Muscle Chapter PDF
Document Details
Uploaded by CompliantSeal
Tags
Summary
This document reviews the microscopic anatomy of skeletal muscle fibers, the sliding filament mechanism of skeletal muscle contraction, excitation-contraction coupling, length-tension relationships, the neuromuscular junction, muscle metabolism, control of muscle tension, motor units, muscle twitch, isotonic and isometric contraction, types of skeletal muscle fibers, smooth muscle physiology. It is a good resource for understanding muscle tissue.
Full Transcript
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.