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Chapter 3

Chapter 3! ~ Pg 67 up to Excitatory Synapses

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Kinesiology & Health Science
KINE 2011
Gillian Wu

FALL 2012 FINAL EXAM STUDY GUIDE KINE2011 The Principles of Neural Connection (Pg. 67- )  Nerve and Muscle  Nerve & Muscle undergo transient & rapid changes in their membrane potential o Tissues: Nerve & Muscle, change resting potential to produce electrical signals when excited.  Neurons: receive a signal, initiate a message, transmit a message (neural communication)  Muscle: initiate muscle contractions, for movement & exercise o Plasma Membrane has a resting potential. Changes in resting potential: electrical signals. Two forms of electrical signals: graded potentials (short-distance signals), action potentials (long distance)  Depolarization and Hyperpolarization  Polarization: charges separated across plasma membrane, membrane has potential. Anytime membrane potential is not 0 mv (either positive/negative direction), state of polarization.  Depolarization: Reduction in magnitude of negative membrane potential; membranes less polarized then at resting conditions. Fewer charges separated under resting conditions. Is a movement in positive (+) direction or upward. (inside is less negative)  Repolarization: Membrane back to resting potential after being depolarized. Repolarization is movement in negative (-) direction/downward.  Hyperpolarization: increase in magnitude of negative membrane potential. Membrane is more polarized then at resting conditions. More negative, more charges separated then at resting, movement in the negative (-) direction/downward.  Graded Potentials  Local/short changes in membrane potential that occur in varying grades/degrees of magnitude/strength (could change from -70 to -60 mV [10 mv graded potential])  Triggering Events o Usually from specific triggering (chemical/mechanically gated channels) o Gated Na+ open leading to inward movement of Na+ down the electrochemical gradient o When graded potential happens, charges (Na+) flow between place of origin of this potential & adjacent regions of plasma membrane that are still at resting potential o The stronger the triggering event, the more gated channels that open, the greater the positive changes entering the cell, the larger the depolarizing/hyperpolarizing graded channel at that 1 FALL 2012 FINAL EXAM STUDY GUIDE KINE2011 point of origin. Also, the longer the duration of the triggering event, the longer the duration of the graded potential  Graded Potentials & Passive Currents o Remainder of membrane still at resting potential o Active Area: Temporarily depolarized region (inside of cell more positive then surroundings) o Inactive Area: The surroundings (still at resting potential) o On both sides of membrane, there’s a charge difference between active/inactive areas  Current: Flow of electrical charges o Direction of current flow is determined by direction positive charges are moving o Amount of current flowing depends on difference in potential between areas & resistance material through which charges are moving o Conductors: low resistance (ICF,ECF) o Insulators: High resistance (lipids)- difference between ion movement & current flow o Graded potential is decremental (gradually decreases). This is because of leaking of charge-carrying ions across the uninsulated parts of the membrane (through open channels) o Progressively diminishes with increasing distance from initial site of origin  Action Potentials  Brief, rapid, large (100 Mv) changes in membrane potential during which potential reverses (inside of excitable cell become more positive then outside)  Conducted throughout the entire membrane in nondecremental fashion (do not diminish in strength as travelling from site of initiation throughout remainder of cell membrane)  Example: nerve cell that brings contraction of muscle cells in your big toe (to wiggle it)  Reversal of Membrane Potential  If of insufficient magnitude, graded potential can generate action potential  Portion of excitable membrane where graded potentials are produced in response to triggering event, doesn’t undergo action potential (fire)  Triggering event: membrane depolarization from resting potential (for action potential)  Depolarization is slow until it reaches threshold potential (-55 to -50 Mv) o Explosive depolarization happens (upward deflection of +30 Mv), Potential reverses (overshoot) 2 FALL 2012 FINAL EXAM STUDY GUIDE KINE2011 o Push potential too far After Hyperpolarization (more negative inside membrane, -80 Mv) before resting potential restored o Membrane repolarizes, dropping