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Handout 2 Sol

Handout 2 Sol - TA Anna Gruzman E-mail [email protected]

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TA: Anna Gruzman Section 2 E-mail: [email protected] 1/18/10 1. Explain why there is no current generated in graph A. Explain what causes the positive and negative current in graph B. A: Background: Conventions of Current: - Positive Current: Out of Cell - Negative Current: Into Cell - Development of the voltage clamp o Squid has an axon that signals its escape muscles and this is a giant axon o Can put in a chamber and squeeze out all of the cytoplasm o Could put a wire inside an axon ( two wires) and measure the membrane potential and the current o Called a voltage clamp because one could fix the membrane potential ( Vm) to any desired level o Since now the driving force is fixed any changes in current will now reflect changes in conductance ( G) o Hodgkin and Huxley worked with the squid axons and in 1952 published a series of papers that gave the mechanism of the action potentials o Voltage graph clamp ( see notes) o Outward current is positive and inward current is negative: conventions of current At rest: zero net current If depolarize to zero mV and maintains this you get current When raised membrane potential - Have stimulus ( higher than threshold) o Sodium channels: Depolarize ( Open quickly) o K+ channels: Hyperpolarize ( Opens slowly) Helpful information on definitions of current: - The terms inward or negative for current in one direction and outward or positive for current in the opposite direction are conventions. An inward movement of positive ions is defined as negative or inward current, whereas outward movement of positive ions produces positive or outward current. Electrically, inward movement of negative ions cannot be distinguished from outward movement of
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positive ions, so inward movement of negative ions produces a positive or outward current.” ---- http://physioweb.med.uvm.edu/cardiacep/EP/handh.htm - Purpose: Keep the Vm constant and see how the voltage changes based on the movements of ions. 2. Explain why at a voltage of 70 and 90 the curve shows only positive (outwards) current and account for the dip that occurs at roughly one ms. A) In graph A the resting membrane potential of a cell is normally around -70mv. Going from -65mV to -130 mv hyperpolarizes the cell and thus does not generate an action potential or open ion channels. Thus there is no net movement of ions and no current. B) In part be there is a depolarization from -65mV to 0 mV. This depolarization results in
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Handout 2 Sol - TA Anna Gruzman E-mail [email protected]

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