Biological Psychology 1-5 PDF

Chapter 1: Biopsychology as Neuroscience The following are a few of the disciplines of neuroscience that are particularly relevant to The human brain is a squishy, wrinkled, biopsychology. walnut-shaped hunk of tissu...

Chapter 1: Biopsychology as Neuroscience The following are a few of the disciplines of neuroscience that are particularly relevant to The human brain is a squishy, wrinkled, biopsychology. walnut-shaped hunk of tissue weighing about 1.3 kilograms. 1. neuroanatomy. The study of the structure of the nervous system. neurons - cells that receive and transmit electrochemical signals. 2. neurochemistry. The study of the chemical bases of neural activity. neuroscience - the scientific study of the nervous system. 3. neuroendocrinology. The study of interactions between the nervous system thinking creatively - thinking in productive, and the endocrine system. unconventional ways. 4. neuropathology. The study of nervous Clinical (pertaining to illness or treatment) system disorders. considerations are woven through the fabric of biopsychology. This text focuses on the interplay 5. neuropharmacology. The study of the between brain dysfunction and biopsychological effects of drugs on neural activity. research, and each major example is highlighted by a clinical implications tab. 6. neurophysiology. The study of the functions and activities of the nervous An evolutionary perspective is the comparative system. approach (trying to understand biological phenomena by comparing them to different species). Neuroplasticity in the past two decades, research has clearly demonstrated that the adult brain is not a static network of neurons: It is a plastic (changeable) organ that continuously grows and changes in response to the individual’s genes and experiences. What Is Biopsychology? Biopsychology - is the scientific study of the biology of behavior. Some refer to this field as psychobiology, behavioral biology, or behavioral Human and Nonhuman Subjects neuroscience. Human advantages: Psychology - is the scientific study of behavior. *They can follow instructions, What Are the Origins of Biopsychology? *They can report their subjective experiences. biopsychology did not develop into a major *Humans are often cheaper. neuroscientific discipline until the 20th century. *Understanding the intricacies of human brain function is that they have human brains. -The publication of The Organization of Behavior in 1949 by D. O. Hebb played a key role in its Nonhuman animals have three advantages: emergence. *Brains and behavior of nonhuman subjects are -Hebb developed the first comprehensive theory of simpler. how complex psychological phenomena, such as perceptions, emotions, thoughts, and memories, *Insights frequently arise from the comparative might be produced by brain activity. approach, the study of biological processes by comparing different species. Neuroscience is a team effort, and biopsychologists are important members of the *Possible to conduct research on laboratory team. animals that, for ethical reasons. Biopsychologists are neuroscientists who bring to Experiments and Nonexperiments their research a knowledge of behavior and of the Experiment is the method used by scientists to methods of behavioral research. study causation, that is, to find out what causes what. Between-Subjects Design - different groups of subjects are tested under each condition. Within-Subjects Design - possible to test the *Psychophysiology same group of subjects under each condition. *Cognitive neuroscience independent variable - measures the outcome in such a way that there is only one relevant *Comparative psychology difference between the conditions being compared. dependent variable - measured by the experimenter to assess the effect of the independent variable. confounded variable - difficult to determine whether it was the independent variable or the unintended difference. Coolidge effect - the fact that a copulating male who becomes incapable of continuing to copulate with one sex partner can often recommence copulating with a new sex partner. lordosis the arched-back, rump-up, tail-diverted posture of female rodent sexual receptivity during each sex test. Quasiexperimental Studies -studies of groups of subjects who have been exposed to the conditions of interest in the real world. Case Studies Korsakoff’s syndrome. The primary symptom of - Studies that focus on a single case or subject. Korsakoff’s syndrome is severe memory loss, which is made all the more heartbreaking. -generalizability major problem with all case studies. Degree to which their results can be -Korsakoff’s syndrome commonly occurs in applied to other cases. alcoholics, it was initially believed to be a direct consequence of the toxic effects of alcohol on the Pure and Applied Research brain. Pure research is motivated primarily by the -Korsakoff’s syndrome is largely caused by brain curiosity of the researcher—it is done solely for the damage associated with thiamine (vitamin B1) purpose of acquiring knowledge. deficiency. -Many scientists believe that pure research will -The thiamine deficient rats displayed memory ultimately prove to be of more practical benefit than deficits and patterns of brain damage similar to applied research. those observed in human alcoholics. Applied research is intended to bring about some direct benefit to humankind. Translational research: research that aims to translate the findings of pure research into useful applications for humankind motor neurons (neurons that control muscles) hypothalamus (a small neural structure at the base of the brain) corpus callosum (the large neural pathway that connects the left and right halves of the brain) What Are the Divisions of Biopsychology? *Physiological psychology *Psychopharmacology *Neuropsychology -The thiamine limits the development of further Chapter 2 Evolution, Genetics, and Experience brain damage and often leads to a slight improvement in the patient’s condition; Is It Physiological, or Is It Psychological? unfortunately, the brain damage that has already Descartes (1596–1650) advocated a philosophy occurred is mostly irreversible. that, in a sense, gave one part of the universe to Scientific Inference: How Do Biopsychologists science and the other part to the Church. He Study the Unobservable Workings of the Brain? argued that the universe is composed of two elements: (1) physical matter, which behaves Scientific inference is the fundamental method of according to the laws of nature and is thus a biopsychology and of most other sciences—it is suitable object of scientific investigation—the what makes being a scientist fun. human body, including the brain, was assumed to be entirely physical, and so were nonhuman -The empirical method that biopsychologists and animals; and (2) the human mind (soul, self, or other scientists use to study the unobservable. spirit), which lacks physical substance, controls To illustrate scientific inference, we have human behavior, obeys no natural laws, and is selected a research project in which you can thus the appropriate purview of the Church. participate. By making a few simple -cartesian dualism, as Descartes’s philosophy observations about your own visual abilities became known, was sanctioned by the Roman under different conditions, you will be able to Church, and so the idea that the human brain and discover the principle by which your brain the mind are separate entities became even more translates the movement of images on your widely accepted. retinas into perceptions of movement. Is it Inherited or is it Learned? For centuries, scholars have debated whether humans and other animals inherit their behavioral capacities or acquire them through learning. This debate is commonly referred to as the nature– nurture issue. John B. Watson, the father of behaviorism. ethology (the study of animal behavior in the wild) instinctive behaviors (behaviors that occur in all like members of a species, even when there seems to have been no opportunity for them to have been learned)-Because instinctive behaviors are not learned, the early ethologists assumed they are entirely inherited. They were wrong, but then so were the early experimental psychologists. Physiological – or - psychological thinking (the assumption that some aspects of human psychological functioning are so complex that they could not possibly be the product of a physical brain). 2 lines evidence against physiological or psychological thinking. 