How are our muscles structured and how do they work?

Striated muscles allow us to move our body. They are usually joined by tendons to our bones, which is why they are also called skeletal muscles. Striated muscles make up a large part of our body weight.

Typical examples include the muscles in our arms and legs. But the muscles in the tongue, larynx and throat, some facial muscles, and the diaphragm are also striated muscles.

We can consciously control the striated muscles using signals sent by our central nervous system.

Under a microscope you can see the horizontal lines on the individual muscle cells that give the muscles their name. The striated muscle cells are also called muscle fibers because of their thread-like form. A single muscle fiber can be up to several centimeters long.

Muscles do their work by contracting, or tensing. The muscle fibers have little “powerhouses” called myofibrils in them that are responsible for the contractions.

Skeletal muscles are made up of the muscle belly and the two ends of the muscle. The muscles are usually connected to the bones by tendons: They transfer the force of the muscle to the bone.

Skeletal muscles are made up of lots of muscle fibers. They are covered on the outside by connective tissue made of solid collagen fibers, the fascia. There is another layer of stabilizing connective tissue below that. The connective tissue fibers turn into tendon-like material at each end of the muscle.

Inside the muscle, every single muscle fiber is encased by loose connective tissue. More rigid connective tissue groups several muscle fibers together into fascicles. Lots of these muscle fascicles are then grouped together by connective tissue into bundles of muscle fascicles.

Blood vessels and nerves that supply and stimulate the muscles run all through the connective tissue. They provide additional tension and stability. The connective tissue allows different muscle fiber groups to shift against one another when we move.

We move when our muscles alternately contract and relax. Muscles get shorter when they contract. That makes them pull on the tendon, and with it the bone, at the end of the muscle.

Many movements need more than one muscle: In fluid movements, two muscles work together with opposing forces. For example, to bend your forearm your biceps (the muscle at the front of the lower arm) contracts. At the same time, the triceps relaxes at the back of the upper arm. The reverse happens if you want to stretch your forearm out again. The triceps contracts and the biceps relaxes.

Smooth muscles are mainly found in the blood vessels, the hair follicles (the layer around the hair roots), the urinary tract, and the gastrointestinal tract. They influence the shape and functioning of our organs. We have no conscious control over our smooth muscles – they work by on their own.

Smooth muscles are slightly contracted even at rest. That state is called resting tension. Nerve signals, hormones, or the stretching of the intestines after we eat cause them to gradually contract. For instance, smooth muscles are responsible for the wave-like movement of the bowels during digestion.

The heart is almost entirely made up of muscles. Like the smooth muscles, we can’t consciously control them. Our heart beats on its own.

The heart’s muscle cells are striated like the skeletal muscles. The muscle cells are not parallel, though. They are intertwined with each other.

There are two kinds of heart (cardiac) muscle cells:

• Specialized muscle cells that generate electrical signals. They set the rhythm that the heart beats at.
• Muscle cells that strongly contract as soon as they receive an electrical signal. We feel that as our heartbeat. They relax after each contraction and wait for the next signal.

Together, these cardiac muscle cells provide the force needed to pump the blood throughout our entire body.