Muscle Chapter PDF

Muscle Chapter:10: page: 328-356  Review of microscopic anatomy of skeletal muscle fiber.  -Sliding filaments mechanisms of skeletal muscle: Contraction cycle, Excitation-contraction coupling, Length-tension relationship.  -neuromuscular junction. ...

Muscle Chapter:10: page: 328-356  Review of microscopic anatomy of skeletal muscle fiber.  -Sliding filaments mechanisms of skeletal muscle: Contraction cycle, Excitation-contraction coupling, Length-tension relationship.  -neuromuscular junction.  -Muscle metabolism.  -Control of muscle tension; motor units, muscle twitch, isotonic & isometric contraction.  -Types of skeletal muscle fibers.  - Smooth muscle physiology Muscular Tissue Muscle Function Movement – Depends on type of muscle tissue – Depends on location of muscle tissue Thermogenesis Protection Posture Maintenance Joint Stabilization Muscle Tissue Characteristics All muscle tissues share basic characteristics 1.Excitability 2.Contractility 3.Elasticity 4.Extensibility  Properties of Muscle Excitability: capacity of muscle to respond to a stimulus Contractility: ability of a muscle to shorten and generate pulling force Extensibility: muscle can be stretched back to its original length Elasticity: ability of muscle to recoil to original resting length after stretched  Types ofMuscle Tissue  Skeletal muscle tissue is primarily attached to bones. It is striated and voluntary.  Cardiac muscle tissue forms the wall of the heart. It is striated and involuntary.  Smooth (visceral) muscle tissue is located in viscera. It is nonstraited (smooth) and involuntary.  constriction of blood vessels and airways, propulsion of foods through the gastrointestinal tract, and contraction of the urinary bladder and gallbadder)  Skeletal Muscle Tissue  Each skeletal muscle is a separate organ composed of cells called fibers.  In addition to muscle fiber, skeletal muscle contains  Connective Tissue  Blood vessels  nerves Muscle Cell or Muscle Fiber or Myofibers Filaments and the Sarcomere  Sarcomere - repeating functional units of a myofibril Sarcomeres: Z – About 10,000 sarcomeres per myofibril, end to end Disk to Z Disk – Each is about 2 µm long Differences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance. – A bands: a dark band; full length of thick (myosin) filament – M line - protein to which myosins attach – H zone - thick but NO thin filaments – I bands: a light band; from Z disks to ends of thick filaments Thin but NO thick filaments Extends from A band of one sarcomere to A band of the next sarcomere – Z disk: filamentous network of protein. Serves as attachment for actin myofilaments – Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment Thick & Thin Myofilaments Overlap  The Proteins ofMuscle  Myofibrils are built of 3 kinds of protein  contractile proteins  myosin and actin  regulatory proteins which turn contraction on & off  troponin and tropomyosin  structural proteins which provide proper alignment, elasticity and extensibility  titin, myomesin, nebulin and dystrophin The Proteins ofMuscle - Myosin The Proteins ofMuscle - Actin  Thin filaments are made of actin, troponin, & tropomyosin  The myosin-binding site on each actin molecule is covered by tropomyosin in relaxed muscle  The thin filaments are held in place by Z lines. From one Z line to the next is a sarcomere.  Mechanism ofContraction  Myosin cross bridges pull on thin filaments  Thin filaments slide inward  Z Discs come toward each other  Sarcomeres shorten.The muscle fiber shortens. The muscle shortens  Notice :Thick & thin filaments do not change in length How Does Contraction Cycle Begin?  Neuromuscular junction (NMJ)  16  Relaxation  Acetylcholinesterase (AChE) breaks down ACh within the synaptic cleft  Muscle action potential ceases  Ca+2 release channels close  Active transport pumps Ca2+ back into storage in the sarcoplasmic reticulum  Calcium-binding protein (calsequestrin) helps hold Ca+2 in SR (Ca+2 concentration 10,000 times higher than in cytosol)  Tropomyosin-troponin complex recovers binding site on the actin Overview: From Start to Finish  Length-Tension Relationship  The forcefulness of muscle contraction depends on the length of the sarcomeres within a muscle before contraction begins.  