Physiology 1.02 - Muscle Physiology

PHYSIOLOGY Muscle Physiology

1.02 June 11, 2013

Katherine Munarriz, M.D. LEARNING OBJECTIVES

1. 2. 3. 4. 5. 6. 7.

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2. Myosin (Thick Filaments) - Comprise the cross-bridges To describe the functional anatomy of muscle - Interact to actin prior and during contracting To discuss role of nerves in muscle contraction - Contain binding site for both actin and ATP To state the sequence of events in contraction - Myosin ATPase To know the sliding theory of contraction  Located at the ATP binding site To differentiate different muscle types  For ATP hydrolysis To differentiate the types of motor units  Fast twitch myosin – rate of ATPase activity is fast To understand the relationship of length-tension,  Slow twitch myosin – rate of ATPase activity is slow frequency-tension, fiber diameter-tension - Myosin Head  Has stored energy due to hydrolysis of ATP to ADP SKELETAL MUSCLE and phosphate ion Skeletal muscle is multinucleated, striated and  Immediately binds with actin at myosin binding site moves voluntarily. once exposed Each muscle is composed of muscle fibers and NOTE: Myosin head doesn’t bind with actin simultaneously, enclosed by an endomysium. Group of fibers are called fascicles and each rather binds successively to have greater distance of actin movement. Simultaneous binding of myosin head will only fascicle is enclosed in perimysium. move the actin filament to a limited degree. Group of fascicles are enclosed by epimysium. It has bundle of fibers called myofibrils that are surrounded by sarcoplasmic reticulum (SR) and invaginated by transverse tubules (T tubules). The portion of the SR nearest the T tubules is called the terminal cisternae, and it is the site of ++ Ca release, which is critical for contraction of skeletal muscle. The longitudinal portions of the SR are continuous with the terminal cisternae and extend along the length of the sarcomere. This portion of the SR ++ contains a high density of Ca pump protein (i.e., ++ Ca -ATPase), which is critical for re-accumulation ++ of Ca in the SR and hence relaxation of the muscle. Each skeletal muscle is innervated by α- motor neuron. A motor unit consists of the motor nerve and all the muscle fibers innervated by the nerve. The neuromuscular junction formed by the α motor neuron is called an end plate. Acetylcholine released from the α motor neuron at the neuromuscular junction initiates an action potential in the muscle fiber that rapidly spreads along its length. CONTRACTILE APPARATUS

1. Actin (Thin Filaments) - Made up of G-actin (actin molecule) and F-actin (actin strand – helix) - Tropomyosin: covers binding sites of actin at resting state - Troponin:  Troponin C – calcium binding protein that when bound to calcium permits myosin and actin interaction  Troponin T – attaches troponin complex to tropomyosin  Troponin I – inhibits interaction of actin and myosin  Troponin T and I – maintain configuration of tropomyosin - Ca++ will bind with troponin-C that moves tropomyosin laterally that will uncover binding site

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END PLATE AND ACTION POTENTIALS End plate - Plasma membrane of muscle fiber Neuromuscular junction - End plate + post synaptic axon terminal End Plate Potential (EPP) - Localized non-propagated potential

HOW AN ACTION POTENTIAL TRAVELS IN A NERVE  

The first point in which the action potential travels down the axon is termed as the axon hillock. It is the first segment of the axon. + A massive influx of Na will cause a depolarizing + event, while an efflux of K will cause a

PHYSIOLOGY

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MUSCLE PHYSIOLOGY

repolarizing event. The repolarizing event causes the regeneration of the action potential. The next depolarizing event occurs on the node of Ranvier and continues on until it reaches the axon terminal. Before an action potential is generated, graded responses (EPSP and IPSP) are occurring all over the neuron. If the pre-synaptic neuron is excitatory, it will release excitatory neurotransmitters, + resulting in the influx of Na but only a small amount such that it there is only a slight increase in the positivity of the cell. Some neurotransmitters are inhibitory, and they come from inhibitory + neurons. They cause the K efflux increasing the negativity of the cell. These EPSPs and IPSPs are summated in the axon hillock such that it is only at the moment when the cell potential reaches the threshold potential that an action potential is generated. Nerve cell RMP = -70mV Threshold potential = -55mV Muscle cell RMP = -90mV Threshold potential = -75mV SEQUENCE OF EVENTS

