Quick question regarding heart rate/stroke volume regulation Watch

Nerdcubed
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Hiya,

I was just revising up on the effects of the parasympathetic and sympathetic nerves on the frequency and force of contraction of the heart and wondered why 2 nerves are required. When there is an increased demand for oxygen, sympathetic stimulation increases and parasympathetic stimulation decreases, increasing cardiac output. The inverse is true when the body is relaxed. However, why does the body utilise 2 nerves? Surely by lowering or raising the strength of just one nerve, you would achieve the same effect?
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Nerdcubed
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Bump.
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Nerdcubed
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carrotstar
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(Original post by Nerdcubed)
Hiya,

I was just revising up on the effects of the parasympathetic and sympathetic nerves on the frequency and force of contraction of the heart and wondered why 2 nerves are required. When there is an increased demand for oxygen, sympathetic stimulation increases and parasympathetic stimulation decreases, increasing cardiac output. The inverse is true when the body is relaxed. However, why does the body utilise 2 nerves? Surely by lowering or raising the strength of just one nerve, you would achieve the same effect?
There might be a more logical answer, but the one that just popped into my head is why should it be one nerve? We still have body hair but we don’t need it. It’s just how we evolved, and it works, so nature hasn’t improved the process. We still have fingernails but we don’t need them. If the body was constantly evolving to solve every tiny problem or illogical function, we probably wouldn’t have negative emotions, people would no longer have life-threatening degenerative illnesses/diseases etc...

Perhaps not the answer you were looking for but I hope that makes sense!
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Nerdcubed
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That makes sense. There hasn't been any selection pressures so the genetic frequency remains constant. I do wonder if there is a deeper physiological answer for my question but for now, that seems like a rational explanation.

Thanks for your contribution.
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moonkatt
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It’s been a while since I did any physiology, but as far as I can remember (and probably an incredibly basic way to explain it), the SA node fires at around 100 bpm, so for various conditions, you need an accelerator (sympathetic) or a brake (parasympathetic) to achieve a suitable heart rate above or below that intrinsic rate.
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macpatgh-Sheldon
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Nerdcubed

Hi, I saw post earlier, but saw 4 -5 answers so assumed it had been answered correctly, so did not open it.

The answer is to do partly as an analogy to other systems in the body e.g. the homeostatic mechanisms: a specific example would be the control of serum glucose levels - we have a hormone (insulin) that lowers glucose, but also one that raises it (glucagon); in addition, we have other hormones with an anti-insulin action, namely adrenaline, growth hormone and thyroxine. The reason for this is that it is imperative to control and modulate certain physiological parameters like glucose level and cardiac output (by changing stroke volume mainly by inotropic effects, and heart rate by chronotropic effects) EXTREMELY PRECISELY just like it is to maintain blood glucose or serum electrolytes. It is only possible to control something VERY VERY ACCURATELY if there are two opposing forces. We humans have copied this principle in control systems in engineering where two opposing forces are used to control a current or other modality very precisely.

A simple illustration of this principle is the action of antagonistic muscles: you can hold out with your arm outstretched a cup of tea and you will not spill it (unless you have a disease like Parkinson's or cerebellar ataxia) BECAUSE YOUR ANTERIOR DELTOID (and upper pectoralis major) placing a flexion movement at the shoulder joint is working in conjunction with your extensors of the arm like the LATISSIMUS DORSI and others - if you had only one muscle acting in that plane, you would defo spill the tea!

In contrast to the idea of others on this post [and with due respect to them for their evaluative thinking], that this might be due a lack of evolutionary pressure, this superb feat of nature is actually quite the opposite!

Hope this helps!

M
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Nerdcubed
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That clears up a lot; thanks a bunch!
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Staf
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Hello, I thought I would add to the answers already given with respect to the heart in particular. As already alluded to in a previous answer, the intrinsic rate of the SA node (which controls how fast the heart beats) is around 100bpm. Now to bring that down to our normal resting heart rate of around 70bpm, a tiny electrical pulse is sent from the brain to the heart at regular intervals (I'm trying to keep this simple to help understanding) via the Vagus nerve. This electrical pulse does not contain any information in itself, it is just either on or off, much like the electrical pulses sent to muscles to make them contract. You're not 'telling' the muscles to contract, you're just sending electrical pulses that make them contract, and so to make the muscle relax, you simply send less electrical pulses. So with that in mind, if the electrical pulses to the heart become even more frequent, the heart rate would go even lower than 70bpm because more electrical stimulation via the Vagus nerve results in more parasympathetic activity.

So, doing the opposite to the above, if we send less electrical pulses via the Vagus nerve to the heart's SA node it will speed up. If no electrical pulses were sent at all, the heart rate would rise back to 100bpm (the intrinsic rate of the SA node). Hopefully this makes sense so far.

Ok, so what if we only had this one nerve sending electrical pulses to the heart? Well, by taking in what we now know from above, the maximum rate the heart can now attain is only 100bpm. Something else is needed to send an electrical pulse to the SA node to get it to fire faster. We can't use the Vagus nerve because if we stimulate that, it will only serve to slow the heart down again. We need another nerve that, when stimulated, makes the heart speed up beyond 100bpm, this is where the sympathetic nerves step in. An increase in electrical activity from the sympathetic side causes the heart to beat above the 100bpm 'intrinsic' rate in order to meet demand.

This is a very simplistic way of how all this works as in reality there is always a balance between parasympathetic and sympathetic activity to enable the heart to quickly react to the requirements of the body.

Hope this helps in addition to the answers you have already got.
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