Chapter 9 Muscle and Muscle Tissues PDF

Preview

Summary

Notes with Professor Longwith ...

Description

Muscles and Muscle Tissue Terminology: -Myology: The scientific study of muscles -Muscle Fibers= Muscle Cells -Myo, Mys, and Sacro: Word roots referring to muscle Three Types of Muscles -Skeletal, Cardiac, and smooth muscle differ in *Microscopic anatomy *Location *Regulation by the endocrine system and the nervous system Characteristics of Skeletal Muscle -Attached primarily to bones -Voluntary (Conscious) control (Usually) -Contracts quickly, tires easily (fatigable) -Allows for wide range of forces to be generated Skeletal Muscle Cells -Long, cylindrical cells -Striated (banded) -Multinucleate Characteristics of Cardiac Muscle -Forms most of heart wall (Myocardium) -Involuntary (unconscious) -Autorhythmicity (contracts without external stimuli) -Fast contraction, non-fatigable -Beats at constant rhythm that can be modified by neural and hormonal signals Cardiac Muscle Cells -Branched cells -Uninucleate (may occasionally be binucleate) -Striated -Intercalated discs Characteristics of Smooth muscle -Found in the walls of hollow internal structures (Digestive, Respiratory, reproductive tracts, blood vessels) -Arrector pili, pupil of the eye, etc. -Involuntary (unconscious) -Long, slow contractions, non-fatigable

Smooth Muscle Cells -Nonstriated=Smooth -Uninucleate Functions of Muscle Tissues -Motion: External and internal body part movements -Posture: Maintain body posture -Stabilization: Stabilize joints- Muscles have tone even at rest -Thermogenesis: Generating heat by normal contractions and by shivering Functional Characteristics -Excitability (Irritability) *Ability to receive and respond to a stimulus -Contractility *Ability of muscle tissue to shorten -Extensibility *Ability to be stretched without damage *Most Muscles are arranged in functionally opposing pairs- as one contracts, the other relaxed, which permits the relaxing muscle to be stretched back -Elasticity *Ability to return to its original shape -Conductivity (Impulse transmission) *Ability to conduct excitation over length of muscle Connective Tissue Wrapping of Skeletal Muscle Tissue -Superficial Fascia: “Hypodermis” -Deep Fascia: Lines body wall and extremities; binds muscle together, separating them into functional groups -Epimysium: Wraps an entire muscle -Perimysium: Subdivides each muscle into fascicles, bundles of 10-100 muscle fibers -Endomysium: Wraps individual muscle fibers Nerve and Blood Supply -Each muscle is supplied by a branch of a motor nerve -Each muscle is supplied by its own arteries and veins -Blood vessels branch profusely to provide each muscle fibers with a direct blood supply Attachments (To bone) -Origin: Part of a muscle attached to the stationary bone -Insertion: Part of a muscle attached to the bone that moves -Attachments are extensions of connective tissue sheaths beyond a muscle, attaching it to other structures -Direct Attachment: Epimysium fused to periosteum

Attachment Structure -Indirect attachment: Connective tissues wrappings gathered into a tendon or aponeurosis which attaches to an origin or insertion on bone *Tendon: Cord (Dense regular connective tissue) *Aponeurosis: Sheet (Dense regular connective tissue) Microscopic Anatomy of a Skeletal Muscle Fiber -Muscle Fibers (cells): Long cylindrical, and multinucleate (individual muscle cells fuse during embryonic development) -Sarcolemma: Cell membrane of a muscle fiber -Sarcoplasm: Cytoplasm of a muscle fiber, right in oxygen-storing myoglobin protein Myofibrils of a skeletal muscle fiber -Myofibrils: Bundles of contractile protein filaments (myofilaments) arranged in parallel, fil ost of the cytoplasm of each muscle fiber; 100’s to 1000’s per cell -Sarcomeres: The repeating unit of contraction in each myofibril Organelles of skeletal muscle fiber -Mitochondria: Provide ATP required for contraction -Sarcoplasmic reticulum(Smooth ER): Stores ca2+ ions which serve as second messengers for contraction Myofilaments -Thin filaments: Actin (plus some tropomyosin & troponin) -Thick Filaments: Myosin -Elastic filaments: Titin (connectin) attaches myosin to the Z discs (very high mol. wt.) Sarcomeres -Components of the muscle fiber with myofilaments arranged into contractile units -The functional unit of striated muscle contraction -Produce the visible banding pattern (striations) -Myofilaments between 2 successive z discs Striations/Sarcomeres -Z discs (lines): Boundary between sarcomeres; proteins anchor the thin filaments; bisects each I band -A (anisotropic) band: Overlap of thick (myosin) filaments and thin filaments -I (isotropic) band: thin (actin) filaments only -H zone: Thick filaments only -M line: Proteins anchor the adjacent thick filaments Myosin Proteins -Rod-like tail with two heads -Each head contains ATPase and an actin-binding site; point to the Z line