1. composed of the many demonstrations that even the most complex psychological changes (e.g., changes in self-awareness, memory, or emotion) can be produced by damage to, or stimulation of, parts of the brain. 2. Composed of demonstrations that some nonhuman species, particularly primate species, possess some abilities that were once assumed to be purely psychological and thus purely human. asomatognosia - a deficiency in the awareness of parts of one’s own body. Asomatognosia typically involves the left side of the body and usually results from damage to the right parietal lobe. MODEL OF THE BIOLOGY OF BEHAVIOR -Why is social dominance an important factor in evolution? One reason is that in some species, dominant males copulate more than nondominant males and thus are more effective in passing on their characteristics to future generations. -Another reason why social dominance is an important factor in evolution is that in some species, dominant females are more likely to produce more and healthier offspring. Courtship Display - An intricate series of courtship displays precedes copulation in many species. The male approaches the female and signals his interest. His signal (which may be olfactory, visual, auditory, or tactual) may elicit a signal in the female, which may elicit another response in the male, and so on, until copulation ensues. species is a group of organisms reproductively isolated from other organisms; that is, the members of a species can produce fertile offspring only by HUMAN EVOLUTION mating with members of the same species. Modern biology began in 1859 with the publication conspecifics (members of the same species) Only of Charles Darwin’s "On the Origin of Species". the suitable exchange of displays between a -Darwin described his theory of evolution—the courting couple will lead to reproduction. single most influential theory in the biological Course on Human Evolution sciences. Evolution of Vertebrate - Complex multicellular -evolve (undergo gradual orderly change) water-dwelling organisms first appeared on earth Darwin presented three kinds of evidence to about 600 million years ago. support his assertion that species evolve: chordates are animals with dorsal nerve 1.He documented the evolution of fossil records cords (large nerves that run along the through progressively more recent geological layers. center of the back, or dorsum); they are 1 of the 20 or so large categories, or phyla 2.He described striking structural similarities among into which zoologists group animal species. living species (e.g., a human’s hand, a bird’s wing, The first chordates with spinal bones to and a cat’s paw) protect their dorsal nerve cords evolved about 25 million years later. 3.He pointed to the major changes that had been brought about in domestic plants and animals by The spinal bones are called vertebrae, and programs of selective breeding. the chordates that possess them are called vertebrates. Darwin argued that evolution occurs through natural selection. He pointed out that the The first vertebrates were primitive bony members of each species vary greatly in their fishes. Today, there are seven classes of structure, physiology, and behavior and that the vertebrates: three classes of fishes, plus heritable traits associated with high rates of survival amphibians, reptiles, birds, and mammals and reproduction are the most likely ones to be passed on to future generations. Evolution of Amphibians Natural Selection - when repeated for generation -About 410 million years ago, the first bony fish after generation, leads to the evolution of species started to venture out of the water. that are better adapted to surviving and -The advantages of life on land were so great that reproducing in their particular environmental niche. natural selection transformed the fins and gills of fitness - in the Darwinian sense, is the ability of an bony fishes to legs and lungs, respectively— and organism to survive and contribute its genes to the so it was that the first amphibians evolved about next generation. 400 million years ago. Evolution and Behavior Amphibians (e.g., frogs, toads, and salamanders) in their larval form must live in Social Dominance - The males of many species the water; only adult amphibians can establish a stable hierarchy of social dominance survive on land. through combative encounters with other males. Evolution of Reptiles About 300 million years ago, reptiles (e.g., lizards, snakes, and turtles) evolved from a branch of amphibians. Reptiles were the first vertebrates to lay shell-covered eggs and to be covered by dry scales. A reptile does not have to spend the first stage of its life in the watery environment of a pond or lake; instead, it spends the first stage of its life in the watery environment of Thinking about Human Evolution a shell-covered egg. Evolution does not proceed in a single line. Evolution of Mammals Although it is common to think of an About 180 million years ago, during the evolutionary ladder or scale, a far better height of the age of dinosaurs, a new class metaphor for evolution is a dense bush. of vertebrates evolved from one line of small We humans have little reason to claim reptiles. evolutionary supremacy. We are the last surviving species of a family (i.e., hominins) females of this new class fed their young that has existed for only a blip of with secretions from special glands called evolutionary time. mammary glands. Evolution does not always proceed slowly and gradually. Whether human evolution Eventually, mammals stopped laying eggs; occurred gradually or suddenly is still a instead, the females nurtured their young in matter of intense debate among the watery environment of their bodies until paleontologists (those who scientifically the young were mature enough to be born. study fossils). The order to which we belong is the order Evolution does not progress to preordained primates. We humans—in our usual perfection—evolution is a tinkerer, not an humble way—named our order using the architect. Latin term primus, which means “first” or Not all existing behaviors or structures are “foremost.” adaptive. Evolution often occurs through changes in developmental programs that Emergence of Humankind lead to several related characteristics, only one of which might be adaptive—the Primates of the tribe that include humans incidental nonadaptive evolutionary by- are the Hominini. products are called spandrels. This tribe is composed of at least six genera (the plural of genus): Australopithecus, Not all existing adaptive characteristics Paranthropus, Sahelanthropus, Orrorin, evolved to perform their current function. Pan, and Homo. Some characteristics, called exaptations, evolved to serve one function and were later Homo is thought to be composed of at least co-opted to serve another. eight species. Seven of those Homo Structures that are similar because they have species are now extinct, whereas Homo a common evolutionary origin are termed sapiens (humans) are not. homologous; structures that are similar but do not have a common evolutionary origin are The first Homo species are thought to have termed analogous. The similarities between evolved from one species of analogous structures result from convergent Australopithecus about 2 to 2.8 million evolution, the evolution in unrelated species years ago. of similar solutions to the same environmental demands. There is now considerable evidence that Homo sapiens mated with the other Homo species (e.g., Homo neanderthalensis) they encountered both within Africa and as they migrated out of Africa. Evolution of the Human Brain modern humans, whom modern humans believe to be the most intelligent of all creatures, do not have members of both sexes indiscriminately the biggest brains. copulate with many different partners The sizes of the brains of during each mating period. acclaimed intellectuals (e.g., - However, the males and females of Einstein) were found to be some species form mating bonds unremarkable, certainly no (enduring mating relationships) with match for their gigantic intellects. particular members of the other sex. - One obvious problem in relating - natural selection has promoted the brain size to intelligence is the fact evolution in mammalian males of the that larger animals tend to have tendency to bond with the females with larger brains, presumably because which they have copulated. larger bodies require more brain - there is selection pressure on female tissue to