Physiology Review Sheet
Striated Muscle Structure/Function, Muscle Performance, Muscle Protein Structure and Energetics, Smooth Muscle, Clinical Examples of Deranged Intramuscular Ca2+ Homeostasis
· Skeletal Muscle Structure [Note: not sure which parts you need to know for Micro and which for Phys]
o myofibril (grouped into muscle fiber (multinucleated individual cell) which are grouped into muscle fascicles)
§ sarcomeres (space between 2 Z bands)
· Z-band
o thin filaments insert into this
· I band
o light band (1/2 on each side of Z)
o actin
o length varies due to filaments sliding
· A band
o dark band
o myosin, actin
o fixed length = length of thick filament
· H zone
o light zone in center of A band
o myosin
· M line
o dark line in center of H zone
o myomesin (M protein)
o connects thick filaments
o myofilaments
§ thin filament – actin
· G-actin (globular)
· F-actin (filamentous)
o 2 strands of F-actin forming a double helix (string of pearls) in muscle
· Tropomyosin
o lies along actin groove
o covers myosin binding sites during low Ca2+ levels
· Troponin
o TnT – binds tropomyosin
o TnC – binds Ca2+ and relieves inhibition of Tm
o TnI – inhibits actin-myosin interaction at low Ca2+
§ thick filament – myosin
· single heavy chain and 2 light chains
· heavy chains
o tail
o role in filament assembly
· globular heads
o S1 = head region
§ ATPase activity
§ actin binding site
o S2 = hinge
· opposite polarity at center leaves central bare zone – no heads
o reason for plateau in force-length curve
· Motor unit = a motor neuron and all the muscle fibers (cells) it innervates
· Mechanics
o isometric: no change in total muscle length
o isotonic: constant load on muscle
o stimulation frequency
§ sg stim = twitch = one nerve fires once and that motor unit contracts once
§ second AP before relaxation is complete begins wave summation – new contraction occurs at greater force than previous one (temporal summation)
§ treppe – high frequency of stimulation generates multiple contractions before any relaxation is complete resulting in a staircase appearance in force/time curve
§ tetanus – treppe summates to smooth even contraction; note: if too high a frequency or for too long, force will decline
· Excitation-Contraction Coupling
o more skeletal muscle structure
§ sarcolemma
· aka plasma membrane of muscle cell
· electrically excitable; propagates APs like a nerve
§ T tubules
· invaginations of sarcolemma (still excitable)
· open to ECF
· at A-I junction for fast skeletal muscle (at Z bands for others)
§ sarcoplasmic reticulum
· stores and releases Ca2+
· lots of Ca2+ pumps (Ca-ATPase) to sequester it in SR in longitudinal regions
o phospholamban
§ SR protein assoc with Ca-ATPase in slow skeletal, cardiac, and smooth muscle
§ inhibits Ca-ATPase
§ inhibited by phosphorylation (so pump can work)
§ useful to increase cardiac contractility with certain drugs
· terminal cisternae
o widened regions near junction with T tubules
o Ca2+ stored here bound to calsequestrin
o immediately next to T tubule = junctional SR
§ triad
· T tubule flanked by SR on both sides
· ryanodine receptor (RyR)
o foot proteins in terminal cisternae separating T tubule and SR membranes
o Ca2+ release channel
· dihydropyridine receptor (DHPR)
o voltage-gated Ca2+ channel (VGCC)
o tons in T tubule adjacent to terminal cisternae
o Contraction sequence of events (overview)
§ motor neuron fires and elicits AP on sarcolemma
§ AP propagates along sarcolemma and into T tubules
§ signal causes conformational change in DHPR of T tubule to open the RyR of the SR à releases Ca2+
§ Ca2+ binds troponin C
§ actin and myosin interact, slide past each other, generate force
§ Ca-ATPase in SR takes up excess Ca2+ inside cell and ends contraction
§ Note: in cardiac muscle, EC [Ca2+] matters – DHPR is a Ca2+ channel in cardiac muscle and Ca2+ influx induces release of Ca2+ from SR via RyR
· Length-Tension Relationship
o
o force of contraction depends on length of sarcomere prior to contraction
o optimum length = max force
o total force – passive force = active force
§ active force stimulated by contraction
§ Note: active force declines at long and short lengths, but passive force continues to increase with length until muscle tears
· Force-Velocity Relationship
o
o preload: initial length of muscle; stretch; determines max force possible
