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Chapter 3

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Western University
Psychology 1000

CHAPTER THREE: THE BIOLOGICAL FOUNDATIONS OF BEHAVIOUR Neurons are nerve cells linked together in circuits (brain contains hundred billions of them at birth) Sculpted by nature to receive, process and send messages  generate electricity that creates nerve impulses.  release chemicals that allow them to communicate with other neurons and with muscles and glands Supported by Glial cells  surround neurons and hold them in place  manufacture nutrient chemicals that neurons need  form myelin sheath around some axons  absorb toxins/waste materials that might damage neurons  send out long fibres to guide newly divided neurons to their targeted place in the brain during prenatal brain development  outnumber neurons about 10-1 blood-brain barrier protects the brain from substances ex. toxins 3 main parts: cell body or soma contains biochemical structures needed to keep the neuron alive  nucleus carries genetic info that determines how cell develops & functions dendrites branch like fibres emerging from the cell body  specialized receiving units like antenna  collect messages from a thousand or more neighbouring neurons and send them on to the cell body, where info is combined and processed axon extending from one side of the cell body  conducts electrical impulses away from the cell body to other neurons/muscles/glands  branches out at its end to form a # of axon terminals—up to several hundred in some cases  each axon may connect with dendritic branches from numerous neurons, making it possible for one neuron to pass messages to thousands of others SO it is easy to see how there can be trillions of interconnections in the brain. Stuructural elements of a typical neuron. Stimulation received by the dendrites or cell body (soma) may trigger a nerve impulse, which travels down the axon to stimulate axons have a fatty myelin sheaths. Some interrupted at intervals by the nodes of Ranvier. The myelin sheath helps to increase the speed of nerve conduction. The electrical activity of Neurons Nerve impulses & nerve activation 1. at rest, the neuron has an electrical resting potential due to the distribution of pos. and neg. 2. when stimulated a flow of ions in and out thru cell membrane reverses the electrical charge of the resting potential, producing an action potential, or nerve impulse 3. neuron is at rest again; the original distribution of ions is restored Neurons are surrounded by body fluids & separated from this liquid environment by a protective membrane that allows certain substances to pass thru ion channels into the cell. Ions electrically charged atoms ←ion channel passageway or channel in the membrane that can open and allow ions to pass through. ←Creating a nerve impulse: The exchange of ions ←Outside the neuron: positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). ←Inside the neuron: large negatively charged protein molecules (anions or A-) and positively charged potassium ions (K+) • High concentration of positive sodium ions in fluid outside the cell + negatively charged protein ions inside= uneven distribution of positive and negative ions that makes the interior cell negative compared to the outside. ←This difference is called the neuron’s resting potential ←(neuron is in a state of polarization at rest) Action Potential (see images on next page)  Alan Hodgkin and Andrew Huxley  When neuron’s axon is stimulated w mild electrical stimulus, interior voltage differential shifted suddenly from -70 mv (inside) to +40mv  Depolarization=this shift from neg. to pos. voltage 1. In a resting state, sodium & potassium channels are closed, concentration of NA+ ions is 10x higher outside the neuron than inside it 2. BUT when a neuron is stimulated efficiently, nearby sodium channels open up. 3. Neg. proteins inside attract pos. neurons to flood into the axon, creating a state of depolarization 4. Int. now becomes pos. (by ~40mv) compared to ext. creating action potential 5. In a reflex action to restore to the resting potential, cell closes its sodium channels 6. Positively charged potassium ions flow out through their channels, restoring neg. resting potential 7. Extra sodium ions flow out of neuron and escaped potassium ions are recovered 8. Creates resulting voltage changes ^From resting potential to action potential Absolute refractory period: Once an action potential occurs on the membrane, effects spread to sodium channels and the action potential flows down the axon to axon terminals. As soon as an impulse passes a point on the axon, there is a recovery period as K+ ions flow out from the inside. The membrane is not excitable and can’t generate another potential, which places a limit on the rate at which nerve impulses can occur. *300imp/sec for humans ←All-or-none law, action potentials occur at a uniform and maximum intensity, or they do not occur at all ←Potential threshold Neg. potential inside axon requires to go from -70mv to -50mv Graded Potentials Changes in the negative resting potential that do not reach the 50 millivolts action, sometimes can add up to trigger action potentials *For neurons to function properly, sodium and potassium ions MUST enter and leave the membrane at the proper rate Myelin Sheath fatty, whitish insulation layer derived from glial cells during development  interrupted at regular intervals when it is extremely thin or absent  unmyelated axins—action potential travels down axon length like a burning fuse  Myelated axins-electrical conduction can skip from node to node (connecting points), and these great leaps from one gap to another account for high speed  Still not as fast as electric speed; though the brain is way more complex than computers, is not nearly as fast as the quick at which they operate.  