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Chapter 4

Chapter 4- The Brain- Source of Mind and Self.docx

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Lawrence Murphy

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The Brain: Source of Mind and Self The Nervous System: A Basic Blueprint  Function of nervous system- to gather and process information, produce responses to stimuli, and coordinate the workings of different cells  Nervous system has billions of cells, divided into two main parts: the central nervous system and the peripheral (outlying) nervous system The Central Nervous System  Central Nervous System (CNS): the portion of the nervous system consisting of the brain and the spinal cord  Receives, processes, interprets, and stores incoming sensory information- information about tastes, sounds, smells, colour, pressure on the skin, the state of internal organs, and so on  Sends out messages to muscles, glands, and internal organs  Spinal cord: a collection of neurons and supportive tissue running from the base of the brain down the centre of the back, protected by a column of bones (spinal column) o Extension of brain o Acts as bridge between brain and parts of body below neck  Spinal cord produces some behaviours on its own without any help from brain o Spinal reflexes o Touching a hot iron, pull hand away before brain has had chance to register o Nerve impulses bring message to spinal cord (hot!), and the spinal cord immediately sends out command via other nerves, telling muscles in arm to contract and pull hand away from iron o Reflexes above neck (sneezing & blinking) involve lower part of brain, not spinal cord  Neural circuits underlying many spinal reflexes linked to neural pathways that run up and won spinal cord, to and from brain o Because of connections, reflexes can be influenced by thoughts and emotions  Erection (inhibited by anxiety or distracting thoughts, initiated by erotic thoughts) o Some reflexes can be brought under conscious control (may be able to keep knee from jerking when tapped)  Men can learn to voluntarily delay ejaculation (spinal reflex) The Peripheral Nervous System  Peripheral Nervous System (PNS): all portions of the nervous system outside the brain and spinal cord; it includes sensory and motor nerves o Handles central nervous system’s input and output  Like receiver to radio  Sensory nerves carry messages from special receptors in skin, muscles, and other internal and external sense organs to spinal cord which sends them to brain  Nerves put us in touch with outside world and activities of own bodies  Motor nerves carry orders from central nervous system to muscles, glands, and internal organs o Enable us to move, cause glands to contract and secrete substances including hormones  Peripheral nervous system divided into somatic (bodily) nervous system and autonomic (self- governing) nervous system  Somatic Nervous System: the subdivision of the peripheral nervous system that connects to sensory receptors and to skeletal muscles; sometimes called the skeletal nervous system o Consists of nerves that are connected to sensory receptors (cells that let you sense the world) and also to skeletal muscles that permit voluntary action o Somatic system active when feeling bug on arm or turning light off or writing name  Autonomic Nervous System: the subdivision of the peripheral nervous system that regulates the internal organs and glands o Regulates functioning of blood vessels, glands and internal (visceral) organs- bladder, stomach, heart o Autonomic system active when you see someone you have a crush on and your heart pounds, hands get sweaty, and cheeks feel hot o Divided into two parts  Sympathetic Nervous System: the subdivision of the autonomic nervous system that mobilizes bodily resources and increases the output of energy during emotion and stress o Acts like accelerator of car, mobilizing body for action and output of energy o Makes you blush, sweat, breathe more deeply, pushes up heart rate and blood pressure o Whirls into action when in situation that requires flight, flee, or cope  Parasympathetic Nervous System: the subdivision of the autonomic nervous system that operates during relaxed states and that conserves energy o More like brake, doesn’t stop body but slows things down and keeps them running smoothly  Two parts work together, but in opposing ways, to adjust body to changing circumstances o If have to jump out of way of speeding motorcyclist, sympathetic nerves increase heart rate, after, parasympathetic slow down again and keep rhythm regular  Neuropsychologists and other scientists study the brain because it is the bedrock of consciousness, perception, memory, and emotion  The function of the nervous system is to gather and process information, produce responses to stimuli, and coordinate the workings of different cells. Scientists divide it into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS, which includes the brain and spinal cord, receives processes, interprets, and stores information and sends out messages destined for muscles, glands, and organs. The PNS transmit information to and from the CNS by way of sensory and motor nerves  The peripheral nervous system consists of the somatic nervous system, which permits sensation and voluntary actions, and the autonomic nervous system, which regulates blood vessels, glands, and internal (visceral) organs. The autonomic system usually functions without conscious control. The autonomic nervous system is further divided into the sympathetic nervous system, which mobilizes the body for action, and the parasympathetic nervous system, which conserves energy Communication in the Nervous System  Nervous system made of neurons: cells that conduct electrochemical signals; the basic unit of the nervous system; also called a nerve cell o Brain’s communication specialists, transmit information to, from, and within