1 Chapter 9: Muscle Tissue Muscle little mouse Types of Muscle Tissue: Skeletal Muscle Cardiac Muscle Smooth Muscle Characteristics: Attaches to skeleton Voluntary control Striated / multi-nucleated Characteristics: Composes heart wall Involuntary control Striated / uni-nucleated Characteristics: Lines visceral organs Involuntary control Non-striated / uni-nucleated Characteristics of Muscle: Prune Belly Syndrome 1) Excitability: Ability to respond to stimulation 2) Contractility: Ability to shorten forcefully 3) Extensibility: Ability to stretch and still contract 4) Elasticity: Ability to resume resting length after contraction Function of Muscle: 1) Produce movement Locomotion / manipulation (skeletal) Blood pressure (cardiac) Propulsion (smooth) 2) Maintain posture / body position 3) Support soft tissue (e.g., abdominal wall) 4) Guard entrance / exit (e.g., lips / anus) 5) Maintain body temperature (e.g., shivering) 6) Store nutrients (e.g., glycogen) Page 1
2 Gross Anatomy of Muscle: Connective Components: A) Tendons (cords) B) Aponeuroses (sheets) C) Fascia (wrap / bind body) Epimysium Connective Tissue Layers: 1) Epimysium: Outside muscle covering Form tendons Compartments 2) Perimysium: Divides muscle into fascicles Contains blood vessels / nerves 3) Endomysium: Surrounds individual muscle fibers and ties them together Satellite cells (stem cells) Endomysium Perimysium Marieb & Hoehn Figure 9.1 Microanatomy of Muscle: Muscle Fiber (cell): Functional Unit of Muscle Sarcolemma: Cell membrane Sarcoplasma: Cytoplasm Triad Transverse Tubules: Network of passageways through fiber Continuous with outside of cell Sacroplasmic Reticulum: Specialized endoplasmic reticulum Contain calcium ions (Ca ++ ) [40,000x] compared to cytoplasm Myofibrils: Cylindrical structures containing contractile fibers Similar to Marieb & Hoehn Figure 9.5 Page 2
3 Microanatomy of Muscle: Myofibrils contain myofilaments (protein): 1) Actin (Thin filament) 2) Myosin (Thick filament) Sarcomere: Repeating units of myofilaments (~ 10,000 / myfilament) Sarcomere Actin Myosin I Band M line Z line A Band Microanatomy of Muscle: Similar to Marieb & Hoehn Figure 9.6 Microanatomy of Muscle: Interactions between the thick and thin filaments of sarcomeres are responsible for muscle contraction Sliding Filament Theory Thick filaments: Composed of many myosin molecules Tails: Attach molecules together Heads: Bind with thin filament Hinge Page 3
Microanatomy of Muscle: Interactions between the thick and thin filaments of sarcomeres are responsible for muscle contraction Sliding Filament Theory Thick filaments: Composed of many myosin molecules Thin filaments: Composed of interwoven actin molecules Tails: Attach molecules together Heads: Bind with thin filament Tropomyosin: Cover active sites Troponin: Bind tropomyosin to actin Microanatomy of Muscle: Interactions between the thick and thin filaments of sarcomeres are responsible for muscle contraction Sliding Filament Theory Neuron Synaptic Knob ACh ACh ACh Neuromuscular Junction: Neuron Muscle fiber 1 connection / muscle fiber T-tubule Sarcolemma Motor End Plate Sarcoplasmic Reticulum Sarcoplasm Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Muscle Contraction Events: Sarcomere of myofibril 1) Acetylcholine (ACh - neurotransmitter) released from synaptic knob 2) ACh binds to receptors on motor end plate; generates action potential 3) Action potential (AP - electrical impulse) conducted along sarcolemma Page 4 4
Neuron Synaptic Knob ACh ACh ACh Neuromuscular Junction: Neuron Muscle fiber 1 connection / muscle fiber T-tubule Sarcolemma Motor End Plate Sarcoplasmic Reticulum Sarcoplasm Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Ca ++ Sarcomere of myofibril Muscle Contraction Events: 4) AP descends into muscle fiber via T-tubules 5) AP triggers release of Ca ++ from sarcoplasmic reticulum 6) Ca ++ initiates cross-bridging (actin / myosin) Cross-Bridging in Action: 1) Ca ++ binds with troponin; exposes active sites on actin Cross-bridging Events: Ca ++ Actin Troponin Tropomyosin Myosin Head Page 5 5
6 Cross-Bridging in Action: 1) Ca ++ binds with troponin; exposes active sites on actin 2) Myosin head (cocked) binds with active site 3) Myosin head pivots pulls actin forward Cross-bridging Events: Cross-Bridging in Action: 1) Ca ++ binds with troponin; exposes active sites on actin 2) Myosin head (cocked) binds with active site 3) Myosin head pivots pulls actin forward 4) binds to myosin head; head detaches and re-cocks Cross-bridging Events: Page 6
