Muscle Tissue and Movement

Contractility, Excitability, Extensibility, and Elasticity

Muscle tissue possesses several unique physiological properties that enable movement and support in the human body.

• Contractility: The ability of muscle to shorten in response to stimulation, producing force (myofilaments: actin and myosin).

• Excitability: The capacity to respond to stimuli, such as nerve signals (also called 'responsiveness').

• Extensibility: The ability to be stretched, allowing muscles to extend opposing muscles.

• Elasticity: The ability to recoil after stretching, returning to original length.

Open/Close Passages

Muscles regulate the opening and closing of internal passageways, such as sphincters in the digestive and urinary tracts, and control the movement of substances through these systems.

• Examples: Sphincters in the gastrointestinal tract, urinary bladder, and blood vessels.

Support of Soft Tissue

Muscles, especially those forming the abdominal wall and pelvic floor, support the weight of internal organs and protect them from injury.

Heat Production and Posture

Muscle activity generates heat, which is essential for maintaining body temperature. Muscles also help maintain posture by stabilizing joints and supporting the skeleton.

Muscle Tissue Structure

• Muscle types: Skeletal, cardiac, and smooth muscle.

• Fibers: Muscle cells are called fibers; they are long, multinucleated, and contain contractile proteins.

• Connective Tissue Layers:

  • Endomysium: Connective tissue surrounding each muscle fiber.

  • Perimysium: Connective tissue surrounding bundles of fibers (fascicles).

  • Epimysium: Connective tissue surrounding the entire muscle; forms tendons and aponeuroses.

Muscle Attachments

• Origin: The fixed attachment point, usually on a bone.

• Insertion: The movable attachment, typically on a bone via tendons or aponeuroses.

• Direct Attachment: Muscle fibers attach directly to bone via short connective tissue.

• Indirect Attachment: Muscle fibers attach via tendons or aponeuroses (sheet-like connective tissue).

Muscle Fiber Structure

• Muscle Fiber: Also called a myofiber, contains myofibrils composed of myofilaments (actin and myosin).

• Sarcolemma: The plasma membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber, containing organelles and myofibrils.

• Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores calcium ions necessary for contraction.

Motor Units and Neuromuscular Junction

A motor unit consists of a motor neuron and all the muscle fibers it innervates. The neuromuscular junction is the site where the motor neuron communicates with the muscle fiber via neurotransmitters (e.g., acetylcholine) released into the synaptic cleft.

Muscle Movement and Disorders

• Muscles only pull and work in pairs or groups to produce movement.

• Muscle tissue can experience various disorders, except for cardiac muscle, which is less prone to some diseases.

• Muscles cannot undergo mitosis but can undergo hypertrophy (increase in size) with use.

Effects of Repeated Stimulation

• Repeated stimulation leads to increased numbers of mitochondria, more glycolytic enzymes, and more myofibrils, enhancing muscle endurance and strength.

Atrophy and Aging

Muscle atrophy (wasting) can result from aging or injury and may be permanent. Muscle mass as a percentage of body mass is typically 36% in females and 42% in males, largely due to the effects of androgens in males.

Summary Table: Muscle Tissue Structure

Key Equations

• Muscle Force: (Force equals mass times acceleration)

Additional info: Some explanations and terminology have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.