Muscle Types Overview

Muscle Types

• Three types: skeletal, smooth, and cardiac; all elongated muscle cells.
• Myofilaments (myo = muscle, sarco = flesh) enable muscle contraction by shortening.
• Muscles are delicate; protected and bundled by connective tissues.

Skeletal Muscle

• Smoothly forms the body's contours with huge, cigar-shaped, multi-nucleated cells.
• Largest muscle fibers can reach up to 30 cm in length and are striated.
• Voluntary activation often occurs through reflexes; fibers are soft and fragile yet bundled to withstand force.
• Enclosed in connective tissue: endomysium, perimysium, and epimysium, which blend with tendons for durability.

Smooth Muscle

• Involuntary muscle found in hollow visceral organs like the stomach, with no striations.
• Characterized by spindle-shaped cells containing a single nucleus.
• Arranged in circular and longitudinal layers; they alternate contractions for processes like peristalsis.

Cardiac Muscle

• Unique to the heart; striated but involuntary, responsible for pumping blood.
• Innervated by the nervous system and cushioned by a small amount of endomysium.
• Features branching cells connected by intercalated discs, facilitating coordinated contractions.
• Contains a pacemaker that regulates heartbeat.

Muscle Function

• The M-line holds adjacent thick filaments together, while myofibrils consist of chains of sarcomeres aligned in a series.
• Thick filaments (myosins) have ATPase enzymes essential for contraction; the heads form cross-bridges with thin filaments (actin).
• Thin filaments are anchored at Z discs and contribute to the bonding pattern with myosin heads.
• Calcium ions trigger contraction by interacting with troponin in skeletal muscles (calmodulin does so in smooth muscles).

Sliding Filament Theory

• Myosin heads generate cross-bridge tensions, allowing thin filaments to slide past thick filaments during contraction.
• As thin filaments slide, Z discs move toward the M-line, shortening the muscle fiber.
• Muscle fibers need to be activated for contraction, stimulated by nerve endings to generate an action potential.

Excitation-Contraction Coupling

• Begins at the neuromuscular junction with the release of acetylcholine, triggering action potential propagation along the sarcolemma.
• Calcium ions are released from the sarcoplasmic reticulum, binding to troponin and exposing myosin-binding sites on actin.
• Myosin forms cross-bridges with actin, initiating contraction.

Cross Bridge Cycle

• Formation: Myosin binds to actin, breaking ATP into ADP and phosphate.
• Power Stroke: Release of ADP and phosphate allows myosin head to pivot, pulling actin filaments.
• Detachment: ATP binding weakens the connection between myosin and actin.
• Cocking: ATP hydrolysis returns the myosin head to a high-energy state for further cycles.

Types of Contraction

• Isotonic Contraction: Muscle length changes to move a load.
  • Example: External and internal intercostals assist in respiration; diaphragm acts as the prime mover for inspiration.

Key Muscle Groups

• Abdominal Muscles: Rectus abdominis, external and internal obliques, and transversus abdominis flex and rotate the trunk and stabilize the pelvis.
• Pelvic Floor Muscles: Levator ani and coccygeus support pelvic viscera and resist pressure changes.
• Urogenital Diaphragm: Deep transverse perineal muscle and external urethral sphincter support pelvic organs and control urination.
• Pectoral Muscles: Pectoralis minor and serratus anterior play roles in scapula movement and stabilization.

Key Muscle Functions

• Gluteal Muscles: Extend and laterally rotate the thigh; important for walking and posture.
• Quadriceps Femoris: Extends the knee; crucial in activities like jumping and running.

Explore the three types of muscles: skeletal, smooth, and cardiac. Learn about their similarities, structures, and functions in the human body. This quiz will test your knowledge on muscle characteristics and classifications.