It's virtually impossible to think about the types of muscles in the human body without being overwhelmed by the beautiful complexity of the muscular systems. Depending on who you ask and exactly how they define a single muscle, human bodies are home to between 650 and 840 named skeletal muscles alone. Naturally, breaking those muscles down into groups helps you grasp some of that enormity and in the big picture, there are three key types of muscles in the human body: Those composed of skeletal muscle tissue, cardiac muscle tissue and smooth muscle tissue.
The Common Factors
Before you can really get to know how the three types of muscles in the human body differ, it helps to know what they have in common. Because the primary function of muscle cells is to allow movement, muscle cells are excitable. And no, not "excitable" in the "14-year-old playing Fortnite" way, but excitable in that they respond to stimulus.
When muscles are attached to two movable objects (primarily, as you'll soon discover, bones), the contraction and expansion of those muscles causes said objects to move. Muscle movement falls into two basic categories:
- Voluntary: These muscles are under conscious control; you decide to move them and they move in turn, like your decision to raise your arm when you know the answer to a question.
- Involuntary: These muscles are not under your conscious control, yet still move. For example, the dilation of your pupils in the dark or contraction in the light is a type of involuntary muscle movement.
All muscles are composed of thousands of tightly bundled elastic fibers, which themselves are made up of tiny protein blocks called myofibrils. As you take in carbohydrates from your food, the glucose from those carbs fuels your muscles. Of course, carbs are just one part of the food pyramid, and muscle tissue requires a healthy balance of minerals, electrolytes, calcium, magnesium potassium, sodium and other food elements to function at their best.
Get to Know Skeletal Muscle
If you guessed that skeletal muscles are the type of muscle tissue attached to the bone, you guessed right. That's not the whole story, though, as some skeletal muscles are also attached to the skin, such as the expressive muscles of the face. Ultimately, this muscle tissue's function is to help your body move, whether by locomotion, posture, facial expression or other intentional gestures.
This type of movement makes skeletal muscle the only one of the three groups that are voluntarily controlled. Your brain consciously "tells" these muscles what to do when you move. In addition to voluntary movement, skeletal muscles help the body produce heat and protect vital organs.
About 40 percent of your entire body is made up of skeletal muscle, so it's little wonder that this type of muscle features all kinds of wonderfully interconnected processes, from the way it allows locomotion to the way its cells are made up.
Skeletal Muscular System and Movement
Skeletal muscle very much relies on the skeleton itself to perform the bodies' most basic movements. Without the 206 bones of the human skeleton pulling against contracting skeletal muscle fibers, you simply wouldn't be able to walk, run, stand or even sit down. Think of bones like levers of a sort, changing the strength and direction of the force generated by the skeletal muscle. This interacting system between bones and skeletal muscle is known as the musculoskeletal system.
But when your skeletal muscles propel your body to move, they're not just helping you cross the street. As these muscles contract and expand again, they generate heat and contribute to thermal homeostatis, the essential process by which warm-blooded mammals maintain internal body temperature within a specific range. While known for their voluntary movement, shivering is actually an involuntary contraction of skeletal muscles that happens when your body detects low temperatures.
Skeletal Muscle Structure
Getting down to the microscopic level, skeletal muscle cells are called myocytes, of which the human body maintains a fairly consistent amount throughout its lifespan. The cells of skeletal muscles feature a distinct striated appearance, or a texture of tiny, scratch-like grooves, dotted with many nuclei. This also explains why skeletal muscles are sometimes called "striated muscles." The cells themselves are cylindrical, while skeletal muscle tissue is composed of bundles surrounded by connective tissue.
Most often, skeletal muscles operate in opposing pairs, like the biceps at the front of your upper arm and the triceps on the back of it. Skeletal muscle formations typically come in four types of basic shapes:
- Spindle: Tapered on each end and wide in the middle (like the biceps)
- Flat: Sheet-like formations, as seen in the diaphragm
- Triangular: A wide bottom that tapers at the top, as in the deltoid muscles
- Circular: A round, ring-like shape with a space in the middle also known as a sphincter, such as the muscles around the mouth or the pupils
Voluntary and Involuntary Muscle Movement
The interplay between the brain, nerves and skeletal muscles, works directly with skeletal muscles to move your body is known as the neuromuscular system. When you think about moving a muscle, here's how your brain and your body work together to make voluntary muscle movement happen:
- First, your brain sends a message to your motor neurons, a special type of branch-like nerve cell connected to your skeletal muscles.
- Your body then releases the chemical acetylcholine from the presynaptic terminals on the tips of those neurons.
- In response to the release of acetylcholine at the muscular junction, the muscle contracts, setting off the reactions that put the body in motion.