back to resting potential (action potential or spike) o Duration: 1 msec in nerve cell o If threshold potential isn’t reached by initial triggered depolarization, no action potential takes place, weak depolarization is called subthreshold potential(all-or-none), stimulus is subthreshold stimulus o Action potential occurs only when triggering stimulus & current that’s generated are sufficient to open threshold potential (threshold stimuli)  Changes in Membrane Permeability  During action potential: changes in membrane permeability to Na+ and K+ take place, letting rapid fluxes of their ions down their electrochemical gradients (these movement carry the current responsible for potential changes that occur during action potential)  Action potential triggered because of: opening and closing of voltage-gated Na+ channels & voltage-gated K+ channels  Voltage-gated Na+ and K+ channels  Voltage-gated membrane channels: proteins that have a number of charged groups  Voltage-gated Na+ has two gates: activation gate and inactivation gate (much faster to respond to change in membrane voltage/potential) o Activation Gate: guards channel by opening/closing like a hinged door o Inactivation Gate: ball-and-chain-like sequence of amino acids (open when ball is free on its chain and closed when ball binds to receptor located at channel opening, blocking the opening)  Both gates must be open to allow passage of Na+, if either gate is closed, it prevents passage  Voltage-gated Na+ in three different conformations: 1. Closed but capable of opening (activation gate closed, inactivation gate open) 2. Open, or activated (both gates open) 3. Closed and not capable of opening, or inactivated (activation gate open, inactivation gate closed)  K+ channel has only one gate which can either be open/closed  Exist in addition to Na+-K+ pump & leak channels for these ions  Changes in permeability and ion movement during an action potential  At resting potential (-70 Mv): all voltage-gated channels are closed and Na+ channels are in configuration with inactivation gate open (closed-but-capable- of-opening con formation)  Membrane starts to depolarize toward threshold (from triggering event) o Open activation gate of some Na+ channels o Both electrical & concentration gradient favor Na+ movement inside cell 3 FALL 2012 FINAL EXAM STUDY GUIDE KINE2011 o Inward movement of Na+ ions depolarizes membrane further o Opening of voltage-gated Na+ channels (positive feedback cycle)  At threshold potential: explosive increase in Na+ permeability (Pna+) o Membrane is 600 times more permeable to Na+ then K+ o Membrane potential’s reversed & tends to get closer (+30 Mv) TO Na+ equilibrium potential o At peak of activation potential, inactivation gates of Na+ start to close  How do Na+ channels close? o When they open rapidly, initiates closing process, inactivation gate can bind to receptors. Closing process is slow compared to opening one  Delay of 0.5 sec, allows for Na+ inward and action potential reaches peak o Channel remains in inactivated configuration until membrane potential is back to its resting value o Three action potential events occur at threshold 1. Rapid opening of Na+ activation gates, permitting Na+ to enter, moving potential from threshold to its positive peak 2. Slow closing of Na+ inactivation gates, stops further Na+ entry after brief time delay, keeps potential rising any further 3. Slow opening of K+ gates, large part responsible for the potential coming down from peak to resting potential  Opening of the voltage-gated K+ channels increase K+ permeability (Pk+) about 300 times the resting Pna+, outward movement down the electrical & concentration gradient  At peak: electrical gradient is outward & repel K+ ions, electrical gradient for K+ is outward unlike the resting potential  Hyperpolarization: due to relatively slow closing of K+ channels Review  Rising phase of action potential (threshold to +30 mv) due to Na+ influx (Na+ entering the cell) by an explosive increase in Pna+ at threshold  Falling Phase (+30 mv to resting potential) brought by K+ efflux (K+ leaving the cell), by marked increase in Pk+ occurring simultaneously with inactivation of Na+ channels at peak of action potential  At potential returns to resting, shifts to Na+ channels (closed-but-capable-of- opening)  Newly opened voltage-gated K+ channels close, returns to resting number of open K+ leak channels  More K+ may leave than is necessary to bring potential to resting  This efflux makes interior of cell even more negative then resting potential, 4 causing after hyperpolarization FALL 2012 FINAL EXAM STUDY GUIDE
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