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Chapter 3

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Dax Urbszat

Pages 81 - 120 Chapter 3: The Biological Bases of Behaviour Communication in the Nervous System  Your nervous system is a complex communication network in which signals are constantly being transmitted, received, and integrated (handles the info) a) Nervous Tissue: The Basic Hardware  Your nervous system is living tissue composed of 2 types of cells: 1. Neurons o Are individual cells in the nervous system that receive, integrate, and transmit info o Come in a tremendous variety of types and shapes that no single drawing can adequately represent them o The soma (cell body) contains cell nucleus and much of the chemical machinery common to most cells o Rest is devoted to handling info o Has # of branched, feeler-like structures  dendritic trees  Each individual branch is a dendrite (are parts of a neuron that are specialized to receive info  Most dendrites receive info from many other cells o Axon – a long, thin fibre that transmits signals away from the soma to other neurons or to muscles or glands  May branch off to communicate with # of other cells  Humans: many axons are wrapped in cells with high concentration of white, fatty substance  myelin  Myelin Sheath – insulating material, derived from glial cells, that encases some axons o Speeds up transmission of signals that move along axons o If axon’s myelin sheath deteriorates  signals may not be transmitted effectively o Loss of muscle control seen with multiple sclerosis disease  due to degeneration of myelin sheaths  Axon ends in a cluster of terminal buttons, which are small knobs that secrete chemicals  neurotransmitters o These chemicals serve as messengers that may activate neighbouring neurons o Synapses – a junction where info is transmitted from 1 neuron to another  Cells send info to dendrites which passes through the soma and along the axon, and is transmitted to dendrites of other cells at meeting points  synapses 2. Glia o Are cells found throughout the nervous system that provide various types of support for neurons o Tend to be much smaller than neurons but outnumber neurons by about 10-1 o Functions: supply nourishment to neurons, help remove neurons’ waste products, and provide insulation around many axons 1 Pages 81 - 120 o Myelin sheath are derived from special types for glial cells o Role: orchestrates the development of the nervous system in human embryo b) The Neural Impulse: Using Energy to Send Information A. The Neuron at Rest: A Tiny Battery o Both inside + outside the neuron  fluids containing electrically charged atoms and molecules  ions o The cell membrane  semi-permeable, permitting movement of some ions o + charged Na and K ions and – charged Cl ions flow back and forth across cell membrane  don’t cross at same rate  Difference in flow rates leads to slightly higher concentration of – charged ions inside cell  Resulting voltage means neuron at rest is a tiny battery  store of potential energy o Resting Potential – is the stable, - charge when cells is inactive (-70mv) B. The Action Potential o As long as voltage neuron remains constant, no messages are being sent o When neuron is stimulated: channels in its cell membrane open, allowing + charged Na ions to rush in  Result: inside becomes more + creating action potential o A very brief shift in neuron’s electrical charge that travels along an axon o The firing of an action potential is reflected in voltage spike o After the firing, the channels in cell membrane that opened to let in Na close up  Result: neuron can’t fire o Some time is needed before they open again o Absolute Refractory Period – minimum length of time after an action potential during which another action potential can’t begin  Followed by a brief relative refractory period  neuron can fire but its threshold for firing is elevated so more intense stimulation is required to initiate AP C. The All-or-None Law o Either the neuron fires or it doesn’t o A stronger stimulus will cause a cell to fire a more rapid volley of neutral impulses than a weaker stimulus will c) The Synapse: Where Neurons Meet A. Sending Signals: Chemicals as Couriers o The 2 neurons don’t actually touch  separated by the synaptic cleft  Synaptic Cleft – a microscopic gap between the terminal button of 1 neuron and the cell membrane of another neuron 2 Pages 81 - 120  Signals have to cross this gap to permit neurons to communicate o The neuron that sends a signal across the gap  pre-synaptic neuron o Neuron that receives the signal  postsynaptic neuron o Arrival of AP at an axon terminal buttons triggers the release of neurotransmitters  Neurotransmitters – chemicals that transmit info from 1 neuron to another  Within most of these chemicals  stored in small sacs (synaptic vesicles)  NTs released when a vesicle fuses with membrane of pre-synaptic cell and its contents spill into synaptic cleft  after the release, NTs diffuse across the synaptic cleft to membrane of the receiving cell  bind with special molecules in postsynaptic cell membrane at various receptor sites  