Definition, Types, Structure, and Functions of Muscles

A Muscle is a soft tissue that enables contraction and tension in an animal’s body. The types of muscle are skeletal, cardiac, and smooth. Muscles in the human body are made up of muscle fibers, connective tissue, blood vessels, and nerves. Muscle tissue consists of specialized cells called muscle fibers, which contract to move the body.

Each muscle type has its own cellular components, physiology, functions, and pathology. Structurally, muscles consist of contractile fibers organized into bundles. The muscle functions include generating force for movement, maintaining posture, and supporting vital physiological processes. In this article, we will discuss the structure, type, and functions of muscles in detail.

What are Muscles?

Muscle Definition: A muscle is a soft tissue made up of cells that contract to produce movement or locomotion in animals. 

The human body has over 600 muscles that makeup about 40–50% of the body’s weight. Muscles consist of parallel bundles of muscle fibers, each containing myofibrils with sarcomeres responsible for contraction. There are three main types of muscles: skeletal muscles are attached to bones and allow voluntary movements; smooth muscles which are present in internal organs control involuntary processes; and cardiac muscles which form the heart ensure rhythmic contractions for circulation. Muscles have special properties like excitability, contractility, extensibility, and elasticity.

Types of Muscles of Human Body

In the human body there are three types of muscles in human body:

• Skeletal or striated muscles
• Cardiac muscles
• Smooth muscles

Based on the muscle action, there are two types of muscles:

• Voluntary muscles
• Involuntary muscles

Skeletal Muscle

Skeletal muscles also known as striated muscles are attached to bones by tendon and are responsible for skeletal movements. They are part of the musculoskeletal system and work with bones, tendons, and ligaments to support weight and move the body. Skeletal muscles are voluntary muscles and are under the direct control of the somatic nervous system.

Structure of the Skeletal Muscle

Skeletal Muscles are made up of thousands of muscle fibers that are bundled together and wrapped in connective tissue. 

• The individual bundles of muscle fibers within a skeletal muscle are referred to as fasciculi.
• The outermost connective tissue sheath that surrounds the entire muscle is called the epimysium.
• Each fasciculus is covered by the connective tissue sheath known as perimysium.
• The innermost sheath surrounding individual muscle fibers is termed endomysium.
• Muscle fibers, in turn, consist of myofibrils containing multiple myofilaments, with actin and myosin being the two most significant ones.
• These myofibrils are organized in a striated pattern to form sarcomeres, the fundamental contractile units of skeletal muscles.

Function of the Skeletal Muscle

Function of the skeletal muscles are:

• Skeletal muscles work to support and maintain the body’s posture and balance against gravity.
• It provide stability to joints, prevents dislocations and ensure proper alignment.
• Muscle contractions generate heat and contributes to regulation of the body temperature.
• Muscles are involved in chewing and swallowing.
• Skeletal muscles, like the diaphragm and intercostal muscles, play a vital role in the respiratory process.

Some major skeletal muscle groups are:

• Upper body: Abdominal, pectoral, deltoid, trapezius, latissimus dorsi, erector spinae, biceps, and triceps.
• Lower body: Quadriceps, hamstrings, gastrocnemius, soleus, and gluteus.

Cardiac Muscle

Cardiac muscle, also known as heart muscle or myocardium, is a specialized, striated muscle found only in the heart. It is involuntary and works autonomously, meaning it is under the control of the autonomic nervous system. Cardiac muscle cells perform highly coordinated actions that keep the heart pumping and blood circulating throughout the body. It contains blood vessels that supply nutrients to cardiac muscle tissue and remove waste products.

Structure of a Cardiac Muscle

Cardiac muscle is a involuntary, striated muscle that form the main tissue wall of the heart. It’s made up of individual cardiac muscle cells that are connected by intercalated discs.

• Cardiac muscle has a striated or striped appearance due to the presence of sarcomeres within its fibers.
• Each cardiac muscle cell also called as cardiomyocytes, contains a single nucleus.
• Intercalated discs present between cardiac muscle cells facilitate rapid communication and coordination. It ensure synchronized contractions of the heart.
• Cardiac muscle fibers exhibit a branching pattern that form a network. It allows for efficient communication and contraction across the heart.
• Cardiac muscles have an extensive network of blood vessels for oxygen and nutrient supply and also contain a significant number of mitochondria to support the energy requirements of continuous contraction.
• Transverse Tubules or T-tubules are present that penetrate into the cell and allow the rapid transmission of action potentials to the interior of the cell.

Function of the Cardiac Muscle

Cardiac muscle is found only in the heart and contains cardiac muscle cells that perform coordinated actions to keep the heart pumping and blood circulating throughout the body. The functions of cardiac muscle are:

• Cardiac muscle function as involuntary muscle, allowing the heart to function without conscious control.
• It contracts rhythmically to ensure the continuous circulation of blood.
• The cardiac muscle’s inherent pacemaker cells regulate the heartbeat, influencing the heart rate and rhythm.
• It supports oxygen and nutrient exchange by pumping blood through the circulatory system.
• Helps regulate blood pressure by controlling the force and volume of blood pumped.
• Cardiac muscle responds to the autonomic nervous system for adjustments in heart rate and contractility.

Smooth Muscle

Smooth muscle is involuntary non-striated muscle that is found in the walls of hollow organs and blood vessels. It is also known as unstriated muscle because it has no cross striations and looks smooth. Smooth muscle is involuntary and often rhythmic in contraction. Smooth muscle is divided into two subgroups: single-unit and multiunit smooth muscle. It is found in the walls of hollow organs, the walls of blood vessels and lymph vessels, and the tracts of the respiratory, urinary, and reproductive systems.

