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Phys Lect 5

What are excitable cells? Cells that can generate action potentials (AP) **Allows for electrical communication between cells.
4 different types of cells that are considered excitable 1.Neurons. 2.Skeletal Muscle. 3.Cardiac Muscle. 4.Smooth Muscle.
How do excitable cells relay information? All neuronal info is carried/encoded as Action Potentials
What is an Action Potential? Transient reversal of Em of approx 100mV. They are regenerative
Action Potential Stimuli 1.Electrical (Heart and some smooth muscle). 2.Mechanical (touch, pressure, pain, stretch). 3.Thermic. 4.Chemical (Neurotransmitter).
What is the most common Action Potential Stimuli? Chemical Neurotransmitter.
What type of stimuli do sensory receptors use? Mechanical
Main Phases of basic AP 1.Stimulus reaches the threshold (-55mV). 2.Depolarization (Em rapidly becomes more positive). 3.Repolarization (rapidly becomes more negative moving back towards Em).
Plateau of AP Seen in Cardiac myocytes ONLY. It occurs after the peak of depolarizatoin, before repolarization.
Hyperpolarization of AP The cell repolarizes past the normal Em. This occurs in neurons. **Also seen in low concentrations of K+
Threshold Around -55mV. If a cell depolarizes to this point due to a stimulus, an AP will be generated. Otherwise no AP will be generated.
Neuronal vs Ventricular Myocyte AP Neuronal: Em (-70mV), AP is a brief 1-2ms spike. Vent Myocyte: Em (-90mV), AP has a spike and dome shape that lasts 300ms.
"All or None" AP Below the threshold, an increase in the stimulus causes an increase in the voltage response but no AP. At the threshold, an AP is triggered. An above-threshold stimulus does not cause an increase in the magnitude of the AP
Does an increase in stimulus change the magnitude of the AP? NO, it just reaches the threshold SOONER
What happens to the amplitude of an AP as it is conducted along an excitable cell? The amplitude of the AP doesnt change with distance. **Other electrical signals (sub threshold or hyperpolarizing stimulus) decay with distance from the stimulus point.
Refractoriness limits the frequency of APs, preventing them from summating with eachother (adding together to make a bigger AP).
Absolute refractory period No AP can be generated no matter how big the stimulus. **This is because the Na+ channels inactive.
Relative refractory period An AP can be generated, but only by a larger than normal stimulus. **This is b/c some of the Na+ channels can now be activated again, but the stimulus has to be large enough to activate all that are available.
What type of exicatable tissue is a long refractory period important? Cardiac myocytes. A longer refractory period prevents arythmias and allows systole.
What causes Depolarization? Increased permeability of Na+ allowing an influx of Na+ and the Em to get closer to Ena.
What causes Repolarizatoin? Increased permeability of K+ allowing an efflux of K+ out of the cell and Em to get closer to Ek.
Ion flow as an electrical current The current flow, I, is dependent on the driving force and the conductance of the membrane ion channels (essentially whether they are open or not) **the current flow through open channels will vary according to the value of the membrane potential
Explain why nernst potentials of K+ and Na+ affect the Current via driving force Large Current: when the membrane potential is far from the Nernst potential for that ion, the driving force is large. Small Current: When the membrane potential is near the Nernst potential for that ion, the driving force is low
What type of current generates depolarization? Rapid activation of a Na+ current which slows near the peak of the AP (indicating that the driving force is much smaller since the Em is approaching the Ena and less ions are moving into the cell).
What type of current generates repolarization? K+ current which is generated more slowly, increasing as the Em moves away from the Ek. It reaches its highest point just after the peak of the AP.
When is the Na+ current the Strongest? It peaks just before the peak of the AP. It has such a rapid increase due to the fact the the Em at threshold (-55mV) is so far from Ena (+70mV).
When is the K+ current the Strongest? It peaks just after the peak of the AP. The K+ current gradually gets stronger as the Em moves closer to the Ena, creating a greater K+ Driving Force.
Na+ to K+ conductance (g) ratio at: Rest gK > gNa **This allows the Em to be -70mV.
Na+ to K+ conductance (g) ratio at: Depolarization gNa+ > gK+ **Na+ channels open at -55mV and there is a much stronger Driving Force for it to move into the cell.
Na+ to K+ conductance (g) ratio at: Repolarization gK > gNa **This is because the Driving Force for K+ is much stronger.
Na+ to K+ conductance (g) ratio at: Peak of AP gK+ = gNa+
The Na+ current is ALWAYS: INWARD **Because of concentration and electrical gradients (140 outside, 10 inside)
The K+ current is ALWAYS: OUTWARD **Because of concentration and electrical gradients. (5 outside, 140 inside)
What returns the concentration of ions back to normal so that AP can continue to be regenerated? Na+/K+ ATPase
Created by: WeeG



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