o afterload: additional load without changing muscle length; determines velocity
o hyperbolic relationship – velocity decreases rapidly with increased afterload
o power = F * V
o max power occurs at 1/3 max isometric force in skeletal muscle
o shorter muscles contract slower
§ not very important in muscle because limited ROM due to skeleton
§ very important in heart where muscle has a large length range
· Assembly of sarcomeres
o in parallel = ↑ force
o series = ↑ velocity, shortening capacity, and tension cost (ATPase)
· Factors influencing total force developed
o [Ca2+]I -- # of activated actin filaments
o # of crossbridges overlapped
o # of crossbridges able to interact limited by speed
o spatial summation
§ due to motor unit firing (not all cells of unit are in same place; they are of same time)
§ depends on:
· twitch duration of fibers (depends on myosin ATPase)
· frequency of firing
· # of motor units recruited
· size of motor units (# of fibers and fiber cross-section)
o recruitment
§ increase # of motor units firing
§ small to large
§ @ highest forces . . . increase force by increasing firing rate
· Sliding Filament Model of Contraction
o tension of muscle fiber is proportional to extent of thick and thin filament overlap
o insert graph B p10
§ at long lengths, less overlap between thick and thin filaments – can’t generate as much force
§ at length = thick filament + 2 thin filaments (3.6 μm) – no overlap – no force
§ at short lengths (?) – double filament overlap causes less ability to generate force
o ATP hydrolysis by mysoin releases heat – also dependent on degree of overlap between thick and thin filament
o Thin filament regulation
§ inhibition of actin-myosin interaction regulated by tropomyosin and troponin
§ regulated by Ca2+ binding of TnC → conformational change to TnT and TnI → moves Tm off actin binding site on myosin
§ [smooth muscle is regulated by thick filament]
o Ca2+ sensitivity:
§ sensitizers change Ca2+ binding affinity of TnC so that lower Ca2+ would affect more TnC and activate more actin
§ phosphorylation of regulatory proteins alter Ca2+ signal transduction
§ different isoforms of Tn and Tm modulate sensitivity
o Crossbridge Cycle (occurs many times during contraction)
§ myosin bound to ATP won’t bind actin
§ myosin hydrolyzes ATP to ADP and Pi and binds actin (use 1 ATP / cycle)
§ releases ADP and Pi (enhanced by actin binding) and undergoes power stroke (still linked to actin = rigor link)
§ binds fresh ATP and dissociates from actin
· Muscle Metabolism
o Fenn effect: isotonic contraction releases more energy than isometric contraction – feedback between mechanical constraints and rate of crossbridge cycling
o Sources of ATP for contraction
§ muscle stores of ATP
· low amounts
· immediately available
§ creatine phosphate
· 3-5x as much as ATP
· very rapid
· Lohman Reaction
o ATP à ADP + Pi ...... + ...... PCr + ADP à Cr + ATP (one step production of ATP)
§ glycogen
· large stores
· can be metabolized by glycolysis (rapid, limited, lots of ATP, but relatively inefficient; make lactic acid) or by oxidative phosphorylation (slower, limited, huge amounts of ATP, efficient)
§ exogenous stores (depends on diet)
· uses oxidative phosphorylation to generate lots and lots of ATP efficiently, but slowly
§ Feedback mechanisms involve
· ADP & Pi
· Ca2+
o activate phosphorylase cascade to produce glucose from glycogen
o increase permeability of sarcolemma to glucose
· increased blood flow
o improve O2 flow
o remove lactic acid
o Recovery of oxygen consumption (oxygen debt)
§ spring or burst of activity
§ must resynthesize high energy phosphates used from PCr
§ ultimately, nearly all ATP used in contraction is resynthesized during ox-phos
· Diversity of Proteins
o Skeletal muscle contractile and regulatory proteins are NOT all the same – isoforms (myosin, actin, Tm, Tn)
o Myosin isoforms (be familiar with varying characteristics)
§ fast glycolytic (FG)
· white
· type IIb
· high levels of fast ATPase
· few mitochondria
· dense SR
· large fiber diameter
· low oxidative enzyme activity
· low mitochondrial ATPase
· high glycolytic activity
· low myoglobin
§ slow oxidative (SO)
· red
· type I
· low levels of slow ATPase
· intermediate # mitochondria
· intermediate SR
· intermediate fiber diameter
· high/intermediate oxidative enzyme activity
· intermediate mitochondrial ATPase