control and regulate them. mammals to behave in ways that will - the facts that large men tend to have induce males to bond to them because larger brains than small men, that men this improves their ability to pass on tend to have larger brains than women, their own heritable characteristics to and that elephants have larger brains future generations. than humans do not suggest anything about the relative intelligence of these POLYGYNY AND POLYANDRY populations. - The pattern of mate bonding that is - It has been informative to consider the most prevalent in mammals is polygyny, evolution of the brain stem separately an arrangement in which one male form from the evolution of the cerebrum mating bonds with more than one (cerebral hemispheres). female. - The strongest evidence in support of the theory that polygyny evolves when females make a far greater contribution to reproduction and parenting than males do comes from studies of polyandry. - Polyandry is a mating arrangement in which one female form mating bonds with more than one male. - Polyandry does not occur in mammals; it occurs only in species in which the contributions of the males to reproduction are greater than those of the females. MONOGAMY -Monogamy is a mate-bonding pattern This figure makes three important in which enduring bonds are formed points about the evolution of the between one male and one female. human brain: -Monogamy is thought to have evolved The brain has increased in size in those mammalian species in which during evolution. each female could raise younger, or Most of the increase in size has more fit young, if she had undivided help. occurred in the cerebrum. An increase in the number of Western cultures promote convolutions—folds on the cerebral monogamy, but are humans surface—has greatly increased the monogamous? Many of our students surface area of the cerebral cortex, believed so until they were asked to the outermost layer of cerebral consider the following three points; tissue. then they were not so sure: Many human cultures do not Evolutionary Psychology: practice monogamy. Understanding Mate Bonding Even in Western cultures most - Evolutionary psychologists try to people bond with more than one understand human behaviors through partner during their lives. consideration of the pressures that led Infidelity is common. to their evolution - In most vertebrate species, mating is totally promiscuous—promiscuity is a THINKING ABOUT EVOLUTIONARY mating arrangement in which the PSYCHOLOGY The foregoing evolutionary theory of mate -Today, we call each inherited factor a gene. bonding has led to several predictions about current aspects of human mate selection. Alleles - The two genes that control the same trait. confirmed several of them, for example: homozygous - Organisms that possess two identical genes for a trait. Men in most cultures value youth and heterozygous - those that possess two different attractiveness (both indicators of fertility) genes for a trait. in their mates more than women do; in contrast, women value power and Chromosomes earning capacity more than men do. Physical attractiveness best predicts REPRODUCTION AND RECOMBINATION which women will bond with men of high -It was not until the early 20th century that genes occupational status. were found to be located on chromosomes—the The major mate-attraction strategy of threadlike structures in the nucleus of each cell. women is increasing their physical attractiveness; in men, it is displaying -Chromosomes occur in matched pairs, and each their power and resources. species has a characteristic number of pairs in Men are more likely than women to each of its body cells; humans have 23 pairs. commit adultery. meiosis - The process of cell division that Fundamental Genetics produces gametes. Mendelian Genetics -In meiosis, the chromosomes divide, and one chromosome of each pair goes to each of the two - He decided to study dichotomous traits, gametes that results from the cell division. As a and he decided to begin his experiments by result, each gamete has only half the usual crossing the offspring of true-breeding lines. number of chromosomes (23 in humans). - Dichotomous traits occur in one form or -and when a sperm cell and an egg cell combine the other, never in combination. For during fertilization (see Figure 2.16), a zygote (a example, seed color is a dichotomous pea fertilized egg cell) with the full complement of plant trait: Every pea plant has either brown chromosomes is produced. seeds or white seeds. STRUCTURE AND REPLICATION - True-breeding lines are breeding lines in which interbred members always produce offspring with -Each chromosome is a double-stranded molecule the same trait (e.g., brown seeds), generation after of deoxyribonucleic acid (dna). generation. -Each strand is a sequence of nucleotide bases - In one of his early experiments, Mendel studied attached to a chain of phosphate and deoxyribose; the inheritance of seed color: brown or white. there are four nucleotide bases: adenine, thymine, guanine, and cytosine. - One trait, which Mendel called the dominant trait, appeared in all of the first-generation offspring. - the other trait, which he called the recessive trait, appeared in about one-quarter of the second- generation offspring. - An organism’s observable traits are referred to as its phenotype. - the traits that it can pass on to its offspring through its genetic material are referred to as its genotype.  color-blindness gene is quite rare, females almost never inherit two of them and thus almost never possess the disorder; in contrast, every male who possesses one color-blindness gene is color blind. Genetic Code and Gene Expression - Structural genes contain the information necessary for the synthesis of proteins. Proteins are long chains of amino acids; they control the physiological activities of cells and are important components of cellular structure. Enhancers - The stretches of DNA that lack - Replication is a critical process of DNA structural genes are not well understood, but it is molecules. Without it, mitotic cell division clear that they include portions called enhancers. would not be possible. - Enhancers are stretches of DNA whose function is - Chromosome replication does not always to determine whether particular structural genes go according to plan; there may be errors. initiate the synthesis of proteins and at what rate. Sometimes, these errors are gross errors. - Control of gene expression by enhancers is an For example, Down syndrome. important process because it determines how a cell - But more commonly, errors in duplication will develop and how it will function once it reaches take the form of mutations. maturity. MUTATIONS - accidental alterations in individual - Transcription factors - Proteins that bind to DNA genes. and influence the extent to which genes are expressed. - However, in rare instances, mutations increase fitness and in so doing contribute to rapid evolution. SEX CHROMOSOMES AND SEX-LINKED TRAITS - autosomal chromosomes - The typical chromosomes, which come in matched pairs. - sex chromosomes— the pair of chromosomes that determines an individual’s sex. - Female mammals have two X chromosomes, and male mammals have one X chromosome and one Y chromosome. - The strand of transcribed RNA is called messenger RNA because it carries the genetic sex-linked traits - Traits influenced by genes on code out of the nucleus of the cell. the sex chromosomes. - Once it has left the nucleus, the messenger RNA - sex-linked traits are controlled by genes on attaches itself to one of the many ribosomes in the the X chromosome because the Y cell’s cytoplasm (the clear fluid within the cell). chromosome is small and carries few genes. - Each group of three consecutive nucleotide bases along the messenger RNA strand is called a codon. In contrast, recessive sex-linked traits occur - Each kind of amino acid is carried to the more frequently in males. The reason is that ribosome by molecules of transfer RNA. recessive sex-linked traits are manifested only in females who possess two of the recessive genes— The process of gene expression involves two one on each of their X chromosomes—whereas the phases: the transcription of the DNA base- traits are manifested in all males who possess the sequence code to an RNA base-sequence code gene because they have only one X chromosome. and the translation of the RNA base-sequence code into a sequence of amino acids. - The classic example of a recessive