7 Re-cock P Cross-Bridging in Action: 1) Ca ++ binds with troponin; exposes active sites on actin 2) Myosin head (cocked) binds with active site 3) Myosin head pivots pulls actin forward 4) binds to myosin head; head detaches and re-cocks Cross-bridging Events: 5) Myosin head binds to active site; Process repeated Re-cock Re-cock P P Page 7
8 Cross-Bridging in Action: Cross-bridging Events: 1) Ca ++ binds with troponin; exposes active sites on actin 2) Myosin head (cocked) binds with active site 3) Myosin head pivots pulls actin forward 4) binds to myosin head; head detaches and re-cocks 5) Myosin head binds to active site; Process repeated 6) Process ends when APs cease ACh broken down by acetylcholinesterase (AChE) Ca ++ returned to sarcoplasmic reticulum (active transport) Martini & Nath Table 10.1 Rigor Mortis Tetanus Muscle Mechanics: Muscle Tension: Force exerted on an object by a contacting muscle What Regulates Muscle Tension Production? A) Single Fiber: Tension Production = # of cross-bridge attachments Too contracted = no room for movement; poor cross-bridge formation Too stretched = no cross-bridge formation All-or-none response (muscle on or off ) Resting length = # of cross-bridges; distance to slide (maximal muscle force; normal resting length) Page 8
Tension Stimulus Muscle Mechanics: Muscle Tension: Force exerted on an object by a contacting muscle What Regulates Muscle Tension Production? B) Whole Muscle: Tension Production = 1) Frequency of stimulation 2) # of muscle fibers activated Muscle Mechanics: Muscle Tension: Force exerted on an object by a contacting muscle What Regulates Muscle Tension Production? B) Whole Muscle Frequency of stimulation: Twitch = Single stimulus-contraction-relaxation sequence Latent Period: Time between stimulus and tension development CP RP Contraction Phase: Period where tension rises to peak level Ca ++ release; cross-bridge formation Relaxation Phase: Period where tension falls to resting level Ca ++ uptake; cross-bridge detachment LP Time Muscle Mechanics: Muscle Tension: Force exerted on an object by a contacting muscle What Regulates Muscle Tension Production? B) Whole Muscle Frequency of stimulation: Twitch = Single stimulus-contraction-relaxation sequence Variation exists in the duration of twitches among muscles Twitches alone are not a useful contraction Martini & Nath Figure 10.15 Page 9 9
Tension Tension Stimulus Stimulus 10 Muscle Mechanics: Muscle Tension: Force exerted on an object by a contacting muscle What Regulates Muscle Tension Production? B) Whole Muscle Frequency of stimulation: Incomplete Tetanus: Rapid cycles of contraction & relaxation Maximum Tension Summation: Addition of twitches to produce a more powerful contraction Time Muscle Mechanics: Muscle Tension: Force exerted on an object by a contacting muscle What Regulates Muscle Tension Production? B) Whole Muscle Frequency of stimulation: Complete Tetanus: Rapid stimulation erases relaxation phase Maximum Tension SR can not reclaim Ca ++ (stimulation too rapid) Most normal muscle contraction involves complete tetanus Time Muscle Mechanics: Muscle Tension: Force exerted on an object by a contacting muscle What Regulates Muscle Tension Production? B) Whole Muscle Number of muscle fibers activated: Motor Unit: A single motor neuron and all the muscle fibers innervated by it Recruitment: Addition of motor units to produce smooth, steady muscle tension (small large motor units) Muscle Tone: Resting tension maintained in muscle Motor unit size dictates control: Fine Control = 1-5 fibers / MU (e.g., eye) Gross Control = 1000 s fibers / MU (e.g., leg) All-or-none response Similar to Marieb & Hoehn Figure 9.13 Page 10
11 Similar to Marieb & Hoehn Figure 9.18 Types of Muscle Contractions: Isotonic Contraction: Isometric Contraction: Tension stabilized; muscle shortens (e.g., lifting / walking) Tension maximized; no muscle shortening (e.g., pushing against wall / standing) Muscle Elongation (following contraction): Passive process: 1) Elastic rebound (fibers / organelles) 2) Antagonistic muscles (e.g., biceps brachii vs. triceps brachii) Muscle Energetics: Muscle fibers use for contraction (~ 600 trillion / second) Only enough reserve to sustain short contraction periods (~ 5 sec) Must be replenished to sustain contraction How does a fiber replenish? high energy 1) Creatine phosphate (CP) reserves: Nitrogen-containing compound bound to phosphate Creatine kinase Converted to creatine CP Limited supply Replaced by aerobic / anaerobic respiration Creatine Phosphate 15 seconds + P Page 11
