What are the Muscles?

A muscle is a simple soft tissue that is found in both humans and animals. The muscle cells comprise protein filaments of myosin and actin that slide past one another, which produces contraction and changes both the shape and length of the cell.

The word “Muscle” is derived from the Latin word “Musculus” which is referred to as a little mouse, because of the shape of certain muscles or the contraction of muscles which looks like a moving mouse.

Muscles in human function by producing motion and force and are primarily responsible for:

  • Circulation of blood cells throughout the entire body

  • Changing and maintaining body posture

  • Locomotion

  • Movement of internal organs, like the contraction of the heart, and the movement of food through the digestive system via peristalsis.

The human muscular system is comprised of more than 600 muscles, which makes up about 40 - 50 percent of total body weight. These muscles are attached to the blood vessels, bones, and other internal organs of our body and are primarily composed of skeletal muscles, tendons, nerves, and tissue. The human muscular system muscles are composed of a kind of elastic tissue.

In our body, every movement is the result of muscle contraction and is found in every organ, including the heart, blood vessels, digestive organs, and more. Muscles function by transferring the substances in these organs throughout the body. There are mainly three types of muscle and are classified based on their structures and movements.

The energy required for muscle functioning is predominantly powered by the oxidation of carbohydrates and fats, and particularly from the stored energy molecules adenosine triphosphate (ATP).

Types of Muscles

Muscles are divided into three types as given below.

  • Skeletal or striated muscles

  • Cardiac muscles

  • Smooth muscles

Depending on the action of muscle, they are further classified as:

  • Voluntary muscles

  • Involuntary muscles


Skeletal Muscles

Skeletal muscle is the muscle tissue attached to the bones and involved in different body parts functioning. These are also known as voluntary muscles because they come under the central nervous system control in the body.

Structure of Skeletal Muscle

Skeletal muscles are a series of muscle fibres combined with muscle cells, which are multinucleated and long.

Skeletal muscles are shaped cylindrically with branched cells attached to the bones by the collagen fibres or elastic tissue called tendons, composed of the connective tissues.

Each skeletal muscle end has a tendon that connects the muscle to bone and directly to the collagenous, the outer covering of a skeletal muscle.

There is a group of muscle fibres, exist below the epimysium, collectively called fascicles. These fibres are surrounded by another protective shield produced from collagen.

Perimysium, a sheath of connective tissue which is surrounding the muscle fibres, allows blood and nerve vessels to make their way with the help of the muscle.

Functions of a Skeletal Muscle

These functions of muscles include:

  • It maintains body posture and regulates the body temperature

  • It is responsible for the body’s erect posture. Sartorius muscles in thighs are responsible for the body movement

  • It is responsible for body movements like an arm extending, breathing, typing, and more

  • These support the body’s entry and exit points

For example, sphincter muscles present in the below-given body parts.

Around the Anus - These muscles reduce the opening size by contraction of the muscles and facilitates defecation.

Around the Urinary Tract - These control the urination by the muscle contraction in the urethra.

Cardiac Muscles

These are self-stimulating and found only in the heart, with an intermediate speed of energy requirement and contraction. This muscle is not a musculoskeletal system’s part.

Cardiac muscles are striated, being responsible for performing muscular involuntary movements, and keeps our heart functioning by circulating and pumping the blood throughout the body. They are involved in relaxation and continuous rhythmic contraction. The interconnected fibres or muscle cells provide flexibility, strength to the cardiac muscle tissue.

Structure of a Cardiac Muscle

Cardiac muscle only exists within the human heart. It is a specialized muscle form evolved to contract continuously and repeatedly, providing blood circulation throughout the body. It has a regular fibres pattern similar to smooth muscles. These comprise branched cylindrical fibres and a centrally located nucleus. The transverse or T-tubules are rich in ion channels and are found in atrial muscle cells.

These are striated muscles having cylindrical-shaped cells, include intercalated discs, and joins neighboring fibres.

Functions of a Cardiac Muscle

The cardiac muscle’s primary function is to regulate the heart functioning by the contraction and relaxation of the heart muscles. The other functions of muscles include:

  • They function as involuntary muscles and involved in the locomotion or the movement

  • The heart comprises a specialized type of cardiac tissue, consists of “pacemaker” cells. These expand and contract in response to electrical impulses from the nervous system.

Smooth Muscles

Smooth muscles are involuntary muscles and non-striated, controlled by the Autonomous Nervous System (ANS). These are found almost in all organ systems like stomach, vessels, bladder, bile ducts, in eyes, sphincters, uterus, and more.

These muscles function by stimulating the contractility of the urinary, digestive, reproductive systems, airways, and blood vessels. The smooth muscle cells are spindle-shaped with a single nucleus. These activate automatically and don’t depend on conscious thought.

Structure of Smooth Muscles

The smooth muscles are spindle-shaped muscle fibres having a single nucleus. The thickness of these muscles ranges between 3-10 µm and its length ranges between 20 to 200 μm, which are shorter than skeletal muscle. These muscles lack filaments, actin, myosin, and produce their own connective tissue.

Functions of a Cardiac Muscle

Similar to all other muscle types, smooth muscles are also involved in relaxation and contraction. The other functions of muscles include:

  • It is involved in the orifices sealing and produces connective tissue proteins like elastin, collagen

  • Transports chyme - a pulpy acidic fluid for the intestinal tube contractions

Smooth muscles are also responsible for:

  • Raising small hairs on our arm

  • Contracting our body sphincters 

  • Fluid movements through organs

FAQs (Frequently Asked Questions)

1. Explain about Voluntary and Involuntary muscles?

Depending on the muscle actions, they are further classified as listed below.

  • Voluntary Muscles

  • Involuntary Muscles

Ans: Voluntary Muscles

Voluntary muscles are long and multinucleated cells, containing sarcomeres arranged into bundles. These are composed of cylindrical fibres and usually attached to the skin and bones. They play an essential role in allowing the body to move by relaxing and contracting, and their actions lie under the control of the somatosensory nervous system mainly. These voluntary muscles also include skeletal muscles.

Involuntary Muscles

Involuntary muscles are branched and striated in cardiac muscle cases. The actions of involuntary muscles are controlled mainly by the body’s autonomic nervous system. These involuntary muscles include smooth and cardiac muscles.

2. Explain the Functions of Skeletal Muscles in brief?

Ans: Let us discuss the functions of the Skeletal Muscle in brief.

  • It regulates body temperature and maintains body posture

  • It is responsible for performing muscular involuntary movements

  • It controls and connects to the motions of the skeleton

  • It is also responsible for body movements like extending the arm, typing, writing, breathing, and more

  • It is responsible for the body’s erect posture. The sartorius muscles in thighs take care of body movement

  • These muscles protect the internal organs and tissues from any injuries and provide support to these delicate tissues and organs

  • These muscles also support the entry and exit points of the body