ELECTRICAL EVENTS: Motor Neuron Action Potential 1. Action potential is initiated and propagates to motor neuron axon terminals. 2. Calcium enters axon terminals through voltagegated calcium channels. 3. Calcium entry triggers release of ACh from axon terminals. 4. ACh diffuses from axon terminals to motor end plate in muscle fiber. 5. ACh binds to nicotinic receptors on motor end plate, increasing their permeability to sodium and potassium 6. More sodium moves into the fiber at the motor end plate than potassium moves out, depolarizing the membrane, producing the end plate potential (EPP). 7. Local currents depolarize the adjacent muscle cell plasma membrane to its threshold potential, generating an action potential that propagates over the muscle fiber surface and into the fiber along the T-tubules. 8. Action potential is transmitted along the sarcolemma of the muscle fiber to the T tubules,

Ca++ is released from the terminal cisternae SR into the myoplasm. MECHANICAL EVENTS: Skeletal Muscle Fiber Contraction 9. Release of Ca++ from the SR raises intracellular [Ca++], which in turn promotes actin-myosin interaction and contraction. 10. The mechanism underlying the elevation in intracellular [Ca++] involves an interaction between bridging protein in the T tubule and the adjacent terminal cisternae of the SR called feet. Because this channel binds the drug ryanodine, it is commonly called the ryanodine receptor (RYR). Most of the RYR molecule appears to be in the myoplasm and spans the gap between the terminal cisternae and the T tubule. At the T-tubule membrane, the RYR is thought to interact with a protein called the dihydropyridine receptor (DHPR). DHPR is an L-type voltage-gated Ca++ channel with five subunits. 11. Release of Ca++ from the terminal cisternae of the SR is thought to result from a conformational change in the DHPR as the action potential passes down the T tubule, and this conformational change in the DHPR, by means of a protein-protein interaction, opens the RYR and releases Ca++ into the myoplasm. 12. When Ca++ is released from the terminal cisternae of the SR, it binds to troponin C, which in turn promotes movement of tropomyosin on the actin filament such that myosin binding sites on actin are exposed. This then allows the "energized" myosin head to bind to the underlying actin 13. The bond between the head of the cross-bridge and the active site of the actin filament causes a conformational change in the head, prompting the head to tilt toward the arm of the cross-bridge. This provides the power stroke for pulling the actin filament. The energy that activates the power stroke is the energy already stored, like a “cocked” spring, by the conformational change that occurred in the head when the ATP molecule was cleaved earlier. 14. Myosin next undergoes a conformational change termed "ratchet action" that pulls the actin filament toward the center of the sarcomere. Myosin releases ADP and Pi during the transition 15. Binding of ATP to myosin decreases the affinity of myosin for actin, thereby resulting in the release of myosin from the actin filament 16. Myosin then partially hydrolyzes the ATP, and part of the energy in the ATP is used to recock the head and return to the resting state. 17. As [Ca++] falls, Ca++ dissociates from troponin C, and the troponin-tropomyosin complex moves and blocks the myosin binding sites on the actin ++ filament. Uptake of Ca++ into the SR (Ca ATPase) is due to the action of SERCA, which stands for sarcoplasmic endoplasmic reticulum calcium ATPase. If the supply of ATP is exhausted, as occurs with death, the cycle stops with the formation of permanent actin-myosin complexes. In this state the muscle is rigid and the condition is termed rigor mortis.

