Muscle tissue is a specialized tissue found in animals which functions by contracting and thereby causing movement.
This contraction is achieved through the interaction of actin and myosin filaments within muscle cells, known as muscle fibers.
Muscle tissue is categorized into three types: skeletal muscle, cardiac muscle, and smooth muscle, each with distinct structures and functions.
Types of Muscle Tissue
1. Skeletal Muscle
Structure: Long, cylindrical, multinucleated fibers with striations from actin and myosin filaments arranged in sarcomeres.
Location: Attached to bones by tendons.
Function: Responsible for voluntary movements, posture maintenance, and heat generation.
Control: Voluntary, via the somatic nervous system.
2. Cardiac Muscle
Structure: Branched cells (cardiomyocytes) connected by intercalated discs with gap junctions and desmosomes; striated like skeletal muscle.
Location: Walls of the heart.
Function: Pumps blood rhythmically and continuously.
Control: Involuntary, regulated by the autonomic nervous system and intrinsic conduction systems.
3. Smooth Muscle
Structure: Spindle-shaped cells with a single central nucleus; lacks striations due to less organized actin and myosin.
Location: Walls of hollow organs (e.g., intestines, blood vessels, bladder, uterus).
Function: Involuntary movements such as peristalsis, vasoconstriction, and contractions during childbirth.
Control: Involuntary, via the autonomic nervous system.
Structure:
Skeletal muscle: Multinucleated, long, cylindrical cells with a striated appearance.
Smooth muscle: Spindle-shaped, uninucleated cells without striations.
Cardiac muscle: Uninucleated, branched cells with striations and intercalated discs.
Microscopic Structure
1. Myofibrils:
Each muscle fiber contains numerous myofibrils, which are the contractile elements made up of repeating units called sarcomeres.
2. Sarcomeres:
The basic functional unit of a myofibril, consisting of actin (thin) and myosin (thick) filaments. The arrangement of these filaments gives skeletal and cardiac muscle their striated appearance.
3. Sarcoplasmic Reticulum:
A specialized form of the endoplasmic reticulum that stores and releases calcium ions, which are essential for muscle contraction.
4. Mitochondria:
Muscle cells contain numerous mitochondria to meet the high energy demands required for contraction.
Location:
Skeletal muscle: Attached to bones by tendons.
Smooth muscle: Walls of internal organs like the stomach, intestines, and blood vessels.
Cardiac muscle: Heart.
Functions:
Movement: Muscle contractions enable body movement, facial expressions, and posture.
Regulation: Smooth muscle contractions regulate blood flow and control the passage of food through the digestive system.
Pumping: Cardiac muscle contractions pump blood throughout the body.