Study Guide – Muscle Physiology
Physiology of Skeletal Muscle 1: Molecular Aspects of Contraction
molecular aspects of contraction
- functions of skeletal muscle
- function: develop tension in order to perform mechanical work (interaction of myosin, actin, and ATP)
- energy from ATP hydrolysis: converted to heat, mechanical work
- review of muscle ultrastructure
- muscle fiber: individual multinucleated muscle cells
- quantity of muscle fibers
- more fibers found in muscles required to develop large tensions
- tension: arithmetic sum of individual tensions of each muscle fiber
- length of muscle fibers
- longer fibers found in muscles that must shorten rapidly or over long distances
- muscles that require both strength and rapid contraction are composed of many long fibers
- sarcolemma: plasma membrane surrounding a muscle cell
- myofibril: individual contractile bands that run across the length of the cell
- composed of sarcomeres lining up end to end
- each myofibril has cross-striations that arise from the highly ordered arrangement of actin and myosin
- sarcomere: basic contractile unit of muscle
- Z line (Z disc): thin band forming the end of each sarcomere
- α-actinin: primary component of Z lines
- desmin: interconnecting Z protein that binds, holds adjacent myofibril Z lines in register
- I band (isotropic band): light-shaded band adjacent to the Z line
- actin: contractile protein forming thin filament (8 nm diameter)
- each thin filament attaches to Z line, extends through I band, and enters A band
- band width varies directly with sarcomere length
- M line: thick, center-most structure of the sarcomere
- binding point of thick filaments
- contains M-line proteins that extend radially to bind adjacent thick filaments
- H zone: lighter-shaded band adjacent to the M line
- myosin: contractile protein forming thick filament
- region of the A band that contains only thick filaments
- zone width varies directly with sarcomere length
- A band (anisotropic band): darkly-shaded zone of overlap between thin filament and thick filament
- located centrally within the sarcomere
- width (1.65 μm) constant regardless of sarcomere length
- cross-bridges: radial extensions of thick filaments towards thin filaments
- cross bridge: terminal part of one myosin molecule, containing ATP and actin-binding sites
- zone of overlap
- each thin filament surrounded by 3 thick filaments
- each thick filament surrounded by 6 thin filaments
- thick filament structure
- myosin: main component of the thick filament, comprised of 2 heavy chains and 4 light chains
- heavy chain (210 kD): comprised of a rod (filament backbone), and 2 sub-domains (cross bridge projection)
- S1: globular head where ATP, actin-binding sites located
- S2: flexible rod linking S1 to backbone
- light chain (20 kD): two are associated with each S1 myosin head; can be reversibly phosphorylated
- assembly
- successive S1 heads project at a length of 14.3 nm, rotated 120º relative to the previous
- axial repeat: 42.9 nm
- polarity: bipolar
- rod domains: associate at the center of the thick filament
- bare zone: comprised entirely of rigid rod domains, devoid of myosin cross bridges (0.2 μm)
- thin filament structure
- actin: 42 kD globular protein
- spontaneously forms thin filaments at physiological conditions, in presence of MgATP
- comprised of a double helix of actin monomers
- tropomyosin: lies in the thin filament groove, conferring rigidity (one per 7 actin monomers)
- troponin: Ca2+-binding protein associated with each tropomyosin; has three subunits
- troponin I
- troponin C: specific protein that binds Ca2+
- troponin T
- skeletal muscle membrane
- transverse tubules: membranous invaginations of the sarcolemma
- extends into the center of each fiber at the A-I junction
- allows action potential to propagate into the center of the fiber
- sarcoplasmic reticulum (SR): membrane system containing high concentrations of Ca2+
- terminal cisternae: membranous sacs at ends of SR that are closely associated with transverse tubules
- lateral SR: physical association of adjacent transverse tubules, where Ca2+ can be resequestered following AP
excitation-contraction coupling
- excitation-contraction (E-C) coupling: events linking surface membrane activity to increased myoplasmic [Ca2+]
- overview of excitation-contraction coupling in skeletal muscle
- action potential propagates along the surface membrane