sex- linked trait is color blindness. Because the Human Genome Project by which these critical interactions took place were unknown. - One of the most ambitious scientific projects of all time began in 1990. Known as the Human At the turn of the century, there was a newly Genome Project, it was a loosely knit collaboration available arsenal of research techniques resulting of major research institutions and individual from the Human Genome Project. research teams in several countries. Despite its youth, epigenetic research has -The purpose of this collaboration was to compile a already amassed an impressive array of map of the sequence of all 3 billion bases that important discoveries. Here are five important compose human chromosomes. advances: The following are three major contributions of Epigenetic investigations of non gene DNA have the Human Genome Project: identified many active areas. Many of these active areas seem to control the expression of nearby Using this new technology, genomes have genes. already been established for many species, including those of many long-extinct species Small RNA molecules have been found to come leading to important insights into evolution. in a variety of different types. Some small RNA molecules have been found to regulate gene The discovery that we humans, the most complex expression, but it is likely that each type of small of all species, have relatively few genes surprised RNA performs a different function. many scholars. Humans have about 20,000 genes; mice have about the same number, and corn has Many epigenetic mechanisms have been many more. Indeed, protein-encoding genes discovered by which gene expression can be constitute only about 1 percent of human DNA. regulated. Two of the most widely studied are DNA Researchers have now generated a nearly methylation and histone remodeling. DNA complete map of the entire set of proteins encoded methylation is the reaction that occurs when a for by our genes: the human proteome. methyl group attaches to a DNA molecule, usually at cytosine sites in mammals. Histone remodeling Many variations in the human genome related to is the reaction that occurs when histones (proteins particular diseases have been identified. However, around which DNA is coiled) change their shape this has proven to be less useful than anticipated: and in so doing influence the shape of the adjacent So many genes have been linked to each disease DNA. Both DNA methylation and histone that it has proven difficult to sort out the interactions remodeling can either decrease or increase among the numerous genes and experience. expression. Compounding the problem is that even when many genes have been linked to a disease, all of them Some epigenetic effects regulate gene expression together often account for only a small portion of its by acting on messenger RNA rather than genes heritability). For example, 18 different gene variants this is called RNA EDITING. Small RNA molecules have been linked to adult-onset diabetes, but these and other proteins have been shown to cleave 18 variants account for only 6 percent of the messenger RNA apart at precise points and heritability of the disease. sometimes to splice sections of new RNA to create a new sequence of bases. Modern Genetics: Growth of Epigenetics. Remarkably, epigenetic changes such as DNA - Epigenetics is the study of all mechanisms methylation and histone remodeling can be induced of inheritance other than the genetic code by particular experiences (e.g., neural activity, and its expression. hormonal state, changes to the environment) that Why did epigenetics rise to prominence so can last a lifetime. quickly at the turn of the century? The presence Transgenerational epigenetics is a subfield of of the following four conditions set the stage: epigenetics that examines the transmission of The Human Genome Project had just discovered experiences via epigenetic mechanisms across that genes constitute only about 1 percent of generations. human DNA. The other 99 percent of DNA had - it has been shown that when mice been widely regarded as junk DNA. experience an odor associated with a In the 1990s, the vast majority of RNA molecules were found to be small—only 1.2 percent were of the large protein-encoding variety. This indicated that protein encoding is only a minor function of RNA. For decades, there had been a general consensus that inheritance was a product of gene- experience interactions, and yet the mechanisms  painful shock, the memory of that vomiting, seizures, hyperactivity, irritability, and experience is passed on to subsequent brain damage. generations through epigenetic mechanisms. The biochemistry of PKU turned out to be reasonably straightforward. PKU homozygotes lack Epigenetics of Behavioral Development: phenylalanine hydroxylase, an enzyme required Interaction of Genetic Factors and Experience for the conversion of the amino acid phenylalanine to tyrosine. Ontogeny is the development of individuals over their life span; Phylogeny, in contrast, is the As a result, phenylalanine accumulates in the body; evolutionary development of species through and levels of dopamine, a neurotransmitter the ages. normally synthesized from tyrosine, are low. The consequence is abnormal brain development. Selective Breeding of “Maze-Bright” and “Maze- Dull” Rats The period, usually early in life, during which a particular experience must occur to have a major Tryon (1934) undermined this assumption by effect on the development of a trait, is the sensitive showing that behavioral traits can be period for that trait. selectively bred. Development of Birdsong -Tryon then mated the females and males that least frequently entered incorrect alleys during training— Studies of the ontogenetic development of birdsong he referred to these rats as maze-bright. And he suggest that this behavior develops in two phases. bred the females and males that most frequently entered incorrect alleys during training—he referred The first phase, called the sensory phase, begins to these rats as maze-dull. several days after hatching. -When the offspring of both the maze-bright and the The second phase of birdsong development, the maze-dull rats matured, their maze- learning sensorimotor phase, begins when the juvenile performance was assessed. males begin to twitter subsongs (the immature songs of young birds), usually when they are Tryon used a cross-fostering control procedure: several months old. He tested maze-bright offspring that had been reared by maze-dull parents and maze-dull -During this phase, the rambling vocalizations of offspring that had been reared by maze-bright subsongs are gradually refined until they resemble parents. the songs of the birds’ earlier adult tutors. - selective breeding based on one behavioral -Most songbird species, such as the widely studied trait usually brings a host of other behavioral zebra finches and white-crowned sparrows, are traits along with it. age-limited learners; in these species, adult songs, once crystallized, remain unchanged for the rest of The researchers reared maze-bright and maze-dull the birds’ lives. In contrast, some species are rats in one of two environments: open-ended learners; they are able to add new songs to their repertoire throughout their lives. (1) an impoverished environment (a barren wire- mesh group cage) or (2) an enriched environment (a wire-mesh group cage that contained tunnels, ramps, visual displays, and other objects designed to stimulate interest) When the maze-dull rats reached maturity, they made significantly more errors than the maze-bright rats only if they had been reared in the impoverished environment. Phenylketonuria: A Single-Gene Metabolic Disorder -PKU was discovered in 1934 when a Norwegian dentist, Asbjörn Fölling, noticed a peculiar odor in the urine of his two intellectually disabled children. It has two major components: the -He correctly assumed that the odor was related to descending motor pathway and the anterior their disorder, and he had their urine analyzed. forebrain pathway. The descending motor High levels of phenylpyruvic acid were found in pathway descends from the high vocal center on both samples. each side of the brain to the syrinx (voice box) on the same side; it mediates song production. The symptoms of PKU include. anterior forebrain pathway mediates song -As a result, the former assumption that learning. monozygotic twins are genetically identical was disproven, and the common practice of referring