Differentiated cells that possess contractile filaments, classified according to morphology and function, contraction may be voluntary or involuntary
General functional characteristics of muscle
Posture maintenance
Movement
Joint stabilization
Heat generation
Special functional characteristics of muscle
Contractility (shorten and generate pulling force)
Excitability (respond to stimuli)
Extensibility (stretch with contraction of opposing muscle)
Elasticity (recoil passively after stretch)
Isotonic
Type of muscle contraction. Muscle Tension using the same weight.
Isometric
Type of muscle contraction. Muscle Tension developed without changing the
Types of muscle tissue
Skeletal
Cardiac
Smooth
Skeletal muscle
Attach to and move skeleton, 40% of body weight, fibers are multinucleated, visible striations, voluntary
Cardiac muscle
Found only in heart wall, myocardium, striated, involuntary
Smooth muscle
Found in hollow visceral organs, no visible striations, involuntary
Skeletal muscle structure
1. Myofibril
2. Muscle fiber
3. Muscle fascicle
4. Skeletal muscle (organ)
Skeletal muscle general concepts and structure
Multinucleated, cytoplasm is sarcoplasm, plasma membrane is sarcolemma. Sarcolemma forms deep tubular invaginations (T-tubules), consists largely of myofibrils, sarcoplasmic reticulum is a modified smooth ER, T-tubules and sarcoplasmic reticulum form a triad
Skeletal muscle myofibrils
Composed of contractile proteins (myofibrils), myofibrils are divided into segments called sarcomeres (contractile/functional units), composed of alternating thick (myosin) and thin (F-Actin) filaments (collectively known as myofilaments), Z line/disc marks sarcomere boundary
Skeletal muscle proteins
Contractile proteins (e.g. Actin and Myosin - generate force during contraction)
Regulatory proteins (e.g. Troponin and Tropomyosin - switch the contraction process on and off)
Structural proteins (e.g. Titin and Dystrophin - align the thick and thin filaments properly, provide elasticity and extensibility, link the myofibrils to the sarcolemma)
Skeletal muscle contraction
1. Action potential at NMJ transmitted along T-tubules to terminal cisterna of SR, release of calcium
2. Ca++ binds to troponin, myosin heads bind to actin, creating cross bridges
3. Cross bridges pull on thin filaments, sarcomere shortens
4. Ca++ goes back into sarcoplasmic reticulum, contraction stops
Skeletal muscle microscopic appearance
Cells are elongated, do not branch, larger diameter, nucleus is multinucleated, flattened and peripherally located, myoblasts are embryonic cells that fuse to develop muscle fibers
Skeletal muscle fiber microscopic appearance
Myofibrils are striated with distinct myofilaments (actin & myosin), alternating light and dark bands, dark bands (A bands) are anisotropic/birefringent, light bands (I bands) are isotropic/do not alter polarized light
Skeletal muscle distribution
Widely distributed, attached to entire skeletal system of the body, external urethral & external anal sphincter, tongue (although unattached to skeletal system), upper 1/3 of the esophagus (lower part is involuntary smooth muscle)
Types of skeletal muscle
Red/Slow (Type I)
Red/Fast (Type IIa)
White/Fast (Type IIb)
Cardiac muscle
Bundles form thick myocardium, cardiac muscle cells are single cells (not called fibers), auto rhythmicity (each cell beats separately even without stimulation), involuntary activation (like smooth muscle), very fatigue resistant
Cardiac muscle microscopic appearance
Spherical centrally located nucleus (branches have no nucleus), with intercalated discs of Eberth (serve as junction between cardiac cells), elongated branches with numerous areolar CT, myofibrils striated with distinct actin & myosin
Cardiac muscle ultrastructure
T-tubules are larger, located at Z-disk, sarcoplasmic reticulum is poorly defined, contributes to dyads, dyads consist of one T-tubule and one terminal cisterna of SR
Muscle types
Skeletal
Cardiac
Smooth
Skeletal muscle
Long cylindrical shape
Multiple peripheral nuclei
Striated
Triad at A-I junction
Voluntary contraction
Cardiac muscle
Branched shape
Single central nucleus
Striated
Dyad at Z disc
Involuntary contraction
Smooth muscle
Short spindle shape
Single central nucleus
No striations
Caveola and some ER
Gap junctions
No sarcomeres
Involuntary contraction
Smooth muscle contraction is initiated by the calcium-activated phosphorylation of myosin rather than calcium binding to troponin
Smooth muscle contraction involves the interaction of sliding actin and myosin filaments, similar to skeletal muscle
Smooth muscle is very fatigue resistant
Smooth muscle contraction is slow and sustained
Smooth muscle is involuntary and cannot be consciously controlled
Functions of smooth muscle
Alter activity of various body parts to meet their specific needs
Contraction of urinary bladder
Move food through intestines (peristalsis)
Peristaltic movement to move feces down digestive system
Contraction in trachea and bronchi to decrease airway size
Constriction and dilation of blood vessels to regulate blood pressure
Constriction, accommodation and dilation of pupil
Uterine contraction during birthing
Expulsion of glandular contents
Arrector pili muscle causes "goosebumps" for thermoregulation
Smooth muscle is distributed in the eye, respiratory tubes, urinary organs, reproductive organs, digestive tubes, walls of blood vessels, and skin
Cardiac muscle is responsible for generating and conducting electrical impulses for the heart, causing the heart to contract and pump blood throughout the body
Components of the cardiac conduction system
Sinoatrial (SA) node
Atrioventricular (AV) node
Bundle of His
Left and right bundle branches
Purkinje fibers
Sinoatrial (SA) node
Regulates heart rate and rhythm
Atrioventricular (AV) node
Delays cardiac impulses from SA node to allow atria to contract and empty first, then relays impulses to the atrioventricular bundle
Atrioventricular bundle of His
Carries cardiac impulses down the septum to the ventricles via the Purkinje fibers
Left and right bundle branches
Carry nerve impulses that cause contraction of the left and right ventricles respectively
Purkinje fibers
Modified cardiac muscle cells specialized for conduction, trigger waves of contraction through both ventricles simultaneously