You reach for a cup, stand up from a chair, or take a breath without thinking about it. Each of those moments runs on the muscular system, and if you have ever opened an anatomy chapter and wondered what the muscular system actually includes, you are in good company. The short answer: it is the organ system made of your skeletal muscles, the parts that pull on bone, hold you upright, and do a few jobs most people never notice.

Search interest backs that curiosity up. In Google Trends data for the US over the past 7 days, top related searches for the seed term "muscular system" were led by "the muscular system" with a score of 100, followed by "what is the muscular system" (37), "muscular system function" (22), "muscular system organs" (15), "functions of the muscular system" (12), and "human muscular system" (11). Top scores are relative interest scaled so the top term equals 100; they are not search counts. Rising terms show growth in search interest compared with the prior period: "what is the main purpose of the muscular system" rose +80%, "interesting facts about the muscular system" rose +70%, "how do the muscular system and nervous system interact with each other?" rose +60%, and "why is the muscular system important" rose +40%. A "Breakout" label would mean growth too large to show as a percentage. The pattern is clear: people want the definition, the functions, and how muscles connect to the rest of the body.

What is the muscular system?

The OpenStax Anatomy and Physiology 2e textbook opens its muscular system chapter with a simple line: the muscular system allows us to move, flex and contort our bodies. That chapter is about skeletal muscles, which the textbook says are so named because most of them move the skeleton.

So when anatomy texts talk about the human muscular system as an organ system, they mean the skeletal muscles. Each skeletal muscle is an organ in its own right. It is built from muscle fibers, wrapped in connective tissue, fed by blood vessels, and wired to nerves. That is the honest answer to "muscular system organs": the organs of this system are the individual muscles themselves.

Cleveland Clinic describes the muscular system as one anatomical group that includes all your muscles, and notes that providers might also count muscles as parts of other systems throughout the body. That is the key nuance to hold onto.

Three tissue types, one system-level view

The OpenStax textbook names three kinds of muscle tissue: skeletal muscle, which moves the skeleton; cardiac muscle, found in the heart and concerned with pumping blood through the circulatory system; and smooth muscle, concerned with various involuntary movements in places like artery walls and internal passageways.

Functions of the muscular system

If you searched "what is the main purpose of the muscular system," the textbook answer starts with movement, but it does not stop there. The OpenStax Anatomy and Physiology 2e textbook lists several jobs for skeletal muscles:

• Movement: skeletal muscles pull on bones to move the body.
• Posture and stability: OpenStax notes that skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture.
• Protecting internal organs: the textbook says skeletal muscles protect internal organs, particularly abdominal and pelvic organs, from external trauma.
• Guarding entrances and exits: skeletal muscles sit at the openings of internal tracts, which is why swallowing, urination, and defecation are under voluntary control.
• Heat production: OpenStax explains that skeletal muscles generate heat as they contract, and that shivering produces extra heat in the cold.
• Breathing: the textbook describes the diaphragm as a sheet of skeletal muscle that has to contract and relax for you to breathe day and night.

What about blood circulation? Textbooks credit that to the heart. OpenStax describes cardiac muscle as the tissue concerned with pumping blood through the circulatory system, and Cleveland Clinic lists pumping blood through your heart and blood vessels among the jobs muscles do. That is why some lists of muscular system function include circulation and others do not. It depends on whether the list counts the heart.

That list is also the plainest answer to "why is the muscular system important." Movement is the headline, but posture, organ protection, controlled openings, heat, and breathing all run through the same organ system.

How many muscles are in the human body?

Cleveland Clinic states that you have more than 600 muscles in your body. Counts vary between references depending on how muscles are grouped and named, so treat "more than 600" as the common figure rather than a fixed number.

Those muscles come with names that explain themselves once you know the code. The OpenStax textbook lists the criteria anatomists use: shape, size relative to nearby muscles, location, attachments, number of origins, and action. The gluteus maximus, gluteus medius, and gluteus minimus are named by size: largest, medium, and smallest. The biceps brachii has two origins, which is what "bi" signals. The sternocleidomastoid is named for its attachments to the sternum, clavicle, and the mastoid process of the skull.

How the muscular system and nervous system interact

This is the fastest-rising question in the cluster, and the textbook answer is direct. OpenStax says skeletal muscle completely depends on signaling from the nervous system to work properly. Every muscle fiber in a skeletal muscle is supplied by a branch of a somatic motor neuron.

