Muscle Tissue PDF

Muscle Tissue Objectives Describe the organization of muscle and the characteristics of skeletal muscle cells. Identify the structural components of the sarcomere. Summarize the events at the neuromuscular junction. Explain the key concepts involved in...

Muscle Tissue Objectives Describe the organization of muscle and the characteristics of skeletal muscle cells. Identify the structural components of the sarcomere. Summarize the events at the neuromuscular junction. Explain the key concepts involved in skeletal muscle contraction. Describe how muscle fibers obtain energy for contraction. Distinguish between aerobic and anaerobic contraction, muscle fiber types, and muscle performance. Identify the differences between skeletal, cardiac, and smooth muscle. Three types of muscle Skeletal—attached to bone Cardiac—found in the heart Smooth—lines hollow organs Characteristics of muscle Excitability Receive & respond to stimuli. Nervous impulse Contractility Actively generate force to shorten. Passively lengthens Extensibility Ability to be stretched by antagonist or gravity. Elasticity Ability to return to originial shape after stretch/contraction Skeletal muscle functions Produce skeletal movement Maintain posture and body position Support soft tissues Guard entrances and exits Maintain body temperature Organization of connective tissues Epimysium surrounds muscle Perimysium sheathes bundles of muscle fibers Endomysium covers individual muscle fibers Tendons or aponeuroses attach muscle to bone or muscle Skeletal Muscle Histology Skeletal muscle fibers Sarcolemma - cell membrane Sarcoplasm - muscle cell cytoplasm 51  Sarcoplasmic reticulum (modified ER) – sequester Ca++ and form Terminal Cisternae T-tubules – invaginations of Sarcolemma, and form Triads with Terminal Cisternae Sarcomeres—regular arrangement of myofibrils – BE ABLE TO DRAW! Mitochondria – generate ATP Myofibrils Comprised of thick and thin filaments Thin filaments Actin – active site for binding with myosin (thick filament) Tropomyosin - covers active sites on actin Troponin - binds to Ca++ during contraction to expose active sites Other thin filaments: Nebulin Thick filaments Bundles of myosin fibers around titan core Myosin molecule heads form cross-bridges with actin during contraction Sliding filament theory Begins at the Neuromuscular Junction: Action potential along motor neuron reaches the synaptic terminal (end bulb) Influx of Ca++ into neuron causes release of ACh into synaptic cleft ACh binds to receptors in the motor end plate – action potential propagates along sarcolemma AChE removes ACh Action potential propagates along sarcolemma and dips into cell interior vi T-tubles Action potential reaches triad, and causes terminal cisternae of SR to relase Ca++ into sarcoplasm Calcium binds to troponin of thin filament Troponin moves, moving tropomyosin and exposing actin active site Myosin head forms cross bridge and bends toward H zone via energy from ATP Sarcomere shortens – muscle contracts New ATP allows release of cross bridge Cross bridge cycling continues as long as there is action potential, Ca++, ACh, and ATP Relaxation normally occurs due to lack of action potential, so Ca++ is uptaken by SR, and troponin/tropomyosin cover binding sites Tension production by muscle fibers All or none principle Amount of tension depends on number of cross bridges formed 52  Twitch Cycle of contraction, relaxation produced by a single stimulus Treppe Repeated stimulation after relaxation phase has been completed Summation Repeated stimulation before relaxation phase has been completed Wave summation = one twitch is added to another Incomplete tetanus = muscle never relaxes completely Complete tetanus = relaxation phase is eliminated Motor units All the muscle fibers innervated by one motor neuron Precise control of movement determined by number and size of motor unit Motor units are progressively recruited to gradually increase tension. Contractions Isometric Tension rises, length of muscle remains constant = no change in joint angle Isotonic Tension rises, length of muscle changes = change in joint angle Concentric contractions = muscle shortening – overcoming gravity Eccentric contractions = muscle lengthening – lowering to ground Muscle Contraction requires large amounts of energy Creatine phosphate (CP) releases stored energy to convert ADP to ATP Aerobic metabolism provides most ATP needed for contraction At peak activity, anaerobic glycolysis needed to generate ATP Energy use and level of muscular activity Energy production and use patterns mirror muscle activity Fatigued muscle no longer contracts Build up of lactic acid Exhaustion of energy resources Recovery period Begins immediately after activity ends Oxygen debt (excess post-exercise oxygen consumption) Amount of oxygen required during resting period to restore muscle to normal conditions Types of skeletal muscle fibers Fast fibers Slow fibers Intermediate fibers 53 Slow Fibers - Type I - Red Half the diameter of fast fibers Take three times as long to contract after stimulation Abundant mitochondria Extensive capillary supply High concentrations of myoglobin resulting in red appearance Can contract for long periods of time- fatigue resistant Fast fibers – Type II - White Large in diameter Contain densely packed myofibrils Large glycogen reserves Relatively few mitochondria Produce rapid, powerful contractions of short duration – fatigue quickly Intermediate fibers Similar to fast fibers, yet somewhat greater resistance to fatigue Fiber type varies between muscles and vary from person to person Consider sprinters vs. marathoners, domestic fowl vs. migratory fowl Muscles contain a mixture of fiber types: Soleus (postural) 87% slow Orbicularis Oculi 15% slow All fibers w/n a motor unit (motor neuron & fibers it serves) are the same Great variability through genetics. Athletes made or born? Leg muscles: Avg. adult = 45% slow Distance runner = 80% slow Sprinters = 23% slow Muscle performance and the distribution of muscle fibers Pale muscles dominated by fast fibers are called white muscles Dark muscles dominated by slow fibers and myoglobin are called red muscles Training can lead to hypertrophy of stimulated muscle Physical conditioning Anaerobic endurance Time over which muscular contractions are sustained by glycolysis and ATP/CP reserves Aerobic endurance Time over which muscle can continue to contract while supported by mitochondrial activities 54 Changes in Muscular Size Muscular Atrophy Decrease in number of myofibrils & overall diameter form disuse Disuse Atrophy cast, bedridden, sedentary Denervation Atrophy cut nerve supply 1/4 original size from 6 mos to 2 yrs Fibers replaced w/ fibrous tissue Irreversible Muscular Hypertrophy Increase in muscle size from increase in number of myofibrils Not from increase in cell number. This is fixed genetically. More forceful contractions Controlled by genetics & hGH (childhood/puberty), testosterone, & training (weight lifting) Cardiac Muscle Tissue Structural characteristics of cardiac muscle Located only in heart Cardiac muscle cells are small One centrally located nucleus Short broad T-tubules Dependent on aerobic metabolism Intercalated discs where membranes contact one another Functional characteristics of cardiac muscle tissue Automaticity Contractions last longer than skeletal muscle No tetanic contractions possible Smooth Muscle Tissue Structural characteristics of smooth muscle Nonstriated Lack sarcomeres Thin filaments anchored to dense bodies Involuntary Functional characteristics of smooth muscle Contract when calcium ions interact with calmodulin Activates myosin light chain kinase Functions over a wide range of lengths Plasticity Multi-unit smooth muscle cells are innervated by more than one motor neuron Visceral smooth muscle cells are not always innervated by motor neurons Neurons that innervate smooth muscle are not under voluntary control 55