neurons structure and nerve impulse
TRANSCRIPT
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NeuronsStructure and Conduction of a Nerve Impulse
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Two coordinating systems which respond to environmental stimuli
Nervous System & Endocrine (hormone) System
Begin with Nervous System (data processing system)
3 interconnected functions input / integration / output
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Basic Organization
• Sensory Input triggered by stimuli – conduction of signals to
processing center
• Integration– interpretation of sensory
signals within processing centers
• Motor output– conduction of signals to
effector cells (i.e. muscles, gland cells)
sensory receptor (sensory input) integration (motor output) effector
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Neuron
• Dendrite - conducts “signal” toward the cell body -- [input zone]– often short, numerous & highly branched– signal comes from sensory cell or neighboring neuron
• Axon - usually a single fiber -- [conducting zone]– conducts signal away from cell body to another neuron or effector cell
• Axon Ending – a cluster of branches (100’s to 1000’s) – each with a bulblike synaptic knob – relays signal to next neuron / effector cell
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Generation - Conduction of Neural Impulses
• Dependent on concentration gradients of Na+ & K+
– [Na+ ] greater outside– [K+ ] greater inside
• Membrane permeability– lipid bilayer bars passage of K+ &
Na+ ions– protein channels and pumps regulate
passage of K+ & Na+
• at rest more Na+ (3) move out than K+ (2) move in
• Na+ ions pumped out leave behind excess negative charge
• Sodium-potassium pump – Na+ out - K+ in (3 Na+ out 1 K+ in)
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Overview of Neural Impulse
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• Maintenance of negative charge within neuron– resting membrane potential about -70 millivolts – [5% voltage of AA battery]
• Dissolved organic molecules [negative charge] kept inside
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• Stimulus causes opening of Na+ gates & closing of K+ gates -
• Threshold [~ +30 mV] – all - or - nothing response
• Action potential localized electrical event
• Changes permeability of region immediately ahead– changes in K+ & Na+ gates– domino effect – propagation of signal
• Intensity of stimuli (i.e. pinch vs. punch) = number of neurons firing
• Speed on impulse based on diameter of axon & amount of myelination [wire for internet]
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Myelin Sheath• Resembles chain of beads
• Prevents ions from flowing through membranes
• Na+ channels highly concentrated at nodes
• Allows signal to travel faster because impulse “jumps” from node of Ranvier to node of Ranvier (with myelin sheath (225 mph / without 11 mph)
• MS destruction of mylin sheath by own immune system (progressive loss of signal conduction, muscle control & brain function)
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Neurons Communicate at Synapses
• Electrical [no synapse]– common in heart & digestive tract - maintains steady, rhythmic
contraction– All cells in effector contain receptor proteins for neurotransmitters
• Chemical - skeletal muscles & CNS– presence of gap (SYNAPTIC CLEFT) which prevents action
potential from moving directly to receiving neuron– ACTION POTENTIAL (electrical) converted to CHEMICAL SIGNAL
at synapse (molecules of neurotransmitter) then generate ACTION POTENTIAL (electrical) in receiving neuron
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Overview of Transmission of Nerve Impulse
• Action potential synaptic knob opening of Ca+ channels neurotransmitter vesicles fuse with membrane release of neurotransmitter into synaptic cleft binding of neurotransmitter to protein receptor
molecules on receiving neuron membrane opening of ion channelstriggering of new action potential
• Neurotransmitter is broken down by enzymes & ion channels close -- effect brief and precise
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Nerve Impulse
• Presynaptic neuron
• Vesicles
• [Calcium channels]
• Synaptic cleft
• Postsynaptic neuron
• Neurotransmitter receptor
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Nerve Impulse
• Action potential synaptic knob
opening of Ca+
channels neurotransmitter
vesicles fuse with membrane
release of neurotransmitter into synaptic cleft
Ca2+
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Nerve Impulse
• Action potential neurotransmitter
vesicles fuse with membrane
release of neurotransmitter into synaptic cleft
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• Action potential binding of
neurotransmitter to protein receptor molecules on receiving neuron membrane
opening of sodium channels
triggering of new action potential
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Neurotransmitters• Catecholamine Neurotransmitters
– Derived from amino acid tyrosine• Dopamine [Parkinson’s], norepinephrine, epinephrine
• Amine Neurotransmitters– acetylcholine, histamine, serotonin
• Amino Acids– aspartic acid, GABA, glutamic acid, glycine
• Polypeptides– Include many which also function as hormones– endorphins
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• Transmission of signals based on MULTIPLE STIMULI– combined excitatory &
inhibitory neurons
• Inhibition in Pre-synaptic neuron – Ca+ channels blocked
• stops release of neurotransmitter
• Inhibition in Post-synaptic neuron– opens Cl- channels
• makes interior more [-]
• increase permeability of K+ ions– makes interior more [-]