Optimal overlap of thick & thin filaments  produces greatest number of crossbridges and the greatest amount of tension  As stretch muscle (past optimal length)  fewer cross bridges exist & less force is produced  If muscle is overly shortened (less than optimal)  fewer cross bridges exist & less force is produced  thick filaments crumpled by Z discs  Normally  resting muscle length remains between 80 to 120% of the optimum  Pharmacologyofthe NMJ  Botulinum toxin blocks release of neurotransmitter at the NMJ so muscle contraction can not occur  bacteria found in improperly canned food  death occurs from paralysis of the diaphragm  Curare (plant poison from poison arrows)  causes muscle paralysis by blocking the ACh receptors  used to relax muscle during surgery  Neostigmine (anticholinesterase agent)  blocks removal of ACh from receptors so strengthens weak muscle contractions of myasthenia gravis  also an antidote for curare after surgery is finished  Muscle Metabolism Production ofATP in Muscle Fibers  Muscle uses ATP at a great rate when active  Sarcoplasmic ATP only lasts for few seconds  3 sources of ATP production within muscle  Creatine phosphate  anaerobic cellular respiration  aerobic cellular respiration Creatine Phosphate Anaerobic Cellular Respiration Aerobic Cellular Respiration  Muscle Fatigue  Inability to contract after prolonged activity  Factors that contribute to fatigue  central fatigue is feeling of tiredness and a desire to stop (protective mechanism)  insufficient release of acetylcholine from motor neurons  depletion of creatine phosphate  decline of Ca+2 within the sarcoplasm  insufficient oxygen or glycogen  buildup of lactic acid and ADP  Control ofMuscle Tension  Motor unit = one somatic motor neuron & all the skeletal muscle cells (fibers) it stimulates (10 cells to 2,000 cells).  Total strength of a contraction depends on how many motor units are activated & how large the motor units are  A twitch contraction is a brief contraction of all the muscle fibers in a motor unit in response to a single action potential. Parts ofa Twitch Contraction  Latent Period (2msec)  Ca+2 is being released from SR  slack is being removed from elastic components  Contraction Period  10 to 100 msec  filaments slide past each other  Relaxation Period  10 to 100 msec  active transport of Ca+2 into SR  Refractory Period  muscle can not respond and has lost its excitability  5 msec for skeletal & 300 msec for cardiac muscle  W ave Summation  Wave summation is the increased strength of a contraction resulting from the application of a second stimulus before the muscle has completely relaxed after a previous stimulus. Types ofSkeletal Muscle Fibers  On the basis of structure and function, skeletal muscle fibers are classified as  slow oxidative (slow-twitch) fibers,  red in color (lots of mitochondria, myoglobin & blood vessels)  prolonged, sustained contractions for maintaining posture  oxidative-glycolytic (fast-twitch A) fibers,  red in color (lots of mitochondria, myoglobin & blood vessels)  split ATP at very fast rate; used for walking and sprinting  fast glycolytic (fast-twitch B) fibers.  white in color (few mitochondria & BV, low myoglobin)  anaerobic movements for short duration; used for weight-lifting  Cardiac Muscle  Cardiac muscle fibers are arranged similarly to skeletal muscle fibers.  Cardiac muscle fibers connect to adjacent fibers by intercalated discs which contain desmosomes and gap junctions.  Cardiac muscle contractions last longer than the skeletal muscle twitch due to the prolonged delivery of calcium ions from the sarcoplasmic reticulum and the extracellular fluid.  Cardiac muscle fibers contract when stimulated by their own autorhythmic fibers.  The continuous rhythmic activity is a major physiological difference between cardiac and skeletal muscle tissue.  Contracts 75 times per min & needs lots of O2.  Larger mitochondria generate ATP aerobically.  Extended contraction is possible due to slow Ca+2 delivery  Ca+2 channels to the extracellular fluid stay open  Smooth Muscle  Two Types:  Visceral (single-unit)  in the walls of hollow viscera & small blood vessels  Autorhythmic  gap junctions cause fibers to contract in unison  Multiunit  individual fibers with own motor neuron ending  found in large arteries, large airways, arrector pili muscles, iris & ciliary body PhysiologyofSmooth Muscle  Contraction starts slowly & lasts longer  No transverse tubules & very little SR  Ca+2 must flows in from outside  The regulator protein that binds calcium ions in the cytosol is calmodulin (in place of the role of troponin in striated muscle);  calmodulin activates the enzyme myosin light chain kinase, which facilitates myosin-actin binding and allows contraction to occur at a relatively slow rate.