- action potential propagates inward along the transverse tubules
- transverse tubule communicates with sarcoplasmic reticulum
- Ca2+ released from the sarcoplasmic reticulum
- myofilaments activated, causing contraction
- E-C coupling in skeletal muscle
- action potential on the surface membrane
- resting potential: negative due to high resting PK, low resting PNa
- ionic basis: similar to that for nerve action potentials
- depolarization: increased permeability to Na+
- repolarization: inactivation of gNa, transient rise in gK
- Ca2+ channels
- found in high concentration within transverse tubules, but extracellular Ca2+ not required for E-C coupling
- instead, the Ca2+ channel protein is an element of the signaling process between T-tubules and the SR
- action potential along the T-tubules
- mechanism: propagation of action potential, as on the surface membrane
- purpose: faster than sarcolemma diffusion processes, allows for synchronous cross-sectional activation
- transverse tubule to SR communication
- DHP receptors: electron-dense structures embedded in the T-tubule membrane
- function: Ca2+ channel that serves as the voltage sensor of E-C coupling
- mechanism: change in orientation of polar amino acids; physically transmitted to the SR junctional feet
- passage of Ca2+ is NOT required for E-C coupling in skeletal muscle
- charge movement: the net movement of charge from one surface of the membrane to the other
- junctional feet: electron-dense structures embedded in the terminal cisternae of the SR
- other names: SR Ca2+ release channel, ryanodine receptor
- ryanodine: binds Ca2+ release channel with high affinity, renders channel inactive (low conductance state)
- once used as an insecticide, but now banned due to wide-scale toxic effects on other organisms
- function: when active, allows outward movement of Ca2+ from the lumen of the ST to the myoplasm
- mechanism: AP in T-tubule alters physical interaction between DHP receptors & junctional feet, opening the junctional feet and releasing Ca2+ into the myoplasm
- release of Ca2+ from the SR
- increased [Ca2+] is responsible for muscle contraction
- myoplasmic Δ[Ca2+]: 0.3 μm to 10 μm
- approxiamtely 1/4 of SR Ca2+ is released into the myoplasm during a single twitch contraction
- predominant mechanism: increase in conductance of the SR Ca2+ release channel via DHP interaction
- Ca2+-induced Ca2+ release (CICR): use of Ca2+ to amplify and release more Ca2+
- Ca2+ released by junctional feet binds Ca2+ release channels
- this serves to open channels through positive feedback, allowing greater [Ca2+] in the myoplasm
- signal termination
- CICR ceases despite continued elevation of [Ca2+]
- mechanism of termination is unknown, but several hypotheses exist
- Ca2+ binds to other sites / another protein; mediates inactivation of the channel (slower than CICR)
- Ca2+ depletion from SR triggers inactivation
- DHP receptor sensor repolarization contributes to SR Ca2+ closure
- E-C coupling in skeletal muscle
- skeletal muscle: DHP receptor is primarily a voltage sensor that activates Ca2+ release from the SR
- upon depolarization, DHP receptor causes opening of junctional feet, causing Ca2+ release and CICR
- extracellular Ca2+ is not needed for skeletal muscle contraction
- cardiac muscle: DHP receptor is a voltage-gated Ca2+ channel that causes an influx of extracellular Ca2+
- amount of extracellular Ca2+ cannot trigger contraction, but it can activate CICR as in skeletal muscle
- in this case, extracellular Ca2+ is REQUIRED for proper cardiac muscle contraction
- re-uptake and storage of Ca2+ by the SR
- localization: occurs both in the terminal cisternae and in the lateral SR
- Ca2+ taken up in the lateral SR is translocated to the terminal cisternae
- in the terminal cisternae, only small Ca2+ is soluble; most is bound to calsequestrin
- calsequestrin: Ca2+ storage glycoprotein; weakly binds up to 60 Ca2+ ions
overview: defects in muscle contraction
- toxins
- curare: blocks ACh binding reversible
- bungarotoxin: blocks ACh binding irreversible
- cobra toxin: blocks ACh binding irreversible
- botulinum toxin: blocks ACh release
- diseases
- myastenia gravis: autoimmune disorder where antibodies raised against ACh receptors at motor endplate
- malignant hyperthermia: uncontrolled prolonged muscle contraction with exposure to halogenated anesthetics or depolarizing muscle relaxants
Physiology of Skeletal Muscle 2: Molecular Basis of Contraction
