. (c) An unusually large axon terminal contacts and surrounds a postsynaptic soma. (d) An unusually large presynaptic axon terminal contacts five postsynaptic dendritic spines. Notice that larger synapses have more active zones. dendrodendritic synapses. The sizes and shapes of CNS synapses also vary widely (Figure 5.7a-d). The finest details of synaptic structure can be studied only under the powerful magnification of the electron microscope (Box 5.2). CNS synapses may be further classified into two general categories based on the appearance of their presynaptic and postsynaptic membrane differentiations. Synapses in which the membrane differentiation on the postsynaptic side is thicker than that on the presynaptic side are called asymmetrical synapses, or Gray’s type I synapses; those in which the mem- brane differentiations are of similar thickness are called symmetrical 109–142_Bear_05_revised_final.indd 117 12/20/14 3:43 AM  118 PART ONE FOUNDATIONS BOX 5.2 PAT H O F D I S C O V E RY For the Love of Dendritic Spines by Kristen M. Harris T he first time I looked through the microscope and saw a dendritic spine, it was love at first sight, and the affair has Adult Rat Glia vesicles simply never ended. I was a graduate student in the new neu- Spine PSD Dendrite robiology and behavior program at the University of Illinois, Apparatus us Axon and it was indeed an exciting time in neuroscience. The 1979 Glia Society for Neuroscience meeting had only about 5,000 at- Spine tendees (attendance is now about 25,000), and the member Mitochondrion number I obtained during my first year of graduate school 1 micron was and remains 2500. I had hoped to discover what a “learned” dendritic spine Figure B looks like by training animals and then using the Golgi stain- ing method to quantify changes in spine number and shape. microscopy (3DEM). I was truly hooked. With 3DEM, one Eagerly, I developed a high-throughput project, preparing the could reconstruct dendrites, axons, and glia, and not only brains from many rats at once, sectioning through the whole measure and count dendritic spines but also see where syn- brains, checking that the silver staining had worked, and then apses formed, what was inside them, and how glia asso- storing the tissue sections in butanol while engaging under- ciated with synapses (Figure B). The 3DEM platform offers graduates to help mount them on microscope slides. To our enormous possibilities for discovery. My life continues to be dismay, we found several months later that all the silver had devoted to uncovering the processes of synapse formation been dissolved out of the cells. There were no cells to see, and plasticity during learning and memory in the brain. and the project died an untimely death. Early in my career, while the revolution of molecular biology I was fortunate, however, to meet Professor Timothy Teyler was sweeping the field, only a rare student or fellow scien- at a Gordon Research Conference. He had recently brought tist shared my enthusiasm for 3DEM. That bias has shifted the hippocampal slice preparation to the United States from dramatically as neuroscientists have come to recognize the Norway and was moving his lab from Harvard to a new medi- importance of understanding how molecules work in consort cal school in Rootstown, Ohio. I was completely enamored by with intracellular organelles in small spaces like dendrites and the experimental control that brain slices might offer, so I de- spines. Furthermore, all maps of neural circuitry must include veloped a Golgi-slice procedure and moved to complete my synapses. These endeavors have drawn scientists from nearly PhD with Teyler. This time, I prepared one slice at a time, and every field, making 3DEM even more exciting as many of the as can be seen in Figure A, the spines were exquisitely visible. imaging and reconstruction processes previously done man- Unfortunately, accurate counts and shape measurements of the ually are being automated. Figure C shows a recent 3DEM tiny spines were just beyond the resolution of light microscopy. rendering, with color-coding of organelles and synaptic com- While I was a graduate student, I talked my way into the ponents. It is indeed thrilling to be part of this growth. New esteemed summer course in neurobiology at the Marine findings abound regarding the plasticity of synapse structure Biological Laboratories in Woods Hole, Massachusetts. during normal changes in brain function and as altered by There I first learned serial-section three-dimensional electron diseases that tragically affect who we are as human beings. Axonal 3D-EM of Dendrite with Synapses (red) and Organelles Bouton Dendritic Dendrites Spine Neuron Cell Body Axons Dendrites Rapid Golgi preparation (Harris, 1980) Figure A Figure C 109–142_Bear_05_revised_final.indd 118 12/20/14 3:43 AM  CHAPTER 5 SYNAPTIC