Involuntary muscle movement, on the other hand, happens automatically. That's where your body's cardiac muscle and smooth muscle systems come in to play. They may take up a whole lot less space and body mass than your skeletal muscles, but they're every bit as important.
Brush Up on Cardiac Muscle
Like skeletal muscles, it's all in the name for cardiac muscles. Of the three types of muscle tissue, this is the type of muscle that makes up the wall of the heart. This essential muscle group is what enables the heart to contract. It's also the easiest muscle group to remember when the test rolls around, because all human bodies contain only one cardiac muscle: the heart itself.
In contrast to the voluntary control you exert over your skeletal muscles, your cardiac muscles are under involuntary control. Because they contract spontaneously and rhythmically (think of the rhythm of your heartbeat), cardiac muscles are known as autorhythmic muscles. Cardiac muscle cells are unique in that they contract in their own rhythm without the need for external stimulation.
Those autorhythmic cardiac muscle cells are called cardiomyocytes and feature a striated appearance, akin to the cells of skeletal muscles. While skeletal muscle cells sport multiple nuclei on their membranes, cardiomyocytes usually just have a single central nucleus and attach to each other via cell junctions called intercalated discs. Cardiac muscle cells form long muscle fibers, while the disc-shaped junctions help cells synchronize their actions even as pressure in the cardiac cycle changes.
Sort Out Smooth Muscle
Now that you know what falls into the skeletal and cardiac muscle groups, consider the smooth muscle group a sort of catch-all bin for the rest. Smooth muscles are the building blocks of hollow visceral organs such as the intestine, airways, urinary tract, uterus, blood vessels and internal parts of the eyes. The heart, the sole domain of cardiac muscles, is the exception here.
Unlike the other types of muscles, smooth muscles typically exist at a cellular level. That means that everyone's body has just about countless amounts of smooth muscles. Similar to the cardiac muscles of the heart, smooth muscles are also under involuntary control. They're controlled by the autonomic nervous system and are sometimes known simply as "involuntary muscles" or "visceral muscles."
The individual cells of smooth muscles are — wait for it — smooth (no striation here, for a change) and feature a distinctive spindle shape, tapered on the edges and wider in the middle. Similar to cardiac muscle cells, smooth muscle cells feature a single nucleus. These muscles form layered sheets and when they contract, they contract in waves.
Examples of Muscle Types
Now that you know how muscle tissue is made up and how muscles interact with the bones and nervous system, it helps to know where exactly these muscles reside in the body. While cardiac muscles are exclusively found in the heart, skeletal and smooth muscle tissue occurs throughout the body. At nearly half your body weight, skeletal muscles can be found from top to bottom, including these examples:
- Deltoids of the front shoulders
- Trapezius of the upper shoulders
- Pectoralis major of the chest
- Pectoralis minor
- Serratus anterior at the sides of the ribs
- Biceps brachii at the front of the upper arm
- Rectus abdominis of the stomach
- Satorius of the thighs
- Rectus femoris of the front upper legs
- Fibularis longus at the sides the shins
- Splenius capitis at the back of the next
- Levator scapulae at the base of the neck
- Latissimus dorsi of the mid-back
- External obliques above the hips
- Gluteus maximus of the buttocks
- Biceps femoris of the rear thighs
Because smooth muscles are so numerous, it's easiest to identify the organ systems that feature smooth muscles when naming smooth muscle examples. These organ systems include:
- Gastrointestinal tract
- Cardiovascular system (including blood vessels and lymphatic vessels)
- Renal system (the urinary bladder)
- Male and female reproductive tracts
- Respiratory tract
- Integumentary system (the erector pili muscles of the skin found around hair follicles)
- Sensory system (such as the ciliary muscles of the eye's iris)
- U.S. National Library of Medicine: Medline Plus: "Types of Muscle Tissue"
- The Library of Congress: "What Is the Strongest Muscle in the Human Body?"
- Cleveland Clinic: "Musculoskeletal System: Normal Structure and Function"
- Oregon State University: Open Oregon State – Open Textbook Sites: "Anatomy and Physiology: Chapter 4, The Tissue Level of Organization: 4.4, Muscle Tissue"
- OSA Publishing: Optica: "Thermal Homeostatis Using Microstructured Phase-Change Materials"
- State Government of Victoria, Australia: Department of Health and Human Services: BetterHealth Channel: "Muscles"
- National Cancer Institute: SEER Training Modules: "Muscle Types"
- Brigham Young University: Bio 264: Anatomy & Physiology: "Muscles – Structure and Function"
- BCcampus: "Anatomy and Physiology: Chapter 11, The Muscular System: 11.2, Naming Skeletal Muscles"
- NCBI: StatPearls: "Physiology, Smooth Muscle"