The Action Potential 9-5-08

The Action Potential 9-5-08 - The Action Potential Dr...

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Unformatted text preview: The Action Potential Dr. McKemy 9/05/2008 Na+ channels open K+ channels open Nomencalture of the Action Potential Myelinated axon of a motor neuron Soma of a motor neuron Soma in the inferior olive Soma of a Purkinje neuron Neuronal differences The squid led the way! The squid led the way! http://www.science.smith.edu/departments/NeuroSci/courses/bio330 /squid.html The Voltage Clamp Technique http://www.science.smith.edu/departments/NeuroSci/courses/bio330 /squid.html Flow of current during voltage steps Identified twotypes of voltagedependent currents. Early and late currents. Different voltagedependencies Current produced at different potentials Early currents are dependent upon Na+ Pharmacology separates the components into Na+ and K+ components Early and late currents Membrane conductance changes underlying the action potential are time and voltagedependent Mathematical reconstruction of the action potential Regenerative nature of the AP In the resting state, most Na+ channels are closed. K+ channels are open. Negative-inside. If Na+ channels open, the resulting influx of Na+ overwhelms the efflux of K+. Net inward movement of (+)charges. n Also leaves behind Cl-. Now the cytosolic face of the membrane has a net (+)-charge. n Depolarized. n Moved towards ENa. Axon contains Na+ and K+ channels that are gated by changes in MP. In addition to resting K+ channels. Coordinated opening and closing of Na+ and K+ channels Refractory Period Propagation of an action potential Role of Na+ and K+ channels Can the AP be made to conduct faster? Role of Na+ and K+ channels Passive current flow in an axon Voltage dissipates because: 1. 2. The cytoplasm has high resistance and is a poor conductor. The membrane is not totally impermeable and some charge (ions) is lost due to leak. Vx = voltage at any distance X along membrane. Vo = voltage change at point of current injection. e = base of natural log. = the length constant of the axon rm = resistance across the membrane. ri = resistance of the intracellular axoplasm. ro = resistance of the extracellular fluid. The length constant Vt: change V at time t. V: V at steady state. t: time after injection. : the membrane time constant = rm cm rm: membrane resistance cm: cytoplasm resistance The time constant Saltatory AP conduction along a myelinated axon Saltatory AP conduction along a myelinated axon Myelinated axons conduct faster Myelinated axons conduct faster Multiple Sclerosis (MS) is an inflammatory autoimmune disorder that destroys the myelin sheath. De-myelination and inflammation along axonal pathways. Symptoms: n Monocular blindness Lesions in the optic nerve n Double vision Lesions in the medial longitudinal fasciculus n Weakness or paralysis Lesions in the corticospinal tracts n Altered somatic sensations Lesions of somatosensory pathways. n Dizziness Lesions in the vestibular pathways Multiple sclerosis and myelination ...
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