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Chapter 4

Chapter 4- The Brain: Source of Mind and Self

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PSYC 1020H
Wolfgang Lehmann

Chapter 4- The Brain: Source of Mind and Self  Brain is about 1,500 grams  William Shakespeare- the brain is “the soul’s frail dwelling house”  Neuropsychologists and neuroscientists study brain and nervous system in order to understand behaviour and what’s possible for this organ The Nervous System: A Basic Blueprint Central Nervous System Central nervous system (CNS) – the portion of the nervous system consisting of the brain and spinal cord  CNS receives, processes, interprets and stores incoming sensory information o Taste, smell, internal organs etc.  Spinal cord runs from base of brain down centre of back o Protected by spinal column o Produces some behaviours on its own- spinal reflexes require no conscious effort  Ex. Jerking your hand away from a hot stove  Spinal reflexes can be influenced by thoughts and emotions 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) Peripheral Nervous System Peripheral nervous system- all portions of the nervous system outside the brain and spinal cord; it includes sensory and motor nerves  Handles central nervous system’s input and output  Sensory nerves carry messages from receptors in skin, muscles and other internal and external sensory organs to the spinal cord, which sends them along to the brain  Motor nerves carry orders from CNS to muscles, glands and internal organs o Enable us to move, cause glands to secrete hormones (chemical messengers) Somatic nervous system- the subdivision of the peripheral nervous system that connects sensory receptors and to skeletal muscles; sometimes called the skeletal nervous system Autonomic nervous system- the subdivision of the peripheral nervous system that regulates the internal organs and glands Sympathetic nervous system- the subdivision of the autonomic nervous system that mobilizes bodily resources and increases the output of energy during emotion and stress Parasympathetic nervous system- the subdivision of the autonomic nervous system that operates during relaxed states and that conserves energy Nervous System Central Peripheral Nervous System Nervous System Brain Spinal Cord Somatic Autonomic Nervous System Nervous System Sympathetic Parasympathetic Nervous System Nervous System Communication in the Nervous System  Neurons (nerve cell)- the brain’s communication specialists, transmitting information to, from and within the central nervous system o Building blocks of nervous system Neuron- a cell that conducts electrochemical signals; the basic unit of the nervous system; also called a nerve cell  Glia (glial cells) hold neurons in place o Make up 90% of the brain’s cells o Provide neurons with nutrients, insulate them, protect the brain from toxic agents, and remove cellular debris when neurons die o Also communicate chemically with each other and with neurons Glia- cells that support, nurture, and insulate neurons, remove debris when neurons die, enhance the formation and maintenance of neural connections, and modify neural functioning The Structure of the Neuron  Neurons have 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 Cell body- the part of the neuron that keeps it alive and determines whether or not it will fire Axon- a neuron’s extending fibre that conducts impulses away from the cell body and transmits them to other neurons  Axons often divide into branches called axon terminals  Many axons, especially large ones, are insulated by a myelin sheath o Constrictions in the sheath called nodes make segments o Helps prevent signal interference between neurons and speeds up nerve impulses Myelin sheath- a fatty insulation that may surround the axon of a neuron Nerve- a bundle of nerve fibres (axons and sometimes dendrites) in the peripheral nervous system  The human body has 43 pairs of peripheral nerves, one in each pair on the left and right sides of the body  Most enter or leave the spinal cord  12 pairs in the head, cranial nerves, connect directly to the brain Neurons in the News 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 cell type  Stem-cell research is hot area in biology and neuroscience  Scientists prefer working with cells from aborted fetuses and embryos that are a few days old, which consist of just a few cells  In Canada, embryonic stem cells (ES) sources must be approved by the Assisted Human Reproduction Agency of Canada, which regulates stem-cell research  Stem cells are useful because they can differentiate into any type of cell  Stem cells have the potential to help people recover from brain damage, disease such as Alzheimer’s, and damage to the spinal cord and other parts of the body How Neurons Communicate  Neurons are separated by a space called the synaptic clef o Axon terminal of one neuron nearly touches the dendrites/cell body of another  This whole site- the axon terminal, cleft, covering membrane of receiving