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I am relatively new to Physiology, and I've just learnt about muscle tone. I however find it difficult to understand the electrical changes that initiate (and/or accompany) muscle tone in smooth muscles . To be specific, I know that action potentials(AP) cause muscle contraction (just like they cause nerve impulse propagation), and I also know that smooth muscle tone is a result of irregular contractions of the muscles at rest. How then can these muscles contract when no AP is generated at rest?

PS: This question has been edited to achieve more clarity.

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Action potentials usually maintain muscle tone in skeletal muscles, but muscle cells are affected by other influences that can affect their contraction and thus the tone of the muscle.

Smooth muscle occurs in the walls of hollow organs, most notably the blood vessels and the GI tract, and in a few other places. Contraction of smooth muscle is generally involuntary and can be stimulated by the APs from the autonomic nervous system, but often for the slow maintenance of muscle tone contraction is controlled by hormones, other external molecules, or external pressure. The mechanisms that cause this contraction generally involve the phosphorylation of myosin light chains. (1) (2)

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  • $\begingroup$ If action potentials can at all maintain muscle tone, why is muscle tone spoken of as passive contraction? (en.m.wikipedia.org/wiki/Muscle_tone). As much as I know, passive processes don't involve ionic changes seen in active ones (such as action potentials). If I am wrong about this can you point out the misconception? $\endgroup$
    – Chemo-Mike
    Sep 25 '20 at 21:36
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Disclaimer: it has passed some time since my studies in muscle physiology.

Muscle have stretch receptors to monitor the change in length. These are connected to the central nervous system, where information is properly processed and the response triggered. This response, as far as I know, propagates through action potential (AP) across neurons, which activate the muscle (transverse tubules, AP, Ca2+). In summary, muscle tone should be regulated by APs and I am not aware of another mechanism of muscle contraction in this case.

Do you have some references where they discuss another mechanism of non-AP muscle tone? I am very curious to know more.

/Emilio

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    $\begingroup$ My several days of research on the mechanism has turned out futile. But this is the statement which prompted my question: "...smooth muscles of some visceral organs maintain a state of partial contraction called tonus or tone. It is due to the tonic contraction of the muscle that occurs without any action potential or any stimulus...". It is from the text: essentials of Medical Physiology by K Sembulingam $\endgroup$
    – Chemo-Mike
    Sep 25 '20 at 13:55
  • $\begingroup$ @Chemo-Mike Pretty important to indicate you are talking about smooth muscle in your question if that's the case. $\endgroup$
    – Bryan Krause
    Sep 25 '20 at 19:18

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