BIO 2401 MUSCLE TISSUE page 1 MUSCLES AND MUSCLE TISSUE. Striations Present or Absent?

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1 BIO 2401 MUSCLE TISSUE page 1 Types of Muscle MUSCLES AND MUSCLE TISSUE Type of Muscle Skeletal Location of Muscle attaches to and covers bony skeleton Striations Present or Absent? present Control of Muscle voluntary control; contracts rapidly, but tires easily; exerts power Unique or Unusual Characteristics multinucleate cells; elaborate SR; present of T-tubules; no gap junctions; uses aerobic & anaerobic respiration Cardiac Smooth heart walls present involuntary control; contractions are steady, but can speed up for short periods of time walls of hollow visceral organs absent involuntary; contractions are slow and sustained so can generate large amount of force uni- or binucleate cells; presence of T-tubules; gap junctions at intercalated discs; no neuromuscular junctions; mainly uses aerobic respiration often arranged in sheets; lack elaborate connective coverings; have poorly developed SR; lack T-tubules; gap junctions; no sarcomeres; calcium binds to calmodulin on thick filament rather than to troponin on thin filament; mainly uses anaerobic respiration Four major functions of a. produces movement (but can only pull) b. maintains posture c. stabilizes joints d. generates heat Four properties of : a. excitability (irritability) - ability to receive and respond to stimuli via generation of an electrical pulse which causes contraction of the cells b. contractility - ability to shorten c. extensibility - ability to be stretched or extended elasticity - ability of a fiber to recoil and resume its resting length

2 BIO 2401 MUSCLE TISSUE page 2 Gross Anatomy of Skeletal Muscle skeletal - discrete organ made up of several s ( fibers, blood vessels, nerve fibers, connective ) connective coverings: epimysium - overcoat of dense irregular c.t. that surrounds entire ; can blend with fascia that lies between adjacent s; large blood vessels and nerves enter here perimysium - layer of fibrous c.t. that surrounds grouped together fibers fascicles - a bundle of fibers; size of fasciluli reflects function small fasciluli give finely controlled movements and have more c.t. large fasciluli are responsible for gross movements and have little c.t. endomysium - within a fascicle; is fine sheath of connective (mostly reticular) that surrounds each fiber tendon - tough collagen fibers that make an indirect connection with aponeurosis - fibrous or membranous sheet that connects a to the bone it moves deep fascia - layer of fibrous c.t. that covers and separates Attachments: origin - attachment of to the nonmoveable bone insertion - attachment of to the moveable bone Microscopic Anatomy of Skeletal Muscle Fiber cell - long, cylindrical, and multinucleate sarcolemma - plasma membrane surface sarcoplasm - similar to cytoplasm but contains a large amount of glycosomes (stored glycogen) and myoglobin (O 2 binding protein); possess myofibrils and sarcoplasmic reticulum myoglobin - red pigment that binds O 2 sarcoplasmic reticulum (SR) - smooth endoplasmic reticulum; its tubules surround each myofibril; regulates intracellular levels of Ca 2+ (stores and releases it on demand) t-tubules - at each A band - I band junction; sarcolemma of cell penetrates into cell's interior to form an elongated tube (transverse tubule or T-tubule); are continuous with sarcolemma so allows conduction of impulses to deepest regions of cell myofibrils - rodlike fibrils that run in parallel fashion and extend over entire length of cell; densely packed (80% cell vol.); are contractile elements of cells (chains of sarcomeres); are striated due to series of repeating dark (A band) and light (I band) bands

