What is the muscular system made of? It is a common homework question, usually paired with a blank diagram and a list of terms like fascicle, tendon and sarcomere. The short answer: the muscular system is a set of organs built from muscle fibers, and those fibers come in three types. Below is the tour, from the whole muscle down to the smallest repeating unit inside it.

Plenty of people are asking. According to Google Trends, for US searches over the past seven days related to the seed term "skeletal system," the top related queries included "skeletal muscle" at 25, "muscular system" at 23, "the muscular system" at 18, "skeletal muscles" at 17 and "skeletal and muscular system" at 14. Among rising queries for that seed, "skeletal muscle sarcomere" was up 40%. For the seed term "cardiovascular system," rising related queries included "muscular system key structures," up 200%, and "muscular system major organs," up 130%.

A quick note on those numbers. Google Trends scores are relative within each seed term: 100 marks the most searched query in that set, and the others are scaled against it. They are not search counts. A rising percentage measures growth in interest, not volume.

This piece sticks to plain anatomy: what the three muscle types are called, where each one sits, how a skeletal muscle is wrapped and attached, and what a sarcomere is. Every definition comes with its source, mainly the National Cancer Institute's SEER Training Modules, the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) and the open textbook Anatomy and Physiology 2e from OpenStax at Rice University.

What the muscular system is, in one definition

The SEER Training Modules put it in one line: "The muscular system is composed of specialized cells called muscle fibers." SEER adds that the predominant function of those fibers is contractibility, and that "Nearly all movement in the body is the result of muscle contraction."

SEER also lists the other jobs anatomy texts assign to the system: maintaining posture such as sitting and standing, contributing to joint stability through tendons that extend over joints, and producing heat as a by-product of muscle metabolism. In SEER's words, "Nearly 85 percent of the heat produced in the body is the result of muscle contraction."

How many muscles are there? Sources round the figure differently. SEER says "There are more than 600 muscles in the body, which together account for about 40 percent of a person's weight." NIAMS, part of the National Institutes of Health, says "There are more than 650 muscles in the human body." Either way, the count is in the hundreds, and both are stated as "more than."

Skeletal, smooth and cardiac: the three muscle types

NIAMS states it plainly: "There are three main types of muscle: skeletal, smooth, and cardiac." SEER's muscle types page describes each one by location, control and how the cell looks.

Skeletal muscle

According to SEER, "Skeletal muscle, attached to bones, is responsible for skeletal movements." Skeletal muscles are under conscious, or voluntary, control, which is why NIAMS notes they are "sometimes called voluntary muscles." SEER describes the fibers as striated, meaning they have transverse streaks, and says the basic unit is "the muscle fiber with many nuclei."

Smooth muscle

SEER places smooth muscle "in the walls of the hollow internal organs such as blood vessels, the gastrointestinal tract, bladder, and uterus." It is under control of the autonomic nervous system and acts involuntarily. SEER describes the cell as "spindle-shaped" with "one central nucleus," and calls it non-striated, which is where the name smooth comes from.

Cardiac muscle

SEER says cardiac muscle is "found in the walls of the heart" and, like smooth muscle, is under control of the autonomic nervous system. SEER describes the cell as rectangular, with one central nucleus like smooth muscle, but striated like skeletal muscle. If you want the vessels that smooth muscle lines, our explainer on arteries, veins and capillaries by name covers them.

Muscular system key structures, from the outside in

Most searches for muscular system key structures come from labeling diagrams of one skeletal muscle. SEER and OpenStax describe the same nested layout, and it reads well from the outside in.

• Epimysium: SEER says "Each muscle is surrounded by a connective tissue sheath called the epimysium."
• Fascicle and perimysium: inside, fibers are grouped into bundles. SEER calls each bundle a fasciculus, wrapped by "a layer of connective tissue called the perimysium." OpenStax calls the bundles fascicles.
• Endomysium: each individual fiber is enclosed by "connective tissue called the endomysium," per SEER.
• Muscle fiber: SEER says "Each skeletal muscle fiber is a single cylindrical muscle cell."
• Myofibril: OpenStax describes the myofibril as running "the entire length of the muscle fiber."
• Sarcomere: the repeating unit inside the myofibril, covered in the next section.

SEER adds that muscles have a rich blood and nerve supply, and that "Before a skeletal muscle fiber can contract, it has to receive an impulse from a nerve cell."

