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Neurotransmission ISAT 351, Spring, 2004 College of Integrated Science and Technology James Madison University
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Neuron Function Neurons (nerve cells) receive, conduct, and transmit signals Neurons carry signals from sense organs to the central nervous system (brain and spinal cord) where they are processed From the central nervous system, neurons convey signals to muscles and glands
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Neuron Structure The cell body contains the nucleus and receives signals from other neurons on branches called dendrites or directly on the cell body The axon conducts signals away from the cell body and divides into many branches at the nerve terminal
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Neurons Signal Signal Propagation Relay to next Reception (electrical) cell (chemical)
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Electrical & Chemical Signal Propagation Electrical Signal Signal propagation within neuron Branched axon terminus amplifies signal Terminus makes synapses with target cells Chemical Signal Propagation between cells Neurotransmitters Relay electrical signal via exo- & endocytosis Targets: Another neuron Dendrite Muscle cell
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Types of Neurons Sensory neurons receive and convert stimuli from the environment into electrical signals Interneurons receive signals from neurons and transmits signals to neurons Motor neurons receive signals from interneurons and stimulate muscle or glands
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Structures are Similar
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Neuron Signals Electric signals transmit information within a cell from the cell body to the axon terminus by an electric impulse called an action potential Chemical signals transmit information from sensory cells, between neurons (synapses), and to specialized cells such as muscle or glands
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Nerve Signals
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Neurons Form Circuits
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Electrical Signal Nerve signals are changes in the electrical potential across the neuron’s plasma membrane (membrane potential) The action potential or nerve impulse can carry a message without signal attenuation Action potentials actively propagate signal via voltage-gated Na + channels Explosion of activity propagated & amplified along membrane
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Electrical Signal Myelin sheath insulates nerve Prevents signal attenuation Promotes signal propagation and amplification Multiple sclerosis involves demyelination
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Electrical Signal = Action Potential Intra- & extracellular [ion] different [K + ] high internally [Na +, Cl - ] high externally Consequences: Unequal distribution of cations and anions Baseline membrane potential changes when ion distribution changes
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++++++++++++++++++ -------- Propagation of Action Potential Resting V 1 V 2 Baseline Membrane Action Potential Propagation : Potential -60mV -40mV Depolarization Wave +---+++++---++++ +++---+++--- ++++---++++++---++ ---+++----+++- Recovery
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So, Depolarizing membrane by about 20 mV triggers action potential
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Voltage-Gated Channels Mediate Action Potential Depolarization causes channels to open and an influx of anions (Na+) causes further depolarization resulting in the action potential. How is the membrane repolarized?
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Three Conformational States Channel inactivated until K + ions repolarize membrane; speeds recovery
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The Action Potential
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Voltage-Gated Channel
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Measurement of Potential
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Propagation Measurement 1 electrode inside, other outside Stimulate & measure as a function of time V 1, V 2, V 3 have identical amplitudes Shape & intensity of potential maintained Zero attenuation as signal propagated
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Consequences All-or-none; neurons are resting or conducting Amplitude constant, so size of action potential not important THE FREQUENCY OF ACTION POTENTIAL FIRINGS CARRY INFORMATION RATE OF PROPAGATION FACILITATED BY MYELIN INSULATION
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Synapses Communicate Between Neurons 10-100 BILLION neurons in human brain 10-100 TRILLION synapses Human forebrain: ratio of synapses:neurons about 40,000:1 Elastic: improve connectivity by using neurons Neurons communicate via neurotransmitters: Electrical-to-chemical-to electrical signal conversion
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Electrical to Chemical Signal Conversion at Synapse
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Synapses The action potential opens voltage-gated Ca + channels at the nerve terminal The increase in Ca + triggers the release of neurotransmitters into the synaptic cleft The neurotransmitter diffuses across the synaptic cleft, binds to the target cell, and triggers an action potential
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Conversion Back to Electrical Signal
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Neurotransmitter Tidbits Certain psychotic drugs (cocaine, morphine) & venoms mimic NT Feel good with dopamine and serotonin Natural reward system appeared early in evolution; reinforce behaviors favorable to survival Prozac et al
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Dopamine Malfunctions Parkinson’s disease Insufficient dopamine due to destruction of cells that synthesize dopamine Motor malfunctions appear after about 70% of neurons destroyed Schizophrenia hallucinations: excessive dopamine Tourette’s syndrome: supersensitive receptors
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Dopamine and Addictions Stimulate feel good effects of dopamine using alcohol, nicotine, marijuana, and amphetamines Amphetamines stimulate secretion Cocaine keeps [dopamine] high Dopamine may be common end-point of addictions; different mechanisms Addicts’ feedback mechanisms impaired Consequence: dopamine deficit
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Use it or lose it! Mental activity over lifetime reinforces synaptic junctions
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Learning and Memory Thousands of nerve terminals synapse on a neuron Combination of synapses determines if action potential is initiated Synaptic pathways provide a mechanism to store, analyze, and recall inputs
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