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Chapter 3

Action Potentials

What is the composition of extracellular and intracellular fluid? Extracellular fluid has high concentration of sodium and potassium ions and low concentration of potassium ions. Intracellular fluid is the inverse.
Why is intracellular fluid more negatively charged than extracellular fluid despite the high concetration of positive potassium ions? There are large proteins in the intracellular fluid that are negatively charged.
What is the resting potential of the cell? -70mV, it is measured when the cell is not processing a message
What are the two principles that govern movement of ions? Diffusion (molecules move from areas of high concentration to areas of low concentration) and electrostatic pressure (attraction and repulsion)
Explain why potassium ions remain in the cell during resting potential. Assume that ions move freely Diffusion pushes potassium ions out, but electrostatic pressure attracts potassium ions in.
Explain why Chloride ions remain in the cell during resting potential. Assume that ions move freely Diffusion pushes chloride ions in, but electrostatic pressure attacts chloride ions out.
Explain the movement of sodium ions during resting potential, assuming ions move freely Diffusion pushes sodium in, and electrostatic pressure attracts sodium in as well.
Although sodium is supposed to be attracted into the cell during rest, sodium concentration in the cell still is low. Why? Most sodium channels are closed at rest. What little sodium leaked into the cell is removed by the sodium-potassium pumps.
During rest, what ion is allowed through by the membrane? Potassium.
The resting potential is supposed to be -93 mV. Why is it -70 mV instead? Positively charged sodium leaks into the cells
What happens in an action potential? 1. Depolarization occurs: intra -5mV 2. Na+ enters cell: intra +40 mV. 3. K+ leaves near peak: intra is -charged 4. Na+ channels close at peak 5. Hyperpolarization: K+ channels close, Na+ channels gradually reopen, Na-K pumps pump Na+ out
What are the differences between sodium and potassium channels during action potential? 1. Sodium channels open very rapidly, potassium channels open very slowly. 2. Sodium channels open only for a while, and reopen only when near resting potential. Potassium channels remain open for longer.
What is the absolute refractory period? A period when no stimulus whatsoever can produce another action potential.
What is the relative refractory period? A period when a cell can only respond to larger than normal input. More depolarization is needed to reach firing threshold.
What is all-or-none? It means that the action potential either fires or it doesn't.
How do neurons respond to different levels of stimulus? Neural firing can vary to reflect stimulus intensity.
What is propogation? When an action potential reproduces itself down the length of an axon from the axon hillock.
What is the refractory period's role in action potentials? It prevents action potentials from occurring in the previous length of axon. Thus, action potentials travel in one direction.
Describe propagation. 1. Sodium ions are moved by diffusion and electrostatic pressure to the adjacent axon segment. 2. Potassium ions in the segment are pushed by the sodium ions into the next axon segment. 3. Depolarization occurs. Threshold reached, firing commences.
How does myelin aid propagation? 1. Myelin prevents leakage of sodium ions as they move through axon -> fast travel 2. At nodes of Ranvier, another actionpotential is generated as concentration of ion channels is high.
What is saltatory conduction? In myelinated axons, the thing where action potential essentially zooms through the segments of axon covered by myelin.
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