TRANSMISSION 119 synapses, or Gray’s type II synapses (Figure 5.8). As we shall see later in the chapter, these structural differences reveal functional differences. Gray’s type I synapses are usually excitatory, while Gray’s type II syn- apses are usually inhibitory. The Neuromuscular Junction. Synaptic junctions also exist outside the CNS. For example, axons of the autonomic nervous system innervate glands, smooth muscle, and the heart. Chemical synapses also occur Asymmetrical Symmetrical membrane membrane between the axons of motor neurons of the spinal cord and skeletal (a) differentiations (b) differentiations muscle. Such a synapse is called a neuromuscular junction, and it has many of the structural features of chemical synapses in the CNS ▲ FIGURE 5.8 Two categories of CNS synaptic (Figure 5.9). membrane differentiations. (a) A Gray’s Neuromuscular synaptic transmission is fast and reliable. An action type I synapse is asymmetrical and potential in the motor axon always causes an action potential in the mus- usually excitatory. (b) A Gray’s type II cle cell it innervates. This reliability is accounted for, in part, by struc- synapse is symmetrical and usually tural specializations of the neuromuscular junction. Its most important inhibitory. specialization is its size—it is one of the largest synapses in the body. The presynaptic terminal also contains a large number of active zones. In addition, the postsynaptic membrane, also called the motor end- plate, contains a series of shallow folds. The presynaptic active zones are precisely aligned with these junctional folds, and the postsynaptic membrane of the folds is packed with neurotransmitter receptors. This structure ensures that many neurotransmitter molecules are focally released onto a large surface of chemically sensitive membrane. Because neuromuscular junctions are more accessible to researchers than CNS synapses, much of what we know about the mechanisms of synaptic transmission was first established here. Neuromuscular junc- tions are also of considerable clinical significance; diseases, drugs, and poisons that interfere with this chemical synapse have direct effects on vital bodily functions. PRIN PRINCIPLES NCIPLE ES OF CHEMICAL S SYNAPTIC YNAPTIC TRAN NSMISSSION TRANSMISSION Consider the basic requirements of chemical synaptic transmission. There must be a mechanism for synthesizing neurotransmitter and packing it into the synaptic vesicles, a mechanism for causing vesicles to spill their contents into the synaptic cleft in response to a presynaptic action poten- tial, a mechanism for producing an electrical or biochemical response to neurotransmitter in the postsynaptic neuron, and a mechanism for remov- ing neurotransmitter from the synaptic cleft. And, to be useful for sensa- tion, perception, and the control of movement, all these things must often occur very rapidly, within milliseconds. No wonder physiologists were ini- tially skeptical about the existence of chemical synapses in the brain! Fortunately, thanks to several decades of research on the topic, we now understand how many of these aspects of synaptic transmission are so efficiently carried out. Here we’ll present a general survey of the basic principles. In Chapter 6, we will take a more detailed look at the indi- vidual neurotransmitters and their modes of postsynaptic action. Neurotransmitters Since the discovery of chemical synaptic transmission, researchers have been identifying neurotransmitters in the brain. Our current understand- ing is that the major neurotransmitters fall into one of three chemical 109–142_Bear_05_revised_final.indd 119 12/20/14 3:43 AM  120 PART ONE FOUNDATIONS ▲ FIGURE 5.9 The neuromuscular junction. The postsynaptic membrane, known as the motor end-plate, contains junctional folds with numerous neurotransmitter receptors. Motor neuron Muscle fibers Myelin Axon Neuromuscular junction Synaptic vesicles Active zone Synaptic cleft Receptors Junctional fold Postsynaptic muscle fiber Presynaptic Motor end-plate terminals (postsynaptic) region categories: (1) amino acids, (2) amines, and (3) peptides (Table 5.1). Some representatives of these categories are shown in Figure 5.10. The amino acid and amine neurotransmitters are all small organic molecules containing at least one nitrogen atom, and they are stored in and released from synaptic vesicles. Peptide neurotransmitters are large molecules—chains of amino acids—stored in and released from secretory granules. As discussed ear- lier, secretory granules and synaptic vesicles are frequently observed in 109–142_Bear_05_revised_final.indd 120 12/20/14 3:43 AM  CHAPTER 5 SYNAPTIC TRANSMISSION 121 TABLE 5.1 The Major Neurotransmitters Amino Acids Amines Peptides Gamma-aminobutyric Acetylcholine (ACh) Cholecystokinin (CCK) acid (GABA) Dopamine (DA) Dynorphin Glutamate (Glu) Epinephrine