Chapter 9 Muscle Structure & Physiology Lecture 16 Visual Anatomy & Physiology First Edition Martini & Ober Chapter 9 Muscle Structure & Physiology Lecture 16 80 min, 36 slides

Lecture Overview Types, characteristics, functions of muscle Structure of skeletal muscle Mechanism of skeletal muscle fiber contraction Energetics of skeletal muscle contraction Skeletal muscle performance Types of skeletal muscle contractions Comparison of skeletal muscle with smooth muscle and cardiac muscle

Muscular System Review - Three Types of Muscle Tissues Skeletal Muscle usually attached to bones under conscious control (voluntary) striated multinucleated Cardiac Muscle wall of heart not under conscious control striated branched Smooth Muscle walls of most viscera, blood vessels, skin not under conscious control not striated

Functions of Muscle Provide stability and postural tone Fixed in place without movement Maintain posture in space Purposeful movement Perform tasks consciously, purposefully Regulate internal organ movement and volume (mostly involuntary) Guard entrances/exits (digestive/urinary) Generation of heat (thermogenesis)

Characteristics of All Muscle Tissue Contractile Ability to shorten with force CANNOT forcibly lengthen Extensible (able to be stretched) Elastic (returns to resting length) Excitable (can respond electrical impulses) Conductive (transmits electrical impulses)

Structure of a Skeletal Muscle Figure from: Hole’s Human A&P, 12th edition, 2010 epimysium (around muscle) perimysium (around fascicles) endomysium (around fibers, or cells) Alphabetical order largest to smallest: fascicle, fiber, fibril, and filament

Skeletal Muscle Fiber (Cell) Fully differentiated, specialized cell – its structures are given special names sarcolemma (plasma membrane) sarcoplasm (cytoplasm) sarcoplasmic reticulum (ER) transverse tubule triad cisternae of sarcoplasmic reticulum (2) myofibril (1-2 µm diam.) T tubules located at A band-I band junctions and encircle the myofibrils within the muscle fiber. Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007 Transverse tubules contain extracellular fluid ( [Na+],  [K+]) Sarcoplasmic reticulum is like the ER of other cells; but it contains  [Ca2+ ]

Structure of the Sarcomere I band A band H zone Z line M line (~ 2µm long) Triads are placed symmetrically around the M lines, with cisternae lying over the A bands. Figure from: Hole’s Human A&P, 12th edition, 2010 The sarcomere is the contractile unit of skeletal (and cardiac) muscle

Structure of the Sarcomere ‘A’ in A band stands for Anisotropic (dArk) ‘I’ in I band stands for Isotropic (LIght) Isotropic – the same in all directions measured; anisotropic – different when measured in different directions. H band (Ger. Helle = light). M line (Ger. Mitte = middle). Z line/disk (Ger. Zwichenscheibe = intercalated line). Zones of non-overlap: I band (thin filaments), and H zone (thick filaments) A sarcomere runs from Z line (disk) to Z line (disk) (From ‘Z’ to shining ‘Z’!) Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Preview of Skeletal Muscle Contraction Major steps: Motor neuron firing Depolarization (excitation) of muscle cell Release of Ca2+ from sarcoplasmic reticulum Shortening of sarcomeres Shortening of muscle/CTs and tension produced T Tubule Sarcoplasmic reticulum Physiology here we come!! Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Grasping Physiological Concepts The steps in a physiological process give you the ‘when’, i.e. tell you when things happen and/or the order in which they happen. For each step in a process, you should MUST ask yourself the following questions - and be sure you get answers! How? (How does it happen?) Why? (Why it happens and/or why it’s important?) What? (What happens?)

Contraction of the Sarcomere When skeletal muscle contracts: - H zones and I bands get smaller - Areas of overlap get larger - Z lines move closer together - A band remains constant BUT – lengths of actin and myosin filaments don’t change How can this be explained? Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Sliding Filament Theory Figure from: Hole’s Human A&P, 12th edition, 2010 Theory used to explain these observations is called the sliding filament theory …

Myofilaments Thin Filaments composed of actin associated with troponin and tropomyosin Thick Filaments composed of myosin cross-bridges Figure from: Hole’s Human A&P, 12th edition, 2010

Mechanism of Sarcomere Contraction Figure from: Hole’s Human A&P, 12th edition, 2010 When you think myosin, think mover: 1. Bind 2. Move 3. Detach 4. Reset Ca2+  troponin myosin  actin

Mechanism of Sarcomere Contraction Figure from: Hole’s Human A&P, 12th edition, 2010 4. Reset 1. Bind 3. Detach 2. Move Cycle repeats about 5 times/sec Each power stroke shortens sarcomere by about 1% So, each second the sarcomere shortens by about 5% What would happen if ATP was not present? …

