I was taught that SA node sets the heart rate because the slope of phase 4 is steeper than AV node or purkinge fibers. So faster depolarization of SA node means AV follows it's rhythm instead of it's own. Now, I am wondering if AV nodes conducts slowly, wouldn't that be a rate limiting factor for setting heart rate ? What's the point of SA node being faster when AV node limits it's conduction speed ? In that case shouldn't AV node or other slower tissues with automaticity set the heart rate ?
SA node being the pacemaker
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rate of depolarization and conduction speed are two different things
the av node conducts slowly compared to the purkinje fibers for example but that doesn't mean that it has to be the rate limiting factor
the av node conducts slowly compared to the purkinje fibers for example but that doesn't mean that it has to be the rate limiting factor
I fail to see the distinction. Doesn't slower depolarizations mean less action potentials per time ?
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let's say you have a pond and you're throwing pebbles in. each time you throw in a pebble, it will make a wave
if you increase the rate at which you're throwing pebbles in, the number of waves per minute will increase
automaticity is increasing so the heart rate goes up
now you decide put in a lot of mud and this makes the waves propagate more quickly
you're still throwing in pebbles at the same rate but the waves are traveling faster so it takes less time for them to reach the edge of the pond
heart rate is the same but it takes a shorter period of time for the impulse to reach the ventricles from the sa node
if you increase the rate at which you're throwing pebbles in, the number of waves per minute will increase
automaticity is increasing so the heart rate goes up
now you decide put in a lot of mud and this makes the waves propagate more quickly
you're still throwing in pebbles at the same rate but the waves are traveling faster so it takes less time for them to reach the edge of the pond
heart rate is the same but it takes a shorter period of time for the impulse to reach the ventricles from the sa node
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It doesn't. The SA node beats at a certain beats per minute. Conduction speed is in milliseconds. If the SA node beats at it's conduction speed continuously without rest, the patient would be dead because the AV node conduction speed (which is slower than the SA node's) would still send the patient into v-fib.
So how is action potential graph related to conduction ? Does conduction involve part of action potential ? Also, when does the the muscles actually contract ? during which phase of action potential.
Start by defining what an action potential is (forget the graph for the moment). Then figure out the process of how an action potential in location A (for example the SA node) triggers contraction somewhere far away (the ventricles). That information (if you want to call it that) has to get there somehow. So, how does depolarization work in muscle? How are the muscle fibers put together?
From what I understood, the depolarization is transmitted through gap junction.
Do some reading in Hurst's The Heart. Here's the simplified version. Depolarization is an electrical signal that can be propagated along a conduction system (I think of it in terms of specific wires or circuits in the heart). There is a difference in the depolarization rate, and the conduction rate. The depolarization rate is what comes into play with the automaticity of the SA or AV nodes. The conduction rate is how fast that signal gets from point A to point B. The SA node automatically "fires" on its own at a certain rate faster than the AV node or the purkinji fibers. That signal is then conducted along the wires to the AV node. The AV delays that signal from being conducted at its original speed to the ventricles. (Why might this be important?) Then it lets the signal on through to the ventricles. There is a conduction delay, slowing of the signal yes. So the signal is conducted at different rates, yes. But it is the generation of that signal that is the difference between the SA and AV nodes. The cells in the AV node have to depolarize too to propagate the signal along. But the times we're talking about for depolarization events (60-100 for the SA node) are a lot longer than it takes for the depolarization, conduction, repolarization, etc. to happen. So, the automatic depolarization events in the SA node happen faster than the automatic depolarization events in the AV node, thus it sets the intrinsic rate. Fun fact, the actual intrinsic rate of the SA node is closer to 100 as seen in transplant patients since they have lost their innervation to keep it down.
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OP, you really just need to understand the difference between the rate of spontaneous depolarization and the rate of conduction of that initial spontaneous depolarization through the rest of the heart. Even though the AV node conducts the depolarization slower than the preceding and following fibers in the pathway, it's rate of spontaneous depolarization is still much slower than the SA node, which is why the SA node is the pacemaker.