to - Monozygotic twins, who developed from monozygotic twins as identical twins should be the same zygote and thus are genetically curtailed. similar. Buccal cells (cells of the lining of the mouth) - Dizygotic twins, who developed from two zygotes and thus are no more similar than any pair of siblings (brothers and sisters). TWINS STUDIES OF THE EFFECTS OF EXPERIENCE ON HERITABILITY Heritability Estimates: Minnesota Study of -heritability estimates depend on the particular Twins Reared Apart conditions and subjects of a particular study. The Minnesota Study of Twins Reared Apart - Turkheimer and colleagues assessed heritability involved 59 pairs of monozygotic twins and 47 of intelligence in two samples of 7-year-old twins: pairs of dizygotic twins who had been reared apart those from families of low socioeconomic status as well as many pairs of monozygotic and dizygotic (SES) and those from families of middle to high twins who had been reared together. SES. The heritability estimates for intelligence in the middle- to high-SES twins were, as expected, -ages ranged from 19 to 68. about 70 percent. -The results of the Minnesota Study of Twins -However, the heritability estimate for intelligence in Reared Apart proved to be remarkably consistent— the twins from low-SES families was only 10 both internally, between the various cognitive and percent. personality dimensions that were studied, and externally, with the findings of other studies. -One major implication of this finding is that it forces one to think of intelligence as developing from the - Heritability estimate is not about individual interaction of genes and experience, not from one development; it is a numerical estimate of the or the other. It seems that one can inherit the proportion of variability that occurred in a particular potential to be of superior intelligence, but this trait in a particular study as a result of the genetic potential is rarely realized in a poverty-stricken variation in that study. environment. - Heritability estimates tell us about the contribution Chapter 3: Anatomy of the Nervous System of genetic differences to phenotypic differences among the participants in a study; they have Divisions of the Nervous System nothing to say about the relative contributions of genes and experience to the development of Central nervous system (CNS) is the division of individuals. the nervous system located within the skull and spine, and the peripheral nervous system (PNS) is the division located outside the skull and spine. TWIN STUDIES OF EPIGENETIC EFFECTS CNS is composed of two divisions: Brain is the part of the CNS located in the skull; the spinal cord is Most studies of epigenetic effects have focused on the part located in the spine. nonhuman species. Peripheral nervous system (PNS) - is the division -Twin studies may provide a route to the answers. located outside the skull and spine. Also composed -The study of epigenetic effects in humans is of two divisions: difficult because experimental manipulation of - The somatic nervous system (SNS) is the human genetic material is not ethical. part of the PNS that interacts with the -The first systematic demonstration of epigenetic external environment. It is composed of differences in human twins was published by Fraga afferent nerves that carry sensory signals and colleagues. from the skin, skeletal muscles, joints, eyes, ears, and so on, to the central nervous -They took tissue samples (blood, skin, muscle) system and efferent nerves that carry from 40 pairs of monozygotic twins, ranging in age motor signals from the central nervous from 3 to 74. Then, they screened the tissues for system to the skeletal muscles. DNA methylation and histone modifications. They found that the twins were epigenetically - The autonomic nervous system (ANS) is indistinguishable early in life, but differences part of the peripheral nervous system that accumulated as they aged, each type of tissue regulates the body’s internal environment. displaying a different epigenetic profile. The autonomic nervous system has two kinds of efferent nerves:  1. The sympathetic nerves are autonomic Choroid plexuses- networks of capillaries, or small motor nerves that project from the CNS in blood vessels that protrude into the ventricles from the lumbar (small of the back) and thoracic the pia mater. (chest area) regions of the spinal cord. Hydrocephalus- is treated by draining the excess 2. Parasympathetic nerves are those fluid from the ventricles and trying to remove the autonomic motor nerves that project from obstruction. the brain and sacral (lower back) region of the spinal cord. Blood–Brain Barrier - This barrier is a consequence of the special structure of cerebral blood vessels. In the rest of the body, the cells that compose the walls of blood vessels are loosely packed. - The blood–brain barrier does not impede the passage of all large molecules. Some large molecules that are critical for normal brain function (e.g., glucose) are actively transported through cerebral blood vessel walls. Cells of the Nervous System - Most of the cells of the nervous system are of two fundamentally different types: neurons and glial cells. Their anatomy Is discussed in the following two sections. Meninges Anatomy of Neurons The brain and spinal cord (the CNS) are the most 1. External Anatomy or Neurons protected organs in the body. They are encased in bone and covered by three protective membranes, -is an illustration of the major external features of the three meninges one type of neuron. - The outer meninx (which, believe it or not, 2. Internal Anatomy of Neurons is the singular of meninges) is a tough membrane called the dura mater (tough -is an illustration of the major internal features of mother). one type of neuron. - Immediately inside the dura mater is the fine 3. Neuron Cell Membrane arachnoid membrane (spider-web-like - composed of a lipid bilayer, or two layers of fat membrane). molecules. - Beneath the arachnoid membrane is a - Some membrane proteins are channel proteins, space called the subarachnoid space, through which certain molecules can pass; others which contains many large blood vessels are signal proteins, which transfer a signal to the and cerebrospinal fluid; then comes the Inside of the neuron when particular molecules innermost meninx, the delicate pia mater bind to them on the outside of the membrane. (pious mother), which adheres to the surface of the CNS. Cell membrane. The semipermeable membrane Ventricles and Cerebrospinal Fluid that encloses the neuron. - Also protecting the CNS is the Dendrites. The short processes emanating from cerebrospinal fluid (CSF), which fills the the cell body, which receive most of the synaptic subarachnoid space, the central canal of the contacts from other neurons. spinal cord, and the cerebral ventricles of the brain. Axon hillock. The cone-shaped region at the junction between the axon and the cell body. - Central canal is a small central channel that runs the length of the spinal cord. Axon. The long, narrow process that projects from the cell body. - Cerebral ventricles - are the four large internal chambers of the brain: the two Cell body. The metabolic center of the neuron; lateral ventricles, the third ventricle, and the also called the soma. fourth ventricle. Myelin. The fatty insulation is around many axons. Nodes of Ranvier. The gaps between sections of These extensions are rich in myelin, a fatty myelin. insulating substance, and the myelin sheaths they form increase the speed and efficiency of axonal Buttons. The buttonlike endings of the axon conduction. branches, which release chemicals into synapses. Schwann cells -second class of glia. Synapses. The gaps between adjacent neurons across which chemical signals are transmitted. -only Schwann cells can guide axonal regeneration (regrowth) after damage. Microglia -Make up a third class of glia. Microglia are smaller than other glial cells—thus their name. -They respond to injury or disease by multiplying, engulfing cellular debris or even entire cells. Astrocytes- Constitute a fourth class of glia. They are the largest glial cells, and they are so named because they are star-shaped (astron means “star”). -Astrocytes have an affinity for blood vessels, and they also make contact with neurons Neuroanatomical Techniques GOLGI STAIN – (Camillo Golgi) Golgi was trying to stain the meninges, by exposing a block of neural tissue to potassium dichromate and silver CLASSES of NEURONS nitrate, when