Muscle tissue Muscle tissue Muscle cells = muscle fibers Myo, mys, sarco Types Skeletal Cardiac Smooth Characteristics Excitability Contractability Extensibility Elasticity Functions Movement Posture Stabilize joints Heat Protection Dialate/constrict pupils Skeletal muscle Connective tissue sheaths of skeletal muscle: epimysim, perimysium, and endomysium Skeletal muscle fiber Sarcolemma Sarcoplasm - Glycosomes- Myoglobin Myofibrils Striations  Microscopic anatomy Nuclei Dark A band Light I band Thin (actin) filament- Consists of 2 strands of actin subunits twisted into a helix plus 2 types of regulatory proteins (troponin and tropomyosin) Z disc H zone: thick filaments only Thick (myosin) filament- Consists of many myosin molecules whose heads protrude at opposite ends of the filament I band: thin filaments only A band- Outer edge of A band: thick and thin filaments overlap- Myosin filament- Actin filament  I band M line: thick filaments linked by accessory proteins Elastic (titin) filament Structure and organizational levels of skeletal muscle Muscle (organ)- Includes epimysium, fascicle, muscle, tendon- Description: consists of hundreds to thousands of muscle cells, plus connective tissue wrappings, blood vessels, and nerve fibers- Connective tissue wrappings: covered externally by epimysium Fascicle (a portion of the muscle)- Includes part of fascicle, perimysium, muscle fiber- Description: a fascicle is a discrete bundle of muscle cells, segregated from the rest of the muscle by a connective tissue sheath- Connective tissue wrappings: surrounded by a perimysium Muscle fiber (cell)- Includes nucleus, endomysium, sarcolemma, myofibril- Description: an elongated multinucleate cell, it has a banded (striated) appearance- Connective tissue wrappings: surrounded by endomysium Myofibril or fibril (complex organelle composed of bundles of myofilaments) - Includes sarcomere- Description: rod-like contractile elements that occupy most of the muscle cellvolume. Composed of sarcomeres arranged end to end, they appear banded and bands of adjacent myofibrils are aligned  Sarcomere (a segment of a myofibril)- Includes a sarcomere, thin (actin) filament, thick (myosin) filament- Description: is the contractile unit, composed of myofilaments made up of contractile proteins Myofilament or filament (extended macromolecule structure)- Includes thick filament, thin filament, head of myosin molecule, actin molecules- Description: are 2 types, thick and thin Thick: contain bundled myosin molecules Thin: contain actin molecules plus other proteins The sliding of the thin filaments past the thick filaments produces muscle shortening Elastic filaments: maintain the organization of the A band and provide for elastic recoil when muscle contraction ends  Muscle contraction Activation Excitation-contraction coupling Action potential Rise in intracellular Ca2+ Contraction Terms/concepts Somatic motor neurons Neuromuscular junction Synaptic cleft Acetylcholine Ion channels- Chemically gated- Voltage gated  Steps 1) action potential arrives at axon terminal of motor neuron 2) voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal 3) Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis 4) acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma  5) Ach binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber Steps 1) at the start, the membrane is completely polarized 2) when an action potential is initiated, a region of the membrane depolarizes. Asa result, the adjacent regions become depolarized 3) when the adjacent region is depolarized to its threshold, an action potential starts there 4) repolarization occurs due to the outward flow of K+ ions. The depolarization spreads forward, triggering action potential 5) depolarization spreads forward, repeating the process  Action potential scan showing changes in Na+ and K+ ion channels Na+ channels open Depolarization due to Na+ entry Na+ channels close, K+ channels open Repolarization due to K+ exit K+ channels close  Steps in E-C Coupling Action potential is propagated along the sarcolemma and down the T tubles Calcium ions are released  Calcium binds to troponin and removes the blocking action of tropomyosin Contraction begins  Cross bridge cycle Cross bridge formation The power (working) stroke Cross bridge detachment Cocking of myosin head  Axons of motor neurons extended from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with musclefibers scattered throughout the muscle  Muscle twitch Myogram showing 3 phases of an isometric twitch Latent period Period of contraction Period of relaxation  Graded muscle response: temporal/wave summation Low stimulation frequency  unfused (incomplete) tetanus High stimulation frequency  fused (complete) tetanus- At higher stimulus frequencies, there is no relaxation at all between stimuli. This is fused (complete) tetanus  Recruitment  Muscle metabolism Direct phosphorylation Coupled reaction of creatine phosphate (CP) and ADP Energy source: CP Anaerobic pathway Glycolysis and lactic acid formation Energy source: glucose Aerobic pathway Aerobic cellular respiration Energy source: glucose, pyruvic acid, free fatty acids from adipose tissue, amino acids from protein catabolism  Smooth muscle Peristalsis Varicosities Diffuse junctions Caveolae Less developed SR, no T-tubles Tropomyosin and calmodulin Relaxed smooth muscle fiber (note gap junctions) Includes nucleus, intermedicate filament, caveolae, gap junctions, dense bodies Contracted smooth muscle fiber Includes nucleus, dense bodies  Define Intermediate filaments (filaments that are on outer layer of smooth muscle and are diagonal, aren’t providing contractile movement, allow contraction of one cellto affect other tissues around it) Dense bodies (all dots along surface among smooth muscle cells, anchor thin andintermediate filaments,  Gap junctions