3.3 Muscular tissue

Muscular tissue is the body's powerhouse, turning chemical energy into movement. It's made of long cells called muscle fibers that work together to make us move, keep us upright, and even generate heat when we're cold.

There are three types of muscle: skeletal, smooth, and cardiac. Each has its own special job in the body. Skeletal muscle moves bones, smooth muscle works in organs, and cardiac muscle pumps blood through the heart.

Muscular Tissue Characteristics

Composition and Specialization

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• Muscular tissue is composed of elongated cells called muscle fibers specialized for contraction
• Muscle fibers generate force through the conversion of chemical energy (ATP) into mechanical energy
• Muscle fibers are multinucleated, with nuclei located peripherally beneath the cell membrane (sarcolemma)

Functions and Regulation

• The primary functions of muscular tissue are to produce movement, maintain posture, stabilize joints, and generate heat
• Muscle fibers contain myofibrils, composed of myofilaments (actin and myosin) that slide past each other during contraction
• Muscle contraction is triggered by nerve impulses or hormones and regulated by calcium ions released from the sarcoplasmic reticulum
• Muscular tissue is highly vascularized to meet its high energy demands and remove metabolic waste products (lactic acid)

Skeletal vs Smooth vs Cardiac Muscle

Skeletal Muscle

• Skeletal muscle is voluntary, striated, and typically attached to bones via tendons
• Responsible for conscious body movements and under the control of the somatic nervous system
• Skeletal muscle fibers are long, cylindrical, and multinucleated
• Contracts and relaxes quickly (biceps brachii, quadriceps femoris)

Smooth Muscle

• Smooth muscle is involuntary, non-striated, and found in the walls of hollow organs and blood vessels
• Responsible for unconscious movements and controlled by the autonomic nervous system
• Smooth muscle fibers are spindle-shaped and uninucleated
• Contracts and relaxes slowly and can maintain prolonged contractions (stomach, intestines, urinary bladder)

Cardiac Muscle

• Cardiac muscle is involuntary, striated, and found only in the heart
• Responsible for pumping blood and is self-excitable, with its own conduction system
• Cardiac muscle fibers are branched, striated, and typically uninucleated
• Has a unique contraction pattern, with a long refractory period to prevent tetanic contractions (atria, ventricles)

Muscle Fiber Structure and Components

Myofibrils and Sarcomeres

• Muscle fibers are composed of myofibrils, long, cylindrical structures that run parallel to the length of the fiber
• Myofibrils are composed of repeating units called sarcomeres, the basic functional units of muscle contraction
• Sarcomeres contain thick filaments (myosin) and thin filaments (actin, troponin, and tropomyosin) that slide past each other during contraction, shortening the sarcomere length
• The arrangement of light and dark bands within a sarcomere gives skeletal and cardiac muscle their striated appearance

Organelles and Membrane Systems

• The sarcoplasmic reticulum surrounds each myofibril and stores and releases calcium ions, which are essential for muscle contraction
• Transverse tubules (T-tubules) are invaginations of the sarcolemma that penetrate the muscle fiber and allow for the rapid transmission of action potentials into the interior of the fiber
• Mitochondria are abundant in muscle fibers and provide ATP for muscle contraction through cellular respiration

Muscular Tissue Locations and Roles

Skeletal Muscle

• Attached to bones and responsible for voluntary movements, maintaining posture, and stabilizing joints (biceps brachii, quadriceps femoris, external oblique muscles)
• Found in the walls of the pharynx, upper esophagus, and external anal sphincter, where it plays a role in swallowing and maintaining continence

Smooth Muscle

• Found in the walls of hollow organs, such as the stomach, intestines, urinary bladder, and uterus, where it is responsible for peristalsis, segmentation, and expulsion of contents
• Found in the walls of blood vessels, where it regulates blood flow and pressure
• The iris and ciliary body of the eye contain smooth muscle, which is responsible for controlling pupil size and accommodation (focusing) of the lens

Cardiac Muscle

• Found only in the heart and responsible for pumping blood through the circulatory system
• The atria and ventricles of the heart are composed of cardiac muscle