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Chapter 3

Chapter 3

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University of Toronto St. George
Dan Dolderman

Chapter 3 The Biological Bases of BehaviourCommunication in the Nervous SystemNervous Tissue The Basic Hardware Nervous system is living tissue composed of cellscells fall into two major categoriesGlia glueprovide structural supportnourishmentinsulation for neurons also involved in the removal of waste help maintain chemical environment of the neurons which promotes more efficient signalling in the nervous systemNeuronsindividual cells in the nervous system that receive integrate and transmitinformationpermit communication within the system sending messages around Neurons composed ofsoma dendrites axon myelin sheath terminal buttons synapsesSomacell bodycontains cell nucleusmuch of the chemical machinery common to most cells Dendritesparts of a neuron that are specialized to receive information branchlikeAxonlong thin fibre that transmits signals away from soma to other neuronsmusclesglandsMyelin sheathinsulating material derived from glial cells that encases some axons speeds up transmission of signals that move along axons Terminal buttonssmall knobs that secrete chemicals called neurotransmittersthese chemicals serve as messengers that may activate neighbouring neurons at the end of axonsSynapsea junction where information is transmitted from one neuron to another SummarizeInformation is received at the dendrites is passed through the soma and along the axon and is transmitted to the dendrites of other cells at meeting points called synapses The Neural Impulse Using Energy to Send Information The Neuron at Rest A Tiny Battery store of potential energy Hodgkin and Huxleythe neural impulse is a complex electrochemical reaction Fluids inside and outside the neuron containing electrically charged ions cell membrane is semipermeable permitting movement of some ions charged sodiumpotassium ions andcharged chloride ions flow across the membrane dont cross at the same rate causes a slightly higher concentration ofions inside the cellresting potentialstable negative charge about 70 mv when the cell is inactiveThe Action Potential When neurons stimulated channels in its cell membrane open briefly allowingcharged sodium ions to rush incharge is lessorcreating an action potentialAction potentialvery brief shift in a neurons electrical charge that travels along an axon after its firing the channels in its cell membrane close upAbsolute refractory periodminimum length of time after an AP where another AP cannot beginfollowed by a brief relative refractory periodneuron can fire but its threshold for firing is elevated more intense stimulation is required to initiate an AP The AllorNone Law Either neurons fire or they dontits action potentials are all the same size Neurons can convey info about the strength of a stimulus by varying the rate at which they fire APsstronger stimulus will cause a cell to fire more rapid neural impulses than a weaker one Various neurons transmit neural impulses at different speedsex thicker axons transmit neural impulses more rapidly than thinner onesdoThe Synapse Where Neurons Meet Sending Signals Chemicals as CouriersSynaptic cleftmicroscopic gap between the terminal button of one neuron and the cell membrane of another neuron the two neurons wont actually touch are separatedSignals must cross this gap to permit neurons to communicate neuron that sends the signal is a presynaptic neuron the neuron that receives the signal is a postsynaptic neuron Arrival of an AP at the terminal buttons triggers the release of neurotransmitterschemicals that transmit info from one neuron to anotherstored in synaptic vesicles within the buttonsNeurotransmitters released when a vesicle fuses with the membrane of the presynaptic cell its contents spill into the synaptic cleftthey diffuse across to the receiving cells membrane may bind with special molecules in the postsynaptic cell membrane at various receptor sitesReceiving Signals Postsynaptic Potentials Postsynaptic Potential PSPa voltage change at a receptor site on a postsynaptic cell membranewhen a neurotransmitter and receptor molecule combine vary in size increase and decrease the probability of a neural impulsein proportion to the amount of voltage changeTwo types of messages that can be sent from cell to cell excitatory and inhibitorydepends on which receptor sites are activated in the postsynaptic neuron Excitatory PSP voltage shift increases likelihood that the postsynaptic neuron will fire APs Inhibitory PSP voltage shift decreases likelihood that the postsynaptic neuron will fire APsEffects produced last only a fraction of a secondthen the neurotransmitters drift away from receptor sites or are inactivated by enzymes that metabolize convert them into inactive formsMost are reabsorbed into the presynaptic neuron through reuptakea process in which neurotransmitters are sponged up from the synaptic cleft by the presynaptic membrane Integrating Signals Neural NetworksNeurons integrate signals arriving at many synapses before deciding whether to fire a impulse Enough excitatory PSPs causes electrical currents to add up voltage reaches the threshold at which an AP will be fired if many inhibitory PSPs also occur tend to cancel the excit effectsThus the state of the neuron is a weighted balance between excitatory and inhibitory influences Neurons are interlinked in complex pathwaysnetworksfire together to perform functions
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