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Chapter 3

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PSYC 100
Russell Day

The Action Potential Action Potential – a sudden reversal in the neuron’s membrane voltage, during which the membrane voltage monetarily moves from -70 millivolts (inside) to +40 millivolts - Depolarization – shift from negative to positive voltage Neuron Action Potential: - Key mechanism: action of sodium and potassium ion channels in the cell membrane - In resting state: neuron’s sodium and potassium channels are closed, and the concentration of Na ions is 10 times higher outside the neuron than inside it o When neuron is stimulate sufficiently, nearby sodium ions flood into the axon, creating a state of depolarization – interior becomes positive in relation to outside, creating the action potential - In reflex action to restore resting potential  cell closes its sodium channels, and positively charged potassium ions flow out through their channels, restoring the negative resting potential o Eventually excess sodium ions flow out of the neuron, and the escaped potassium ions are recovered - Once action potential occurs at any point on the membrane, its effects spreads to adjacent sodium channels and the action potential flows down the length of the axon to the axon terminals o Immediately after an impulse passes a point along the axon, there is a recovery period as K+ ions flow out of the interior – absolute refractory period  During this period the membrane is not excitable and cannot generate another action potential  This places upper limit on the rate at which nerve impulses can occur – for humans limit is about 300 impulses It’s all or nothing – feature of action potential - All-or-none Law – action potentials occur at a uniform and maximum intensity, or they do not occur at all o Negative potential inside the axon has to be changed from -70 millivolts to about - 50 millivolts (the action potential threshold by the influx of sodium ions into the axon before the action potential will be triggered o Graded potentials - Changes in the negative resting potential that do not read the - 50 millivolts action potential threshold  Only under certain circumstances can graded potentials cause by several neurons add up to trigger an action potential in the postsynaptic neuron - For a neuron to function properly, sodium and potassium ions must enter and leave the membrane at just the right rate o Drugs that alter this transmit system can decrease or prevent neural functioning The Myelin Sheath – - Myelin Sheath – a fatty whitish insulation layer derived from glial cells during development o Cover axons that transmit information throughout the brain and spinal cord 1 o Sheath is interrupted at regular intervals by the nodes of Ranvier – where myelin is either extremely thin or absent In unmyelinated axons, the action potential travels down the axon length like a burning fuse o In myelinated axons, electrical conduction can skip from node to node, and these ‘great leaps’ from one gap to another account for high conduction speeds of more than 300 km per house o Increased efficiency of neural transmission that results is partly responsible for the gains that infants exhibit in muscular coordination as they grow older o Damage to the myelin coating can be seen in people who suffer from multiple sclerosis  Occurs when the person’s immune system attacks the myelin sheath  Damage to myelin sheath disrupts the delicate-timing of nerve impulses, resulting in jerky, uncoordinated movements and in the final stages – paralysis How Neurons Communicate: Synaptic Transmission - Nervous system action requires transmission of nerve impulses from one neuron to another - Synapse – a functional (but not physical) connection between a neuron and its target - Synaptic Cleft – tiny gap or space between the axon terminal of one neuron and the dendrite of the next neuron - Otto Leowi – neurons released chemicals and it was these chemicals that carried the message from one neuron to the next cell in the circuit Neurotransmitters - Neurotransmitters – produced by neurons and are chemical substances that carry messages across the synapse to either excite other neurons or inhibit their firing - Chemical communication involves five steps o 1) Synthesis – chemical molecules are formed inside the neuron. Then molecules are stored in chambers called synaptic vesicles within the axon terminals -when an action potential comes down the axon, these vesicles move to the surface of the axon terminal and the molecules are released into the flid-filled space between the axon of the sending (presynaptic) neuron and the membrane of the receiving (post synaptic) neuron - the molecules cross the synaptic space and bind to receptor sites – large protein molecules embedded in the receiving neurones cell membrane - receptor site have a specially shaped surface that fits a specific transmitter molecule o 2) Storage - o 3) Release o 4) Binding o 5) Deactivation Excitation, Inhibition, and Deactivation - Binding of a transmitter moleculte to the receptor sit produces a chemical reaction that can have one of two effects 2 o 1) reaction will depolarize (excite) the postsynaptic cell membrane by stimulating the inflow of sodium or other positively charged ions  Excitatory transmitters – Neurotransmitters that create depolarization  This stimulation, alone or in combination with activity at other excitatory synapses on the dendrites or the cell body, may exceed the action potential threshold and cause the postsynaptic neuron to fire an action potential o 2) Chemical reaction created by the docking of a neurotransmitter at its receptor site will hyperpolarize