Muscular Tissue – Definition, Types, Structure, and Function
Muscular tissue is a specialised group of cells (muscle fibres) that helps different parts of the body to move by contracting and relaxing. It is mainly made up of proteins such as actin and myosin. In this article, we will discuss what muscular tissue is, its properties, and structure, the three major muscular tissue types, and their roles in our body. We will also look at a few additional insights to help you understand the topic better.
What is Muscular Tissue?
Muscular tissue is one of the four major tissues found in the human body (the others being epithelial, connective, and nervous tissues). It is responsible for generating force and producing motion, whether that motion is related to body movement, organ function, or the pumping action of the heart.
Muscular Tissue Location
Skeletal muscles are found attached to bones through tendons.
Smooth muscles are present in the walls of internal organs (e.g., stomach, intestine, blood vessels).
Cardiac muscles are located exclusively in the heart.
Properties of Muscular Tissue
Contractility – Muscles can shorten or contract when stimulated.
Extensibility – Muscles can be stretched without damage.
Elasticity – Muscles can return to their original shape after stretching.
Excitability – Muscles can respond to nerve impulses or chemical signals.
These properties enable muscles to perform diverse functions, from voluntary movements such as walking to involuntary functions like the beating of the heart.
Structure of Muscular Tissue
Muscular tissues are organised into bundles and supported by connective tissues:
Epimysium: A tough outer layer of connective tissue that surrounds the entire muscle. It connects to tendons, which in turn attach to bones.
Fascicles: Long bundles of muscle fibres grouped together. Each fascicle is surrounded by another connective tissue layer called the perimysium.
Endomysium: A thin layer of connective tissue that surrounds each individual muscle fibre.
Inside each muscle fibre:
Sarcolemma is the membrane that encloses the muscle fibre.
Sarcoplasm is the cytoplasm of the muscle cell, containing proteins and organelles.
Sarcoplasmic reticulum is a network of membranes involved in storing and releasing calcium ions essential for muscle contraction.
Myofibrils are thread-like structures within the muscle fibre that contain the contractile proteins (actin and myosin).
When the muscle receives a signal (nerve impulse), actin and myosin filaments slide past each other, causing the muscle to contract. This is explained by the Sliding Filament Theory. Explore more about this concept in [Sliding Filament Theory].
Types of Muscular Tissue
1. Skeletal Muscle Tissue
Location: Attached to bones of the skeleton.
Features:
Also called striated muscular tissue because of its light and dark band pattern.
Multinucleated cells with elongated mitochondria for energy.
Voluntary control (movement is consciously controlled).
Helps in locomotion, posture, and overall movement of the body.
Comprises about 40% of body mass in humans.
Function: Enables movement of bones, supports posture, and generates heat through contractions.
Explore more about Skeletal Muscles to understand how these muscles bring about movement.
2. Smooth Muscle Tissue
Location: Walls of internal organs such as the stomach, intestines, blood vessels, and other hollow structures.
Features:
Non-striated and spindle-shaped cells with a single nucleus.
Involuntary control is governed by the autonomic nervous system.
Actin and myosin are arranged in a random pattern (hence no striations).
Function: Regulates the movement of internal organs (e.g., peristalsis in the digestive tract) and controls the diameter of blood vessels and airways.
3. Cardiac Muscle Tissue
Location: Found exclusively in the heart.
Features:
Striated like skeletal muscle but operates involuntarily.
Cells are uninucleated and branched, joined by special junctions called intercalated discs.
Intercalated discs contain desmosomes for structural support and gap junctions for electrical coupling.
Function: Pumps blood throughout the body via rhythmic contractions. The coordinated heartbeat is driven by electrical signals.
Learn more about the structure and function of the heart in Human Heart.
Muscular Tissue Function
Movement: Muscular tissue helps in moving our body parts (skeletal muscles), pushing food through the digestive system (smooth muscle tissue), and circulating blood (cardiac muscle tissue).
Maintaining posture: Skeletal muscles contract slightly to hold the body upright and stable.
Generating heat: Muscle contractions produce heat that helps maintain body temperature.
Supporting organ function: Smooth muscles help in blood flow regulation, air passage regulation, and movement of substances within organs.
