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What are Involuntary Muscles?

By Greer Hed
Updated Mar 03, 2024
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Involuntary muscles are muscles that are not controllable consciously, and instead contract due to unconscious impulses sent by the autonomic nervous system or certain specialized cells or hormones. Both smooth muscle and cardiac muscle can be classified as involuntary muscles. Smooth muscle is comprised of spindle-shaped cells that have no striations and is found in numerous locations throughout the human body. Cardiac muscle is striated rather than smooth, and is found only within the walls of the heart.

Smooth muscles are involuntary muscles composed of thick and thin protein filaments that are homologous to the organelles known as myofibrils found in skeletal muscles. The thin filaments are composed of a globular protein called actin, while the thick ones are made up of a motor protein called myosin. Smooth muscles require extracellular calcium ions to contract: the ions activate a nucleotide called Adenosine triphosphate (ATP), which then activates the myosin filaments. The myosin filaments attach to the actin filaments in a process known as the crossbridge cycle, which causes the thick and thin filaments to slide over each other and contract. When the myosin filaments release the actin filaments, the muscle relaxes.

There are many types of smooth muscle in the human body. Smooth involuntary muscles control the iris of the eye, which contracts and expands involuntarily in accordance with changing light levels. The process of peristalsis, which creates a wave that pushes food through the esophagus and small intestine, is also controlled by involuntary muscles. Smooth muscle can also be found in the respiratory tract, the reproductive systems of both women and men, the ciliary muscle of the eye, and the urinary bladder. Most blood and lymphatic vessels in the human body are lined with smooth muscle cells, allowing them to constrict and dilate.

Cardiac muscle is also sometimes considered to be involuntary muscle. However, cardiac muscle shares features of both smooth and skeletal muscle tissue. It is striated like skeletal muscle tissue, but its contractions are involuntary, like those of smooth muscle tissue. Cardiac muscle is unique in that is particularly invulnerable to fatigue.

Contractions of cardiac muscles are controlled by the nerve impulses delivered by a group of cells located in the right atrium of the heart called the sinoatrial node. These contractions push blood through the four chambers of the human heart, the atria and ventricles. They also move blood throughout the veins and arteries of the circulatory system. These contractions, like those of smooth muscles, are initiated by calcium ions that come from outside the muscle cell.

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Discussion Comments
By anon302260 — On Nov 08, 2012

The question is, why do we twitch?

By croydon — On Apr 19, 2012

@Mor - Well, the lungs are controlled by the body in both involuntary and voluntary ways. You can hold your breath, but you usually can't kill yourself by doing that, and it takes quite a lot of effort to even hold it for a minute.

And you keep breathing in your sleep without needing to concentrate on it.

I think if you could control your own heart beat it would work like that. The difference between voluntary and involuntary muscles is that there is no need for us to consciously control the involuntary ones.

There are only a few times I can imagine needing to stop the digestive process or slow my heart beat and fiddling about with either of those processes would be kind of dangerous and probably nowhere near as efficient as my body can do by itself.

So, I'll stick to eating at the right times and breathing slowly to calm my heart I think.

By Mor — On Apr 19, 2012

@bythewell - I suspect what actually happens in those cases is that the person manipulates the parts of their body that they can control rather than the muscle tissue that has involuntary regulation of contraction.

For example, if someone wants to slow their heart beat they only need to breathe more slowly and calmly for a while and that will slow it.

It won't slow it right down, but sleep will and if they put themselves into a kind of trance, that could be the same thing.

I'm not saying it's not difficult, or even that it can all be explained by science at the moment, but I don't think there's anything mystical about it and I very much doubt they are actually controlling their involuntary muscles. There simply isn't a brain connection for people to be able to do that, and for a very good reason.

If people could control their heart beat voluntarily it would be way too easy to kill them. One little mistake and you'd quickly be dead.

By bythewell — On Apr 18, 2012
I've always been fascinated by the idea of involuntary muscles, or more specifically, when people try to control those muscles.

Like the monks who are supposed to be able to slow their heart beat down to almost nothing so that they can hibernate, or the people who can make their muscles produce more heat than usual so that they can survive in really cold weather (although that's controlling an involuntary reaction in muscles rather than specifically controlling involuntary muscles).

I'm not sure how they do it, but it almost seems like a super power and I have always wanted to learn how to do it myself.

I guess it takes years of practice though and I don't think I'm patient enough to do it when it comes right down to it.

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