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Chapter 3

Chapter 3 PS101.docx

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Carolyn Ensley

PS101 Chapter 3 – The Biological Bases of Behaviour Week 4 Communication in the Nervous System -Behaviours depends on rapid information processing. Information travels almost instantaneously from your eye to your brain, from your brain to the muscles off our arm and hand, and from your palms back to our brain -Your nervous system is a complex communication network in which signals are constantly being transmitter, received, and integrated -It handles information Nervous Tissue: The Basic Hardware -The cells in the nervous system fall into two major categories: glia and neurons -Neurons are individual cells in the nervous system that receive, integrate, and transmit information -They are the basic links that permit communication within the nervous system -They communicate with other neurons -A small amount of them receive signals from outside the nervous system or carry messages from the nervous system to the muscles that move the body -The soma, or cell body contains the cell nucleus and much of the chemical machinery common to most cells -The rest of the neuron is to handle information -Neurons have individual branches called dendrites -Dendrites are the parts o a neuron that are specialized to receive information -From the dendrites, information flows into the cell body and then travels away from the soma along the axon -The axon is a long, thin fibre that transmits signals away from the soma to other neurons or to muscles or glands -The myelin sheath is insulating material, derived from glial cells, that encases some axons -The myelin sheath speeds up the transmission of signals that move along axons -Multiple sclerosis is due to a degeneration of myelin sheaths -The axon ends in a cluster or terminal buttons, which are small knobs that secrete chemicals called neurotransmitters -The point at which neurons interconnect are called synapses, a junction where information is transmitted from one neuron to another -Information is received at the dendrites, passed through the soma and along the axon, and is transmitted to the dendrite of other cells at meeting points called synapses Glia -Glia are cells found throughout the nervous system that provide various types of support for neurons -Glia is literally glue -Outnumber neurons 10-1 -Glia may also send and receive chemical signals -They may be implicated in diseases -May also play an important role in memory formation -Play a crucial role in the experience of chronic pain -Although glial cells may contribute to information processing in the nervous system, the bulk of this crucial work is handled by the neurons PS101 Chapter 3 – The Biological Bases of Behaviour Week 4 The Neural Impulse: Using Energy to Send Information -What happens when a neuron is stimulated? The Neuron at Rest: A Tiny Battery -Neural impulse is a complex electrochemical reaction -Both inside and outside the neuron are fluids containing electrically charged atoms and molecules called ions -Positively charged sodium and potassium ions and negatively charged chloride ions flow back and forth across the cell membrane, but they do not cross at the same rate -The resting potential of a neuron is its stable, negative charge when the cell is inactive The Action Potential -As long as the voltage of a neuron remains constant, the cell is quiet and no messages are being sent -When he neuron is stimulated, channels in its cell membrane open, briefly allowing positively charged sodium ions to rush in -An action potential is a very brief shift in a neuron’s electrical charge that travels along an axon -The firing of an action potential is reflected in the voltage spike -After the firing of an action potential, the channels in the cell membrane that opened to let in sodium close up -The absolute refractory period is the minimum length of time after an action potential during which another action potential cannot begin -This is not very long and is followed by a brief relative refractory period – where the neuron can fire, but its threshold for firing is elevated, so more intense stimulation is required to initiate an action potential The All-or-None Law -Either the neurons ire or they don’t -Weaker stimuli do not produce smaller action potentials -Neurons can convey information about the strength of a stimulus -They do so by varying the rate at which they fire action potentials -Various neurons transmit neural impulses at different speeds The Synapse: Where Neurons Meet Sending Signals: Chemicals as Couriers -The neurons don’t actually touch -They are separated by the synaptic cleft, a microscopic gap between the terminal button of one neuron and the cell membrane of another neuron -Signals have to cross this gap to permit neurons to communicate -The arrival of an action potential at an axons terminal buttons triggers the release of neurotransmitters – chemicals that transmit information from one neuron to another -Within these buttons, most of the chemicals are stored in small sacs, called synaptic vesicles Receiving Signals: Postsynaptic Potentials -When a neurotransmitter and a receptor molecule combine, reactions in the cell membrane cause a postsynaptic potential (PSP), a voltage change at a receptor site on a postsynaptic cell membrane PS101 Chapter 3 – The Biological Bases of Behaviour Week 4 -They do not follow the all-or-none law – they are graded -Two types of messages can be sent from cell to cell: excitatory and inhibitory -An excitatory PSP is a positive voltage shift that increases the likelihood that the postsynaptic neuron will ire action potentials. An inhibitory PSP is a negative voltage shit that decreases the likelihood that the postsynaptic neuron will fire action potentials -The direction of the voltage shift depends on which receptor sites are activated in the postsynaptic neuron -Reuptake, a process in which neurotransmitters are sponged up from the synaptic cleft by the presynaptic membrane Integrating Signals: Neural Networks -A neuron may receive a symphony of signals from thousands of other neurons -A neuron must integrate signals arriving at many synapses before it decides whether to fire a neural impulse -The firing of a single neuron is not enough to create the twitch of an eyelid in sleep, let alone a conscious impression…Millions of neurons must fire in unison to produce the most trifling ought -Most neurons are interlinked in complex chains -The elimination of od synapses appears to play a larger role in the sculpting o neural networks than the creation of new synapses -The elimination of old or less-active synapses is called synaptic pruning -Neurons do not act alone in influencing behaviour but that they are linked in complex networks or cell assemblies -One neuron stimulating another neuron repeatedly produces changes in the synapse learning has taken place Neurotransmitters and Behaviour -There are nine well-established, classic transmitters, about 40 additional neuropeptide chemicals that function as least part time, and a handful or novel neurotransmitters -Just as a key has to fit a lock to work, a transmitter has to fit into a receptor site for binding to occur -Specific transmitters can deliver signals only at certain locations on cell membranes Acetylcholine -It is a transmitter between motor neurons and voluntary muscles – every move you make depends on it -It also contributes to attention, arousal, and memory -The activity of Ach may be influe
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