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Biological Sciences
BIOL 1412
Paul Messing

BIOL1412 01 17 2014 Neuronal Physiology E. Action Potential (AP) (Fig. 11.4) * a nerve impulse (signal) * a large change in MP that propagates along an axon with no change in intensity * initiates at trigger zone eg. axon hillock of multipolar + bipolar neurons; just past dendrites of unipolar neurons 1) = GP - membrane potential at the axon hillock reaches - 55mV 2) 3) 4) = Action potential 2) depol. phase (Fig 11.11) a) voltage-gated Na+ channels respond to MP change (i.e. GP) and open - greatly increases Na+ permeability b) as gates open more Na+ diffuses in (further changing MP) => causes even more Na+ gates to open (more Na+ rush in = positive feedback mechanism) c) Na+ diffuses in causing depol. to +30 mV (inside membrane becomes +ve) (Fig. 11.10) 3) repolarization phase (repol.) a) Na+ channels close become inactivated (decreased Na+ permeability) ⇒ Na+ movement returns to resting levels b) voltage-gated K+ channels open (increased permeability) therefore, K+ diffuses out (+ve charges (K+) move out - decreases MP) 4) After-hyperpol. phase (below the RMP) a) K+ channels are slow to close and remain open longer than necessary b) Na+ channels are reactivated - can respond to stimuli at this point * once K+ channels close ⇒ MP returns to RMP (Fig. 11.10) * NOTE: Na+/K+ -ATPase always working to maintain gradients * takes 10 000s of APs to cause a measurable change in [ion] in the cell F. Refractory Periods of an AP: (Fig. 11.14) 1) Absolute Refractory Period (prevents AP summation) * NOAP can be generated, regardless of stimulus size results f
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