CNS  Neurons held in place by glia: cells that support, nurture, and insulate neurons, remove debris when neurons die, enhance the formation and maintenance of neural connections, and modify neuronal functioning ; also called glial cells (from Greek word for “glue”) o Make up 90% of brain’s cells o Communicate chemically with each other and other neurons; without glial cells, neurons could not function properly o Help determine which neural connections get stronger or weaker, suggests that they play a vital role in learning and memory  Neurons building blocks of nervous system though in structure, more like snowflakes than blocks, exquisitely delicate and differing from one another greatly in size and shape o In a giraffe, neuron that runs from spinal cord to hind leg may be 3 metres long o In human brain, microscopic ~100 billion The Structure of the Neuron  Three main parts: dendrites, a cell body, and an axon  Dendrites: a neuron’s branches that receive information from other neurons and transmit it toward the cell body; means “little tree” in Greek o Act like antennas, receiving messages from as many as 10 000 other nerve cells and transmitting these messages toward the cell bod o Also do preliminary processing of messages  Cell Body: the part of the neuron that keeps it alive and determines whether or not it will fire o Shaped like sphere or pyramid, contains biochemical machinery for keeping neuron alive o Plays key role in determining whether the neuron should “fire”- transmit a message to other neurons- depending on inputs from other neurons  Axon: a neuron’s extending fibre that conducts impulses away from the cell body and transmits them to other neurons; means “axle” in Greek o Commonly divide at the end into branches called axon terminals o Dendrites and axons give each neuron a double role: a neuron is first a catcher, then a batter  Myelin Sheath: a fatty insulation that may surround the axon of a neuron o Made up of glial cells o Constrictions in this covering, called nodes, divide it into segments, with make it look like a string of sausages o One purpose is to prevent signals in adjacent cells from interfering with each other o Another is to speed up conduction of neural impulses  Nerve: a bundle of nerve fibres (axons and sometimes dendrites) in the peripheral nervous system o Human body has 43 pairs of peripheral nerves; one nerve from each pair is on the left side of the body and the other is on the right o Most of these nerves enter or leave the spinal cord, but 12 pairs in the head, the cranial nerves, connect directly to the brain Neurons in the News  Severed axons in spinal cord can regrow if treated with certain nervous system chemicals  Neurogenesis: the production of new neurons from immature stem cells  Stem Cells: immature cells that renew themselves and have the potential to develop into mature cells; given encouraging environments, stem cells from early embryos can develop into any cell type o Stem cells involved in learning and memory seem to divide and mature throughout adulthood o Physical exercise, effortful mental activity, and an enriched environment promote the production and survival of new cells, whereas again and stress can inhibit their production and nicotine can kill them  Stem-cell research one of hottest areas in biology and neuroscience because scientists prefer working with cells from aborted fetuses and from embryos that are a few days old (just a few cells) o Fertility clinics store many such embryos because several “test tube” fertilizations are created for every patient who hopes to become pregnant; eventually, the extra embryos are destroyed o In Canada, embryonic stem cells (ES) can be obtained only from sources approved by the Assisted Human Reproduction Agency of Canada (regulatory body for stem-cell research) o ES cells are especially useful as they can differentiate into any type of cell, from neurons to kidney cells, whereas those from adults are more limited and harder to keep alive o Recently, scientists have successfully reprogrammed cells from adult organs, most notably skin cells, to become stem cells o Like ES cells, these “induced pluripotent stem (iPS) cells” seem capable of giving rise to all types of cells although it is still unclear whether they will prove equally versatile  Noted success in animals o Mice with spinal cord injuries regained much mobility after being injected with stem cells derived from human fetal brain tissue o Microscopic analysis showed that most of the cells turned into either neurons or a particular type of glial cell o When the rat brain is treated with certain chemicals and exercise, it can regenerate new neurons and gain improvements in memory How Neurons Communicate  Don’t directly touch each other, separated by minuscule space called synaptic cleft, where one axon terminal of our neuron nearly touches a dendrite or the cell body of another  Synapse: the site where transmission of a nerve impulse from one nerve cell to another occurs; it includes the axon terminal, the synaptic cleft, and receptor sites in the membrane of the receiving cell  Neuron may have synaptic connections with a great many others because a neuron’s axon may have hundreds or even thousands of terminals o Number of synapses in nervous system runs into trillions or even quadrillions  Speak to each other or muscles and glands in an electrical and chemical language  When nerve cell is stimulated, a change in electrical potential occurs between the inside and outside of cell o Sudden, momentary inflow of positively charged sodium ions across cell’s membrane, followed by outflow of positively charged potassium ions o Result is action potential: a brief change in electrical voltage that occurs between the inside and the outside of an axon when a neuron is stimulated; it serves to produce an electrical impulse  If axon is unmyelinated, action potential as each point in axon gives