Due to leaky ion channels and electrogenic ion pumps in membrane o As membrane

Due to leaky ion channels and electrogenic ion pumps

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Due to leaky ion channels and electrogenic ion pumps in membrane o As membrane potential rises, bursts of APs are created o This increases intracellular Ca  contraction (strength and duration depend on time course of Ca changes) Can affect frequency or amplitude of contractions, baseline ( tonic ) tension RESULTS Increased contraction: Ach, eserine, atropine, passive stretch Decreased contraction: epinephrine Action Potential Propagation in Peripheral Nerves NEURONS Cell body (soma), cytosplasmic extensions (dendrites- receive electrical impulses and carry them toward soma), axon (carries impulses away from soma) Ability to carry electrical signals is related to difference in electrical potential across surface membrane  membrane potential - determined by 2 factors: o (1) Relative concentrations of ions in and around neuron o (2) Relative permeability of membrane to ions o Goldman eqn ION CHANNELS Voltage-gated Na and K channels responsible for creating and propagating an action potential o Threshold value usually 15-30 mV above resting potential ACTION POTENTIAL Above threshold, v-g Na channels open and membrane moves toward equilibrium potential for Na o Upstroke of AP V-g K channels are opening simultaneously As Na channels begin to close, membrane pot. moves back towards equil potential for K Increased K permeability causes membrane to “overshoot” resting membrane potential o Generating after-hyperpolarization
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V-g K channels close and axon returns to resting membrane potential Given neuron does not vary in size of AP, but does vary in frequency of AP Dealing w/long nerve so many axons o Different fibers have different diameters  different resistances to voltage stimulus o A given stimulus may not be strong enough to initiate an AP in all fibers REFRACTORY PERIODS Absolute refractory – After depolarization of a section of a membrane, Na channels close and cannot be opened (they are inactivated). o Prevents incoming current of Na from reopening channels o Prevents backwards propagation of the AP o Sets upper limit on frequency at which a neuron can generate APs Relative refractory – after absolute, some (but not all) Na channels at rest and can be re- excited o Requires greater voltage stimulus to activate enough channels for AP o In lab, responses observed were an average of responses from population of neurons RESULTS Observed CAP, stimulus response; threshold=0.5 V; max=4 V Temporal summation – 2 stimuli; as ISI increased, CAP decreased (log) Refractory period- reached when ISI reduced to 0.5 ms Conduction velocity= d/t where d=distance between second stimulating and first recording electrode and t=time from onset of stimulus to start of CAP Neuronal Control of Skeletal Muscle SKELETAL MUSCLE Largest tissue in vertebrate body; contractions provide voluntary movement Muscle divided into long, cylindrical cells called muscle fibers which contain myofibrils o 1-2 micrometers in diameter; contain groups of interdigitating thick (myosin) and thin (actin) filaments
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