Muscle Energetics: Muscle fibers use for contraction (~ 600 trillion / second) Only enough reserve to sustain short contraction (~ 2 sec) Must be replenished to sustain contraction How does a fiber replenish? 2) Anaerobic Metabolism (Glycolysis): Lactate Pyruvate Primary source of during times of peak activity Inefficient (2 / glucose) Acid buildup leads rapidly to muscle fatigue Glucose (Glycogen) Anaerobic Respiration Creatine Phosphate 1-2 minutes 15 seconds + P Muscle Energetics: Muscle fibers use for contraction (~ 600 trillion / second) Only enough reserve to sustain short contraction (~ 2 sec) Must be replenished to sustain contraction How does a fiber replenish? 3) Aerobic Metabolism (Aerobic Respiration): CO 2 + H 2 O Pyruvate Glucose Primary source of during periods of rest (95% of ) (Glycogen) Efficient (36 / glucose) Can use multiple energy sources (carbohydrates, lipids, proteins) Page 12 12
13 Aerobic Respiration Anaerobic Respiration Creatine Phosphate Weeks 1-2 minutes 15 seconds + P Energy Use at Different Levels of Activity: At Rest: Aerobic metabolism predominates Fuel = Fatty acids surplus: 1) Glycogen reserves 2) Recharge CP Energy Use at Different Levels of Activity: At Moderate Activity: Aerobic metabolism predominates Fuel = Glycogen required for muscle contraction Page 13
Energy Use at Different Levels of Activity: At Peak Activity: Anaerobic metabolism predominates Fuel = Glycogen required for muscle contraction acquired from CP (initially) Muscle Fatigue: Fibers lose ability to contract despite neural stimulation High Intensity Activity: Fatigue = Ionic imbalances Decoupling of electrical signaling and muscle contraction Low Intensity Activity: Fatigue = Energy reserves depleted Energy reserves include glycogen, lipids, proteins Muscle Recovery: Return of fibers to pre-activity conditions A) Lactate removed (blood liver) or recycled (glycogen) B) Aerobic respiration replenishes energy reserves Oxygen debt: Amount of oxygen needed to restore body to pre-activity conditions Muscle fibers, liver, sweat glands C) Excess heat is lost via blood flow to skin Process can take days to weeks Muscle Performance: Power: Maximum amount of tension produced by a muscle vs. Endurance: Amount of time a muscle can perform an activity Factors Determining Muscle Performance: 1) Muscle Fiber Type: A) Fast Fibers (most common) Contract in 0.01 s or less (after stimulation) Large diameter (densely packed myofibrils) Powerful ( sarcomere # = tension) Rely on anaerobic respiration Large glycogen reserves Fatigue rapidly B) Slow Fibers 3x slower contract; ½ the diameter Sustained contractions ( endurance) Rely on aerobic metabolism: Extensive capillaries ( O 2 supply) Myoglobin: Binds O 2 ( O 2 storage) Large # of mitochondria ( O 2 usage) Page 14 14
15 The distribution of muscle fibers impacts muscle performance: White Muscle: Muscle dominated by fast fibers (e.g., turkey breast) Red Muscle: Muscle dominated by slow fibers (e.g., turkey leg) Human muscles a mixture of the two fiber types (Genetically determined) Fast Fibers Slow Fibers Marathoners 18% 82% Avg. Human 55% 45% Sprinters 64% 37% Physical Conditioning: 1) Anaerobic Endurance Sustained, powerful muscle contractions (e.g., weight lifting) Training = Frequent, brief, intense workouts Muscle Hypertrophy: Enlargement of muscle muscle fiber size, not overall # 2) Aerobic Endurance Continual contraction of muscle over time (e.g., jogging) Training = Sustained low levels of muscle activity mitochondria; capillaries Property Skeletal Muscle: Cardiac Muscle: Smooth Muscle: Filament Organization Sarcomeres along myofibrils Sarcomeres along myofibrils Scattered in sarcoplasm Control Mechanism Neural Automaticity (pacemaker cells) Automaticity, neural, hormonal Calcium Source Sarcoplasmic reticulum SR / across sarcolemma Across sarcolemma Contraction Rapid onset; tetanus can occur; rapid fatigue Slower onset; no tetanus; fatigue-resistant Slow onset; tetanus can occur; fatigue-resistant Energy Source Aerobic / Anaerobic metabolism Aerobic metabolism Aerobic metabolism Page 15