Enkephalins (Enk) Glycine (Gly) Histamine N-acetylaspartylglutamate (NAAG) Norepinephrine (NE) Neuropeptide Y Serotonin (5-HT) Somatostatin Substance P Thyrotropin-releasing hormone Vasoactive intestinal polypeptide (VIP) COOH COOH CH2 CH2 CH2 CH2 NH2 CH COOH NH2 CH NH2 CH2 COOH (a) Glu GABA Gly HO O CH3 OH + CH3 C O CH2 CH2 N CH3 HO CH CH2 NH2 CH3 (b) ACh NE Carbon Oxygen Nitrogen Hydrogen Arg Pro Lys Pro Gln Gln Phe Phe Gly Leu Met Sulfur (c) Substance P ▲ FIGURE 5.10 Representative neurotransmitters. (a) The amino acid neurotransmitters gluta- mate, GABA, and glycine. (b) The amine neurotransmitters acetylcholine and nor- epinephrine. (c) The peptide neurotransmitter substance P. (For the abbreviations and chemical structures of amino acids in substance P, see Figure 3.4b.) 109–142_Bear_05_revised_final.indd 121 12/20/14 3:43 AM  122 PART ONE FOUNDATIONS the same axon terminals. Consistent with this observation, peptides often exist in the same axon terminals that contain amine or amino acid neu- rotransmitters. As we’ll discuss in a moment, these different neurotrans- mitters are released under different conditions. Different neurons in the brain release different neurotransmitters. The speed of synaptic transmission varies widely. Fast forms of synaptic transmission last from about 10–100 msec, and at most CNS synapses are mediated by the amino acids glutamate (Glu), gamma-aminobutyric acid (GABA), or glycine (Gly). The amine acetylcholine (ACh) medi- ates fast synaptic transmission at all neuromuscular junctions. Slower forms of synaptic transmission may last from hundreds of milliseconds to minutes; they can occur in the CNS and in the periphery and are medi- ated by transmitters from all three chemical categories. Neurotransmitter Synthesis and Storage Chemical synaptic transmission requires that neurotransmitters be synthe- sized and ready for release. Different neurotransmitters are synthesized in different ways. For example, glutamate and glycine are among the 20 amino acids that are the building blocks of protein (see Figure 3.4b); consequently, they are abundant in all cells of the body, including neurons. In contrast, GABA and the amines are made primarily by the neurons that release them. These neurons contain specific enzymes that synthesize the neurotransmit- ters from various metabolic precursors. The synthesizing enzymes for both amino acid and amine neurotransmitters are transported to the axon termi- nal, where they locally and rapidly direct transmitter synthesis. Once synthesized in the cytosol of the axon terminal, the amino acid and amine neurotransmitters must be taken up by the synaptic vesicles. Concentrating these neurotransmitters inside the vesicle is the job of transporters, special proteins embedded in the vesicle membrane. Quite different mechanisms are used to synthesize and store peptides in secretory granules. As we learned in Chapters 2 and 3, peptides are formed when amino acids are strung together by the ribosomes of the cell body. In the case of peptide neurotransmitters, this occurs in the rough ER. Generally, a long peptide synthesized in the rough ER is split in the Golgi apparatus, and one of the smaller peptide fragments is the active neurotransmitter. Secretory granules containing the peptide neurotransmitter bud off from the Golgi apparatus and are carried to the axon terminal by axoplasmic transport. Figure 5.11 compares the synthesis and storage of amine and amino acid neurotransmitters with that of peptide neurotransmitters. Neurotransmitter Release Neurotransmitter release is triggered by the arrival of an action poten- tial in the axon terminal. The depolarization of the terminal membrane causes voltage-gated calcium channels in the active zones to open. These membrane channels are very similar to the sodium channels we discussed in Chapter 4, except they are permeable to Ca2⫹ instead of Na⫹. There is a large inward driving force on Ca2⫹. Remember that the internal calcium ion concentration—[Ca2⫹]i—at rest is very low, only 0.0002 mM; therefore, Ca2⫹ will flood the cytoplasm of the axon terminal as long as the calcium channels are open. The resulting elevation in [Ca2⫹]i is the signal that causes neurotransmitter to be released from synaptic vesicles. The vesicles release their contents by a process called exocytosis. The membrane of the synaptic vesicle fuses to the presynaptic membrane at the active zone, allowing the contents of the vesicle to spill out into the synaptic cleft (Figure 5.12). Studies of a giant synapse in the squid nervous 109–142_Bear_05_revised_final.indd 122 12/20/14 3:43 AM  CHAPTER 5 SYNAPTIC TRANSMISSION 123 Synaptic Precursor Active peptide vesicles peptide neurotransmitter Nucleus 3 4 1 2 3 Secretory granules Ribosome Golgi apparatus Rough ER Precursor molecule 1 Synthesizing enzyme (a) Neurotransmitter ▲ FIGURE 5.11