rise to new action potential at next point; thus, impulse travels down axon somewhat as fire travels along fuse of firecracker  In myelinated axons, process different o Because of myelin, sodium and potassium ions cannot cross cell’s membrane except at brakes (nodes) between the myelin’s “sausages” o Action potential “hops” from one node to the next  More precisely, positively charged ions flow down axon at a fast rate, causing regeneration of the action potential at each node o Arrangement allows impulse to travel faster than it could if the action potential had to be regenerated at every point along axon  When neural impulse reaches axon terminal’s buttonlike tip, must get its message across synaptic cleft to another cell o At this point, synaptic vesicles, tiny sacs in tip of axon terminal, open and release few thousand molecules of a chemical called a neurotransmitter: a chemical substance that is released by a transmitting neuron at the synapse and that alters the activity of a receiving neuron o Like sailors carrying a message form one island to another, these molecules then diffuse across synaptic cleft o When reach other side, neurotransmitter molecules bind briefly with receptor sites, special molecules in the membrane of the receiving neuron’s dendrites (sometimes cell body), fitting these sites like a key and a lock o Changes occur in receiving neuron’s membrane and ultimate effect is either excitatory (voltage shift in positive direction) or inhibitory (voltage shift in negative direction), depending on which receptor sites have been activated o If effect is excitatory, probability that receiving neuron will fire increases; if inhibitory, probability decreases o Inhibition in nervous system extremely important  Without, could not sleep or coordinate movements  Excitation of nervous system would be overwhelming, producing convulsions  What any neuron does at any given moment depends on the net effect of all the messages being received from other neurons  Only when cell’s voltage reaches certain threshold will it fire  Thousands of messages, both excitatory and inhibitory, may be coming into cell, and receiving neuron must arrange them  Message that reaches final destination depends on rate at which neurons are firing, how many are firing, what types are firing, where neurons are located, and degree of synchrony among different neurons o Doesn’t depend on how strongly individual neurons are firing however, because a neuron always either fires or doesn’t- like turning on light switch, the firing of a neuron is an all-or-none event The Plastic Brain  Plasticity: the brain’s ability to change and adapt in response to experience- for example, by reorganizing or growing new neural connections  Plasticity vividly demonstrated in cases of people with brain damage who have experienced remarkable recoveries; ex. Individuals who cannot recall simple words after a stroke but are speaking normally within months, or ones who cannot move an arm after a head injury but regain full use after physical therapy; brains apparently rewired themselves to adapt to damage  Experiment- blinded people for 5 days, in beginning, visual areas in brains were quiet during tasks requiring hearing or touch but by day 5, these areas were lighting up during the tasks. When blindfolds were removed, visual centres quieted down o Visual areas of brain apparently possess computational machinery necessary for processing nonvisual information, but machinery remains dormant until circumstances require its activation  Brain is dynamic organ o Circuits continually modify in response to information, challenges, and changes in environment Chemical Messengers in the Nervous System Neurotransmitters: Versatile Couriers  Neurotransmitters make it possible for one neuron to excite or inhibit another  Exist not only in brain but also in spinal cord, peripheral nerves, and certain glands  Through effects on specific nerve circuits, substances can affect mood, memory and well-being  Nature of effect depends on level of neurotransmitter, its location, and type of receptor it binds with  Types of neurotransmitters: o Serotonin affects neurons involved in sleep, appetite, sensory perception, temperature regulation, pain suppression, and mood o Dopamine affects neurons involved in voluntary movement, learning, memory, emotion, pleasure or reward, and, possible, response to novelty o Acetylcholine affects neurons involved in muscle action, cognitive functioning, memory, and emotion o Norepinephrine affects neurons involved in increased heart rate and the slowing of intestinal activity during stress, and neurons involved in learning, memory, dreaming, waking in sleep, and emotion o GABA (gamma-aminobutyric acid) is the major inhibitory neurotransmitter in the brain o Glutamate is the major excitatory neurotransmitter in the brain; it is released by about 90% of the brain’s neurons  Harmful effects can occur when neurotransmitter levels are too high or too low o Abnormal GABA levels can cause sleep, eating, and convulsive disorders (epilepsy) o People with Alzheimer’s lose brain cells responsible for producing acetylcholine and other neurotransmitters- helps account for devastating memory problems o Loss of cells that produce dopamine- responsible for tremors and rigidity of Parkinson’s o In multiple sclerosis, immune cells overproduce glutamate- damages or kills glial cells that normally make myelin  Possible that something about a disorder leads to abnormal neurotransmitter levels instead of the other way around o Ex. Although drugs that boost or decrease levels of particular neurotransmitters are sometimes effective in treating certain mental disorders- doesn’t necessarily mean that abnormal neurotransmitter levels cause the disorder  Aspirin can relieve a headache, but headaches aren’t caused by a lack of aspirin  Well-balanced diet is best brain food Endorphins: The Brain’s Natural