dendrite or cell body, is the called the synapse 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  Neurons communicate with each other or to muscles and glands using an electrical and chemical language o The nerve cell is stimulated, causing a change in electrical potential between the inside and outside of the cell o There is a sudden inflow of positive sodium ions, followed by a sudden outflow of positive potassium ions o This causes a change in electrical voltage called action potential 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  In myelinated axons, action potential is regenerated at each node and nerve impulses travel faster o Nerve impulses are slower in babies because their myelin sheaths are not yet fully developed  When neural impulses reach the axon terminal’s tip, they must transfer the message across the synaptic clef o Synaptic vesicles, tiny sacs in the tip of the axon terminal, release a few thousand molecules of a neurotransmitter Neurotransmitter- a chemical substance that is released by a transmitting neuron at the synapse and that alters the activity of a receiving neuron  Neurotransmitter molecules bond with receptor sites, special molecules in dendrites or the cell body, that neurotransmitters enter like a key into a lock  Change in the neuron membrane is: o Excitatory- voltage shift in a positive direction, increasing probability to fire o Inhibitory- voltage shift in a negative direction, decreasing probability to fire  The net effect of all the messages being received determines whether or not the neuron will fire o It DOES NOT depend on the strength of the neuron’s firing- it either fires or it doesn’t, there is no degree of firing 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  The brain is flexible in adapting to new experiences  Ex. Some blind people use visual areas of the brain to take part in activities involving hearing Chemical Messengers in the Nervous System Neurotransmitters: Versatile Couriers  Neurotransmitters exist not only in the brain, but also in spinal cord, peripheral nerves, and certain glands  They affect mood, memory and well-being through effects on specific nerve circuits  Some examples 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 possibly response to novelty o Acetylcholine- affects neurons involved in muscle action, cognitive functioning, memory, and emotion o Norepinephrine- affects neurons involved in increasing heart rate and the slowing of intestinal activity during stress, and neurons involved in learning, memory, dreaming, waking from sleep, and emotion o GABA (gamma-amino butyric acid)- the major inhibitory neurotransmitter in the brain o Glutamate- 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 are implicated in sleep and eating disorders and convulsive disorders, including epilepsy o Alzheimer’s disease is associated with the loss of brain cells producing acetylcholine; it is related to the memory problems o Loss of cells producing dopamine causes the tremors and rigidity seen in Parkinson’s disease o In multiple sclerosis, immune cells overproduce glutamate, damaging or killing the glial cells that produce myelin  Many regularly ingested substances affect neurotransmitters o Tryptophan, an amino acid found in protein-rich foods, is a building block of serotonin 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 pain reduction, pleasure, and memory and are known technically as endogenous opioid peptides  Effects similar to natural opiates- reduce pain, promote pleasure  Endorphins also play a role in appetite, sexual activity, blood pressure, mood, learning and memory  Some function as neurotransmitters, but most alter their effects by limiting or prolonging them  Endorphin levels rise when an animal or person is afraid or under stress- making pain bearable gives species an evolutionary advantage Hormones- Long-Distance Messengers Hormones- chemical substances, secreted by organs called glands, that affect the functioning of other organs Endocrine glands- internal organs that produce chemicals and release them into the bloodstream  Hormones are produced primarily by endocrine glands  They are released directly into the bloodstream and carried to organs and cells that may be far from their point of origin  Hormones have dozens of jobs, from promoting bodily growth to aiding digestion to regulating metabolism  Neurotransmitters and hormones are not always chemically distinct o A particular chemical may have multiple classifications, ex. Norepinephrine Melatonin- a hormone secreted by the pineal gland, that is involved in the regulation of daily biological rhythms  Secreted by the pineal gland deep within the brain  Helps regulate daily biological rhythms and promotes sleep Oxytocin- a hormone, secreted by the pituitary gland, that stimulates uterine contractions during childbirth, facilitates the ejection of milk in nursing, and seems to promote, in both sexes, attachment and trust in relationships  Secreted by the pituitary gland, another small