3 BIO 2401 MUSCLE TISSUE page 3 sarcomeres - region of myofibril between 2 successive Z-disks; smallest contractile unit of fiber; composed of myofilaments made up of contractile proteins Molecular Composition of Myofilaments in Skeletal Muscle thick myofilaments - extends length of A band (dark) and contains bundled myosin molecules (~200); are maintained in parallel by attachment to disk-like zone (M bands) myosin - molecule with rodlike head terminating in 2 globular heads and chain of polypeptides tail; heads contain ATP binding sites and ATP enzymes and have 2 smaller light polypeptide chains; heads link thick and thin myofilaments together during contraction (cross-linking) thin myofilaments - extend across I band and part way into A band; composed mostly of actin molecules that are polymerized into long filaments (coil back on themselves); the filaments are stiffened by 2 strands of tropomyosin; also have troponin molecules that bind to actin and tropomyosin actin - bear the active sites to which myosin cross-bridges attach during contraction Molecular Composition of Myofilaments in Smooth Muscle interdigitating thick and thin filaments arranged diagonally to spiral down long axis of smooth cells thick filaments are longer; contain actin-gripping heads along entire length thin filaments lack troponin cells are electrically coupled so all cells making up a sheet respond in unison Structure & Components of Neuromuscular Junction of Skeletal Muscle 1) motor neuron - part of somatic (voluntary) division of nervous system; resides in brain or spinal cord; axons of these neurons travel to fibers (1 per fiber); terminate at motor end plate axon terminals - terminus of axon in which are synaptic vesicles with neurotransmitter; when action potential reaches here, opens Ca 2+ gates which causes vesicles to move toward membrane and fuse to release neurotransmitter neurotransmitter - usually acetylcholine 2) synaptic cleft space between axon terminal and folded section of sarcolemma; filled with a gell-like extracellular substance; neurotransmitter diffuses across and attaches to receptors on sarcolemma to cause a polarization change in cell 3) cell motor end plate (sarcolemma below axon terminal) trough-like portion of sarcolemma that is highly folded to provide increased surface area for receptors membrane receptors for neurotransmitter millions present; reversibly bind to neurotransmitter; binding causes Na + channels to open and cell to become less negative; can result in an action potential being generated across the sarcolemma

4 BIO 2401 MUSCLE TISSUE page 4 4) smooth lacks neuromuscular junctions; have innervating fibers with varicosities that release neurotransmitter into wide synaptic cleft Function of Neuromuscular Junction: motor neuron branches as it enters each axonal ending forms a neuromuscular junction with single fiber junction = axonal terminal, synaptic cleft, sarcolemma of 1 fiber serves to electrically isolate skeletal cells from each other Overview of Sliding Filament Theory: contraction of cell is via activation of myosin s cross-bridges so they can latch onto myosin-binding sites on actin in thin filaments contraction is initiated by action potential thin filaments are then pulled (slide) past thick filaments steps: 1. action potential when generated, intracellular levels of Ca 2+ rise in cell intracellular Ca 2+ comes from the cisternae of the SR SR has an ATP dependent Ca 2+ pump to restore low intracellular Ca 2+ levels after repolarization 2. Ca 2+ binds to troponin; tropomyosin molecule moves from myosin binding site on actin 3. cross-bridge attachment activated myosin heads are attracted to exposed binding sites on actin; bind at these sites 4. power stroke bound myosin head pivots, which pulls on thin filaments (slides it towards center of sarcomere); ADP + P i are release from myosin head 5. cross-bridge detachment new ATP molecule binds to myosin head; myosin loosens hold on actin and detaches; actin sites for myosin are blocked by tropomyosin and troponin 6. cocking of myosin head ATP molecule is hydrolyzed to ADP + P i ; provides energy to return myosin head to its high energy state position for next round of attachment; ADP + P i remain attached to head causes shortening of only 1% of a ; must occur frequently to result in 30-35% shortening typical of contracting required for fiber contraction: 1. Ca ATP Changes in Skeletal Muscle due to Exercise: A. Changes in Response to Aerobic Conditions Aerobic = endurance exercise (stamina); e.g., running, biking, swimming 1. increased capillaries to skeletal 2. increased # mitochondria in skeletal

5 BIO 2401 MUSCLE TISSUE page 5 3. increase in amount of myoglobin 4. improves efficiency of metabolism and neuromuscular coordination 5. improves gastrointestinal mobility 6. strengthens skeleton 7. hypertrophies heart 8. clears fatty deposits from blood vessels 9. improves gas exchange in lungs Result: more efficient metabolism greater endurance, strength, resistance to fatigue B. Changes in Response to Anaerobic Conditions Anaerobic = resistance exercise (strength); e.g., weight lifting 1. increase in size of cells (hypertrophy) 2. increase in mitochondria, myofilaments, myofibrils, glycogen (if vigorously stressed) 3. increase in amount of connective between cells Result: increase in bulk increased strength