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Chapter 3

Chapter 3 Notes

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PSYC 1010
Rebecca Jubis

Readings- Chapter 3 The Biological Bases of Behaviour Nervous Tissue: The Basic Hardware Nervous system is living tissue composed of cells Cell are either glia or neurons Neurons: individual cells in the nervous system that receive, integrate & transmit information Every neuron looks different Soma or cell body: contains the cell nucleus & much of the chemical machinery common to most cells Neurons have a bunch of branched, tree-like structures called dendrites Dendrites: the parts of a neuron that are specialized to receive information Axon: a long, thin fibre that transmits signals away from the soma to other neurons or to muscles or glands; they can be as long as a metre Axons are wrapped by white, fatty substance called myelin sheath Myelin sheath: insulating material, derived from glial cells, that encases some axons Myelin sheath speeds up the transmission of signals that move along axons If an axons myelin sheath deteriorates, signals may not be effectively transmitted sometimes lead to multiple sclerosis Axon ends in a clutter of terminal buttons Terminal buttons: small knobs that secrete chemicals called neurotransmitters; these chemicals are like messengers Synapses: a junction where information is transmitted from one neuron to another Overall, information is received at the dendrites, passed through the soma, along the axon, & is transmitted to the dendrites of another cell at meeting points called Glia Glia: cells found throughout the nervous system that provide various types of support for neurons They tend to be smaller than neurons but they outnumber neurons (10-1) Glial cells take up about 50% of the brains volume www.notesolution.com Glial cells supply nourishment to neurons, help remove neurons waste products, & provide insulation around many axons Glial also help the development of the nervous system in the human embryo Glia may also send & receive chemical signals Can be implicated in many diseases; amyotrophic lateral sclerosis (ALS), & Parkinsons disease Glial cells also play an important role in memory formation, & can lead to Alzheimers disease Impaired neural-glial communication might also contribute to schizophrenia & mood disorders The Neuron at Best: A Tiny Battery Hodgkin & Huxley learned that the neural impulse is a complex electrochemical reaction Inside & outside the neuron are fluids containing electrically charged atoms & molecules called ions + charged sodium & potassium ions & - charged chloride ions flow back & forth across the cell membrane. But dont cross at the same rate Usually a slightly higher concentration of charged ions inside the cell Resting potential of a neuron: its stable, - charge when the cell is inactive At rest, its resting potential is at -70 millivolts The Action Potential When the neuron is stimulated, channels in its cell membrane open, allowing + charged sodium ions to rush in which causes an action potential Action potential: a very brief shift in a neurons electrical charge that travels along an axon Absolute refractory period: the minimum length of time after an action potential during which another action potential cant begin This downtime usually lasts 1-2 milliseconds; followed by a brief relative refractory period; the neuron can fire, but its threshold for firing is elevated; more intense stimulation is required to initiate an action potential The All-or-None Law Neural impulse is all or nothing; not halfway www.notesolution.com Neurons have control over how strong the stimulus is A stronger stimulus will cause a cell to fire a more rapid volley of neural impulses than a weaker stimulus Thicker axons transmit neural impulses quicker than thinner ones Can move up to 100 metres per second The Synapse: Chemicals as Couriers During the synapse part, the two neurons dont actually touch, they are divided by a synaptic cleft Synaptic cleft: a microscopic gap between the terminal button of one neuron & the cell membrane of another neuron The neuron that sends a signal across the gap is called the presynaptic neuron; the neuron that receives the signal is called the postsynaptic neuron Neurotransmitters: chemicals that transmit information from one neuron to another Most of these chemicals are stored in synaptic vesicles The neurotransmitters are released when a vesicle fuses with the membrane of the presynaptic cell; its contents spill into the synaptic cleft After release, neurotransmitters diffuse across the synaptic cleft to the membrane of the receiving cell There, they mix in the postsynaptic cell membrane Receiving Signals: Postsynaptic Potentials When a neurotransmitter & a receptor molecule combine, reactions cause a postsynaptic potential (PSP) Postsynaptic potential: a voltage change at a receptor site on a postsynaptic cell membrane They dont follow the all or nothing law; they are graded They vary in size & they increase or decrease the probability of a neural impulse in the receiving cell to the amount of voltage change Two types of messages can be sent from cell to cell excitatory or inhibitory Excitatory postsynaptic potential: a positive voltage shift that increases the likelihood that the postsynaptic neuron will fire action potentials. www.notesolution.com Inhibitory postsynaptic potential: a negative voltage shift that decreases the likelihood that the postsynaptic neuron will fire action potentials The direction & nature of the postsynaptic potential depends on which receptor sits are activated in the postsynaptic neuron The excitatory or inhibitory effects only last a fraction of a second The neurotransmitters drift away from the receptor sits or are converted into inactive forms Reuptake: a process in which neurotransmitters are sponged up from the synaptic cleft by the presynaptic membrane Reuptake allows synapses to recycle their materials Integrating Signals: Neural Networks Neurons must mix signals arriving at many synapses before it decided whether to fire a neural impulse If enough excitatory postsynaptic potentials occur in a neuron, an action potential can be fired If many inhibitory postsynaptic potentials occur, it will cancel the effects of a excitatory postsynaptic potential The state of a neuron is balanced between excitatory & inhibitory influences Our perceptions, thoughts & actions depend on patterns of neural activity in networks The elimination of old synapses appear to play a larger role in the sculpting of neural networks than the creation of new synapses Nervous system often forms more synapses than needed & then gradually eliminates the less active ones Neurotransmitters on Behaviour Neurotransmitters are fundamental to behaviour There are 9 transmitters, about 40 neuropeptide chemicals that function as neurotransmitters & a bunch of new novel transmitters Specific neurotransmitters work at specific kinds of synapses A transmitter has to fit into a receptor site for binding to occur; like a lock & key Acetylcholine (Ach) The only transmitter between motor neurons & voluntary muscles www.notesolution.com
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