xAVx
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Hi,

I'm currently studying Physiology and was wondering if anybody could give me a bit of explaination to this which i found rather confusing. It is with regards to the parasympathetic and sympathetic nervous system.

I was reading the Sherwood Physiology book From Cells to Human, and under the eye chapter, it states that..."The cilliary muscles are controlled by the autonomic nervous system, and sympathetic stimulation causes the muscles to relax while parasympathetic stimulation causes it to contract"

Based on this interpretation, it means the sympathetic nervous system causes the cillary muscles to relax. If that's the case, sympathetic nervous system is also responsible for causing the lens to be flattened and weak. This allows us to see far.

But isn't the sympathetic nervous system also known as the fight or flight response and is the system that is in use in times e.g. when the body is in exercise and hence not really the default mode that the body is in? That would be weird because based on my understanding it would mean that the body in its relaxed mode can only see near and not far.

Which would then not really make sense else as the body age people wouldn't be able to see far (instead of getting prebyopia).

I think i'm rather confused about the concepts here and hope someone would be able to explain it to me!

Thanks!
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KA125
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You're over complicating this soooooooooo much


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Hype en Ecosse
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(Original post by xAVx)
Hi,

I'm currently studying Physiology and was wondering if anybody could give me a bit of explaination to this which i found rather confusing. It is with regards to the parasympathetic and sympathetic nervous system.

I was reading the Sherwood Physiology book From Cells to Human, and under the eye chapter, it states that..."The cilliary muscles are controlled by the autonomic nervous system, and sympathetic stimulation causes the muscles to relax while parasympathetic stimulation causes it to contract"

Based on this interpretation, it means the sympathetic nervous system causes the cillary muscles to relax. If that's the case, sympathetic nervous system is also responsible for causing the lens to be flattened and weak. This allows us to see far.

But isn't the sympathetic nervous system also known as the fight or flight response and is the system that is in use in times e.g. when the body is in exercise and hence not really the default mode that the body is in? That would be weird because based on my understanding it would mean that the body in its relaxed mode can only see near and not far.

Which would then not really make sense else as the body age people wouldn't be able to see far (instead of getting prebyopia).

I think i'm rather confused about the concepts here and hope someone would be able to explain it to me!

Thanks!
The sympathetic nervous system isn't solely a "fight or flight" function - its sole purpose isn't to deal with fear/stress response. And the parasympathetic nervous system isn't solely a "rest and digest" function: these are just little rules of thumbs to help you remember the functions that they regulate. There are, of course, exceptions where functions of these two divisions of the autonomic nervous system don't make sense when placed under that rule of thumb.
The body's "default mode" IS a combination of sympathetic and parasympathetic activity, and the former isn't only active during "fight or flight" (an easy example to see how this is is the way beta-blockers decrease heart rate: that wouldn't work unless the SNS had constant input).

Regarding your example, I think you should read up on "the accommodation reflex" to learn about the triggers of this SNS/PSNS activity in the eye. The parasympathetic nervous system is responsible for most contraction responses in the eye (pupillary reflexes, accommodation reflex). I have no idea why it's evolved that way, but it's likely to be a very evolutionarily old trait that's highly conserved given the location of the Edinger-Westphal nucleus.
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xAVx
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(Original post by Hype en Ecosse)
The sympathetic nervous system isn't solely a "fight or flight" function - its sole purpose isn't to deal with fear/stress response. And the parasympathetic nervous system isn't solely a "rest and digest" function: these are just little rules of thumbs to help you remember the functions that they regulate. There are, of course, exceptions where functions of these two divisions of the autonomic nervous system don't make sense when placed under that rule of thumb.
The body's "default mode" IS a combination of sympathetic and parasympathetic activity, and the former isn't only active during "fight or flight" (an easy example to see how this is is the way beta-blockers decrease heart rate: that wouldn't work unless the SNS had constant input).

Regarding your example, I think you should read up on "the accommodation reflex" to learn about the triggers of this SNS/PSNS activity in the eye. The parasympathetic nervous system is responsible for most contraction responses in the eye (pupillary reflexes, accommodation reflex). I have no idea why it's evolved that way, but it's likely to be a very evolutionarily old trait that's highly conserved given the location of the Edinger-Westphal nucleus.
Thanks so much for the clear answer

Another question i have that wasn't covered during lectures and my tutor isn't replying to my emails

I understand resting membrane potential of neurons is -70mV. Is the same true for skeletal muscle cells? Because from the various diagrams in the myology chapter, it seems the resting potential is actually -90mV. If that's the case does refractory period apply to muscle cells? (bearing in mind that aboslute refractory period is at -90mV where the membrane is hyperpolarized) Because if they do then there's the problem of not being able to contract the muscle continuously?? (No action potential being able to be generated in skeletal muscle cell -> no activation of dihydropyridine receptors -> no activation of ryanodine receptors -> no Ca2+ entry into muscle fibres, no subsequent contraction)




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Hype en Ecosse
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(Original post by xAVx)
Thanks so much for the clear answer

Another question i have that wasn't covered during lectures and my tutor isn't replying to my emails

I understand resting membrane potential of neurons is -70mV. Is the same true for skeletal muscle cells? Because from the various diagrams in the myology chapter, it seems the resting potential is actually -90mV. If that's the case does refractory period apply to muscle cells? (bearing in mind that aboslute refractory period is at -90mV where the membrane is hyperpolarized) Because if they do then there's the problem of not being able to contract the muscle continuously?? (No action potential being able to be generated in skeletal muscle cell -> no activation of dihydropyridine receptors -> no activation of ryanodine receptors -> no Ca2+ entry into muscle fibres, no subsequent contraction)




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The first thing to understand is that resting membrane potentials aren't fixed levels. Neuronal RMPs tend to vary from -55mV to -70mV, and muscle cells tend to vary from -70mV to -90mV. The "refractory period" doesn't have one threshold that applies to all excitable tissues, for example, cardiomyocytes have an effective refractory period while in positive membrane voltages. What's true for nerve cells isn't necessarily true for other excitable tissues. I've never heard of absolute refractory periods in muscle (otherwise, how would tetany be possible?).
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(Original post by Hype en Ecosse)
The first thing to understand is that resting membrane potentials aren't fixed levels. Neuronal RMPs tend to vary from -55mV to -70mV, and muscle cells tend to vary from -70mV to -90mV. The "refractory period" doesn't have one threshold that applies to all excitable tissues, for example, cardiomyocytes have an effective refractory period while in positive membrane voltages. What's true for nerve cells isn't necessarily true for other excitable tissues. I've never heard of absolute refractory periods in muscle (otherwise, how would tetany be possible?).
I've never heard of it either :P Was wondering if there was, some of these questions sounds kinda stupid to be asked in PBL either Doesn't help that it's counted for grades. One last stupid question!! To be clarified If the skeletal muscles are innervated by the somatic nervous system, it also means to say that it is controlled by the SNS. This doesn't mean that there aren't any autonomic neurons present right? Because if thats the case there wouldn't be reflex actions? (Because those are involuntary?) would the alpha motor neuron and gamma neuron be considered to be part of the autonomic nervous system then?

I know these are really major issues, but its like a constant bug i kinda want the answer to.

Thanks so much!
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