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Action Potential: Two phases At threshold potential, membrane permeability to sodium suddenly changes in the region of stimulation. 1. Depolarization : As sodium channels open, sodium ions rush in, and the membrane potential changes and becomes depolarized.

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  1. Action Potential: Two phases At threshold potential, membrane permeability to sodium suddenly changes in the region of stimulation. 1. Depolarization: As sodium channels open, sodium ions rush in, and the membrane potential changes and becomes depolarized. Rapid decrease polarization of membrane Inside cell becomes more + (outside more -) How? Opening of (2 Voltage-gated) ion channels 20,000 Na+ rush in = + charge inside K + flows out = 2. Repolarization: At the same time, potassium channels open to allow potassium ions to leave the cell, the membrane becomes repolarized, and resting potential is reestablished. Restore membrane to resting state 9 - 1

  2. repolarization depolarization Threshold is reached at -55 mV. All-or-None…once threshold is reached an AP must be fired. hyperpolarization

  3. Action Potential

  4. Hyperpolarizing Phase Occurs when repolarization OVERSHOOTS Why? K+ channels are wide open Pumps come into play to balance out. Refractory Period after an AP the channels of the neuron are inactive period of inactivity lasts a few milliseconds cell can't be triggered Na+ channels are inactive and unable to transport Na + depolarization during refractory period DOES NOT trigger channel opening

  5. All-or-None Response Principle Either the stimulus is large enough to cause an AP or it will not occur. Large stimulus still creates the same ‘size’ AP. Weak stimulus = no AP.

  6. Nerve Impulse A. A nerve impulse is conducted as action potentials local current travels down the fiber/axon. B. Impulse Conduction: 1. Unmyelinated fibers conduct impulses over the entire surface. 2. Myelinated fibers conduct impulses from node of Ranvier to node of Ranvier, because ionic movement is inhibited beneath myelin sheath. This is called saltatory conduction. Saltatory conduction is up to 50x faster than conduction on unmyelinated neurons. Conserves energy because Na-K Pump is not needed as extensively because only nodes of Ranvier are depolarized and repolarized Diameter and length of axon can affect speed (the greater the diameter the greater density of voltage gated) 9 - 6

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