The meeting point is called the neuromuscular junction, which OpenStax defines as the site where a motor neuron's terminal meets the muscle fiber. Here is the sequence the textbook describes:

1. The motor neuron's axon terminal releases a neurotransmitter called acetylcholine.
2. Acetylcholine diffuses across a minute space called the synaptic cleft.
3. It binds to receptors on the muscle fiber's membrane at a region called the motor end-plate.
4. That signal triggers the fiber to contract. When the neuron stops releasing acetylcholine, the fiber relaxes.

OpenStax adds that, unlike skeletal muscle, cardiac and smooth muscle can be influenced by the nervous system to some degree but also respond to other stimuli, such as hormones. For the signaling side of this partnership, our guide to what the nervous system is covers the neurons and pathways in more detail.

How the muscular system works with the skeleton

Muscles move the body by pulling on bone, and the connection runs through tendons. OpenStax explains that the collagen in a muscle's connective tissue layers intertwines with the collagen of a tendon, so tension from contracting fibers passes through the tendon to pull on bone.

The textbook describes the body as a lever system: bones act as stiff levers, the articular ends of bones inside synovial joints act as fulcrums, and the effort comes from contracting skeletal muscle. Each muscle has two ends. The origin attaches to a fixed, stabilized bone. The insertion is the moveable end attached to the bone being pulled.

Muscles also work in teams. In OpenStax's example of lifting a cup, the biceps brachii is the prime mover, or agonist. The brachialis assists as a synergist. The triceps brachii is the antagonist, with the opposite action: the biceps brachii flexes the forearm, whereas the triceps brachii extends it. For the bones on the other side of each tendon, see our guide to what the skeletal system is.

Interesting facts about the muscular system

The rising search interest in anatomy facts deserves real ones. Each of these comes from a named reference:

• A single skeletal muscle fiber can be long. The OpenStax textbook notes fibers reaching lengths up to 30 cm (11.8 in) in the sartorius of the upper leg.
• Those long fibers carry many nuclei, because they form from fused embryonic cells called myoblasts, according to OpenStax.
• Skeletal muscles are even used during sleep, OpenStax points out, with the diaphragm as the example.
• Cleveland Clinic notes that your heart is the only organ that is also a muscle, and that cardiac muscle does not exist anywhere else in your body.
• Each muscle is wrapped in layers of connective tissue. OpenStax names them the epimysium around the whole muscle, the perimysium around bundles of fibers called fascicles, and the endomysium around each fiber.

One system, three connections

The search data shows people asking about the muscular system from several angles: what it is, what it does, and how it links to the rest of the body. Those questions fit together. The muscular system is the set of skeletal muscles. Its functions are movement, posture, organ protection, controlled openings, heat, and breathing. And it never works alone. The nervous system sends the signal across the neuromuscular junction, and the skeletal system gives the pull something to act on through tendons and levers.

Our take on the muscular system

We like anatomy that explains itself, and the muscular system does. Every muscle name is a clue. Every movement is a lever. Every contraction starts with a signal from a nerve.

We think the most useful habit is to stop memorizing muscles one at a time and start asking three questions: where does it attach, what does it pull, and what tells it to fire? Answer those and almost any muscle falls into place.

Why we carry it at PRSPR

We wrote this anatomy guide for our Learn library, and it sits alongside the Essentials Bundle. This article is about how the muscular system is organized, nothing more. Here is what the listing states about the bundle itself:

• Name: Essentials Bundle.
• Format: a four-product bundle.
• What is in it: Magnesium Glycinate, Black Seed Oil, Vitamin D3 + K2, and Creatine Monohydrate.
• Price: $109.99.

It is made in a GMP-certified facility and third-party batch tested. You can browse it with our other products in the Vitamins & Supplements collection.

See Essentials Bundle at PRSPR

FAQ

What is the muscular system?

It is the organ system made of the body's skeletal muscles. The OpenStax Anatomy and Physiology 2e textbook says the muscular system allows us to move, flex and contort our bodies.

What are the functions of the muscular system?

OpenStax lists movement, maintaining posture, protecting abdominal and pelvic organs, guarding the openings of internal tracts, and generating heat. The diaphragm, a skeletal muscle, also drives breathing. Pumping blood is credited to the heart's cardiac muscle.

How many muscles are in the human body?

Cleveland Clinic states that you have more than 600 muscles in your body. Exact counts vary with how muscles are grouped.

How do the muscular system and nervous system interact with each other?

A motor neuron meets each skeletal muscle fiber at the neuromuscular junction. According to OpenStax, the neuron releases acetylcholine across the synaptic cleft, it binds at the motor end-plate, and the fiber contracts.