he noticed an amazing thing. Golgi *multipolar neuron; most neurons are multipolar stain permits an excellent view of the silhouettes of the few neurons that take up the stain, it provides * unipolar neuron- neuron with one process no indication of the number of neurons in an area extending from its cell body is classified NISSL STAIN- The first neural staining procedure *bipolar neuron- neuron with two processes to overcome this shortcoming. The most common extending from its cell body dye used in the Nissl method is cresyl violet. *interneurons – Neurons with a short axon or no ELECTRON MICROSCOPY – A neuroanatomical axon at all. technique that provides information about the details of neuronal structure. Their function is to integrate neural activity within a single brain structure, not to conduct signals from NEUROANATOMICAL TRACING TECHNIQUES one structure to another. -two types: anterograde (forward) tracing methods and retrograde (backward) tracing NEURONS AND NEUROANATOMICAL methods. STRUCTURE -Anterograde tracing methods are used when an In the central nervous system, clusters of cell investigator wants to trace the paths of axons bodies are called nuclei (singular nucleus); in the projecting away from cell bodies located in a peripheral nervous system, they are called ganglia particular area. (singular ganglion). -Retrograde tracing methods work in reverse; they are used when an investigator wants to trace Glia: The Forgotten Cells the paths of axons projecting into a particular area. -It has been reported that there are 10 times as many glia as neurons, but this is incorrect. In the Directions in the Vertebrate Nervous System human brain, there are roughly equal numbers of neurons and glia. The vertebrate nervous system has three axes: anterior–posterior, dorsal–ventral, and medial– lateral. Oligodendrocytes Anterior - means toward the nose end (the -are glial cells with extensions that wrap around anterior end), posterior means toward the tail end the axons of some neurons of the central nervous (the posterior end); these same directions are system. sometimes referred to as rostral and caudal, respectively.  Dorsal - means toward the surface of the back or Mesencephalon – The two divisions of the the top of the head (the dorsal surface), and ventral mesencephalon are the tectum and the tegmentum. means toward the surface of the chest or the bottom of the head (the ventral surface). *tectum (roof) - is the dorsal surface of the midbrain. In mammals, the tectum is composed of Medial - means toward the midline of the body, two pairs of bumps, the colliculi (little hills). The and lateral means away from the midline toward posterior pair, called the inferior colliculi, have an the body’s lateral surfaces. auditory function. The anterior pair, called the superior colliculi, have a visual-motor function, specifically to direct the body’s orientation toward or Anatomy of the Central Nervous System away from particular visual stimuli. SPINAL CORD – comprises two different areas, *tegmentum - is the division of the mesencephalon an inner H-shaped core of gray matter and a ventral to the tectum. surrounding area of white matter. *Gray matter is composed largely of cell bodies *The periaqueductal gray is the gray matter and unmyelinated interneurons, situated around the cerebral aqueduct, the duct connecting the third and fourth ventricles; *white matter is composed largely of myelinated *The substantia nigra (black substance) axons. (It is the myelin that gives the white matter its glossy white sheen.) *red nucleus The two dorsal arms of the spinal gray matter are Diencephalon called the dorsal horns, and the two ventral arms are called the ventral horns. *thalamus - is the large, two-lobed structure that constitutes the top of the brain stem. Sensory relay Pairs of spinal nerves are attached to the spinal nuclei—nuclei that receive signals from sensory cord— one on the left and one on the right—at 31 receptors, process them, and then transmit them to different levels of the spine. Each of these 62 the appropriate areas of sensory cortex. spinal nerves divides as it nears the cord, and its axons are joined to the cord via one of two roots: *hypothalamus is located just below the anterior the dorsal root or the ventral root thalamus (hypo means “below”). It plays an important role in the regulation of several motivated Dorsal root ganglia – All dorsal root axons, behaviors (e.g., eating, sleep, and sexual behavior). whether somatic or autonomic, are sensory (afferent) unipolar neurons with their cell bodies * pituitary gland – The literal meaning of pituitary grouped together just outside the cord to form. gland is “snot gland”; it was discovered in a gelatinous state behind the nose of an unembalmed cadaver and was incorrectly FIVE MAJOR DIVISION OF THE BRAIN assumed to be the main source of nasal mucus. Dangles from it on the ventral surface of the brain Myelencephalon – the most posterior division of the brain, is composed largely of tracts carrying *Optic chiasm is the point at which the optic signals between the rest of the brain and the body. nerves from each eye come together. -reticular formation – interesting part of the myelencephalon from a psychological perspective. -reticular formation is referred to as the reticular Telencephalon – the largest division of the human activating system because parts of it seem to play a brain, mediates the brain’s most complex functions. role in arousal. It initiates voluntary movement, interprets sensory input, and mediates complex cognitive processes Metencephalon – like the myelencephalon, such as learning, speaking, and problem solving. houses many ascending and descending tracts and part of the reticular formation. These structures *CEREBRAL CORTEX – cerebral hemispheres are create a bulge, called the pons, on the brain stem’s covered by a layer of tissue. Mainly composed of ventral surface. small, unmyelinated neurons, it is gray and is often referred to as the gray matter. *The pons is one major division of the metencephalon; the other is the cerebellum. *convolutions have the effect of increasing the amount of cerebral cortex without increasing the Cerebellum - is the large, convoluted structure on overall volume of the brain. the brain stem’s dorsal surface. It is an important sensorimotor structure; cerebellar damage *The large furrows in a convoluted cortex are called eliminates the ability to precisely control one’s fissures. movements and to adapt them to changing conditions. *and the small ones are called sulci (singular callosum; it encircles the dorsal thalamus sulcus). (cingulate means “encircling”). *The ridges between fissures and sulci are called *FORNIX – the major tract of the limbic system, gyri (singular gyrus). also encircles the dorsal thalamus; it leaves the dorsal end of the hippocampus and sweeps *longitudinal fissure – the largest fissures, cerebral forward in an arc coursing along the superior hemispheres are almost completely separated. surface of the third ventricle and terminating in the *these hemisphere-connecting tracts are called septum and the mammillary bodies (fornix means cerebral commissures. The largest cerebral “arc”). commissure, the corpus callosum. *SEPTUM- midline nucleus located at the anterior *These fissures partially divide each hemisphere tip of the cingulate cortex. into four lobes: the frontal lobe, the parietal lobe Basal ganglia the temporal lobe, and the occipital lobe. About 90 percent of human cerebral cortex is *caudate- in a posterior direction and then in an neocortex (new cortex); that is, it is six-layered anterior direction, it’s the tail-like. Each caudate cortex of relatively recent evolution. forms an almost complete circle; in its center, connected to it by a series of fiber bridges, is the Pyramidal cells are large multipolar neurons with putamen. pyramid-shaped cell bodies, a large dendrite called an apical dendrite that extends from the apex of the *STRIATUM - caudate and the putamen, which pyramid straight toward the cortex surface, and a both have a striped appearance. very long axon. * GLOBUS PALLIDUS - The remaining structure of Stellate cells are small starshaped interneurons. the basal ganglia is the pale circular structure. (FIGURE 3.25) Chapter 4 Neural Conduction and Synaptic Transmission Resting Membrane Potential -membrane potential is the difference in electrical charge between the inside and the outside of a cell. -To record a neuron’s membrane potential, it