regulation of cross-bridge interaction
- troponin: Ca2+-binding protein (low affinity) that regulates the physical position of tropomyosin
- tropomyosin: protein that sits in actin groove, blocking binding sites of myosin
- regulation
- altered calcium: between inactive and contracting muscle, [Ca2+] changes from <0.5>
- low [Ca2+]: troponin Ca2+-binding sites unoccupied; tropomyosin blocks myosin binding
- high [Ca2+]: troponin Ca2+-binding sites occupied; tropomyosin moved out of the way, allowing myosin binding
cross-bridge cycle
- by conformation, starting at the extended and unbound conformation
- actin, myosin·(ADP + Pi): extended, unbound
- dissociated cross bridges and myosin projecting at a 90° angle
- can be reversed
- actin–myosin·(ADP + Pi): extended, weakly bound
- occurs in the presence of Ca2+, which causes tropomyosin to move and open up binding sites
- can be reversed
- *actin–myosin·(ADP) [1]: extended, strongly bound
- upon binding weakly, Pi is immediately given off, increasing strength of binding and beginning power stroke
- can be reversed in the presence of high amounts of Pi
- *actin–myosin·(ADP) [2]: contracted, strongly bound
- cross bridge contracts, causing movement
- can be reversed
- *actin–myosin: contracted, strongly bound; rate limiting, power stroke, rigor complex
- rate-limiting: ADP dissociates at a rate of 1-3 s-1
- power stroke: loss of product decreases likelihood of reversal
- rigor complex: after death, when ATP is lacking, cross bridges become frozen in this conformation
- *actin, myosin·(ATP): contracted, unbound
- association with ATP causes myosin dissociation from actin
- upon ATP hydrolysis, will extend and return to resting conformation (extended, unbound)
- by reaction, starting at rigor complex
- protein release: ATP binds myosin head, causing it to release from the actin filaments
- conformational change: ATP à ADP + Pi, causing a conformational extension
- Pi release: Pi falls off, allowing myosin to bind actin (beginning of power stroke)
- ADP release: ADP falls off, protein resumes original conformation (end of power stroke)
sliding filament mechanism
- contraction: activation of cross bridge cycling (includes resistance during muscle extension)
- sliding filament mechanism: Z lines drawn closer together by myosin pulling thin filaments along thick filaments
twitch and tetanic contractions of skeletal muscle
- definitions
- tension: force exerted on an object by a muscle
- load: force exerted by an object on a muscle
- muscle contraction
- isotonic: load <>
- isometric: load = tension; constant length
- eccentric: load > tension; muscle lengthening
- twitch contractions
- twitch: unitary all-or-none contraction response of a muscle
- varies with number of activated fibers
- varies under extreme conditions (e.g. fatigue)
- process
- single maximal electrical stimulus applied to muscle
- sufficient Ca2+ released to saturate Ca2+ binding sites on troponin-C, and cross bridges begin to activate
- SR begins resequestering Ca2+, and Ca2+ dissociates quickly
- typically, a single Ca2+ transient will not allow all cross bridges to bind to actin
- as such, the twitch tension typically does not represent the maximum tension the muscle can develop
- tension peak
- response vs. time
- action potential: most rapid rise and fall
- intracellular [Ca2+]: peak lags behind action potential
- tension: peak lags behind intracellular [Ca2+]
- rate of relaxation
- quantification
- single cross bridge cycle: 1-3 sec-1
- twitch: 300 ms
- during a twitch, some cross bridges will bind during the transient increase in [Ca2+]
- as Ca2+ is resequestered, tropomyosin will move back in place, but will be blocked by bound myosin heads
- once myosin has become unbound from actin, tropomyosin will move into place and prevent further binding
- thus the rate of relaxation (tension peak) is dependent on cross bridge release
- tetanic contractions
- tetanus: any contraction in which there is summation of the force responses to successive stimuli
- sufficiently fast successive stimuli will cause myoplasmic Ca2+ transients to summate
- fused tetanus: contraction in which summation of forces is at maximum
- at high enough stimulation, Ca2+ will be maintained at saturating levels
- time is not a limiting factor for binding, so the maximum number of cross bridges will bind to actin
- post-tetanic potentiation