Neuromuscular Junction site where axon and muscle fiber communicate motor neuron motor end plate synaptic cleft synaptic vesicles neurotransmitters The neurotransmitter for initiating skeletal muscle contraction is acetylcholine (ACh) Synaptic cleft is about 60 – 100 nm wide. About 50 million ACh receptors on a muscle cell. Deficiency of these leads to muscle paralysis in myasthenia gravis. Ca2+ Ca2+ SR Ca2+ Ca2+ Ca2+ Figures from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Stimulus for Contraction: Depolarization nerve impulse causes release of acetylcholine (ACh) from synaptic vesicles ACh binds to acetylcholine receptors on motor end plate generates a muscle impulse muscle impulse eventually reaches sarcoplasmic reticulum (via T tubules) and Ca2+ is released acetylcholine is destroyed by the enzyme acetylcholinesterase (AChE) Each action potential causes the release of the contents of about 60 synaptic vesicles (about 10,000 molecules of ACh). Botulinum toxin (Clostridium botulinum) blocks exocytosis of ACh from synaptic vesicles. Curare binds to and blocks ACh receptors. Anticholinesterase agents, e.g., neostigmine, slow removal of ACh from the synaptic cleft. Defective ryanodine receptors (quick to open, slow to close) release too much Ca in skeletal muscle and may be stimulated to release Ca by succinyl choline or halothane anesthetics and may give rise to uncontrollable, extensive muscle contractions causing malignant hyperthermia. Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Linking of nerve stimulation with muscle contraction is called excitation-contraction coupling

Summary of Skeletal Muscle Contraction Relaxation Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Modes of ATP Synthesis During Exercise Muscle stores enough ATP for about 4-6 seconds worth of contraction, but is the only energy source used directly by muscle. So, how is energy provided for prolonged contraction? Muscle contains about 5 millimoles of ATP and 15 millimoles of CP per Kg of tissue. This is enough to power about 1 min of brisk walking or 6 seconds of sprinting or fast swimming. (ATP and CP) Continual shift from one energy source to another rather than an abrupt change Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Energy Sources for Contraction Figure from: Hole’s Human A&P, 12th edition, 2010 Creatine phosphate 2) Glycolysis (30 sec – 2 min) 3) Aerobic respiration stores energy that quickly converts ADP to ATP CP + ATP provide about 10-15 seconds of energy Creatine is obtained from the diet (milk, meat, fish) and is also made by the kidneys, liver, and pancreas. Creatine is spontaneously and slowly catabolized to creatinine, which is excreted by the kidneys (and so used as an indication of kidney function). myoglobin stores extra oxygen

Oxygen Debt (The Cori Cycle) Oxygen debt – amount of extra oxygen needed by liver to convert lactic acid to glucose, resynthesize creatine-P, and replace O2 removed from myoglobin. when oxygen is not available glycolysis continues pyruvic acid converted to lactic acid (WHY?) liver converts lactic acid to glucose Figure from: Hole’s Human A&P, 12th edition, 2010 (The Cori Cycle)

Muscle Fatigue Inability to maintain force of contraction Commonly caused by decreased blood flow ion imbalances accumulation of lactic acid relative decrease in ATP availability decrease in stored ACh Cramp – sustained, involuntary contraction It seems that most forms of exercise-associated cramps result from an abnormally sustained activity of the nerve cells in the spinal cord which control skeletal muscle, the alpha motor neurons. Fatigue appears to be the central factor in exercise associated muscle cramps (EAMC). Fatigue enhances the input to the alpha motor neurons from the main receptors in the muscles (muscle spindles) and inhibits the input from the receptors in their tendons (Golgi tendon organs) that signal tension. Because the spindle signals excite alpha motor neurons, while those from tendon organs are inhibitory, these fatigue effects can combine to promote uncontrolled activity in the relevant regions of the spinal cord. It is a common experience that cramp may be precipitated by contraction of the muscle from an already shortened position, and this of course is when the tension signal from its tendon is weakest.

Length-Tension Relationship Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Maximum tension in striated muscle can only be generated when there is optimal overlap between myosin and actin filaments

Muscular Responses Threshold Stimulus minimal strength required to cause contraction in an isolated muscle fiber Recording a Muscle Contraction (myogram) latent period period of contraction period of relaxation refractory period all-or-none response A single twitch Typical latent period is about 2 ms. Entire twitch lasts about 70 – 100 msec. An individual muscle fiber (cell) is either “on” or “off” and produces maximum tension at that resting length for a given frequency of stimulation

Treppe, Wave Summation, and Tetanus Wave (Temporal) Summation Treppe (10-20/sec) Little/no relaxation period Complete Tetanus (>50/sec) Incomplete Tetanus (20-30/sec) Tetany is a sustained contraction of skeletal muscle Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Treppe, Wave Summation, and Tetanus all involve increases in maximum tension generated in a muscle fiber after re-stimulation The difference among them is WHEN the muscle fiber receives the second and subsequent stimulations: Treppe – stimulation immediately AFTER a muscle cell has relaxed completely. Wave Summation – Stimulation BEFORE a muscle fiber is relaxed completely Incomplete tetanus – partial relaxation between stimuli Complete tetanus – NO relaxation between stimuli