is necessary to position the tip of one electrode inside the neuron and the tip of another electrode outside the neuron in the extracellular fluid. -The intracellular electrodes are called microelectrodes; their tips are less than onethousandth of a millimeter in diameter—much too small to be seen by the naked eye. neuron’s resting potential - steady membrane Limbic System and the Basal Ganglia potential of about −70 mV. Limbic system is a circuit of midline structures that ions - the salts in neural tissue separate into circle the thalamus (limbic means “ring”). Major positively and negatively charged particles. structures of the limbic system include the amygdala, the fornix, the cingulate cortex, and the -There are many different kinds of ions in neurons, septum. but this discussion focuses on only two of them: sodium ions and potassium ions. The *AMYGDALA – the almond-shaped nucleus in the abbreviations for sodium ions (Na+) and potassium anterior temporal lobe (amygdala means “almond” ions (K+) are derived from their Latin names: natrium (Na) and kalium (K). The plus signs *CINGULATE CORTEX – the large strip of cortex indicate that each Na+ and K+ ion carries a single in the cingulate gyrus on the medial surface of the positive charge. cerebral hemispheres, just superior to the corpus  Both EPSPs and IPSPs are graded responses. ion channels - unequal distributions of Na+ and K+ ions are maintained even though there are specialized pores. Weak signals elicit small postsynaptic potentials, and strong signals elicit large ones. There is substantial pressure on Na+ ions to enter the resting neurons. This pressure is of The transmission of postsynaptic potentials two types. has two important characteristics. 1. electrostatic pressure - from the resting 1. it is rapid—so rapid that it can be assumed to be membrane potential: Because opposite charges instantaneous for most purposes. It is important attract, the -70 mV charge attracts the positively not to confuse the duration of EPSPs and IPSPs charged Na+ ions into resting neurons. with their rate of transmission; although the duration of EPSPs and IPSPs varies considerably, 2. pressure from random motion for Na+ ions to all postsynaptic potentials, whether brief or move down their concentration gradient. enduring, are transmitted at great speed. -Transporters are commonly referred to as 2. the transmission of EPSPs and IPSPs is sodium–potassium pumps. decremental: EPSPs and IPSPs decrease in amplitude as they travel through the neuron, just -Since the discovery of sodium−potassium pumps, as a sound wave loses amplitude (the sound several other classes of transporters (mechanisms grows fainter) as it travels through air. Most EPSPs in the membrane of a cell that actively transport and IPSPs do not travel more than a couple of ions or molecules across the membrane) have millimeters from their site of generation before they been discovered. fade out; thus, few travel very far along an axon. Integration of Postsynaptic Potentials and Generation of Action Potentials The receptive areas of most neurons are covered with thousands of synapses, and whether a neuron fires is determined by the net effect of their activity. More specifically, whether a neuron fires depends on the balance between the excitatory and inhibitory signals reaching its axon. axon hillock - the conical structure at the junction between the cell body and the axon. The action potential generated in axon hilock. Generation and Conduction of Postsynaptic but they are actually generated in the adjacent Potentials section of the axon, called the axon initial segment. When neurons fire, they release from their terminal buttons chemicals called neurotransmitters, which If the sum of the depolarizations and diffuse across the synaptic clefts and interact with hyperpolarizations reaching the axon initial specialized receptor molecules on the receptive segment at any time is sufficient to depolarize the membranes of the next neurons in the circuit. membrane to a level referred to as its threshold of excitation. -They may depolarize the receptive membrane (decrease the resting membrane potential, from The action potential (AP) is a massive but −70 to −67 mV, for example) momentary—lasting for 1 millisecond—reversal of the membrane potential from about −70 to about or +50 mV. -they may hyperpolarize it (increase the resting membrane potential, from −70 to −72 mV, for example). Excitatory postsynaptic potentials (EPSPs) - Postsynaptic depolarizations. Because they increase the likelihood that the neuron will fire. Inhibitory postsynaptic potentials (IPSPs) - Postsynaptic hyperpolarizations. Because they decrease the likelihood that the neuron will fire.  It shows the three possible combinations of Refractory Periods spatial summation. Absolute refractory period - There is a brief period of about 1 to 2 milliseconds after the initiation of an action potential during which it is impossible to elicit a second one. Relative refractory period—the period during which it is possible to fire the neuron again but only by applying higher-than- normal levels of stimulation. Refractory period is responsible for two important characteristics of neural activity. 1. It is responsible for the fact that action potentials normally travel along axons in only one direction. Because the portions of an axon over which an action potential has just traveled are left momentarily refractory, an action potential cannot reverse direction. It illustrates temporal summation. It shows how postsynaptic potentials produced in 2. The refractory period is responsible for the fact rapid succession at the same synapse sum to form that the rate of neural firing is related to the a greater signal. intensity of the stimulation. If a neuron is subjected to a high level of continual stimulation, it fires and Ionic Basis of Action Potentials then fires again as soon as its absolute refractory period is over—a maximum of about 1,000 times The resting membrane is relatively impermeable to per second. Na+ ions and because those few that do pass in are pumped out. But things suddenly change when Axonal Conduction of Action Potentials the membrane potential of the axon is depolarized to the threshold of excitation by an EPSP. The conduction of action potentials along an axon differs from the conduction of EPSPs and IPSPs in The rapid change in the membrane potential two important ways. associated with the influx of Na+ ions then triggers the opening of voltage-activated potassium 1. The conduction of action potentials along an channels. At this point, K+ ions near the membrane axon is nondecremental; action potentials do not are driven out of the cell through these channels— grow weaker as they travel along the axonal first by their relatively high internal concentration membrane. and then, when the action potential is near its peak, 2. action potentials are conducted more slowly than by the positive internal charge. After about 1 postsynaptic potentials. millisecond, the sodium channels close. -If electrical stimulation of sufficient intensity is This marks the end of the rising phase of the applied to the terminal end of an axon, an action action potential and the beginning of repolarization potential will be generated and will travel along the by the continued efflux of K+ ions. axon back to the cell body; this is called antidromic conduction. -Axonal conduction in the natural direction— from cell body to terminal buttons—is called orthodromic conduction. It illustrates the timing of the opening and closing of the sodium and potassium channels during an action potential. The sodium–potassium pumps play only a minor role in the reestablishment of the resting potential. CONDUCTION IN MYELINATED AXONS The following are some properties of cerebral neurons that are not shared by motor neurons: Nodes of Ranvier—the gaps between adjacent myelin segments. Many cerebral neurons fire continually even when they receive no input. -When an action potential is generated in a myelinated axon, the signal is conducted Axons of some cerebral neurons can actively passively—that is, instantly and decrementally— conduct both graded signals and action potentials. along the first segment of myelin to the next node of Ranvier. Action potentials of different classes of cerebral neurons vary greatly in duration, amplitude, and -Myelination increases the speed of axonal frequency. conduction. Because conduction along the myelinated segments of the axon is passive, it