What Are Muscles? Definition, Types, and Key Roles in the Body
In this article, students will learn about Muscles in our body, types, structure and functions of Muscles. Let's first know what muscle is.
A muscle is a soft tissue found in both humans and animals. The word muscle is derived from the Latin word “musculus” meaning little mouse as some Muscles resemble the shape of a small mouse or the contraction of Muscles look like a moving mouse.
Muscles are made up of protein filaments of myosin and actin that slide past each other and produce expanding and contracting movements of Muscles. This kind of movement changes both the shape and length of the muscle cells. Muscles function in humans by producing motion and force and help in circulation of blood, changing and maintaining body pressure, movement of internal organs like the movement of food down the digestive tract, and also helps in locomotion.
The human muscular system comprises more than 600 Muscles and makes up about 40-50% of our overall body weight. Muscles are essentially attached to blood vessels, bones and other internal organs. Muscles are composed of special kinds of elastic tissue, tendons, nerves and tissues.
The functioning of Muscles helps in making movements of every kind in the body and internal organs by transferring substances from these organs throughout the body. Muscles function by using up energy by oxidation of carbohydrates and fats, especially from the stored energy molecules adenosine triphosphate (ATP).
Types
There are three types of Muscles found in the body:
Cardiac Muscles
Smooth Muscles
Skeletal Muscles
Out of these three types, cardiac Muscles are involuntary Muscles which means that the movement of cardiac Muscles cannot be controlled by us according to our will.
Skeletal Muscles
Skeletal Muscles are voluntary Muscles and are attached to the bones and involved in different body parts functioning. They come under the central nervous system control of the body. Skeletal Muscles are long and multinucleated. They are cylindrically shaped with branched cells which are attached to the bones by collagen fibres and tendons, which are composed of connective tissues.
The function of Skeletal muscle can be summed up briefly as follows:
The primary function of Skeletal muscle is contraction. And thus, contraction helps produce heat in our body.
Skeletal muscle helps in maintaining the body posture and joint position by conveying information about the degree of muscle length and stretch to the central nervous system.
Skeletal Muscles produce ATP and store glucose in the form of glycogen. Glycogen can be converted into glucose when energy is needed.
Cardiac Muscles
Cardiac Muscles are found only in the heart and are involuntary like movement. They are made up of cylindrical-shaped cells. These are striated Muscles responsible for keeping the heart moving by circulating and pumping blood throughout the body. The interconnected Muscles provide flexibility and strength to the Cardiac muscle tissue and are involved in rhythmic relaxation and contraction of the heart Muscles . Cardiac Muscles are characterised by branched cylindrical fibres and a centrally located nucleus.
Cardiac Muscles are primarily found in humans and regulate rhythmic relaxation and contractions of the heart Muscles for pumping blood. The Cardiac Muscles have a specialised type of tissue called “pacemaker” cells that expand and contract by responding to electrical impulses of the central nervous system.
Smooth Muscles
Smooth Muscles are also involuntary Muscles that are non-striated and are present in major organs whose movements are not controlled by the will, such as the stomach, vessels, bladder, uterus, etc. They are spindle-shaped with a single nucleus. These Muscles are shorter than skeletal Muscles with a length ranging between 20 to 200 μm and thickness between 3-10 µm. These Muscles produce their connective tissue and lack actin, myosin and filaments.
Smooth Muscles perform various tasks in our body which can be given as follows:
Smooth Muscles maintain the diameter of arteries and thereby maintain blood pressure. Arteries have thick walls due to Smooth muscle present there, which helps them carry blood away from the heart.
In the digestive tract, Smooth muscle maintains the peristaltic movement and forces food through the digestive tract.
They are present in the eye responsible for shrinking the size of the pupil.
In the respiratory system, Smooth muscle helps air go from the trachea to the lungs.
In reproductive systems Smooth muscle helps sperm to move along the male reproductive system, during menstruation Smooth muscle helps to expel the uterine lining.
Smooth muscle in the uterus helps women to push out her baby during childbirth.
