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Lecture 9

Week/Lecture 9: Neuronal Electrophysiology.pdf

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BIOL 171

Neuronal Electrophysiology Electrical Signals in Neurons neurons produce two types of electric signals propagated action potentials that can travel long distances local graded potentials that do not spread from their site of formation different in size; don't spread far down axon Potential Difference separation of charge (positive and negative separated; like battery ends) measured in volts (V) in living cells: charges (ions) are separated by semi-permeable membrane 2 positively charged ions to create potential difference (Na+ and K+) ions flow through ion channel in the cell membrane (charged particles cannot cross membrane) potential differences are measured in millivolts (mV) Membrane Ion Channels passive (leakage) channels ions constantly leaking across membrane, along with concentration gradient open and close randomly- no stimulus required cells generally contain more K+ channels than Na+ channels (facilitated passive diffusion) active (gated) channels open or closed; needs activity to open voltage-gated channels are opened/closed by voltage changes in near vicinity of membrane (-70mV to -50mV) ligand-gated channels are opened/closed by chemical stimuli binding to channel (hormones, neurotransmitters, ions, Ach) mechanically-gated channels are opened by mechanical/physical stimulation: shape change (vibration, touch, stretching) Resting Membrane Potential potential difference measured across the cell membrane of an unstimulated (resting) nerve cell normally -60mV to -70mV resting membrane potential = -70mV result of different composition of ICF (K+ and proteins) compared to ECF (Na+ and Cl-) membrane has many more K+ leak channels than Na+ leak channels more + charges leaving cell than entering; so inside is more negative than outside determined by the relative permeability of membrane to Na+ vs K+ (25% more permeable to Na+) Potassium-Sodium pump helps even charge out by pumping in 2 K+ and pumping out 3 Na+ Graded Potentials small local deviations from RMP hyperpolarization when membrane potential becomes more negative "more polarized": millivolt value getting lower depolarization when membrane potential becomes more positive "less polarized": millivolt value moving higher Action Potentials "nerve impulse" brief, transient, local period of membrane depolarization membrane potential changes from -70 mV to +30 mV and back again phases: depolarization RMP becomes more positive ex: -70 mV --> -40 mV ex: 0 mV--> +30 mV repolarization RMP becomes more negative by sending out potassium cannot send out Na+, because requires stimulus to go against concentration gradient as soon as Na+ gates close, K+ gates open still more K+ inside and Na+ outside cell after movement return of depolarized membrane potential towards RMP ex: +30 mV --> -70 mV (after-)hyperpolarization getting more negative (overshoot resting potential) because K+ channel opens slower, gives time for Na+ ions to enter; both stimuli occur simultaneously; by the time K+ channel decides to close, a little extra K+ has leaked out of the cell (hyperpolarization: a little more negative than needed) Concentration difference drives action potential. Restoration of ion equilibrium concentration difference drives action potential must restore original concentrations of Na+ and K+ in order to enable future AP's Threshold Potential -55 mV point at which a local potential becomes a self-propagating action potential graded potentials are mini AP's that don't reach threshold potential "all or nothing" law: once threshold potential is reached, full action potential ensues- no half AP's- all the way to +30 mV All-or-none Phenomenon if the threshold potential is exceeded, an action potential is generated if the threshold potential is NOT exceeded, NO action potential is generated all action potentials are identical Refractory Period time following the onset of an action potential when it is impossible/difficult to produce a second action Absolute refractory period: impossible for new AP Na+ channels inactivated (must return to their resting state before they can be reopened) even very strong stimulus will not elicit another AP Relative refractory period: possible for new AP Na+ channels have closed, but K+ channels are still open a suprathreshold stimulus will elicit an AP Sodium gates voltage-gated Na+ channels have 2 gates: activation gate inactivation gate in a resting neuron, inactivation gate is open, activation gate is closed
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