- post-tetanic potentiation (PTP): peak twitch tension is increased proceeding after a period of tetanic stimulation
- sustained Ca2+ binds calmodulin, activating myosin light chain kinase (MLCK)
- phosphorylation of MLC2 increases electronegativity of myosin S1, driving it away from the thick filament
- this brings it closer to actin, making cross bridge binding more likely
- strength of PTP is less than the tetanic stimulation, but greater than a single twitch after relaxed conditions
Physiology of Skeletal Muscle 3: Mechanical Properties of Muscle
- muscle mechanics: study of tension-generating properties, responses of muscle to changes in load or length
- history
- early work consisted of models involving simple springs and dashpots
- in 1957, A. F. Huxley proposed side pieces extending from the thick filament to actin
- elecron microscopy confirmed this hypothesis
length-tension relationship
- total tension: sum of active and passive tensions at that length
- active: tension created by myosin binding to actin
- passive: tension created by titin, which creates an elastic force to help reform extended muscle
- length dependence of tension
- active tension: rises and falls somewhat parabolically around a length of maximum active tension
- passive tension: rises somewhat exponentially from the length of maximum active tension
- total tension: initially follows active tension
declines momentarily during a period of overlap of active and passive tension
ultimately follows passive tension closely as active tension rapidly declines
resting tension
- resting tension: indefinitely-maintained passive tension developed at sarcomere lengths > 2.5 μm
- increases exponentially with sarcomere length
- at 3.6 μm, resting tension is as large as tetanic tension at 2.45 μm
- titin: muscle protein responsible for elasticity and passive tension
- largest muscle proteins in all mammalian cells
- stretch from Z disc to M line
- collagen: also has somewhat of an effect as it gives cells much of their rigidity
active length-tension relationship
- overview
- based on isometric tetanic tension vs. length
- peaks between 2.25 and 2.45 μm, indicating an optimal length for contraction
- at lengths above or below, tension decreases, indicating less actin to myosin interactions
- single fibers vs. whole muscle
- length-tension relationship in single fibers can vary slightly
- effect: rounds the corners of the relationship shown in whole muscle
- active length-tension relationship
- ascending limb
- length: 1.30 to 1.65 μm (steep part), 1.65 to 2.25 μm (shallow part)
- etiology: steep: thick filaments are forced against the Z line, an elastic structure
shallow: thin filaments pass through the bare zone, overlap on opposite side, block binding
- effect: with increasing length, tetanic tension increases steeply, then sharply, from 0 to maximum
- plateau
- length: 2.25 to 2.45 μm
- etiology: at these lengths, overlap between thick and thin filaments is constant and maximal
- effect: tetanic tension stays constant over the given lengths
- descending limb
- length: 2.45 to 3.90 μm
- etiology: at lengths above 2.45 μm, overlap between thick and thin filaments decreases linearly
above 3.90 μm, there is no overlap, and thus no active tension
- effect: with increasing length, tetanic tension decreases linearly from maximum to 0
force-velocity relationship
- force-velocity relationship: velocity of muscle lengthening vs. power
- definitions
- Vmax: velocity under zero load, limited only by ADP ejection from myosin head (varies by muscle)
- Po: steady isometric tension developed under tetanic stimulation
- power relationship: 
- force-velocity relationship: load <>o
- effect: muscle shortens at a given velocity
- hyperbolic relationship 
- inverse relationship between force and velocity means power is greatest at intermediate tension (~0.3 Po)
- there, efficiency is greatest, as 40-45% of chemical energy is converted to mechanical work
- force-velocity relationship: load > Po
- effect: muscle shortens at a negative velocity (i.e. lengthens, giving resistance to slow the opposing force)
- relationship
- starting at zero, gradual decrease (increase in negative velocity) to ~2 Po
- above 2 Po, negative velocity drops rapidly, apparently to protect against damage due to large loads
variations in contraction with fiber type
- classification of skeletal muscle fibers
- red: I high oxidative metabolism slow twitch