Many cerebral neurons do not display action occurs instantly, and the signal thus “jumps” along potentials. the axon from node to node. The dendrites of some cerebral neurons can Saltatory conduction - The transmission of action actively conduct action potentials. potentials in myelinated axons. (saltare means “to skip or jump”) Synaptic Transmission: Chemical THE VELOCITY OF AXONAL CONDUCTION Transmission of Signals among Neurons -Conduction is faster in large-diameter axons, 1. Structure of Synapses and— as you have just learned—it is faster in those that are myelinated. -Mammalian motor neurons (neurons that synapse on skeletal muscles) are large and myelinated; thus, some can conduct at speeds of 100 meters per second (about 224 miles per hour). -small, unmyelinated axons conduct action potentials at about 1 meter per second. - The maximum velocity of motor neuron action potentials was found to be about 100 meters per second in cats and was then assumed to be the same in humans. It is not. The maximum velocity of conduction in human motor neurons is about 60 meters per second. CONDUCTION IN NEURONS WITH AXONS axodendritic synapses—synapses of axon -Action po tentials are the means by which axons terminal buttons on dendrites. conduct all-or-none signals nondecrementally over dendritic spines (nodules of various shapes that relatively long distances. are located on the surfaces of many dendrites) -many neurons in mammalian brains either do not axosomatic synapses— synapses of axon have axons or have very short ones, and many of terminal buttons on somas (cell bodies). these neurons do not normally display action potentials. Conduction in these interneurons is -axoaxonic synapses are particularly important typically passive and decremental. because they can mediate presynaptic facilitation and inhibition. The Hodgkin-Huxley Model in Perspective -The preceding account of neural conduction is based heavily on the Hodgkin-Huxley model, the theory first proposed by Hodgkin and Huxley in the early 1950s -The Hodgkin-Huxley model was a major advance in our understanding of neural conduction. - The Hodgkin-Huxley model was based on the study of squid motor neurons. Motor neurons are simple, large, and readily accessible in the PNS— squid motor neurons are particularly large. The synapses depicted in Figures 4.7 and 4.8 are directed synapses—synapses at which the site of neurotransmitter release and the site of When a neuron is at rest, synaptic vesicles that neurotransmitter reception are in close proximity. contain small-molecule neurotransmitters tend to congregate near sections of the presynaptic Nondirected synapses - are synapses at which membrane that are particularly rich in voltage the site of release is at some distance from the site activated calcium channels of reception. One type of nondirected synapse is depicted in Figure 4.9. -The exocytosis of small-molecule neurotransmitters differs from the exocytosis of neuropeptides. Small-molecule neurotransmitters are typically released in a pulse each time an action potential triggers a momentary influx of Ca2+ ions through the presynaptic membrane; in contrast, neuropeptides are typically released gradually in response to general increases in the level of intracellular Ca2+ ions, such as might occur during a general increase in the rate of neuron firing. -Any molecule that binds to another is referred to as its ligand. The different types of receptors to which a particular neurotransmitter can bind are called the Synthesis, Packaging, and Transport of receptor subtypes for that neurotransmitter. Neurotransmitter Molecules -One advantage of receptor subtypes is that they There are two basic categories of neurotransmitter enable one neurotransmitter to transmit different molecules: small and large. kinds of messages to different parts of the brain. -The small neurotransmitters are of several types Ionotropic receptors are associated with ligand- -large neurotransmitters are all neuropeptides. activated ion channels; Neuropeptides - are short amino acid chains metabotropic receptors are associated with composed of between 3 and 36 amino acids; in signal proteins and G proteins effect, they are short proteins. (guanosinetriphosphate−sensitive proteins); -Small-molecule neurotransmitters are typically synthesized in the cytoplasm of the terminal button and packaged in synaptic vesicles by the button’s Golgi complex. -It was once believed that each neuron synthesizes and releases only one neurotransmitter, but it has been clear for some time that many neurons contain two neurotransmitters—a situation generally referred to as coexistence. Release of Neurotransmitter Molecules -The subunit may move along the inside surface of Exocytosis—the process of neurotransmitter the membrane and bind to a nearby ion channel, release. thereby inducing an EPSP or IPSP; or it may trigger the synthesis of a chemical called a second messenger (neurotransmitters are considered to be the first messengers) Autoreceptors - are metabotropic receptors that have two unconventional characteristics: They bind to their neuron’s own neurotransmitter molecules, and they are located on the presynaptic, rather than the postsynaptic, membrane. Their usual function is to monitor the number of neurotransmitter molecules in the synapse, to reduce subsequent release when the levels are high, and to increase subsequent release when they are low. Reuptake, Enzymatic Degradation, and -The first three are common in the proteins we Recycling consume, whereas GABA is synthesized by a simple modification of the structure of glutamate. Two mechanisms terminate synaptic messages and keep that from happening. These two -Glutamate is the most prevalent excitatory message-terminating mechanisms are reuptake by neurotransmitter in the mammalian central nervous transporters and enzymatic degradation system. GABA is the most prevalent inhibitory neurotransmitter. MONOAMINE NEUROTRANSMITTERS -Monoamines are another class of small-molecule neurotransmitters. monoamine (one amine) -Monoamine neurotransmitters are slightly larger than amino acid neurotransmitters, and their effects Reuptake is the more common of the two tend to be more diffuse. deactivating mechanisms. -present in small groups of neurons whose cell Other neurotransmitters are degraded (broken bodies are, for the most part, located in the brain apart) in the synapse by the action of enzymes— stem. proteins that stimulate or inhibit biochemical reactions without being affected by them. There are four monoamine neurotransmitters: dopamine, epinephrine, norepinephrine, and Glia, Gap Junctions, and Synaptic serotonin. Transmission They are subdivided into two groups, Gap junctions are narrow spaces between catecholamines and indolamines, on the basis of adjacent cells that are bridged by fine, tubular, their structures. cytoplasm-filled protein channels, called connexins. -gap junctions connect the cytoplasm of two adjacent cells, allowing electrical signals and small -Dopamine, norepinephrine, and epinephrine are molecules (e.g., second messengers) to pass from catecholamines. one cell to the next. -Each is synthesized from the amino acid tyrosine. -Gap junctions are sometimes called electrical Tyrosine is converted to l-dopa, which in turn is synapses. converted to dopamine. -Neurons that release norepinephrine are called noradrenergic; -those that release epinephrine are called adrenergic. -Cerebral gap junctions occur between all classes Two reasons for this naming. One is that of cerebral cells; however, the majority of them epinephrine and norepinephrine used to be called seem to occur between cells of like kind. adrenaline and noradrenaline, respectively, by many scientists, until a drug company registered Neurotransmitters Adrenalin as a brand name. The other reason will The Roles and Functions of Neurotransmitters become apparent to you if you try to say norepinephrinergic. *Amino Acid Neurotransmitters Acetylcholine (abbreviated Ach) is a small- -The four most widely studied amino acid molecule neurotransmitter that is in one major neurotransmitters are glutamate, aspartate, respect like a professor who is late for a lecture: glycine, and gamma-aminobutyric acid (GABA). UNCONVENTIONAL NEUROTRANSMITTERS How Drugs Influence Synaptic Transmission One class of unconventional neurotransmitters, the (1) synthesis of the neurotransmitter, (2) soluble-gas neurotransmitters, includes nitric storage in vesicles, oxide and carbon monoxide.