Cardiac Physiology Question

[g2dent]

Quick question:

Would an increase in sympathetic activation of SA node cells lead to an increase in stroke-volume?

For example, wouldn't increased sympathetic activity from exercise increase SA activity which would increase amount of contractions which would then strengthen the heart and then make contractions stronger?

If you can provide any sources to show this would be great. Thanks for the help in advance!

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[m.z]

Yes it would. Sympathetic stimulation of the SA node would cause an increase in heart rate and force of contraction allowing for greater volume of blood being pumped per heart beat. One of the reasons why professional athletes have a relatively low resting heart rate is that their heart muscles become stronger with all the training that they do, and pump a greater amount of blood per heart beat (better efficiency).

 

[g2dent]

Yes it would. Sympathetic stimulation of the SA node would cause an increase in heart rate and force of contraction allowing for greater volume of blood being pumped per heart beat. One of the reasons why professional athletes have a relatively low resting heart rate is that their heart muscles become stronger with all the training that they do, and pump a greater amount of blood per heart beat (better efficiency).

My professor says that the SA node does not have anything to do with ventricular muscle contraction. Besides the fact that SA node leads to eventual ventricle contraction are there any sources that show that the ventricle increases in contraction strength as a result of increased SA activity?

 

[SuperSneaky24]

I think they are coupled together. While the SA node doesn't necessarily cause the ventricle to contract stronger, sympathetic stimulation as a whole causes the heart rate and stroke volume to increase, thus affecting the SA node and ventricles independently via different sympathetic fibers.

 

[DrknoSDN]

My professor says that the SA node does not have anything to do with ventricular muscle contraction. Besides the fact that SA node leads to eventual ventricle contraction are there any sources that show that the ventricle increases in contraction strength as a result of increased SA activity?

I'd have to agree with the professor. Changing SA node behavior won't alter stroke volume directly. It will increase cardiac output which is just (HR * stroke volume) but shouldn't change anything related to muscle behavior such as contractility or ejection fraction.

Over-stimulation as in SA tachycardia would decrease stroke volume because it would be changing preload from incomplete ventricle filling.
Google Tachycardia Stroke Volume

Since the cardiac muscle is all connected with gap junctions and the AP's are all or nothing, why would increase firing rate increase how forcefully the muscle contracts?

 

[m.z]

The SA node aka the sinoatrial node aka the pacemaker of the heart sets the rate of contraction for the rest of the heart. If the SA node fails, the AV node picks up the slack and does about 40 beats per minute. If you're going to sympathetically stimulate the SA node, your going to sympathetically stimulate the rest of the heart.

"Stimulation via sympathetic fibers causes an increase in the SA node rate (therebyincreasing the heart rate and force of contraction). Sympathetic fibers can increase the force of contraction because in addition to innervating the SA and AV nodes, they innervate the atria and ventricles themselves."
Taken directly from Wikipedia.

 

[DrknoSDN]

The SA node aka the sinoatrial node aka the pacemaker of the heart sets the rate of contraction for the rest of the heart. If the SA node fails, the AV node picks up the slack and does about 40 beats per minute. If you're going to sympathetically stimulate the SA node, your going to sympathetically stimulate the rest of the heart.

"Stimulation via sympathetic fibers causes an increase in the SA node rate (therebyincreasing the heart rate and force of contraction). Sympathetic fibers can increase the force of contraction because in addition to innervating the SA and AV nodes, they innervate the atria and ventricles themselves."
Taken directly from Wikipedia.

Let me rephrase that:
"Sympathetic fibers can increase the force of contraction because they innervate the atria and ventricles themselves."... That's how you should be reading that sentence.
"In addition to innervating the SA and AV nodes", means that the SA and AV nodes don't have anything to do with contractility.

Larger Quote: http://en.wikipedia.org/wiki/Sinoatrial_node

• Stimulation of the vagus nerves (the parasympathetic fibers) causes a decrease in the SA node rate (thereby decreasing the heart rate). Parasympathetic fibers cannot change the force of contraction, however, because they only innervate the SA node and AV node (which control heart rate only)***
• Stimulation via sympathetic fibers causes an increase in the SA node rate (thereby increasing the heart rate and force of contraction). Sympathetic fibers can increase the force of contraction because in addition to innervating the SA and AV nodes, they innervate the atria and ventricles themselves.

So no, the SA node does not alter stroke volume, it's just a pacemaker.

 

[a1medicine]

Dito, take a look at AFIB (atrial fibrillation) , random increased hr although SV remains with out change

 

[a1medicine]

 

[m.z]

Let me rephrase that:
"Sympathetic fibers can increase the force of contraction because they innervate the atria and ventricles themselves."... That's how you should be reading that sentence.
"In addition to innervating the SA and AV nodes", means that the SA and AV nodes don't have anything to do with contractility.

Larger Quote: http://en.wikipedia.org/wiki/Sinoatrial_node

• Stimulation of the vagus nerves (the parasympathetic fibers) causes a decrease in the SA node rate (thereby decreasing the heart rate). Parasympathetic fibers cannot change the force of contraction, however, because they only innervate the SA node and AV node (which control heart rate only)***
• Stimulation via sympathetic fibers causes an increase in the SA node rate (thereby increasing the heart rate and force of contraction). Sympathetic fibers can increase the force of contraction because in addition to innervating the SA and AV nodes, they innervate the atria and ventricles themselves.

So no, the SA node does not alter stroke volume, it's just a pacemaker.

Why don't you reread what I wrote. "If you're going to sympathetically stimulate the SA node, your going to sympathetically stimulate the rest of the heart."