Quick Quiz on Muscular Tissue
1. Which muscular tissue is found in the heart?
A. Smooth
B. Cardiac
C. Skeletal
D. Striated
Answer: B. Cardiac
2. Which property allows muscles to return to their original shape after being stretched?
A. Contractility
B. Extensibility
C. Elasticity
D. Excitability
Answer: C. Elasticity
3. What is the membrane enclosing a muscle fibre called?
A. Endomysium
B. Sarcolemma
C. Sarcoplasm
D. Epimysium
Answer: B. Sarcolemma
4. Which type of muscle is voluntarily controlled?
A. Smooth
B. Cardiac
C. Skeletal
D. All of these
Answer: C. Skeletal
Related Topics
FAQs on Muscular Tissue
1. What exactly is muscular tissue?
Muscular tissue is a specialised type of body tissue composed of cells that have the unique ability to shorten or contract. This contraction is what generates force to produce movement, whether it's the voluntary movement of your limbs or the involuntary beating of your heart.
2. What are the primary functions of muscular tissue in the body?
Muscular tissue performs several crucial functions to keep the body working. Its main roles include:
- Producing Movement: Enabling actions like walking, running, and lifting by pulling on bones.
- Maintaining Posture: Constantly working to keep your body upright and stable.
- Circulating Substances: Pushing blood through arteries (cardiac muscle) and moving food through the digestive system (smooth muscle).
- Generating Heat: Muscle contractions produce heat, which is essential for maintaining a stable body temperature.
3. What are the three main types of muscular tissue?
The human body has three distinct types of muscular tissue, each with a specific job:
- Skeletal Muscle: This is the tissue attached to your skeleton that you control consciously to move your body.
- Smooth Muscle: Found in the walls of internal organs like the stomach and blood vessels, this tissue works automatically without your conscious thought.
- Cardiac Muscle: This unique tissue is found only in the heart and is responsible for pumping blood.
4. How can you tell the difference between skeletal, smooth, and cardiac muscles?
You can distinguish them by their structure and how they are controlled. Skeletal muscle is voluntary (under conscious control) and appears striped, or striated. Smooth muscle is involuntary (works automatically) and looks smooth, not striped. Cardiac muscle, found in the heart, is also striated like skeletal muscle but is involuntary and its cells are branched and interconnected.
5. What happens inside a muscle cell to make it contract?
Muscle contraction is driven by a process known as the sliding filament theory. Inside each muscle cell are two types of tiny protein filaments: actin and myosin. When a muscle receives a signal from a nerve, these filaments slide past each other, causing the entire cell to shorten. This collective shortening of millions of cells results in the contraction of the whole muscle.
6. Why are skeletal muscles voluntary while cardiac and smooth muscles are involuntary?
The difference in control comes down to the part of the nervous system that manages them. Skeletal muscles are controlled by the somatic nervous system, which governs conscious, deliberate actions. On the other hand, smooth and cardiac muscles are regulated by the autonomic nervous system, which handles vital, automatic functions like your heartbeat and digestion without you needing to think about them.
7. Why doesn't the heart muscle get tired like arm or leg muscles do?
Cardiac muscle is exceptionally resistant to fatigue because it is built for endurance. Its cells contain a very high number of mitochondria, which are the energy factories of the cell. This, combined with a constant, rich supply of oxygenated blood, ensures the heart can continuously produce enough energy to beat rhythmically for an entire lifetime without rest.
8. Where can you find smooth muscle tissue working in the body?
Smooth muscle tissue is the automatic worker found inside many of your hollow organs. For example, it's the muscle in the walls of your stomach and intestines that pushes food along, the muscle that controls blood pressure by contracting or relaxing your arteries, and the muscle in your bladder that controls urination.
9. Are all striated (striped) muscles under our conscious control?
No, and this is a key distinction. While it is true that the skeletal muscles we control are striated, the cardiac muscle of the heart is also striated. However, cardiac muscle is completely involuntary. Therefore, the presence of stripes does not automatically mean a muscle is voluntary; its location and function determine its control.
10. Why do skeletal muscle cells have multiple nuclei?
Skeletal muscle fibres are formed during development when many individual precursor cells, called myoblasts, fuse together to create one long, continuous cell. Each of these myoblasts contributes its nucleus to the final fibre. Having multiple nuclei allows the large muscle cell to efficiently produce the proteins needed for powerful contractions and to repair itself when damaged.