Motor Unit single motor neuron plus all muscle fibers controlled by that motor neuron ** Contraction in a single muscle fiber is an “all or none” phenomenon (but remember that the tension will not always be maximal) Motor units differ in the number of muscle fibers innervated by a single motor neuron, e.g., in the eye 3-6 muscle fibers/neuron while the gastrocnemius has about 1,000 muscle fibers/neuron. Figure from: Hole’s Human A&P, 12th edition, 2010

Recruitment of Motor Units recruitment - increase in the number of motor units activated to perform a task whole muscle composed of many motor units as intensity of stimulation increases, recruitment of motor units continues until all motor units are activated

Sustained Contractions smaller motor units recruited first larger motor units recruited later produces smooth movements muscle tone – continuous state of partial contraction

Types of Contractions concentric – shortening contraction isotonic – muscle contracts and changes length eccentric – lengthening contraction isometric – muscle “contracts” but does not change length Muscles can generate about 3-4 Kg/cm2 (about 50 lbs/sq in.). Quadriceps has as much as 16 sq. in. of muscle -> about 800 lbs of tension. Figure from: Hole’s Human A&P, 12th edition, 2010

Types of Skeletal Muscle Fibers Slow Oxidative (SO) (GO REDSOX!) Fast Oxidative-Glycolytic (FOG) Fast Glycolytic (FG) Alternate name Slow-Twitch Type I Fast-Twitch Type II-A Fast-Twitch Type II-B Myoglobin (color) +++ (red) ++ (pink-red) + (white) Metabolism Oxidative (aerobic) Oxidative and Glycolytic Glycolytic (anaerobic) Strength Small diameter, least powerful Intermediate diameter/strength Greatest diameter, most powerful Fatigue resistance High Moderate Low Capillary blood supply Dense Intermediate Sparse SO fibers are smallest, FG fibers are largest, FOG fibers are intermediate in size. Within a motor unit, all of the skeletal muscle fibers are the same type. All fibers in any given motor unit are of the same type

Smooth Muscle Fibers Compared to skeletal muscle fibers shorter single nucleus elongated with tapering ends myofilaments organized differently no sarcomeres, so no striations lack transverse tubules sarcoplasmic reticula not well developed exhibit stress-relaxation response Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Types of Smooth Muscle Single-unit smooth muscle visceral smooth muscle sheets of muscle fibers that function as a group, i.e., a single unit fibers held together by gap junctions exhibit rhythmicity exhibit peristalsis walls of most hollow organs, blood vessels, respiratory/urinary/ reproductive tracts Multiunit Smooth Muscle fibers function separately, i.e., as multiple independent units muscles of eye, piloerector muscles, walls of large blood vessels

Smooth Muscle Contraction Resembles skeletal muscle contraction interaction between actin and myosin both use calcium and ATP both depend on impulses Different from skeletal muscle contraction smooth muscle lacks troponin smooth muscle depends on calmodulin two neurotransmitters affect smooth muscle acetylcholine and norepinephrine hormones affect smooth muscle have gap junctions stretching can trigger smooth muscle contraction smooth muscle slower to contract and relax smooth muscle more resistant to fatigue

Cardiac Muscle only in the heart muscle fibers joined together by intercalated discs fibers branch network of fibers contracts as a unit (gap junctions) self-exciting and rhythmic longer refractory period than skeletal muscle (slower contract.) cannot be tetanized fatigue resistant has sarcomeres Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Review Three types of muscle tissue Muscle tissue is… Skeletal Cardiac Smooth Muscle tissue is… Contractile Extensible Elastic Conductive Excitable

Review Functions of muscle tissue Muscle fiber anatomy Provide stability and postural tone Purposeful movement Regulate internal organ movement and volume Guard entrances/exits Generation of heat Muscle fiber anatomy Actin filaments, tropomyosin, troponin Myosin filaments Sarcomere Bands and zones

Review Muscle contraction Muscular responses Sliding filament theory Contraction cycle (Bind, Move, Detach, Release) Role of ATP, creatine Metabolic requirements of skeletal muscle Stimulation at neuromuscular junction Muscular responses Threshold stimulus Twitch – latent period, refractory period All or none response Treppe, Wave summation, and tetanus

Review Table from: Hole’s Human A&P, 12th edition, 2010

Review Muscular responses Fast and slow twitch muscle fibers Recruitment Muscle tone Types of muscle contractions Isometric Isotonic Concentric Eccentric Fast and slow twitch muscle fibers Slow Oxidative (Type I) (think: REDSOX) Fast Oxidative-glycolytic (Type II-A) Fast Glycolytic (Type II-B)