- intermediate: IIa high oxidative metabolism slow twitch
- white: IIb low oxidative metabolism fast twitch
TABLE: Distinguishing Skeletal Muscle Types
|
| red (I) | intermediate (IIa) | white (IIb) |
| myosin isoforms (Vmax) | slow | fast | fast |
| myosin ATPase | low | high | high |
| muscle color | red | red | white |
| myoglobin content | high | high | low |
| oxidative enzymes | high | high | low |
| mitochondrial content | high | high | low |
| glycolytic activity | low | medium | high |
| rate of fatigue | slow | intermediate | fast |
- muscle characteristics
- pH dependence: determines numbering (I, IIa, IIb); histological assessment, not commonly used anymore
- myosin isoform: determines rate of ADP ejection, and thus ATPase activity and muscular Vmax
- myoglobin content: determines color, where those high O2 need correlates with redness
- intermediate fibers
- rely on both anaerobic and aerobic metabolism
- most useful in middle distance running events
- type of muscle most effectively skewed during training
- predominantly red or white muscle body type determined primarily by genetics
- effective training tends to skew intermediate muscles more in one direction or the other
- energy metabolism
- creatine phosphate: high energy phosphate reserve; used in first few seconds of contraction
- aerobic metabolism: oxidative phosphorylation; predominant in red muscles
- anaerobic metabolism: glycolysis; predominant in white muscles
- fatigue: muscle weakness due to sustained use
- mechanism: poorly understood, but a variety of hypotheses have been suggested
- conduction failure
- build-up of H+
- build-up of ADP, Pi
- central command failure (“you giving up, punk?”)
- muscle resistance to fatigue
- fast: fatigue rapidly due to use of glucose, poor efficiency of ATP generation
- slow: fatigue slowly due to anaerobic oxidation, high efficiency in ATP generation
motor units
- motor unit: motor neuron, its axon and collaterals, and all muscle fibers the neuron innervates
- safety factor of neuromuscular transmission is high
- all muscle fibers of a motor unit respond with a twitch when the motoneuron discharges an action potential
- size variance
- extra-ocular muscles
- each motoneuron innervates ~3 muscle fibers
- advantage: fine gradation of muscle contraction
- limb muscles
- each motoneuron innervates ~1500 muscle fibers
- advantage: more efficient contraction when fine gradation is less necessary
- fiber variance
- characteristics
- all constituent fibers of a motor unit are of a similar type (red, intermediate, white)
- muscles (collections of motor units) may contain a mixture of fiber types
- motor units comprised of red muscle fibers
- small, slowly-conducting axons
- few muscle fibers per motoneuron
- fatigue-resistant
- motoneurons fire almost continuously at a low rate
- motor units comprised of white muscle fibers
- large, fast-conducting neurons
- many muscle fibers per motoneuron
- fatigue rapidly
- motoneurons fire in rapid, interrupted bursts
Physiology of Skeletal Muscle 4: Stretch Reflexes and the γ Motor Loop
the muscle spindle
- muscle spindle: encapsulated, fusiform sensory organs sensitive to mechanical stretch
- fiber types
- extrafusal muscle fibers: force-generating fibers, innervated by α-fibers
- intrafusal muscle fibers: sensory structures within muscles, innervated by γ-motor neurons
- intrafusal muscle fibers
- structure
- sensory structures present along intrafusal fibers
- ends are contractile, but not force-generating; instead, regulate sensor function during extrafusal contraction
- innervation and receptor types: intrafusal muscle fibers
- annulospiral receptors: group Ia afferents, sensitive to muscle length and rate of change during shortening
- flowerspray receptors: group II afferents, sensitive to muscle length only
- γ-motor neurons: efferent innervation to contractile ends of intrafusal fibers
- stimulation causes mechanical stretch on middle fiber regions (sensor stretching)
- sensor stretch generates an action potential that is propagated to the spinal cord
- intrafusal muscle fiber types
- nuclear bag fibers
- afferent: annulospiral receptors
- efferent: γ-motor neurons
- nuclear chain fibers
- afferent: annulospiral receptors, flowerspray receptors
- efferent: γ-motor neurons
sensory information due to passive stretch of intrafusal muscle fibers
- general function of intrafusal fibers
- variations in length stretch receptors provide information about:
