Muscle Tissue Overview

Muscle Tissue

• Three types of muscle tissue: skeletal, cardiac, and smooth
• Muscle tissue is responsible for movement through contraction using calcium
• All muscle cell parts include the prefix "sarco-", such as sarcoplasm, sarcoplasmic reticulum, and sarcolemma

Skeletal Muscle Tissue

• Striated, multinucleated (fusion of many cells)
• Movement is voluntary and organized into long muscle fibers
• Muscle fibers can be 1 mm to 30 cm in length and are 10-100 µm in diameter
• Satellite cells are "reserve cells" and are responsible for limited regeneration after damage

Organization of Skeletal Muscle

• Endomysium: very thin layer of connective tissue that surrounds individual muscle fibers; contains capillaries and nerve endings
• Perimysium: thin connective tissue layer surrounding a bundle of muscle fibers; acts as a functional unit where fibers work together and are innervated by one nerve
• Epimysium: dense connective tissue that surrounds the entire muscle; contains nerves, blood, and lymph vessels

Skeletal Muscle Fiber Organization

• Muscle fibers have highly organized myofibrils, which are long bundles of protein filaments
• Thick filaments: myosin proteins
• Thin filaments: actin proteins (includes troponin and tropomyosin)
• Sarcomere: the functional unit of muscle fibers, extending from Z-line to Z-line
• Contraction: Actin and myosin slide past each other, bringing Z-lines closer together

Contraction of a Muscle Fiber

• Contraction is initiated by a signal from the neuromuscular junction (NMJ) that releases acetylcholine (ACh)
• ACh depolarizes the sarcolemma (T-tubules), causing the release of calcium (Ca2+) from the sarcoplasmic reticulum (SR)
• Ca2+ initiates the contraction cycle
• T-tubules are infoldings of the sarcolemma
• Branched sarcoplasmic reticulum (SR) helps regulate calcium levels
• Triad is formed by a T-tubule and two SRs
• Mitochondria provide energy for contraction

Cardiac Muscle Tissue

• Cells are branched, striated, and connected via intercalated discs
• Surrounded by endomysium and many capillaries
• Groups of cardiac muscle cells are surrounded by perimysium, which is continuous with the connective tissue of the heart: endocardium and epicardium
• Signal transfer is similar to skeletal muscle but less organized
• Contains many mitochondria (up to 40%) for aerobic respiration
• Primarily uses fatty acids as energy source

Cardiac Intercalated Discs

• Strong adhesion through desmosomes
• Communication through gap junctions

Smooth Muscle Tissue

• Individual cells with many gap junctions
• Endomysium contains basal lamina and collagen fibers, transferring contractile forces
• Slow contraction using actin and myosin
• No striations
• Cells pack closely together in layers with different alignments
• Dense bodies serve as attachment points (similar to Z-lines)
• Uses calmodulin and MLCK instead of troponin and tropomyosin

Muscle Tissue and Movement

• Skeletal muscle tissue: movement is initiated by motor nerves releasing acetylcholine (ACh)
• Cardiac muscle tissue: movement is initiated by the autonomic nervous system and the pacemaker heart
• Smooth muscle tissue: movement is initiated by the autonomic nervous system, hormones, and secretions