FAQs on Muscles: Types, Structure, and Functions Explained
1. What are the three main types of muscles found in the human body?
The human body has three main types of muscles, each with a distinct structure and function:
- Skeletal Muscle: These are voluntary muscles attached to bones by tendons. They are responsible for all conscious movements like walking, lifting, and typing. They appear striated or striped under a microscope.
- Smooth Muscle: Found in the walls of internal organs like the stomach, intestines, and blood vessels, these are involuntary muscles. They manage processes like digestion and blood pressure regulation without conscious control.
- Cardiac Muscle: This is an involuntary, striated muscle found exclusively in the heart. Its rhythmic and powerful contractions are responsible for pumping blood throughout the body.
2. How are voluntary and involuntary muscles different in terms of control and function?
The primary difference lies in their control mechanism. Voluntary muscles, such as the skeletal muscles in your arms and legs, are controlled by your conscious thoughts via the somatic nervous system. You decide when to move them. In contrast, involuntary muscles, like the smooth muscle in your digestive tract and the cardiac muscle of your heart, operate automatically. They are regulated by the autonomic nervous system to manage essential life-sustaining functions without you having to think about them.
3. What is the basic structure of a skeletal muscle?
A skeletal muscle is a complex, hierarchical structure. The entire muscle is wrapped in a sheath of connective tissue called the epimysium. Inside, the muscle is organized into bundles of muscle fibres called fascicles, each wrapped in perimysium. Each fascicle contains individual muscle fibres (cells), which are enclosed by endomysium. Within each fibre are long cylindrical organelles called myofibrils, which contain the contractile units, sarcomeres, made of actin and myosin filaments that slide past each other to cause contraction.
4. What are the most important functions of the muscular system?
The muscular system performs several vital functions beyond just movement. Its key roles include:
- Mobility: Enabling all gross and fine motor movements, from running to writing.
- Posture and Stability: Maintaining body posture and stabilising joints through constant muscle tension.
- Circulation: The cardiac muscle pumps blood, and smooth muscles in arteries regulate blood flow and pressure.
- Respiration: The diaphragm is a skeletal muscle essential for breathing.
- Digestion: Smooth muscles propel food through the digestive system via peristalsis.
- Heat Generation: Muscle contractions, including shivering, produce a significant amount of the body's heat to maintain temperature.
5. Why are cardiac muscles considered unique compared to skeletal and smooth muscles?
Cardiac muscle is unique due to its hybrid characteristics. Like skeletal muscle, it is striated, allowing for strong, forceful contractions. However, like smooth muscle, it is involuntary and can contract rhythmically and tirelessly throughout a lifetime. A key structural feature is the presence of intercalated discs, which connect individual cells. These discs contain gap junctions that allow electrical signals to pass rapidly from cell to cell, enabling the entire heart to contract in a coordinated, wave-like fashion.
6. How do muscles get the energy they need to contract?
Muscles require a constant supply of energy in the form of Adenosine Triphosphate (ATP) to function. This ATP is generated through several pathways. For short, intense bursts of activity, muscles use stored ATP and another high-energy molecule called creatine phosphate. For sustained activity, muscles break down glycogen (stored glucose) through aerobic respiration (with oxygen) or anaerobic respiration (without oxygen). Anaerobic respiration is faster but less efficient and produces lactic acid, which contributes to muscle fatigue.
7. How do muscles work in pairs to create movement at a joint?
Muscles can only pull; they cannot push. Therefore, to create movement back and forth at a joint, they work in antagonistic pairs. When one muscle, the agonist (or prime mover), contracts to cause a movement, its opposing muscle, the antagonist, relaxes. For example, to bend your elbow, your biceps (agonist) contracts, while your triceps (antagonist) relaxes. To straighten the elbow, the roles reverse: the triceps contracts, and the biceps relaxes. This coordinated action ensures smooth and controlled motion.
8. What is the role of connective tissue within a muscle's structure?
Connective tissue is crucial for a muscle's organisation and function. It provides a structural framework by bundling muscle fibres into progressively larger units (fibres into fascicles, fascicles into the whole muscle). This allows the force generated by individual fibres to be transmitted effectively throughout the muscle and to the bone via the tendon. Furthermore, it provides pathways for nerves and blood vessels to reach the muscle fibres, supplying them with oxygen, nutrients, and signals to contract while removing waste products.