- muscle length
- joint angle
- variations in velocity stretch receptors provide information about:
- rate of change of muscle length
- rate of change of joint angle
- this information allows CNS to anticipate the direction and velocity of limb movement
- essential for CNS control of limb position, coordinated motion of the body
- receptor encoding
- annulospiral receptors
- stretch: increase frequency during linear stretch, fall off (at higher frequency than original)
- tap: rapidly increase frequency, stop, and continue at original frequency
- release: stop during release, fall back to normal
- flowerspray endings
- stretch: gradually increase frequency during linear stretch, maintain at that frequency
- tap: mildly increase frequency, quickly continue at original frequency
- release: gradually decrease frequency during linear stretch, fall back to normal
- γ-motor neurons: increasing the range of intrafusal fiber sensors
- relaxed muscle: spindle fiber sensitive to stretch of muscle
- contracted muscle
- at sufficiently short lengths, without other mechanisms, spindles would slacken and reduce to zero
- however, with shorter muscle lengths, γ-motor neurons contract in response to top-down CNS instruction
- this allows sensory input and encoding over a wider range of muscle lengths
control of muscle length
- stretch reflex (myotactic reflex)
- stretch reflex: monosynaptic reflex that operates to control muscle length
- reflex elements
- receptor: muscle spindles
- afferent pathway: type Ia afferent neuron
- integrator: synapse
- efferent pathway: α-motor neuron
- effector: muscles
- activation
- passive stretch of the intrafusal muscle causes AP propagation along the Ia neuron
- Ia neuron synapses with the α-motor neuron, which causes contraction of the extrafusal fibers
- overall: passive intrafusal stretch activates a pathway that counters by active extrafusal contraction
- deactivation: loss of passive stretch eliminates “contraction” signal
- effects of the γ-motor loop
- activation
- CNS excitation of γ-motor neurons stretches sensory endings in the central regions of intrafusal fibers
- Ia neuron synapses with the α-motor neuron, which causes contraction of the extrafusal fibers
- overall: CNS-directed stretch of intrafusal sensors results in contraction of extrafusal fibers
- deactivation: loss of CNS-induced stretch eliminates “contraction” signal
- function: feedback control of skeletal muscle
- co-activation of the γ-motor loop and α-motor neurons
- motor systems
- α-motor neurons
- function: few synapses, rapid movement
- drawbacks: low feedback control; causes “spindle pause” during contraction
- spindle pause: spindle stops firing during contraction
- γ-motor neurons
- function: more synapses, high feedback control
- drawbacks: less responsive, slower movement
- most movements involve simultaneous co-activation of both systems, giving both speed and control
- allows estimation of load size, with fine-tuning by feedback
- not perfect: gross overestimation or underestimation of a load will override fine control
Smooth Muscle Physiology
smooth muscle structure
- anatomy
- shape: thin (2-20 μm), spindle shaped, with a single central nucleus
- dense bodies: analogous to Z lines of striated muscles
- join to thin filaments, sarcolemma
- α-actinin: primary component
- organization: spiraled along the length of the cell
- contrasts the repetitive linear arrangement of skeletal muscle
- thick and thin filaments only in parallel when the cell is highly stretched
- primary component of force is axial
- E-C coupling
- structures
- sarcoplasmic reticulum: present
- transverse tubules: absent
- E-C coupling: two mechanisms
- trans-sarcolemmal Ca2+ currents in cells that undergo an action potential
- IP3 release in response to hormone or receptor binding on the sarcolemma
- connections to other cells
- gap junctions: intercellular communication
- gap junction: specialized areas of closed apposition of the sarcolemma of adjacent cells
- contain discrete bridge structures that mediate electrical and chemical communication
- function: allow synchronization of contractile activity within smooth muscle tissue
- CT struts, collagen fibers: structural connection and tissue-level force transmission
mechanical properties of smooth muscle
- similarities: contractile proteins
- myosin: mechanoenzyme that splits ATP, generating force and/or movement