TRANSCRIBERS: Maja, Eli, Trisha, Rissa, Kenan, Eunika, Von, Catie

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PHYSIOLOGY

MUSCLE PHYSIOLOGY

CROSS-BRIDGE CYCLE

*hyperkalemic periodic paralysis ++ c. Defective Ca RYR ++ Continued Ca release

*defective hyperthermia MECHANISMS OF RELAXATION Decrease cytosolic Ca++cause muscle to relax ATP is needed by SERCA---70% of what we eat is used for the maintainance of these pumps *In Cardiac Muscles Ca++-Na+ exchanger – uses secondary active transport (antiport)  Na+ enters the cell while Ca++ is released in the extracellular, decreasing intracellular fluid Ca+ thus causing relaxation Mechanics of Skeletal Muscle Contraction Summation – adding together of individual twitch contractions to increase intensity of overall muscle contraction

MECHANISM Myosin hydrolyses ATP to ADP and phosphate ion. ↓ In the presences of elevated Ca++, myosin head binds with actin. ↓ Myosin releases ADP and phosphate ion. ↓ Immediately after the release, there occurs the “ratchet” movement called the powerstroke. ↓ Contraction takes place. (tension) ↓ ↓ Myosin binds with another no ATP is available ATP and detaches with ↓ actin. (Relaxation) ↓ prolonged binding and Cycle repeats again contraction will occur

Spatial Summation – increasing the number of motor units contracting simultaneously. - Small motor neurons in spinal cord are more excitable than larger ones so naturally they are excited first. - Different motor units are driven asynchronously so contraction alternates among motor units one after the other providing smooth contraction even at low frequencies of nerve signals.

Temporal Summation – increasing frequency of contraction. - In increasing the frequency, there will come a time that new contraction overlaps with the first contraction producing larger strength of contraction.  Tetany - when frequency reaches a certain level where contractions occur rapidly and appears smooth and continuous. It is where intracellular ++ [Ca ] increases and is maintained throughout the period of stimulation and the amount of force developed greatly exceeds that seen during a twitch. Any additional increase in frequency after tetany has no effect in contraction because - Active Tension – caused by myosin pulling actin, enough calcium ions are maintained in sarcoplasm created by myofilaments even between action potentials so that full - Passive Tension – caused by stretching of connective contractile state is sustained without allowing any tissue, nothing to do with contractile elements relaxation between action potentials. - ATP – is responsible for both contraction and relaxation of muscle fibers  Incomplete tetany - at intermediate stimulus ++ - Myotonia – disease associated with unavailability of frequency, intracellular [Ca ] returns to baseline ATP during contraction just before the next stimulus. However, there is - If Ca++ level is low after contraction, relaxation will gradual rise in force. occur. NOTE: Myosin is always ready for binding. It has MUSCLE TONE stored/potential energy and it is only waiting for active - Dependent on stretch reflexes sites to be open. - Response to passive stretch MECHANISMS THAT PROLONG CONTRACTION Table 1. Dynamic Stretch Reflex vs Static Stretch Reflex ++ Type of Factors that prolong cytosolic Ca DYNAMIC STATIC Stretch Reflex a. Increased frequency of AP Sudden rapid b. Defective Na inactivation Spontaneous stretch of MS Continued Na influx  muscle membrane will be Stimulus firing of static (muscle depolarized  conformational change in DHPR γ-MN spindle) leading to RYR activation

TRANSCRIBERS: Maja, Eli, Trisha, Rissa, Kenan, Eunika, Von, Catie

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PHYSIOLOGY Receptor Afferent N Center Efferent N Effectors Response

Muscle spindle Group Ia L2 – L4 α-MN and dynamic γ-MN Knee extensors and intrafusals Knee extension

MUSCLE PHYSIOLOGY Muscle spindle Group II L2 – L4 α-MN and static γ-MN Intrafusal and extrafusal Basal contraction

TWITCH CONTRACTION 3 PHASES: 1. Latent: includes events up to binding of myosin heads to actin  increase tension; 2 ms 2. Contraction: cross-bridge formation  further increase tension; 15 ms ++ 3. Relaxation: Ca reuptake; tropomyosin covers actin; 25 ms SKELETAL MUSCLE DYNAMICS 1. Length – tension relationship 2. Frequency – tension relationship 3. Load-tension and load-velocity relationships 4. Motor unit size and precision of muscle movement

length is decreased toward L0, the amount of overlap increases, and contractile force progressively increases. As sarcomere length decreases below 2 μm, the thin filaments collide in the middle of the sarcomere, and the actinmyosin interaction is disturbed and hence contractile force decreases. Frequency – Tension Relationship Higher frequency> prolonged increase in Ca conc.> increase in number of cross bridges> increase in tension