Opiates  Endorphins: chemical substances in the nervous system that are similar in structure and action to opiates; they are involved in paid reduction, pleasure, and memory and are known technically as endogenous opioid peptides; also play role in appetite, sexual activity, blood pressure, mood, learning, and memory  Some act as neurotransmitters but most act primarily by altering effects of neurotransmitters- ex. Limiting or prolonging effects  Endorphin levels shoot up when under stress  In infancy, contact with mother stimulates flow of endorphins, strengthens infant’s bond with her  “Endorphin rush” also occurs in early stages of passionate love between adults, accounting for the feeling of euphoria that “falling” for someone creates Hormones: Long-Distance Messengers  Hormones: chemical substances, secreted by organs called glands, that affect the functioning of other organs  Produced primarily in endocrine glands: internal organs that produce hormones and release them into the bloodstream  Have dozens of jobs from promoting bodily growth to aiding digestion to regulating metabolism  Neurotransmitters and hormones are not always chemically distinct  A particular chemical like norepinephrine may belong to more than one classification depending on where it’s located and what function it’s performing  Hormones of particular interest to psychologists: o Melatonin, which is secreted by the pineal gland deep within the brain, helps to regulate daily biological rhythms and promotes sleep o Oxytocin, which is secreted by another small gland in the brain, the pituitary gland, enhances uterine contractions during childbirth and facilitates the ejection of milk during nursing. Psychologists are interested in this hormone because along with another hormone, vasopressin, it contributes to relationships in both sexes by promoting attachment and trust o Adrenal hormones, which are produced by the adrenal glands (organs that are perched right above the kidneys), are involved in emotion and stress. These hormones also rise in response to other conditions, such as heat, cold, pain, injury, burns, and physical exercise, and in response to some drugs, such as caffeine and nicotine. The outer part of each adrenal gland produces cortisol, which increases blood-sugar levels and boosts energy. The inner part produces epinephrine (adrenaline) and norepinephrine. When adrenal hormones are released in your body, activated by the sympathetic nervous system, they increase your arousal level and prepare you for action. Adrenal hormones also enhance memory. o Sex hormones, which are secreted by tissue in the gonads (testes- men, ovaries- women) and also by the adrenal glands, include three main types, all occurring in both sexes but in differing amounts and proportions in males and females after puberty. Androgens (most important of which is testosterone) are masculinizing hormones produced mainly in the testes but also in the ovaries and the adrenal glands. Androgens set in motion the physical changes males experience at puberty- ex. Deepened voice, facial and chest hair- and causes pubic and underarm hair to develop in both sexes. Testosterone also influences sexual arousal in both sexes. Estrogens are feminizing hormones that bring on physical changes in females at puberty, such as breast development and the onset of menstruation, and that influence the course of the menstrual cycle. Progesterone contributes to the growth and maintenance of the uterine lining in preparation for a fertilized egg, among other functions. Estrogens and progesterone are produced mainly in the ovaries but also in the testes and the adrenal glands  Most common belief about the nonsexual effects of sex hormones- that fluctuating levels of estrogen and progesterone make most women “emotional” before menstruation- is far from conclusively supported by research Nervous-System Chemicals and Their Effects Type Function Effects Where Produced Examples Neurotransmitters Enable neurons to Diverse, Brain, spinal cord, Serotonin, excite or inhibit depending on peripheral nerves, dopamine, each other which circuits are certain glands norepinephrine activated or suppressed Endorphins Usually modulate Reduce pain, Brain, spinal cord (several varieties, the effects of promote pleasure; not discussed) neurotransmitters also linked to learning, memory, and other functions Hormones Affect functioning Dozens, ranging Primarily in Epinephrine, of target organs from promotion of endocrine glands norepinephrine, and tissues digestion to estrogens, regulation of androgens metabolism  Neurons are the basic units of the nervous system. They are held in place by glial cells, which nourish, insulate, and protect them, and enable them to function properly. Each neuron consists of dendrites, a cell body, and an axon. In the peripheral nervous system, axons (and sometimes dendrites) are collected together in bundles called nerves. Many axons are insulated by a myelin sheath that speeds up the conduction of neural impulses and prevents signals in adjacent cells from interfering with one another  Research has disproven two old assumptions: that neurons in the human central nervous system cannot be induced to regenerate and that no new neurons form after early infancy. In the lab, neurons have been induced to regenerate. And scientists have learned that stem cells in brain areas associated with learning and memory continue to divide and mature throughout adulthood, giving rise to new neurons. A stimulating environment seems to enhance this process of neurogenesis  Communication between two neurons occurs at the synapse. Many synapses have not yet formed at birth. During development, axons and dendrites continue to grow as a result of both physical maturation and experience with the world, and throughout life, new learning results in new synaptic connections in the brai
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