What Is Muscle Made Of?
Muscle is made up of proteins called actin and myosin, which the actual percentage of each may differ depending on the type of muscle.

Muscle is made up of proteins called actin and myosin. Depending on the type of muscle (skeletal muscle, smooth muscle, or cardiac muscle), the actual structure and percentage of actin and myosin may differ.

  • A muscle unit is a complex physiological structure that consists of a structure called a sarcomere, a nerve fiber supplying the sarcomere and surrounding connective tissue. 
  • As you age, the muscle unit may contain fat, leading to age-related muscle loss and weakness.

Myoglobin, an iron and oxygen-binding protein, can be found in both your heart (cardiac) and skeletal muscles. It collects oxygen that the muscle cells use to generate energy. 

  • Myoglobin is released into the bloodstream during a heart attack or significant muscle damage.
  • Myoglobin levels in your blood increase two to three hours after you notice the first signs of muscle injury.
  • Troponin is another protein specific to cardiac muscle and smooth muscle cells.

Your body has about 600 muscles, and they are divided into three types of muscles, each with its importance and distinctive functions in the body:

  1. Skeletal muscle
  2. Cardiac muscle
  3. Smooth muscle

What is a skeletal muscle?

Skeletal muscles are also known as striated muscles because they contain small stripes of both dark and light bands. They constitute 30 to 40 percent of the total weight of the body.

All skeletal muscles are voluntary muscles, which means you will have complete conscious control over the muscle. You may move the muscle according to your desire.

Skeletal muscle and its component tendons connect the muscles to the bones in the body. 

  • Skeletal muscle keeps all the bones aligned in a place and provides shape to the body.
  • Ligaments are another form of muscle that are tougher than normal muscles; they connect to the muscles on both ends.

The main functions of skeletal muscles include:

  • Play a role in bodily movements
  • Provide body posture and shape
  • Support and maintain the position of the bones, joints, and internal organs
  • Protect other organs of the body from external forces
  • Maintain optimum body temperature

With continuous physical activity, the muscle fiber may get microscopic injuries and you feel fatigued. You might need some rest from the activity for the muscles to repair back and gain some strength.

Apart from excess physical activity or any direct physical injury to the muscles, certain medical conditions weaken the integrity of the muscles and cause life-threatening diseases. These are known as myopathies, which include

  • Myasthenia gravis: It is an autoimmune disease in which communication between the muscles and nerves is lost. This leads to severe weakness of the muscles, making it difficult for you to walk, breath, speak, hold up eyelids or chew.
  • Rhabdomyolysis: As muscle tissue is physically injured due to an injury, a severe drug reaction, or sepsis, certain chemicals are released into the bloodstream. These chemicals are toxic and cause severe damage to other organs.
  • Muscular dystrophy: It is a genetically inherited disease in which the defective gene breaks down the skeletal muscle fibers, thus weakening the muscle. There are various types of muscular dystrophies.
  • Tendonitis: It is inflammation of the tendons that connect the muscle with bones. Overuse of tendons causes tendonitis and pain to the muscles.
  • Sarcopenia: It is a gradual loss of muscle mass with aging and causes issues with mobility and balance. By the age of 80 years, you may lose up to 50 percent of your total muscle. Hormonal changes, obesity, and other medical conditions may speed up the process of sarcopenia.
  • Strain: Overstretch or overuse of the muscle may cause muscle strain or pull. Severe strain might result in a partial or whole muscle tear.


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What is a cardiac muscle?

Your heart is a muscular structure made up of cardiac muscle, also known as heart muscle. 

  • The heart muscle is a striated but an involuntary muscle, which means we have no control over the movements of the cardiac muscle.
  • The myocardium contains myocytes (heart muscle cells) that undergo periodic rhythmic contraction and relaxation responsible for circulating blood throughout the body.
  • Internally, the myocardium is divided into four chambers to accommodate the proper flow of the blood.

Inflammation of the myocardium is known as myocarditis in which the cells get damaged and swell due to accumulation of fluid in response to an injury, a viral infection, or a reaction to a drug. This hinders the pumping action of the myocardium, causing symptoms such as pain, shortness of breath, and even heart failure.

Troponin is an important protein present in the cardiac muscle, and as the cardiac muscle is damaged, troponin is released into the bloodstream. Detection of troponin levels in the blood indicates injury to the cardiac muscles.

What is a smooth muscle?

Smooth muscle is also known as a nonstriated muscle because the bands that are seen in skeletal and cardiac muscles are absent. Smooth muscle is seen lining the walls of hollow organs such as the blood vessels, urinary tract, urinary bladder, and other organs such as skin, eye, respiratory system, digestive system, and others.

Smooth muscle fibers are spindle-shaped organs that have a single nucleus. They range in length from 30 to 200 meters (thousands of times shorter than skeletal muscle fibers) and manufacture their connective tissue (endomysium).

Smooth muscle fibers have actin and myosin contractile proteins, as well as thick and thin filaments.

The location and functions of smooth muscle include:

  • Gastrointestinal tract: Lines the whole digestive system and other organs of the digestive system and helps with the movement of food and release of digestive juices
  • Cardiovascular system: Lines the inner walls of the blood vessel and lymphatic vessels; blood flow and pressure are maintained at optimum levels
  • Genitourinary system: Lines the urinary bladder and urinary tract and regulates the flow of urine
  • Genital system: Lines the uterus and both male and female reproductive tracts and causes contraction of the uterus in women and release of sperm in men
  • Respiratory tract: Lines the bronchus and other structures of the lungs and maintains sufficient diameter of the bronchiole
  • Integumentary system: Present in the skin and raises hair with erector pili muscle
  • Special senses: Present in the ciliary muscle and iris of the eye, causes dilation and constriction of the pupil and changes the lens shape

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Medically Reviewed on 1/19/2022
Image Source: iStock Images

Skeletal Muscle: https://biologydictionary.net/skeletal-muscle/

Cardiac Muscle: https://biologydictionary.net/cardiac-muscle/

Smooth Muscle: https://biologydictionary.net/smooth-muscle/