Load- Velocity Relationship

A skeletal muscle contracts rapidly when it contracts against no load. When load is applied, velocity of contraction decreases. If load and maximum force that Length- Tension Relationship Measures tension developed during isometric muscle exerts is equal, velocity becomes zero, no contractions when the muscle is set to fixed lengths contraction results. It measures velocity of shortening of isotonic contractions when the muscle is challenged (preload) with different afterloads (the load against which the Passive Tension - when a muscle at rest is stretched, muscle must contract). it resists stretch by a force that increases slowly at first and then more rapidly as the extent of stretch increases. Active tension – tension will be maximum when there is maximum overlap of thick and thin filaments. When muscle is stretched to greater lengths, number of cross bridges reduced because there is less overlap. When muscle length is decreased, thin filaments collide and tension is reduced.

Load- Tension Relationship

Specifically, contractile force increases as muscle length is increased up to a point (designated L0 to indicate optimal length). As the muscle is stretched beyond L0, contractile force decreases. At a very long sarcomere length (3.7 μm), actin filaments no longer overlap with myosin filaments, so there is no contraction. As muscle

- ↑ load  ↑ number muscle fibers recruited to the effort  ↑ number cross-ridges  ↑ force or tension on muscle fibers - Types of stimuli/load: o Submaximal - progressively stronger stimuli, produce actin potentials in axons of additional motor units o Maximal - produces action potentials in axons of all motor units of muscle o Supramaximal - greater stimulus than maximal; has not additional effect because all motor units are already recruited - Maximal rate of work

TRANSCRIBERS: Maja, Eli, Trisha, Rissa, Kenan, Eunika, Von, Catie

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PHYSIOLOGY

MUSCLE PHYSIOLOGY

ISOTONIC VS ISOMETRIC Isotonic Contraction

o Corticospinal tract (CST) neurons (upper motor neuron) (first neurons in the pathway that brings about the contraction) goes from the brain to the spinal cord (lower motor neurons) synapses to muscle fibers - Light load: 1 upper motor neuron 2 lower motor neuron - Heavier load: recruit additional motor neurons enough to carry the load o Producing appropriate tension in muscle cells 7. Maximal tension that can be produced (in optimal length) – from this point muscle begins to lengthen, the shortening velocity decreases until it becomes zero *Eccentric Contraction – lengthening of muscle when load is heavy

PRELOAD VS AFTERLOAD Preload – given to a non-contracting muscle o Establishes the initial length of muscle (prior to contraction) o ↑ preload  ↑ active tension produced by contracting muscle o ↑ preload  ↑ muscle length up to optimal length  ↑ tension 1. Load is held constant Afterload 2. Muscle contracts, produces tension and shortens o Resistance that muscle has to overcome to 3. Load < tension the muscle can develop (produce contract (load resisted by contracting muscle) tension greater than the load you carry) o Load given when muscle is already contracting o No load – greatest shortening velocity o More appropriate in cardiac muscles o Shortening velocity may reach negative when muscles lengthen NOTE: If you have a normal afterload, then you will have a normal tension generated by the muscle Isometric Contraction specifically the heart. When the ventricles of the heart contracts and you have a normal afterload then you will have a normal tension of ventricular myocardium. In cardiac muscles, the afterload will be anything that impedes the flow of blood through the aortic valve. Anything that increases the afterload will decrease the shortening of myocardial fibers during systole, therefore decreasing the stroke volume. MECHANISMS OF WHOLE MUSCLE TENSION Tension developed by each fiber: a. Frequency of action potential - ↑ frequency, ↑ tension developed - Temporal summation: ↑ action potential, ↑ frequency, prolonging contraction b. Fiber initial length - ↑ length, ↑ tension developed - 90-110% or 2-2.2 micrometer c. Fiber diameter (Hypertrophy vs Atrophy) - ↑ diameter, ↑ cross-bridge cycles that 1. Length was kept at constant length throughout the can be formed specially in experiment hypertrophied muscles 2. Muscle contracts, produces tension, but does not - spatial summation – recruiting muscle shorten fibers 3. Load = tension the muscle can develop (up to a d. Fatigue certain point) - Lower tension 4. In the experiment, the subject was given different II. Number of active fibers: load that produced different initial length: a. Number of fibers per motor unit - Spatial summation: recruiting muscle o Increase in initial length of sarcomere for each added fibers weight or load to the muscles, optimal length is at 2.0b. Number of active motor units that have been 2.2 micrometers – maximal tension produced recruited 5. You cannot develop the tension for particular load – - Spatial summation: recruiting muscle lengthening contractions occur in muscle 6. How one can carry a load producing appropriate tension in fibers skeletal muscle cells? - same number of fibers per motor unit I.