Damage done to myelin coating leads to immune system attacking the myelin sheath (MS) Synaptic Transmission how neurons communicate  The nervous system operates as a giant communications network, and its action requires the transmission of nerve impulses from one neuron to another[  Famous Spanish anatomist Santiago Ramon y Cajal and British scientist Charles Sherrington, argued that neurons were individual cells that did not make actual physical contact with each other, but communicated at a synapse, a functional (but not physical) connection between a neuron and its target  chemical neurotransmission Neurons released chemicals that carried the message from one neuron to the next cell in the circuit  synaptic cleft, a tiny gap or space, between the axon terminal of one neuron and the dendrite of the next neuron  If the action potential does not cross the synapse, what does? What carries the message and how does it affect the next neuron in the circuit? Neurotransmitters chemical substances that carry messages across the synapse to either excite other neurons or inhibit their firing 1. Synthesis-chemical molecules are formed inside the neuron 2. Storage-molecules are then stored in chambers called synaptic vesicles within the axon terminals 3. Release-when an action potential comes down the axon, these vesicles move to the surface of the axon terminal and the molecules are released to a fluid filled space between the axon of the sending (presynaptic) neuron and the membrane of the receiving (postsynaptic) neuron. 4. Binding-the molecules cross the synaptic space and bind (attach themselves) to receptor sites: large protein molecules embedded in the cell membrane; fit a specific transmitter molecule like a lock&key 5. Deactivation Binding can have 1 of 2 effects on the postsynaptic neuron a) Excitation –excitatory neurotransmitters depolarize neurons membrane, increase likelihood of action potential  reaction will depolarize (excite) the postsynaptic cell membrane by stimulating inflow of sodium/other +ly charged ions. Excitatory transmittors-Neurotransmitters that create depolarization. The stimulation, alone/in combination w activity at other excitatory synapses on the dendrites or cell body may exceed the action potential threshold and cause the postsynaptic neuron to fire an action potential b) Inhibition-inhibitory neurotransmitters hyperpolarize neurons membrane, decrease likelihood of action potential  chemical reaction created by docking of neurotransmitter will hyperpolarize the postsynaptic membrane by stimulating ion channels that allow + charged potassium ions to flow out of the neuron or - charged ions to flow into the neuron, making the membrane even more neg. (ex. changing from -70 to -72mv) Hyperpolarization makes it more difficult for excitatory transmitters at other receptor sites to depolarize the neuron to its action potential threshold of -55mv. Transmittors that create hyperpolarization are thus inhibitory in their function. A given neurotransmitter can have an excitatory effect on some neurons and an inhibitory influence on others. Deactivation  Every neuron is constantly bombarded with excitatory and inhibitory neurotransmitters from other neurons  Interplay of these influences determines whether the cell fires an action potential  Perfect balance between excitatory and inhibitory processes must be maintained if the nervous system is to function properly.  Inhibition allows a fine tuning of neural activity and prevents an uncoordinated discharge of the nervous system ex. seizure ^many neurons fire off action potentials in a runaway fashion Once a Neurotransmitter molecule binds to its receptor, it continues to activate/inhibit the neuron until it’s shut off (deactivated) Deactivation occurs in two major ways 1. chemicals in the synaptic space break the transmitter molecules down into their chemical components 2. reuptake-transmitter molecules are reabsorbed into the presynaptic axon terminal. When the receptor molecule is vacant, the postsynaptic neuron returns to its former resting state, awaiting the next chemical stimulation *Psychoactive drugs can alter chemical neurotransmission. Drug’s psychological effects are determined by which chemical transmitter it targets. Specialized Transmitter Systems  Brain is divided into systems that are uniquely sensitive to certain messages through use of chemical transmitters.  Transmitter molecules can assume many shapes. Because the various systems in the brain recognize only certain chemical messengers, they are protected from “crosstalk” from other systems.  Each substance has a specific excitatory or inhibitory effect on certain neurons.  2 widespread neurotransmitters are simple amino acids, glutamate, (glutamic acid), and gamma-amino-butyric acid (GABA)  glumate and GABA found throughout central nervous system; have some role in mediating virtually all behaviours  Glumate=excitatory, important role in mechanisms for learning and memory o Can’t fix learning/memory by enhancing glumate activity; overactivation could cause seizure activity within brain  GABA=inhibitory, important for motor (muscle), sleep, and anxiety control o Drugs to treat anxiety enhance GABA activity o Ex. alcohol makes brain more sensitive to GABA o GABA induced inhibition cause drunkness  acetylcholine (ACh) which is involved in memory and muscle activity o Reductions in ACh weaken or deactivate neural circuitry that stores memories o Drugs that block the action of ACh, therefore, can prevent muscle activation, resulting in muscular paralysis o Example: botulism, a serious type of food poisoning that can result from improperly canned food  Dopamine excitatory transmitter  Serotonin enhances mood, eating, sleep and sexual behaviour  Endorphins reduce pain and increase feeling of well-being  Neuromodulators Table 3.1: several of the more important transmitters that have been linked to psychological phenomena Drugs and the Brain Drugs affect consciousness and behaviour by influencing the activity of neurons. Most psychoactive drugs produce effects by either increasing or decreasing actions of neurotransmitters. Agonists may 1. Enhance a neurons ability to synthesize, store or release neurotransmitters 