gland in the brain  Enhances uterine contractions during childbirth and facilitates ejection if milk in nursing  Along with vasopressin, contributes to relationships in both sexes by promoting attachment and trust Adrenal hormones- hormones that are produced by the adrenal glands and that are involved in emotion and stress  Produced by adrenal glands (organs perched right above the kidney)  Involved in emotion and stress  Levels also rise in response to conditions such as heat, cold, pain, injury, burn, and physical exercise, and in response to some drugs, such as caffeine and nicotine  Outer part of adrenal glands produce cortisol, which increases blood-sugar levels and boosts energy  Inner part of adrenal glands produces epinephrine (adrenaline) and norepinephrine  When released in the body, activated by the sympathetic nervous system, adrenal hormones increase your arousal level and prepare you for action  Also enhances memory Sex hormones- hormones that regulate the development and functioning of reproductive organs and that stimulate the development of male and female sexual characteristics; they include androgens, estrogens, and progesterone  Sex hormones are secreted by tissue in the gonads (testes in men, ovaries in women) as well as by the adrenal glands  There are three main types: o Androgens (most importantly testosterone) are masculinizing hormones produced mainly in the testes, as well as ovaries and adrenal glands  Set in motion physical changes of males in puberty (deepened voice, facial and chest hair) and cause pubic and underarm hair to develop in both sexes  Testosterone influences sexual arousal in both sexes o Estrogens are feminizing hormones that bring on physical changes in females at puberty (breast development and menstruation) and influence the course of the menstrual cycle o Progesterone contributes to the growth and maintenance of the uterine lining in preparation for a fertilized egg, as well as other functions o Both estrogens and progesterone are produced mainly in the ovaries but also in the testes and adrenal glands Nervous-System Chemicals and Their Effects Type Function Effects Where Produced Examples Neurotransmitters Enable neurons to excite Diverse, depending Brain, spinal cord, Serotonin, dopamine, or inhibit each other on which circuits are peripheral nerves, norepinephrine activated or certain glands suppressed Endorphins Usually modulate the Reduce pain, Brain, spinal cord (Several varieties, not effects of promote pleasure; discussed in this text) neurotransmitters also linked to learning, memory, and other functions Hormones Affect functioning of Dozens, ranging Primarily in Epinephrine, target organs and tissues from promotion of endocrine glands norepinephrine, digestion to estrogens, androgens regulation of metabolism Mapping the Brain  Scientists perform case studies on patients who have had a part of the brain damaged or removed because of disease or surgery  The lesion method involves damaging or removing sections of brain in animals and then observing the effects Electrical and Magnetic Detection  The brain can be probed with devices called electrodes  Electrodes detect the electrical activity of millions of neurons in particular regions of the brain and are widely used in research and medical diagnosis  Electrodes are connected by wires to a machine that translates the electrical energy from the brain into wavy lines on a moving piece of paper or visual patterns on a screen o That is why electrical patterns in the brain are known as “brain waves” o Different wave patterns associate with sleep, relaxation, mental concentration Electroencephalogram (EEG) – a recording of neural activity detected by electrodes  A brain-wave recording is called an EEG  Can tell that something is happening, but not what is happening or who is doing it  Computer technology can be combined with EEG technology to get a clearer picture of brain activity patterns associated with specific events and mental processes  For more precise information, researchers use needle electrodes, very thin wires or hollow glass tubes that can be inserted into the brain o Only the skull and membranes covering the brain need to be anesthetized o A human or animal can be awake and not feel pain during the procedure o Microelectrodes are so fine they can be inserted into single cells Transcranial magnetic stimulation (TMS) – a method of stimulating brain cells, using a powerful magnetic field produced by a wire coil placed on a person’s head; it can be used by researches to temporarily inactivate neural circuits and is also being used therapeutically  A large current is produced through a wire coil placed on a person’s head  This procedure causes neurons under the coil to fire, and can be used to produce motor responses, or to briefly inactivate an area and observe the effects on behaviour  Drawback- when neurons fire, many others become active as well- hard to pinpoint  Found that
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