The contraction of muscles begins in the mind (brain).

TRANSCRIBERS: Maja, Eli, Trisha, Rissa, Kenan, Eunika, Von, Catie

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PHYSIOLOGY

MUSCLE PHYSIOLOGY

ENERGY SOURCES

Table 3. Skeletal Muscle Types Motor Unit Classification Type I (Slow) Type II/ Fast Characteristics Oxidative Glycolysis phosphorylation Properties of nerve Cell diameter Small Large Conduction Fast Very fast velocity Excitability High Low Properties of muscle cells Number of Few Many fibers Fiber diameter Moderate Large Force of unit Low High Metabolic Oxidative Glycolytic profile Contraction Moderate Fast velocity Fatiguability Low High

TYPES OF MUSCLE CELL METABOLISM:

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Aerobic Glycolysis - Oxidative Glycolysis  Anaerobic Glycolysis - Creatine Phosphatase SOURCES:  Carbohydrates/glucose  Fatty acids  Oxygen ATP produced will be used for:  Myosin-ATPase  contraction  Ca ATPase  relaxation Main source of energy in prolonged aerobic exercises  Blood glucose and FFA, at rest and lowintensity E - Blood glucose: 4 mins - Liver glycogen: 18 mins - Muscle glycogen: 70 mins - TAG in adipose tissue: 4000 min or 66 hours Table 2. Types of Muscle Fibers and Major Fuel Sources Used by a Sprinter and by a Marathon Runner

SIZE PRINCIPLE OF RECRUITMENT - controlled by the CNS Weak Stimuli ↓ Activate neurons with low threshold ↓ Small motor units (slow) recruited first

Strong Stimuli ↓ Activate neurons with high threshold ↓ Small motor units (slow) recruited first

- The small motor units are still recruited first, because small motor units have lower thresholds to synaptic activation. - Recruited successively: first is Type I  Type IIA  Type IIB FATIGUE Physiochanges: - Not due to lactate (lactate concentration increases in exercise and peaks during fatigue) - Not enough release of Ach - Depletion of ATP, glycogen, creatinine phosphate - Build-up of ADP inhibits cross bridge cycle - Depletion of potassium ions in ICF and accumulation of it in ECF - Decrease release of calcium ions in SR - Increase in protons (decrease in pH) changes protein conformation

CARDIAC MUSCLE 2 Major Types of Muscle: 1. Atrial and Ventricular muscle 2. Excitatory and Conductive muscle fibers  SA (sinoatrial) and AV (atrioventricular) nodes  Purkinje fibers Myocardium Important structures o Central nucleus