Structure of Smooth Muscle

Smooth muscle is characterised by the absence of striations, the spindle-shaped cells, and the presence of coordinated sheets of cells that allow involuntary, slow, and sustained contractions.

• Smooth muscle fibers are spindle-shaped, with a single nucleus, and range from 30–200 micrometers in length. 
• They are thousands of times shorter than skeletal muscle fibers and produce their own connective tissue, called endomysium.
• Smooth muscles contain thick and thin filaments that are not arranged into sarcomeres, giving it a non-striated pattern. 
• Smooth muscle cytoplasm contains a large amount of actin and myosin, which are the main proteins involved in muscle contraction.
• Gap junctions may be present between smooth muscle cells, enabling communication and coordination of contractions and forms sheets of tissue.

Functions of Smooth Muscle

Function of the smooth muscles are:

• Smooth muscle important for involuntary contractions of organs, such as the digestive tract, that allow processes like peristalsis.
• It controls the diameter of blood vessels and influences the blood flow and blood pressure.
• Smooth muscle is involved in various reproductive processes, including the movement of sperm through the reproductive tract and the contractions of the uterus during childbirth.
• It regulates the diameter of bronchioles in the respiratory passages, controlling airflow into the lungs.
• It contributes to the contraction of the bladder during urination and the movement of urine through the urinary tract.
• Smooth muscles forms sphincters, muscular rings that control the opening and closing of passages between organs, such as the pyloric sphincter in the stomach and the anal sphincters.

Voluntary Muscle

Voluntary muscles can be controlled consciously and are attached to bones and control movement of the limbs, head, neck, and body. They are controlled by neuromuscular signals from the brain that cause muscle fibers to contract. Voluntary muscles are divided into two categories: superficial and deep.

Involuntary Muscles

Involuntary muscles are muscles that are controlled by the autonomic nervous system (ANS) and are not affected by voluntary nerve activities. They are spindle-shaped, uninucleated, and striated. Involuntary muscles include smooth muscles and cardiac muscles. Involuntary muscles are present in the organs such as the stomach, intestines, bladder, bronchi, blood vessels, and arrector pili. 

Difference Among Skeletal, Smooth, and Cardiac Muscles

The differences among skeletal, smooth, and cardiac muscles are given below:

Characteristic	Skeletal Muscle	Smooth Muscle	Cardiac Muscle
Location	Attached to bones	Walls of internal organs, blood vessels	Walls of the heart
Shape of Cells	Elongated, cylindrical	Spindle-shaped	Branched, cylindrical
Nuclei	Multinucleated	Single, centrally located	Single, centrally located
Striations	Present (striated appearance)	Absent (non-striated)	Present (striated appearance)
Voluntary/Involuntary	Voluntary	Involuntary	Involuntary
Controlled by	Somatic nervous system	Autonomic nervous system	Autonomic nervous system
Contraction Speed	Rapid and forceful	Slow and sustained	Rhythmic and coordinated
Cell Arrangement	Bundled in parallel	Sheets or layers	Branched network
Mitochondria Density	Moderate to high	Moderate	High
Intercalated Discs	Absent	Absent	Present
Function	Body movement, posture, heat production	Organ contractions, peristalsis, blood vessel regulation	Pumping blood throughout the circulatory system
Example Locations	Biceps, quadriceps	Stomach, intestines, blood vessels	Heart

Conclusion: An Overview, Types, Structure and Functions Of Muscles

Muscles are soft tissues that can contract and produce movement in the body. They are made up of muscle cells, which contain protein filaments that slide past each other to produce contraction. The muscle tissue contract to produce movement in the body. There are three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones and are responsible for voluntary movement. Smooth muscles are found in organs and blood vessels and are responsible for involuntary movement. Cardiac muscles are found in the heart and are responsible for pumping blood. The primary function of the muscles are movement, maintaining posture, circulation and also play a role in respiration, digestion, and metabolism.

FAQs – An Overview, Types, Structure and Functions Of Muscles

What are the 3 Types of Muscles?

Three type of muscles are Skeletal, Smooth and Cardiac muscle.  Each muscle type has its own cellular components, structure, and functions.

What is the Main Function of Muscles?

The main function of muscles is to move the body and maintain posture and body position.

What are the 6 Main Muscles in Your Body?

The six main muscles in the human body include the deltoids, pectorals, biceps, quadriceps, hamstrings, and gastrocnemius, each involves various movements and functions.

What is Cardiac Muscle?

Cardiac muscle, also known as myocardium or heart muscle, is a type of muscle tissue that makes up the heart’s thick middle layer.

Why is Skeletal Muscle Different from Both Smooth and Cardiac Muscle?

Skeletal muscle is voluntary and associated with body movement, whereas smooth and cardiac muscles are involuntary and control internal organ functions.

What are the 4 Properties of Muscles?

The four properties of muscular tissue are extensibility, elasticity, excitability, contractility. The human body has three major muscle types: skeletal, cardiac, and smooth muscle.

What is the Structure of a Muscle?

Muscles consist of bundles of muscle fibers, each containing myofibrils composed of sarcomeres, the basic contractile units, surrounded by connective tissue layers (epimysium, perimysium) for support and coordination of muscle contractions.

Where are Skeletal Muscles?

The human body has approximately 640 skeletal muscles. They are present in the tongue, diaphragm, eye socket, upper esophagus, throat, anus, and urethra.

What is the Difference Between Skeletal, Cardiac and Smooth Muscle?

Skeletal muscles are voluntary and responsible for body movement, cardiac muscles are involuntary and form the heart, while smooth muscles are involuntary and control internal organ functions.