Muscular system major organs

People searching muscular system major organs often expect a short list like the heart or the lungs. Anatomy texts frame it differently. OpenStax says "Each skeletal muscle is an organ that consists of various integrated tissues." So the organs of this system are the individual muscles themselves, the hundreds of them counted above.

What a skeletal muscle sarcomere is

OpenStax defines it directly: "The sarcomere is the functional unit of the muscle fiber." It also gives the boundaries: "The sarcomere, the region from one Z-line to the next Z-line, is the functional unit of a skeletal muscle fiber." OpenStax notes the Z-lines are also called Z-discs, and puts each sarcomere at about 2 micrometers long.

Inside each skeletal muscle sarcomere are two kinds of protein strands. OpenStax calls the actin strands "the thin filament of the sarcomere" and the myosin strands "the thick filament of the sarcomere," because myosin has more mass. The striped look of skeletal muscle under a microscope, OpenStax explains, comes from the ordered arrangement of these actin and myosin myofilaments. That is the same striation SEER uses to tell skeletal and cardiac muscle apart from smooth muscle.

How skeletal muscles attach: tendons

SEER explains that the three connective tissue wrappings extend past the fleshy part of the muscle to form either a "thick ropelike tendon" or a broad, flat sheet called an aponeurosis. These form indirect attachments from muscles to the periosteum of bones, the bone's outer covering. NIAMS defines a tendon as "A special kind of tissue that connects muscles to bones."

SEER describes the usual arrangement: "Typically a muscle spans a joint and is attached to bones by tendons at both ends." It adds that "One of the bones remains relatively fixed or stable while the other end moves as a result of muscle contraction." SEER's muscle groups page uses the terms origin and insertion for those two attachment points in its naming examples.

How skeletal muscles get their names

SEER says most skeletal muscles are named for a feature of the muscle, often combining several. Examples from its list: size ("maximus (large)," "brevis (short)"), shape ("deltoid (triangular)"), fiber direction ("rectus (straight)"), location ("pectoralis (chest)"), number of origins ("biceps (two heads)") and action ("flexor (to flex a structure)").

The skeletal and muscular system as a pair

Anatomy texts often teach the skeletal and muscular system together, and SEER gives the reason in its skeletal system module: "Bones work together with muscles as simple mechanical lever systems to produce body movement." SEER lists tendons as part of the skeletal system too, alongside bones, cartilage and ligaments, so the tendon sits right at the border between the two.

Our take

We like starting with the names. Most confusion about the muscular system is confusion about which muscle type someone means. Skeletal is attached to bone and voluntary. Smooth lines hollow organs and is involuntary. Cardiac is in the heart wall, involuntary and striated.

Then the nesting: muscle, fascicle, fiber, myofibril, sarcomere. Learn the wrappings that go with each layer and a labeling diagram stops being a guessing game.

For the frame the muscles pull on, see what the skeletal system is: bones, joints and marrow by name. For why "lean muscle" shows up in body-composition talk, our piece on lean body mass vs skeletal muscle mass sorts out the terms. Questions about your own body belong with a clinician.

Why we carry it at PRSPR

This piece connects to Natures BBL through vocabulary. This article is anatomy: names for tissues, layers and attachments. Natures BBL is a bundle, one of the multi-product sets in our Bundles collection, listed at $75.98.

Like everything we carry, it is made in a GMP-certified facility and third-party batch tested, so what's on the label is what's in the bottle.

To read the full listing, see Natures BBL at PRSPR.

FAQ

What is the muscular system?

The SEER Training Modules define it as a system "composed of specialized cells called muscle fibers," whose predominant function is contractibility.

What are the three types of muscle?

Skeletal, smooth and cardiac, per NIAMS. SEER places skeletal muscle on bones, smooth muscle in the walls of hollow internal organs, and cardiac muscle in the walls of the heart.

What is a skeletal muscle sarcomere?

OpenStax defines it as the region from one Z-line to the next Z-line, the functional unit of a skeletal muscle fiber, about 2 micrometers long, containing thin actin and thick myosin filaments.

What connects skeletal muscles to bones?

Tendons. NIAMS calls a tendon "A special kind of tissue that connects muscles to bones," and SEER says a muscle typically spans a joint with a tendon at each end.