TRANSCRIBERS: Maja, Eli, Trisha, Rissa, Kenan, Eunika, Von, Catie

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PHYSIOLOGY

MUSCLE PHYSIOLOGY

o Intercalated disks – found between 2 myocardial cells Cardiac Action Potential o Gap junctions  1 cardiac action potential can last up to 200  Creates syncytium (pass of ions bet. cells) ms o Capillaries  Maximum HR of a person is 300 bpm  Well-developed  5 action potentials in a sec x 60 secs =  For O2 for ATP production 300 o T-tubules  Well-developed to ensure contraction occur Contraction: ++ simultaneously  Influx of Ca during action potential triggers ++ Ca release from SR  contraction Nodal Cells  Depolarization of nodal cells is achieved by Relaxation: ++ + ++ the opening of Ca channels and closing of K  Reaccumulation of Ca by the SR and channels extrusion of Ca++ from the cell  Calcium ions – responsible for depolarization Spread of depolarization via gap junction SMOOTH MUSCLE 2 Major Types: 1. Multi-unit Smooth Muscle  Discrete, separate smooth muscle fibers without gap junctions  Innervated by a single nerve ending  Contract independently of the others and their control is mainly exerted by nerve signals  Ex: ciliary and iris muscle of the eye 2. Unitary Smooth Muscle  Several (hundred to thousand) smooth muscle fibers that contract as a single unit; with gap junctions  Ions can flow freely from one muscle cell to the next

Ca++ entry via L-type Ca++ Channels (DHPR) > triggers Ca++ induced Ca++ release from SR via RYR Contraction: ++  Influx of Ca during action potential triggers ++ Ca release from SR  contraction Relaxation: ++  Reaccumulation of Ca by the SR and ++ extrusion of Ca from the cell

 Caveola (instead of T-tubules) - nice in-pouches that increase the area enough to cause Ca++ to influx to the smooth muscle  Dense body - Equivalent of Z-lines

Beta1 Agonist 1. Increased rate of contraction 2. Increased force of contraction / peak tension 3. Increased rate of relaxation – by increased Contractile Mechanism in Smooth Muscle: SERCA activity 1. Chemical Basis  Contains both actin and myosin filaments Digitalis – interact with each other the same way  Increased myocardial contractility + + + that they do in skeletal muscle  Inhibits Na K ATPase  Increased ICF Na ++ +  Does not contain troponin complex (unlike Ca pump  Increased Na influx in skeletal muscle)  Contractile process: activated by calcium  ATP degraded to ADP  energy for Phospholamban contraction  Inhibits SERCA in the unphosphorylated state 2. Physical Basis  But when phosphorylated by PKA, its ability to  No striated arrangement of actin and be inhibitory is lost > thus activators of PKA myosin; instead they have dense bodies (beta adrenergic agonist ex. Epinephrine) may enhance cardiac muscle relaxation rate

TRANSCRIBERS: Maja, Eli, Trisha, Rissa, Kenan, Eunika, Von, Catie

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PHYSIOLOGY 

MUSCLE PHYSIOLOGY

Dense bodies: serve the same role as the  Intermittent activity – like a muscle twitch in Z discs in skeletal muscle skeletal muscle ++  Attached to the cell membrane and a. High transient ꜛICF Ca (multi-unit) dispersed inside the cell b. High levels of MLCK (myosin light chain  Transmission of force of contraction: kinase) membrane-dense bodies of adjacent cells bonded together by intercellular Tonic Contraction and the Latch State ++ protein bridges a. Low sustained increased ICF Ca (visceral b. High levels of MLCK Relaxation ++ a. Decrease in ICF Ca b. ꜛ level of MLCK  Relaxation greatly depends on the amount of active myosin phosphatase; not on the ++ level of ICF Ca

Tonic Contraction  Continuously active  Only 1/10 to 1/300 (or 0.3 – 10%) as much energy is required to sustain the same tension of contraction in smooth muscle as in skeletal muscle  Resulted from slow attachment and detachment cycling of the crossbridges and because only 1 molecule of ATP is required for each cycle (regardless of its duration)  Energy utilization: important to overall energy economy of the smooth muscles which maintain tonic muscle contraction almost indefinitely  Ex. Intestines, urinary bladder, gallbladder and other viscera Latch Phenomenon  Condition of tonic contraction during which force is maintained at low energy expenditure Phasic Contraction

TRANSCRIBERS: Maja, Eli, Trisha, Rissa, Kenan, Eunika, Von, Catie

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