- actin: globular protein arranged into fibers, providing a tract for binding
- differences
- increased contraction time
- contraction time: T1/2 of several seconds, much slower than that of skeletal muscle
- time required to increase [Ca2+]i membrane Ca2+ currents or IP3
- time required for phosphorylation of the myofilaments by myosin light-chain kinase (E-C coupling)
- release time: T1/2 of several seconds, also slower than in skeletal muscle
- time required to clear Ca2+ from the myoplasm
- time required for myosin light-chain phosphorylase to dephosphorylate myosin
- decreased shortening velocity
- Vmax: 0.25 muscle lengths, characteristic of the expressed myosin isoform
- varies with myoplasmic [Ca2+]
- greatest at [Ca2+] that results in complete phosphorylation of myosin light chains
- ability to generate tension depends on strength of the stimulus and thus myoplasmic [Ca2+]
- wide length-tension relationship
- can generate maximal tension over a wider range of lengths
- broader plateau of active tension vs. length
- muscle length tends to start lower, allowing tension to still increase as length increases
- even at 2.5X resting length, can still generate near maximal tension (skeletal: 0 at 1.5X L0)
excitation-contraction coupling
- contraction: regulated by elevated myoplasmic [Ca2+], which is required for myosin light chain phosphorylation
- elevating [Ca2+]: three mechanisms
- voltage-gated Ca2+ channels
- depolarization: transient increase in gCa (NOT gNa)
- repolarization: inactivation of gCa, transient increase in gK
- Ca2+ influx: mediates Ca2+-induced Ca2+ release (CICR)
- influx through sarcolemma is insufficient to induce contraction
- instead, Ca2+ works to amplify the Ca2+ signal via release sarcoplasmic reticulum (SR) stores
- ligand-gated receptors
- channel-based: G-protein coupled to Ca2+ channel, influx triggers CICR
- messenger-based: G-protein coupled to IP3 production, IP3 stimulates Ca2+ release from the SR
regulation of cross-bridge interaction with actin
- relaxed smooth muscle: myosin cross-bridges are unable to interact with actin
- myosin is non-phosphorylated, and held rigidly from completing cross bridge cycles
- upon phosphorylation of a myosin light chain (MLC2), head becomes more flexible
- activation of smooth muscle: myosin light-chain kinase (MLCK)
- regulation: Ca2+/calmodulin-dependent
- calmodulin: Ca2+-binding molecule, structurally analogous to troponin-C
- smooth muscle lacks actin-associated troponin, but has calmodulin as an analogous regulatory molecule
- after elevation of [Ca2+] (μmol levels), calmodulin binds Ca2+ (1:4 ratio)
- calmodulin-(Ca2+)4 complex binds, activates MLCK, forming an active kinase that phosphorylates MLC2
- this imparts flexibility, electronegativity on the cross bridge, allowing it to bind actin
- myosin light-chain kinase (MLCK)
- skeletal muscle: aids in post-tetanic potentiation, but is not necessary for contraction
- smooth muscle: essential for contraction; non-phosphorylated myosin is unable to bind actin
- relaxation of smooth muscle: myosin light-chain phosphatase
- myosin light-chain phosphatase: dephosphorylates MLC2, thereby deactivating smooth muscle myosin
- regulation: inhibited in the presence of Ca2+
- mechanism: in absence of stimulation, [Ca2+] returns to resting levels, upregulating phosphatase
- effect: dephosphorylation causes deactivation of cross-bridges, rendering them unable to interact with actin
latch state
- latch state: smooth muscle state in which tension is maintained with very little use of metabolic energy
- acute stimulation: [Ca2+], MLC2 phosphorylation, Vmax, and isometric tension all rapidly increase
- sustained stimulation
- [Ca2+], MLC2 phosphorylation, Vmax fall to only slightly above resting values
- tension is maintained for longer periods of time (at the expense of flexibility)
- cycling vs. tension
- cross-bridge cycling: requires phosphorylation of MLC2, brought on by elevated [Ca2+]
- tension maintenance: requires only slight elevation of [Ca2+]
- mechanism (hypothesized)
- during sustained stimulation, myosin light-chain phosphorylase cleaves phosphate prior to completion of cycle
- rate of ADP release (rate-limiting step) is significantly reduced, and ATP consumption is much slower
classification of smooth muscle tissues
- multi-unit smooth muscle
- comparison: more similar to skeletal muscle
- examples: large arterial walls
pilomotor muscles attached to hair follicles
