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# Phys Exam 1: Ch 5

### Membrane Potential

Define Polarity Separation of charge
Define Potential Measurement of the separation of charge (measurement of polarity), measured in volts
Define volts Unit of measure for potential
What is the Nernst Equation used for? Review the equation from slides. Expresses the relationship of membrane potential to concentrations of a univalent ion
What is the Goldman-Hodgkin-Katz Equation used for? Review the equation from slides. Expresses the membrane potential for multiple univalent ions.
If potential is negative after calculating the Nernst equation, are the ions entering or leaving the cell? If the calculations are positive? Negative result = ions leave the cell Positive result = ions enter the cell
What are the 4 components that make up the resting membrane potential? 1. Diffusion potential of Na; 2. Diffusion potential of K; 3. leakage potential of Na & K; 4. the effects of the Na/K pump
Describe the Na/K pump The Na/K pump requires cleavage of 1 ATP for each 3 Na ions EXPORTED and 2 K ions IMPORTED
To which ion - Na or K - are the leak channels more permeable? ~100x more permeable to K
Practice calculating the diffusion potential for Na using the Nernst equation using standard values. What is the value? Do the same for K. What is the value? Na = +61mV K = -94mV
Practice calculating the net diffusion potential of Na and K using the G-H-K equation using standard values. What is the value? Net diffusion of Na & K = -86.17mV
Define EMF The electromotive force, measured in mV. This is the term for the value of the Nernst and G-H-K equations.
What are standard values of K inside and outside of the cell? Inside = 140mEq/L; outside = 4 mEq/L
What are standard values of Na inside and outside of the cell? Inside = 14 mEq/L; outside = 142 mEq/L
What determines the resting membrane potential? Ddetermined primarily by the diffusion potential of K because the rate of diffusion of K is much more rapid than Na.
What is the action of the Na/K pump? What is the difference (in mV) that this pump sets up? Pumps 3 Na out of the cell, 2 K into the cell. -4mV
What is the resting membrane potential (in mV)? What the two components (in mV) that add up to this value? Resting membrane potential = -90mV; -86 mV from diffusion potentials of Na & K, -4mV from the Na/K pump
Define action potential (AP) A rapid change in the membrane potential that is spread along the nerve membrane
Describe the conditions of the resting membrane before an AP occurs [Na] is higher OUTSIDE the neuron, [K] is higher INSIDE the neuron; Net resting potential is -90mV; Voltage-gated ion channels are closed
Define depolarization (Part 1 of an AP) A decrease in the difference in polarization across the membrane.
Describe the process of depolarization 1. Stimulation briefly opens voltage-gated Na channels; 2. The influx of Na neutralizes the negatively-charged interior of the neuron
Describe the cycle of the voltage-gated Na & K channels when a positive charge is applied (see slide for image) 1. When + charge is applied to Na channel, the activation gate opens. 2. A fraction of a millisecond later the activation gate closes. 3. The Na gate stays closed until the local voltage returns to -90mV
Define repolarization (Part 2 of an AP) The re-establisment of the resting membrane potential
Describe the process of repoalrization 1. As the intracellular potential reaches neutrality, the voltage-gated K channels open. 2. K flows out, carrying positive charge out of the cell and restoring the negative resting potential.
Review the image for an action potential (on slides) The Na channels briefly open, Na ions flow in, intracellular voltage inc & depolarization occurs. As the cell interior becomes + charged, voltage-gated K channels open, repolarization occurs until the cell interior has returned to the resting potential.
Describe the difference between Na, K, and Ca voltage-gated channels Na channels close very quickly after opening (~0.5ms) and stay closed until local voltage is negative. K channels close relatively slowly (~1.0ms). Ca channels are slow to open and slow to close (over ~0.4sec); are found in cardiac & smooth muscle.
Describe why APs only travel in one direction Voltage-gated Na channel proteins recover from activation relatively slowly, and only in very negative polarity. Thus,only "fresh" Na channels along the axons are forward of the AP.
What is the conduction velocity of an AP on a bare neuron? ~1m/sec on a bare neuron
How does the body increase the speed of an AP? What is the term for this process? 1. Adjacent nervous system cells wrap an axon w/layers of lipid-rich membrane: myelin sheath. 2. The presence of the myelin sheath blocks almost all ion channels, allowing for faster signal transmission: saltatory conduction
What is saltatory conduction? The myelin sheath covers most ion channels, so enough Na ions can only enter the axon at regular gaps (Nodes of Ranvier). Ion current flows to the next available Na channels @ next node.
How much faster is saltatory conduction than conduction velocity on a bare neuron? Flow is ~50x faster, so signal is ~50x faster, or 50m/sec on myelinated axons (50m/sec = 1m/20msec)
Which cell forms the myelin sheath in the peripheral nervous system? Schwann cells
Which cell forms the myelin sheath in the central nervous system? Oligodendrocytes
Created by: hclark86