-Tails point to the M line -Splitting ATP releases energy which causes the head to “ratchet” and pull on actin fibers Thick (Myosin) Membrane -Each thick filament contains many myosin units woven together Thin (Actin) Myofilaments *2 G actin has a binding site for myosin -Each G actin has a binding site for myosin -Two tropomyosin filaments spiral around the actin strands -Troponin regulatory proteins (“switch molecules”) may bind to actin and tropomyosin && have Ca2+ binding sites Muscle Fibers Triads -Triads: 2 terminal cisternae + 1 T tubule -Sarcoplasmic reticulum (SER): Modified smooth ER, stores Ca2+ ions -Terminal cisternae: Large flattened sacs of the SER -Transverse (T) tubules: Inward folding of the sarcolemma Regulation of Contraction & The Neuromuscular Junction *The Neuromuscular Junction: -Where motor neurons communicate with the muscle fibers -Composed of an axon terminal & Motor End plate *Axon terminal: end of the motor neuron’s branches (axon) *Motor end plate: Specialized region of the muscle cell plasma membrane adjacent to the axon terminal Neuromuscular Junction -Synapse: Point of communication is a small gap -Synaptic cleft: Space between axon terminal & motor end plate -Synaptic vesicles: Membrane-enclosed sacs in the axon terminals containing a neurotransmitter -Neurotransmitter: Chemical messenger that travels across the synapse, e.g., acetylcholine, ACh) -Acetylcholine (ACh) receptors: Integral membrane proteins which bind ACh Generation of an Action Potential (Excitation) -Binding of neurotransmitter (ACh) causes the ligand- gated Na+ channels to open -Opening of the Na+ channels depolarize the sarcolemma (cell membrane) Generation of an Action Potential -Initial depolarization causes adjacent voltage-gated Na+ channels to open; Na+ ions flow in, triggering an action potential -Action potential: Large transient depolarization of the membrane potential mi

*-70 is resting action potential *Transmitted over the entire sarcolemma (and down the T tubules) -Repolarization: Return to polarization due to the closing voltage-gated Na+ channels and the opening of voltage gated K+ channels. -Refractory period: Time during membrane repolarization when the muscle fiber cannot respond to a new stimulus (few milliseconds) -All-or- none response: Once an action potential is initiated it results in a complete contraction of the muscle cell Excitation-Contraction Coupling -The action potential (excitation) travels over the sarcolemma, including T-tubules -DHP receptors serve as voltage sensors on the T-tubules and cause ryanodine receptors on the SR to open and release Ca2+ ions Sliding Filament Model of Muscle Contraction -Thin and thick filaments slide past each other to shorten each sarcomere and thus, each myofibril -Cumulative effect is to shorten the muscle Calcium (Ca2+) -The “on-off switch”: allows myosin to bind to actin -Binds to troponin Calcium movements inside muscle fibers -Action potential causes release of Ca2+ ions (from the cisternae of the SR) -Ca2+ combines with troponin, causing a change in the position of the tropomyosin, allowing the actin to bind to myosin and be pulled -Ca2+ pumps on the SR remove calcium ions from the sarcoplasm with them stimulus ends The Power Stroke and ATP 1.) Cross Bridge attachment *Myosin binds to actin 2.) The working stroke *Myosin changes shape (pulls actin towards it); releases ADP +Pi 3.) Cross bridge detachment *Myosin bids to new ATP; releases actin 4.)” Cocking” of the myosin head *ATP hydrolyzed (split) to ADP + pi; provides potential energy for the next stroke The Ratchet Effect -Repeat steps 1-4: *The “Ratchet action” repeats the process, shortening the sarcomeres and myofibrils, until Ca2+ ions are removed from the sarcoplasm or the ATP supply is exhausted

Excitation-Contraction Coupling 1.) The action potential (excitation) travels over the sarcolemma, including T-tubules 2.) DHP receptors serve as voltage sensors on the T-tubules and cause ryanodine receptors on the SR to open and release Ca2+ ions 3.) Ca2+ binds to troponin causing tropomyosin to move out of its blocking position 4.) Myosin forms cross bridges to actin, the power stroke occurs, filaments slide, muscle shortens 5.) Calsequestrin and calmodulin help regulate Ca2+ levels inside muscle cells Destruction of Acetylcholine -Acetylcholinesterase: Enzyme that breaks down acetylcholine which is located in the neuromuscular junction *Prevents continuous excitation -Many drugs and disease interfere with events in the neuromuscular junction *Myasthenia gravis: Loss of function at ACh receptors *Curare (Poison arrow toxin): Binds irreversibly to and blocks the ACh receptors Muscle Contraction -One power stroke shortens a muscle about 1% -Normal muscle contraction shorten a muscle about 35% *Cross bridge (ratchet effect) cycle repeats -Continue repeating power strokes, continue pulling -Increasing overlap of fibers; Z lines come together *About held the myosin molecules are attaches at any time -Cross bridges are maintained until Ca2+ levels decrease Rigor Mortis in Death -Ca2+ ions leak from SR causing binding of actin and myosin and some contraction of the molecules -Lasts ~24 hours, then enzymatic tissue disintegration eliminates it in another 12 hours Skeletal Muscle Motor Units -Motor Unit=Motor neuron + Muscle fibers to which it connects (synapses) -The size varies *Small-2 muscle fibers *Large-100’s muscle fibers -Individual muscle cell/fiber Myogram -Myogram: Recording of muscle contraction -Stimulus: Nerve impulse or electrical charge -Twitch: A single contraction of all the muscle fibers in a motor unit (one nerve signal) -Latent period: Delay between stimulus and response

