Muscular System Chapter 9

Three types of Muscle Tissue  Skeletal  Smooth  Cardiac

Connective Tissue Covering  Fascia (dense CT) covers each skeletal muscle  May Become a tendon or aponeuroses (broad flat fibrous sheet)  Aponeurosis Attaches muscle to muscle

Tendon vs Ligament

Layers of CT that enclose and spearate the parts of a muscle:

 Epimysium: outer layer  Perimysium: extends inward from epimysium and separates muscle into fasicles  Endomysium: covers each muscle fiber in the fasicle

 Deep Fascia: surrounds and penetrates the muscles  Subcutaneous fascia: deep fascia is continuous with subcutaneous (just beneath the skin)  Subserous fascia: continuous with subcutaneous; forms CT layer of the serous membrane covering organs

Skeletal Muscle Fiber

Terms:  Sarcolemma: muscle cell membrane  Sarcoplasm: cytoplasm  Myofibrils: threadlike structures; made of proteins actin and myosin  Myosin: thick  Actin: thin  Sarcomere: functional unit of a muscle

Sarcomere structure: Z line to Z line

 I bands: light; made of thin actin  Z lines: hold myosin in place with protein called titin  A bands: thick myosin overlapping thin actin  H Zone: consists of thick myosin  M line: made of proteins that hold the myosin in place

Actin & Myosin Cross bridges

 Actin: thin filaments twisted into a helix; have binding sites for the cross bridges of myosin  Myosin: two twisted protein strands with globular parts called cross bridges

Actin: troponin-tropomyosin complex

Sarcoplasmic Reticulum (ER)  Network of membranous channels that surround each myofibril (in sarcoplasm)  Cisternae: enlarged areas of sarcoplasmic reticulum  Transverse tubules (T-tubules)are continuous with the sarcolemma and contain extracellular fluid

Triad: Cisternae of SR & T Tubule

Muscle Structure Review  Muscle (covered w/ epimysium)  Fasicles (covered w/perimysium)  Muscle fibers=cells (covered w/endomysium)  Myofibrils covered w/sarcoplasmic reticulum  Filaments: actin and myosin

Neuromuscular Junction

Muscle Fiber Contraction  1. nerve impulse travels down the motor neuron axon  2. Motor neuron vesicles release the neurotransmitter acetylcholine (ACh).  3. ACh binds to ACh receptors on the motor end plate.  4. The sarcolemma is stimulated, and a muscle impulse travels over the surface of the muscle fiber and deep into the fiber through the T tubules

 5. The muscle impulse reaches the sarcoplasmic reticulum, and calcium channels open.  6. Calcium ions diffuse from the SR cisternae into the sarcoplasm and bind to troponin molecules  7. Tropomyosin molecules move and expose specific sites on actin.  8. Actin and myosin form linkages.

 9. Thin (actin) filaments are pulled toward the center of the sarcomere by myosin cross-bridges.  10. The musle fiber shortens and contracts.

Muscle Fiber Relaxation  1. Acetylcholinesterase (enzyme) decomposes Ach, and the muscle fiber membrane is no longer stimulated.  2. Calcium ions are actively transported into the SR.  3. ATP (attached to cross-bridges) breaks linkages between actin and myosin w/O breakdown of the ATP. Tropomyosin rolls back into its groove, preventing cross bridge attachment.

 4. Breakdown of ATP “cocks” the cross-bridges  5. Troponin and tropomyosin molecules inhibit the interaction b/w myosin and actin.  6. Muscle fiber remains relaxed, yet ready until stimulated again.

 AS long as ATP and Calcium ions are present, the contraction cycle continues.  When Ca is low-the muscle remains relaxed.  ATP is needed for muscle contraction (Ca Pump) and muscle relaxation.

Sliding Filament Model  Sarcomeres shorten (fg. 9.11)  H zones and I bands narrow  Z lines move closer together

 Muscle contraction Muscle contraction  muscle contraction muscle contraction

Energy Sources for Contraction  ATP molecules  ATP must be regenerated  Creatine phosphate-stores energy released from mitochondria(Fg. 9.12)  Also glucose (cellular respiration)

Oxygen Supply and Cellular Respiration  Anaerobic respiration-2 ATP per glucose  Aerobic respiration-38 ATP per glucose  Hemoglobin in RBC’s delivers Oxygen to cells  Myoglobin in muscle cells stores some temporary oxygen

Oxygen Debt  During strenuous exercise, O deficiency may develop and lactic acid may accumulate as a result of anaerobic respiration  The amount of O needed to convert accumulated lactic acid to glucose and to restore supplies of ATP and creatine phosphate is called oxygen debt

Muscle Fatigue & Cramp  Muscle loses ability to contract  Usually due to accumulation of lactic acid (low pH prevents muscle fibers from responding to stimulation)  Cramp-sustained contraction; decreased electrolyte concentration, motor neurons trigger uncontrolled stimulation  Athletes produce less lactic acid than non-athletes

Athletes:  Aerobic training stimulates new capillaries to extend within the muscles (more O and nutrients)  And adds more mitochondria (more ATP)

Heat Production  Muscles are an important source of body heat  Most of the energy released by Cellular Respiration is lost as heat