these sites are specifically “tuned” to recognize and respond to some NTs only B. Receiving Signals: Postsynaptic Potentials o When NTs and receptor molecule come, reactions in cell membrane case postsynaptic potential (PSP)  Postsynaptic Potential – a voltage change at a receptor site on a postsynaptic cell membrane  Don’t follow all-or-none law  Are graded (they vary in size and increase/decrease the probability of a neural impulse in receiving cell in proportion of the amount of voltage change  2 types of messages can be sent from cell to cell: 1) Excitatory (PSP)  A + voltage shift, increases likelihood that postsynaptic neuron will fire action potentials 2) Inhibitory (PSP)  A – voltage shift, decreases likelihood that postsynaptic neuron will fire action potentials  Then, NTs drift away from receptor sites or inactivated by enzymes that metabolize (convert) them into inactive forms  Most are reabsorbed into presynaptic neuron through reuptake (process in which NTs are sponged up from synaptic cleft by presynaptic membrane  Reuptake: allows synapses to recycle their material C. Integrating Signals: Neural Networks o Neurons must integrate signals arriving at many synapses before it “decides” whether to a fire a neural impulse 3 Pages 81 - 120  If enough excitatory PSPs occur in a neuron, electrical currents can add up, causing cell’s voltage to reach threshold at which an AP will be fired  If any inhibitory PSPs also occur, they will cancel effect of excitatory PSPs = equilibrium o Nervous System usually forms more synapses than needed and then gradually eliminate the less active ones o 1 neuron stimulating another neuron repeatedly produces changes in synapses d) Neurotransmitters and Behaviour  Nervous system relies on chemical couriers to communicate info between neurons (NTs) o NTs are fundamental to behaviour  play key role in everything from muscle movements to moods and mental health  Specific NTs work at specific kinds of synapses o Recall: NTs deliver their messages by binding to receptor sites in postsynaptic membrane o Binding process operates like a lock and key o Thus, NTs can only deliver signals at certain locations on cell membranes o Purpose: this variety and specificity reduces cross-talk between densely packed neurons, making NS’s communication more precise *** Summary Chart of NTs 1. Acetylcholine (ACh)  Agonist – a chemical that mimics action of a NT  Antagonist – a chemical that opposes the action of a NT  Ex. some chemicals bind to receptors but fail to produce PSP (key slides into lock but doesn’t work) Result: they block the action of a natural NT by occupying its receptor sites, rendering them unusable 2. Monoamines  Includes 2 NTs: dopamine, norepinephrine, and serotin  Neurons using these NTs regulate many aspects of everyday behaviour  Dopamine: used to control voluntary movements  Serotonin: role in regulation of sleep and wakefulness and eating behaviour 3. GABA and Glutamate  Consists of amino acids: Gamma-Aminobutyric Acid (GABA) and glycine (work in the brain)  Produce inhibitory postsynaptic potentials 4 Pages 81 - 120  ACh & NE (versatile) can produce either excitatory or inhibitory PSPs depending on synaptic receptors they bind to  GABA contribute to regulation of anxiety in humans  Glutamate always has excitatory effects  Contribute to learning and memory  A subset of glutamate circuits play key role in long-term potentiation  involves durable increases in excitability at synapses along specific neural pathway 4. Endorphins  Internally produced chemicals that resemble opiates in structure and effects  Widely distributed throughout body and contribute to modulation of pain e) The Peripheral Nervous System  Made up of all those nerves that lie outside the brain and spinal cord  Nerves – are bundles of neuron fibres (axons) that are routed together in the peripheral nervous system A. The Somatic Nervous System o Made up of nerves that connect to voluntary skeletal muscles and to sensory receptors o Nerves carry info from receptors to skin, muscles, joints to CNS and that carry commands from CNS to muscles; require 2 kinds of nerve fibres: 1) Afferent Nerve Fibres  Axons that carry info inward to CNS from periphery of body 2) Efferent Nerve Fibres  Axons that carry info outward from CNS to periphery of body o Contains “two-way streets” with income (afferent) and outgoing (efferent) lanes o Lets you feel the world and move around in it B. The Autonomic Nervous System o Made up of nerves that connect to heart, blood vessels, smooth muscles, and glands o Is separate system, although governed by CNS o Controls: automatic, involuntary, visceral functions that people don’t normally think about (ex. heart rate, digestion, perspiration) o Mediates much of physiological arousal that occurs when people experience emotions (ex. walking home at night, and someone follows you  heart rate and breathing speed up) 5 Pages 81 - 120 o Can be subdivided: 1) Sympathetic Division  Creates fight-or-flight response  Slows digestion processes + drains blood from periphery, lessening bleeding in case of an injury  Send signals to adrenal