-Contraction phase: Tension or shortening occurs -Relaxation Phase: Relaxation or lengthening Muscle Twitches -All or none rule: All the muscle fibers of a motor unite contract all the way when stimulated Graded Muscle Responses -Force of muscle contraction varies depending on need. How much tension is needed? *Twitch does not provide much force *Contraction force can be altered in 3 ways 1.) Changing the frequency of stimulation (temporal summation) 2.) Changing the stimulus strength 3.) Changing the muscle’s length Temporal Summation -Contractions repeated before complete relaxation, leads to progressively stronger contractions *Unfused tetanus: Frequency of stimulation allows only incomplete relaxation *Fused tetanus: Frequency of stimulation allows no relaxation Treppe: Staircase effect - “Warming up” of a muscle fiber Multiple Motor Unit summation (Recruitment) -The stimulation of more motor units leads to more forceful muscle contraction The Size Principle -As stimulus intensity increases motor units leads with larger fibers are recruited Stretch: Length- Tension relationship -Stretch (Sarcomere length) determines the number of cross bridges *Finish on D2L Contraction of a Skeletal Muscle -Isometric Contraction: Muscle does not shorten -Tension increases -Isotonic Contraction: -Tension does not change -Muscle length shortens Muscle Tone -Regular small contractions caused by spinal reflexes -Respond to tendon stretch receptor sensory input -Activate different motor units over time -Provide constant tension development

-Muscles are firm Muscle Metabolism -Energy availability *Not much ATP is available *ATP is needed for cross bridges and Ca2+ removal *Maintaining ATP *Finish on D2L Direct Phosphorylation-Creatine Phosphate system -CrP stored in cell -Allows for rapid ATP replenishment -Only a small amount available (10-30 seconds worth) Anaerobic Glycolysis- Lactic Acid System -Anaerobic system-No O2 required -Very inefficient, does not create much ATP -Only useful in short term situations (30 sec-1min) -Produces lactic acid as a by-product Aerobic System -Uses oxygen for ATP production -Oxygen comes from RBC’s in the blood and the myoglobin storage depot -Uses many substrates: carbs, lipids, proteins -Good for long term exercise -May provide 90-100% of the needed ATP during these periods Oxygen Deficit -Amount of oxygen needed to restore muscle tissue to the pre-exercise state -Muscle O2, ATP, CP, and Glycogen levels, and a normal Ph must be restoring Factors Affecting the Force of Contraction 1.) Number of Muscle fibers contracting 2.) Size of the muscle 3.) Frequency of stimulation 4.) Degree of muscle stretch *Finish on D2L Muscle Fiber Type: Speed of Contraction -Slow: Contract slowly, have slow acting myosin ATPases and are fatigue resistant -Fast: Contact quickly, have fast myosin ATPases, and have moderate resistance to fatigue -Fast glycolytic fibers: Contract quickly, have fast myosin ATPases and are easily fatigues

Smooth Muscle Tissue -When the longitudinal layer contracts, the organ dilates and contracts -When the circular layer contracts, the organ elongates Smooth Muscle Contractions -Peristalsis-Alternating contractions and relaxations of smooth muscles that squeeze substances through the lumen of hollow organs -Segmentation-Contractions and relaxations of smooth muscle that mix substances *Finish on D2L Contraction of Smooth Muscle -Some smooth muscle cells -Act as pacemakers and set the contractile pace for whole sheets of muscle -Are *Finish on D2L Smooth Muscle Tissue -Contracts under the influence of *Autonomic nerves *Hormones *????? Developmental Aspects of the Muscular System *Finish on D2L Regeneration of Muscle Tissue *Finish on D2L Developmental Aspects: After Birth -Muscular developmental reflects neuromuscular coordination -Developmental occurs head-to-toe and proximal to distal -Peak natural neural control of muscles is achieved by mid Development Aspects: Male and Female -There is a biological basis for greater strength in men than in women -Women’s skeletal mu -Differences due to male sex hormone testosterone -With more muscle mass, men are stronger than women -Body strength per unit muscle mass Developmental Aspects: Age Related -With age, connective tissue increases, and muscle fibers decrease -Muscles become stringier and more sinewy -By age 80, 50% of muscle mass is lost

-Regular exercise reverses sarcopenia -Aging of the cardiovascular system affects every organ in the body -Atherosclerosis may block distal arteries, leading to intermittent claudication and causing serve pain in leg muscles Homeostatic Imbalances -Muscular dystrophy: Group of inherits muscle-destroying diseases where muscles enlarge due *Finish on D2L...