the postsynaptic membrane by stimulating ion channels that allow positively charged ions, like chloride, to flow into the neuron  This makes the membrane potential even more negative (ex changing it from -70 millivolts to -72 millivolts)  Hyperpolarization makes it more difficult for excitatory transmitters at other receptor sites to depolarize the neuron to its action potential threshold of -55 millivolts  Transmitters 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 - Every neuron is constantly bombarded with excitatory and inhibitory neurotransmitters from other neurons, and the interplay of these influences determines whether the cell fires an action potential - Action of an inhibitory transmitter from one presynaptic neuron may prevent the postsynaptic neuron from reaching the action potential threshold, even if it is receiving excitatory stimulation from several other neurons at the same time - Balance between excitatory and inhibitory processes must be maintained if the nervous system is to function properly - Process of inhibition allows a fine-tuning of neural activity and prevents an uncoordinated discharge of the nervous systems, as occurs in a seizure, when large numbers of neyrons fire off action potential in a runaway fashion - On a neurotransmitter molecule binds to its receptor, it continues to activate or inhibit the neuron until it is shut off, or deactivated o Deactivation occurs in 2 major ways:  1) some transmitter molecules are deactivated by other chemicals located in the synaptic space that break them down into their chemical components  2) deactivation mechanism is reuptake, in which the 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 - most commonly used, and abused, psychoactive drugs influence one of these steps in chemical neurotransmitter’s receptor, binding to the receptor in place of the neurotransmitter, or one of the steps in the synthesis or release of the neurotransmitter - drugs can also alter synaptic transmission by influencing how the transmitter is cleated from the synaptic cleft after it has been released 3 - a drug’s exact psychological effects are determined not by its actions at the synapse, but by which specific chemical transmitter it targets Specialized Transmitter Systems - brain divided into systems that are uniquely sensitive to certain messages - 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 - two widespread neurotransmitters are simple amino acids: o 1) glutamate (glutamic acid)  Both are found throughout the CNS and so have some role in the mechanisms involved in learning and memory  Over-activation of glutamate activity will induce seizure activity within the brain, especially within the cerebral cortex (because its is an excitatory neurotransmitter o 2) Gamma-aminobutyric acid (GABA)  Inhibitory neurotransmitter  Especially important for motor control and the control of anxiety  The symptoms of intoxication reflect the progressive inhibition of brain function with increasing GABA-induced inhibition - Best understood neurotransmitter – acetylcholine (Ach) – is involved in memory and muscle activity o underproduction of Ach is thought to be an important factor in Alzheimer’s disease – a degenerative brain disorder involving profound memory impairment that afflicts between 5-10% of all people over 65 years of age o Reductions in ACh weaken or deactivate neural circuitry that store memories o It is an excitatory transmitter at the synapses where neurons activate muscle cells o Drugs that block the action of AChc an prevent muscle activation, resulting in muscular paralysis - Neurotransmitter dopamine mediates a wide range of functions, including motivation, reward, and feelings of pleasure; voluntary motor control; and control of thought processes o In Parkinson’s disease, one group of dopamine-producing neurons degenerate and die o As dopamine is lost in the affected brain areas, there is a concomitant loss of voluntary motor control o Symptoms of Parkinson’s disease are most commonly treated with a drug (L-DOPA) that increases the amount of dopamine within the brain o Treatment of emotionally disturbed people has been revolutionized by the development of psychoactive drugs that operate by either enhancing or inhibiting the actions of transmitters at the synapse o Antipsychotic drugs attach to dopamine receptors and block dopamine from having its effect  Such blockade of dopamine is effective in treating symptoms of schizophrenia, like disordered thinking, hallucinations, and delusions 4 - Depression – involves abnormal sensitivity to serotonin – a neurotransmitter that influences mood, eating, sleep, and sexual behaviour o different mechanism occurs in the treatment of depression o antidepressant drugs increase serotonin activity in several ways o Drug, Prozac, blocks the reuptake of serotonin from the synaptic space, allowing serotonin molecules to remain active and exert their mood-altering effects on depressed patients o Other antidepressant drugs work on a different deactivating mechanism  They inhibit the activity of enzymes in the synaptic space that deactivate serotonin by breaking it down into simpler chemicals – which prolongs serotonin activity at the synapse - Endorphins – are neurotransmitters that reduce pain and increase feelings of well-being o They bind to the same receptors as the ones activated by opiate drugs, like opium and morphine, which produce similar psychological effects o The ability of people to continue to function despite severe injuries is due to release of endorphins and their abilities to act as analgesics - Most neurotransmitters have their excitatory or inhibitory effects only on specific neurons that have receptors for them - Others, called neuromodulators, have a more widespread and generalized influence on synaptic transmission o These substances circulate through the brain and either increase or decrease (modulate) the sensitivity of thousands, or millions, of neurons to their specific transmitters o They play role in functions like eating, sleep and stress – so some neurotransmitters have very specific effects whereas other have more general effects on neural activity The Nervous System - It is the body’s master control centre - 3 major types of neurons carry out the system’s input, output, and integration function o 1) Sensory Neurons – carry input messages from the sense organs to the spinal cord and the brain o 2) Motor Neurons – transmit output impulses from the brain and spinal cord to the body’s muscles and organs o Interneurons – link the input and output functions which is far outnumber sensory motor neurons , perform connective or associative functions within the nervous system  Ex: allow us to recognize a tune by linking the sensory input from the song we’re hearing with the memory of that song stored everywhere in the brain  Activity of interneurons makes possible the complexity of our higher mental functions, emotions, and behavioural capabilities - Nervous System can be broken down into several interrelated subsystems but 2 major divisions are: o 1) Central Nervous System – consists of all the neurons in the brain and spinal cord 5 o 2) Peripheral Nervous System – composed of all the neurons that connect the central nervous system with the muscles, glans, and sensory receptors The Peripheral Nervous System - The Peripheral Nervous System – contains all the neural structures that lie outside the brain and spinal cord o Its specialized neurons help to carry out the input and 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 - It has 2 major divisions: o 1)Somatic nervous system o Autonomic Nervous System The Somatic Nervous System - Somatic Nervous System – consists of o the sensory neurons that are specialized to transmit messages from the eyes, ears, and other sensory receptors  axons group together like the many strands of a rope to form sensory nerves (and motor neuron axons combine to form motor nerves)  inside the brain and spinal cord, nerves are called tracts o Motor Neurons – sends messages from the brain and spinal cord to the muscles that control brain and spinal cord to the muscles that control our voluntary movements The Autonomic Nervous System - Autonomic Nervous System - regulates body’s internal environment o Controls the glands and the smooth (involuntary) muscles that form the heart, the blood vessels, and the lining of the stomach and intestine o Is concerned with involuntary functions like respiration, circulation, and digestion  Also involved in aspects of motivation, emotional behaviours, and stress responses o Consists of 2 subdivisions that affect the same organ or gland in opposing ways:  1) Sympathetic Nervous System – has an activation or arousal function and tends to act as a total unit  Ex: in stressful situation, it simultaneously speeds heart so it can pump blood to muscles and dilates pupils so more light can enter eyes and you have better vision etc.  Is called fight-or-flight response  2) Parasympathetic Nervous System – is far more specific in its opposing actions  Slows down body processes and maintains or returns you to a state of rest  These two (sympathetic and parasympathetic) work together to maintain equilibrium in our internal organs  They maintain homeostasis – a delicately balanced or constant internal state 6 The Central Nervous System - This system contains the spinal cord, which connects most parts of the peripheral nervous system with the brain The Spinal Cord - Most nerves enter and leave the CNS by way of the spinal cord - Spinal cord’s neurons are protected by the vertebrae (bones of the spine) o Motor nerves exit the spinal cord’s front side - Spinal Reflexes – are simple stimulus-response sequences o Can be triggered at the level of the spinal cord without any involvement of the brain  Ex: touching something hot The Brain - Is the most complex structure in the known universe - Is the most active energy consumer of all your body organs - Accounts for 2% of total body weight and consumes about 20% oxygen you use in a resting state - Brain never rests, its rate of energy metabolism is relatively constant day and night Unlocking the Secrets of the Brain Neuropsychological tests - measures verbal and non-verbal behaviours that are known to be affected by particular types of brain damage - are used in clinical evaluations of people who may have suffered brain damage through accident or disease - are also important research tools - provide much information about brain-behaviour relations Destruction and Stimulation Techniques - researchers can produce brain damage under carefully controlled conditions in which specific nervous tissue is destroyed with electricity, with cold or heath, or with chemicals - Researchers can also surgically remove some portion of the brain and study the consequences - Human can be studied when an accident or a disease produces a specific lesion or when abnormal brain tissue must be surgically removed - Alternative to destroying neurons: stimulating them – which produces opposite effects (typically) o A specific region of the brain can be stimulated by a mild electric current or by chemicals that excite neurons o Electrodes can be permanently implanted so they can stimulate individual neurons - In chemical stimulation studies, a tiny tube is inserted into the brain so that a small amount of the chemical can be deliv
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