2. Mimic the action of a neurotransmitter by binding with and stimulating postsynaptic receptor sites 3. make deactivation of neurotransmitters more difficult Antagonist may 1. reduce a neurons ability to synthesize, store or release neurotransmitters 2. prevent a neurotransmitter from binding with a postsynaptic neuron by fitting into and blocking the receptor sites on the postsynaptic neuron Alcohol  as an agonist alcohol stimulates inhibitory transmitter GABA’s activity, depressing neural activity  as an antagonist decreases glutamate (excitatory transmitter)  effect=slowing of neural activity of normal brain functions (clear thinking, emotional control, motor coordination) Caffeine  agonist increases activity of neurons and other cells  as an antagonist reduces transmitter adenosine’s activity, producing higher rates of cellular activity Nicotine  agonist for excitatory transmitter Ach  chemical structure is similar to Ach to allow it to fit into Ach binding sites and create action potentials  stimulates dopamine activity (important chemical mediator for motivation and reward) at other receptor sites Amphetamines  increasing activity of excitatory neurotransmitters dopamine and norepinephrine by 1. causing neurons to release greater amounts of ^^neurotransmitters 2. inhibit reuptake, allowing dopamine and norepinephrine to keep stimulating postsynaptic neurons  this boosts arousal and mood  cocaine produces similar effects by blocking reuptake of norepinephrine and dopamine Rohypnol and GHB –date rapes  suppress general neural activity by enhancing GABA  lead to respiratory depression, loss of consciousness, coma and even death  decreases neurotransmission in areas of brain that involve memory (causes users to forget what happened) The Nervous System (Body’s master control center) 3 MAJOR types of neurons carry out systems input, output & integration functions: Sensory neurons carry input messages from the sense organs to the spinal cord & brain. Motor neurons transmit output impulses from the brain and spinal cord to the body’s muscles and organs Interneurons link the input and output functions. -far outnumber sensory and motor neurons -activity makes possible the complexity of our highter mental functions, emotions and behavioural capabilities -perform connective/associative functions w nervous system ex. interneurons allow us to recognize a tune by linking the sensory input from the song we’re hearing w the memory of that song stored elsewhere in the brain Nervous system: composed of sensory neurons, motor neurons and interneurons (associative neurons) Two major divisions Central Nervous system consists of all the neurons in the brain & spinal cord Peripheral Nervous system composed of all the neurons that connect the central nervous system with the muscles, glands and sensory receptors (all the neural structures that lie outside of the brain & spinal cord) Specialized neurons help carry out the input & output functions that are necessary for us to sense what is going on inside and outside our bodies and to respond with our muscles and glands. Two Major divisions Somatic nervous system sensory neurons that are specialized to transmit msgs from eyes, ears, & other sensory receptors, and the motor neurons that send msgs from brain and spinal cord to muscles that control our voluntary movements. Allows you to sense and respond to your environment  Axons of sensory neurons combine together to form sensory nerves  Motor axons combined form motor nerves *nerves in brain =tracts Automatic nervous system controls glands and smooth muscles from the bodily organs. Largely concerned with involuntary functions ex. respiration, circulation, digestion. Also involved with motivation, emotional behaviour, stress responses. Two Subdivisions Sympathetic nervous system speeds up body processesactivation and arousal.  speeds heart to pump more blood  dialates pupils  improves vision  slows digestive system so blood can run to muscles  increases respiration rate  mobilizes body to confront stressorfight-or-flight response Parasympathetic nervous system slows down body processesmaintains or returns you to a state of rest  slows down heart rate, etc.  decreases blood pressure  The two systems work together to balance out the works of our internal organs to maintain homeostasis (equilibrium-delicatelly balanced/constant internal state) The Central nervous system (CNS) the Spinal Cord connects most parts of the peripheral nervous system with the brain  nerves enter and leave the CNS by way of spinal cord  spinal cords neurons are protected by vertebrae-bones of the spine  cross-section view (butterfly structure) consists of grey coloured neuron cell bodies & interconnections  surrounding grey matter are white coloured myelinated axons that connect various levels of the spinal cord with each other and w higher centres of brain  sensory nerves enter back side of spinal cord (along its length)  motor nerves enter spinal cord’s front side  spinal reflexes can be triggered at the level of the spinal cord w/o any involvement of the brain ex. touching a hot iron… o sensory receptors in your skin trigger nerve impulses in sensory nerves that flash into spinal cord and synapse with interneurons o interneurons then excite motor neurons that send impules to the hand to pull away from the hot object o other interneurons carry the “hot” message all together up the spinal cord to the brain  you don’t have to wait for the brain to tell you what to do in such emergencies; getting messages to and from the brain takes slightly longer, so the spinal cord reflex system significantly reduces reaction time (saving us from tissue damage in the above case) Healing the Nervous system • axon repair • brain grafts • transplantation of neural stem cells the Brain • 1.4 kg of fat, protein and fluid inside the skull • most complex structure in the universe & the only one that can wonder about itself • most active energy consumer of all your body organs • account
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