glands, triggering release of hormones that ready the body for exertion (energy) 2) Parasympathetic Division  Conserves bodily resources  Activates processes that allow body to save and store energy  Ex. slow heart rate, reduce blood pressure, promote digestion f) The Central Nervous System  Consists of brain and spinal cord  Protected by enclosing sheaths  meninges  Surrounded by cerebrospinal fluid o Cerebrospinal Fluid (CSF) – nourishes brain and provides a protective cushion for it o The hollow cavities in brain that are filled with CSF  ventricles A. The Spinal Cord o Connects brain to rest of body through PNS o Sends signals from brain to motor neurons that move body’s muscles B. The Brain o Contains billions of interacting cells that integrate info from inside and outside the body, coordinates body’s actions, and enable humans to talk, think, remember, plan, create, and dream Looking Inside the Brain: Research Methods  Geography/structure of brain can be mapped out relatively easily by examining and dissecting brains removed from animals or from deceased humans  Neuroscientists – investigators who conduct research on brain or NS a) Electrical Recordings  Electroencephalograph (EEG) – a device that monitors electrical activity of brain over time by means of recording electrodes attached to surface of scalp o Recordings translated into line tracings  brain waves b) Lesioning  Involves destroying a piece of the brain  Done by inserting an electrode into a brain structure and passing a high- frequency electric current through it to burn the tissue and disable the structure c) Electrical Stimulation of the Brain (ESB) 6 Pages 81 - 120  Involves sending a weak electric current into a brain structure to stimulate (activate) it  Current is delivered through an electrode (different from one in leisioning) d) Transcranial Magnetic Stimulation (TMS)  A new technique that permits scientists to temporarily enhance or depress activity in a specific area of the brain  A magnetic coil mounted on small paddle is held over a specific area of a subject’s head o Coil creates a magnetic field that penetrates to a depth of 2cm o By varying timing/duration: can increase/decrease excitability of neurons in local tissue e) Brain-Imaging Procedures  Allows researchers to look into the brain  Computerized Tomography (CT Scan): a computer-enhanced x-ray of brain structure o Multiple x-rays are shot from many angles and the computer combines the readings to create a vivid image of a horizontal slice of the brain  Positron Emission Tomography (PET scan): can examine brain function, mapping actual activity in the brain over time o Provide a colour-coded map indication which areas of the brain become active when subjects clench their fist etc. o can study activity of specific NTs  Magnetic Resonance Imaging (MRI): uses magnetic fields, radio waves, and computerized enhancement to map out brain structure o Much better than CT scans; produces 3D pictures of brain that have remarkably high resolution o fMRI (Funtional Magnetic Resonance): newer MRI; monitors blood flow and oxygen consumption in brain to identify areas of high activity The Brain and Behaviour a) The Hindbrain  Includes cerebellum and 2 structures found in lower part of brainstem: medulla and pons  Medulla: attaches to spinal cord; is in charge of largely unconscious but vital functions (ex. circulating blood, breathing, reflexes  sneezing, coughing)  Pons: includes a bridge of fibres that connects brainstem with cerebellum o Contains several clusters of cell bodies involved with sleep and arousal  Cerebellum: large and deeply folded structure located adjacent to the back surface of the brainstem o Critical to the coordination of movement and to the sense of equilibrium/physical balance o Role: organizing the sensory info that guides muscular movements 7 Pages 81 - 120 b) The Midbrain  Segment of the brainstem that lies between hindbrain and forebrain  Contains an area concerned with integrating sensory processes (ex. vision & hearing)  An important system of dopamine-releasing neurons o Involved in performance of voluntary movements  Reticular formation: occurs through hindbrain & midbrain o Contributes to modulation of muscle reflexes, breathing, pain perception o Regulation of sleep and arousal c) The Forebrain  Largest and most complex region of brain, encompassing a variety of structures including: thalamus, hypothalamus, limbic system, and cerebrum A. The Thalamus: A Way Station o A structure in the forebrain through which all sensory info (except smell) must pass to get to cerebral cortex o Made up of clusters of cell bodies/somas  Each concerned with relaying sensory info to a particular par of cortex o Role: integrates info from various senses B. The Hypothalamus: A Regulator of Biological Needs o Found near base of forebrain that is involved in regulation of basic biological needs o Hypo = under o Contains clusters of cells  Function: control autonomic NS o Serves as a vital link between brain and endocrine system C. The Limbic System: The Seat of Emotion o Loosely conn
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