· low/intermediate glycolytic activity
· high myoglobin
§ fast oxidative glycolytic (FOG)
· red
· type IIa
· high levels of fast ATPase
· lots of mitochondria
· dense SR
· small fiber diameter
· intermediate/high oxidative enzyme activity
· high mitochondrial ATPase
· intermediate/low glycolytic activity
· high myoglobin
· Cardiac Muscle
o structure
§ striated, sarcomeres
§ similar to SO skeletal muscle
§ sarcolemma with T tubules
· DHP receptor is a voltage sensor AND a Ca2+ channel – Ca2+ induced Ca2+ release (CICR) – not voltage gated as in skeletal
· Na+- Ca2+ exchanger
o forward: Ca2+i exits, Na+e enters
o reverse: Ca2+e enters (mechanism of ouabain and digitalis → inhibits Na+ pump →increases [Na+]i reverses pump and increases Ca2+i
§ lots of mitochondria and lower PCr
o function
§ myocytes electrically coupled
· no recruitment. . . vary [Ca2+]i to regulate force
§ mechanism for force generation the same for skeletal muscle
· less myofilaments in parallel – less force/unit cross sectional area
· energy cost is less → slower cross bridge cycling rate
· Smooth muscle
o structure
§ spindle-shaped cells
§ proteins
· actin/myosin in scattered arrangement (no sarcomeres)
· fewer myosin filaments per actin
· dense bodies containing -actinin anchor actin
· intermediate filaments connect dense bodies to cytoskeleton
· poorly developed SR – can store Ca2+
· no troponin
o energetics
§ sustains contraction longer without fatigue & lower O2 consumption
· latch state: maintain force with reduced crossbridge cycling velocity
§ force-length similar to skeletal
§ oxidative contraction
· low energy requirements (supply=demand)
· low PCr pool (not needed)
· no oxygen debt
§ glycolysis for membrane function
· lactate is produced under fully oxygenated conditions
· fuels membrane pumps (ATPases)
· metabolic compartmentation (have anaerobic and aerobic going on at same time)
o Smooth Muscle Excitation-Contraction Coupling
§ General
· many types of Ca2+ channels and membrane receptors
· no fast Na+ channels
· AP carried via Ca2+ channels, and Ca2+ acts as second messenger
· automaticity
o pacemaker potentials
o slow waves (APs occur in bursts)
§ oscillations in Nai-Ko pump
· act as stretch receptors (in GI tract, bladder, uterus, some blood vessels)
· neurotransmitters can activate
§ mechanism of [Ca2+] I elevation → contraction
· Ca2+ entry via voltage-dependant channels and receptor operated channels
· Ca2+ release from SR via Ca2+ or IP3
o consequence of Ca2+ channels opening in PM
o G-protein cascade with DAG or IP3 directly open Ca2+ in SR
§ angiotensin II acts via G-protein activated phospholipase
· DAG and phosphorylation of PK-C activates slow Ca2+ channels – triggers release from SR
· IP3 acts as 2nd messenger activating SR Ca2+ channels
· reversed Na+/ Ca2+ exchange (follows gradient)
· inhibition of SERCA (SR Ca2+ reuptake pumps)
§ mechanism of smooth muscle relaxation (lowering [Ca2+]i) – favored by high [Ca2+]i
· SERCA
· Na+/ Ca2+ exchanger in forward direction
· sarcolemma Ca2+ ATPase channels
· inhibition of sarcolemma Ca2+ channels
§ transduction of Ca2+ signal at level of contractile filaments
· activation
o Ca2+ binds calmodulin (free in cytosol)
o Ca2+/calmodulin activates MLCK
§ phosphorylates MLC 20
§ activates ATPase and allows crossbridge formation
§ sliding filament
· relaxation
o MLCP (phosphatase) (may regulate latch state)
o dephosphporylates MLC 20
§ modulation of Ca2+ sensitivity
· Ca2+ entry blockers
· inhibit binding of Ca2+ /calmodulin to MLCK → less MLC phosphorylation
· stimulation of MLCP
· Malignant Hyperthermia
o clinical features
§ potentially fatal
§ triggered by anesthetics (ether, or any of the –thanes) or muscle relaxants (succinyl choline)
§ hyperthermia and muscle rigidity
§ family history
§ priming factors: stress, youth, prolonged surgery
o Muscle Disorder
§ a mild reaction increases creatine kinase
§ survivors of severe reactions have rhambdomyolysis (severe destruction of muscle)
§ abnormal sensitivity to halothane or caffeine
o Porcine Stress Syndrome
§ stress induced MH in pigs
§ provides excellent clinical model (pathophysiology, treatment, molecular bio)
o Molecular biology
§ defect in RyR (triggers open and keep open)
§ increase in Ca2+ ATPase activity (trying to pump Ca2+ back into SR)
§ increase in myosin ATPase activity