The Basics of Muscle

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No matter how weak and flabby you may feel, you've got muscles. Not only would you otherwise be completely unable to move, but your intestines wouldn't pass food along, your heart wouldn't beat, your blood pressure would never change, and the hairs on your skin would never stand on end. Almost anything the body does that requires significant movement is done with muscle fibers.

There are three main muscle types: cardiac, skeletal, and smooth.

 The key differences lie in how the fibers of these muscles are arranged. Overall, though, these muscles have more in common than not.

For the remainder of this article, when in doubt, I'm discussing skeletal muscle, which stretches over joints to help us move our bodies around.

The Structure of Muscle Fibers

Under the microscope, muscles look like a bunch of fibers. In skeletal muscle, these fibers align with each other, so that when they shorten, the muscle applies force in one direction. Each of these fibers is itself made up of even smaller fibers called myofibrils. Myofibrils contain overlapping strands of the proteins actin and myosin. In actuality, the myosin molecule looks something like a hammer, with a head and a long body that resembles the hammer’s handle. In a series of chemical reactions, requiring both calcium and adenosine triphosphate (ATP), the head of myosin attaches to the actin and moves along it.

This increases the amount of overlap between the actin and myosin strands, shortening the overall muscle as it contracts.

If you interweave your fingers just at the tips, you could picture the right fingers as being strands of myosin, and the left as actin. Now slowly bring your hands together so the strands overlap further, and you get a very rough model of how increasing overlap between these molecules causes muscle contraction.

 First, though, the muscles need to receive a signal that it's time to contract.

How the Brain Signals the Body to Action

The brain communicates with the muscles through nerve fibers. The nerves that send signals to muscles are called motor neurons. Each nerve branches out along the muscle and sends signals to different parts. Some neurons can innervate a very large number of muscle fibers, such as in the thigh. Others, such as those that control fine movements of the face and hands, may only innervate up ten fibers, which allows for more subtle movements.

Each motor neuron releases the neurotransmitter acetylcholine at the level of the muscle. The neurotransmitter is normally stored in little vesicles, like tiny balloons or closed pockets with the acetylcholine inside. When the chemical is released from the end of the nerve fiber, the acetylcholine crosses a short gap called the neuromuscular junction, and then causes an electrochemical shift in the membrane of the muscle, leading to a complex calcium-mediated reaction that ultimately leads to muscle contraction.

Motor Units and Muscle Fiber Types

The combination of the individual motor neuron and all the parts of the muscle with which it communicates is called a motor unit. There are many different types of motor units:

  • Slow twitch fatigue resistant units— these respond to lower levels of activation. While these are highly resistant to fatigue and can be used for hours, they don’t produce as much power. These units rely heavily on oxygen. 
  • Fast-twitch fatigue resistant units— These are weaker than the above and respond to higher levels of activation. The range of fatigue resistance varies from fairly high to intermediate, with a maximum duration of use between a minute to 30 minutes. These have some degree of oxygen use, but also the ability to operate without oxygen for some period.
  • Fast-twitch fatigable units—these are very forceful in contraction and require the highest level of activation from the neuron. The fastest type of muscle fibers have a low ability to use oxygen, instead relying on a process called glycolysis.

The force of muscle contraction is increased by increasing the number of motor units involved. Usually, this happens in a set order, with fatigable units activated last in order to save energy. This order can be reversed under certain conditions, like pain, which activate fast fibers first, allowing for an immediately strong reaction.


So in brief, the brain sends signals to a muscle to contract along a motor nerve, which is branched out across a bunch of different areas in the muscle. Nerves attached to slow twitch units fire first, then as required force increases, other nerves fire that are more fatigable but also more powerful. Each time the nerve fires, acetylcholine crosses the neuromuscular junction and stimulates an electrical shift that causes actin and myosin to increasingly overlap, decreasing the muscle length and moving the body.


Huned Patwa and Hajime Tokuno, Diseases of the Muscle, in Comprehensive Board Review in Neurology, ed. Mark Borsody, 2012

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