- contraction: initiated by action potentials in nerve fibers innervating the muscles
- regulation: innervation by autonomic nervous system, conducting via varicosities
- varicosities: regions of swelling along axonal membrane; membrane-bound vesicles stored
- vesicles are released into extracellular space during AP (as opposed to discreet synapse)
- because there can be numerous varicosities along an axon, one axon can influence many cells
- this also causes considerably greater delay between nerve stimulation, contractile response
- receptors: autonomic: receptors located along the entire membrane, respond to neurotransmitters
metabolic: some receptors responsive to certain hormones, causing IP3 production and Ca2+ release
- potentiation: membrane depolarization analogous to that of PSP in skeletal muscle
- single unit smooth muscle
- comparison: more similar to cardiac muscle
- examples: walls of small arteries and veins
gut smooth muscle
reproductive tract smooth muscle
- contraction: depolarization initiated by pacemaker cells, propagated via gap junctions to non-pacemaker cells
- gap junctions: synchronize electrical, mechanical connectivity between adjacent cells
- pacemaker potential: slow depolarization of Em due to gradual increase in gCa
- non-pacemaker cells fire at the same frequency
- regulation: spontaneous electrical activity within the muscle, as regulated by:
- autonomic innervation
- circulating hormones pharmaceutical agents
- mechanical stretch
depolarizing agents increase frequency of contraction
hyperpolarizing agents decrease frequency of contraction
TABLE: Summary of Comparisons Between Muscle Types
|
| skeletal | cardiac | smooth |
| excitation mechanism | neuromuscular transmission | pacemaker potentials electronic depolarization via gap junctions | synaptic transmission hormone-activated receptors electrical coupling pacemaker potentials |
| electrical activity of muscle cell | action potential spikes | action potential spikes | action potential spikes, plateaus graded membrane potential changes slow waves |
| Ca2+ sensor | troponin | troponin | calmodulin |
| E-C coupling | T-tubule DHP receptor (L-type Ca2+ channel) coupled to SR Ca2+ release channel (junctional foot) | T-tubule DHP receptor (L-type Ca2+ channel) causes Ca2+ influx, CICR | voltage-gated Ca2+ influx, CICR ligand-gated Ca2+ influx, CICR ligand-gated IP3 metabolism, CICR |
| contraction termination | acetylcholinesterase ACh breakdown | action potential repolarization | myosin light-chain phosphatase |
| twitch duration | 20-200 msec | 200-400 msec | 200 msec, sustained |
| regulation of force | frequency, multifiber summation | regulation of Ca2+ entry | MLCK phosphorylation, dephosphorylation balance |
| metabolism | oxidative, glycolytic | oxidative | oxidative |
- ACh: acetylcholine
- CICR: calcium-induced calcium release
- DHP: dihydropyridine
- IP3: inositol 1,4,5-triphosphate
- MLCK: myosin light-chain kinase
- SR: sarcoplasmic reticulum
Diseases of Skeletal Muscle
strategies for identifying genetic diseases
- classical approach
biochemical defect (function) à protein à gene à sequence à mutation
- positional cloning (reverse genetics)
map genetic defect à identify gene à sequence à structure? à function?
non-dystrophy muscle diseases
- metabolic diseases: impair the energy-producing (ATP) machinery of the muscle cell
- phosphorylase deficiency
- maltase deficiency
- carnitine acyltransferase deficiency
- myotonias and periodic paralyses: disorders of muscle excitation caused by ion channel defects
- myotonia congenita
- symptoms: transient, uncontrollable contraction of voluntary muscles (experienced as muscle stiffness)
- etiology: reduced sarcolemmal Cl- activity causing hyperexcitability
- paramyotonia congenita
- symptoms: identical to myotonia congenita, but exacerbated by cold and activity
- etiology: increased Na+ channel activity
- hyperkalemic periodic paralysis
- symptoms: periodic paralysis, with or without myotonia, exacerbated by high [K+]o
- etiology: decreased Na+ inactivation causing prolonged membrane depolarization
- hypokalemic periodic paralysis
- symptoms: episodic muscle weakness associated with low [K+]o
- etiology: mutations in the S4 voltage sensor of the dihydropyridine (DHP) receptor
- myotonic dystrophy
- symptoms: progressive, multisystemic muscle disease that becomes increasingly severe in transmission
most common form of muscle disease in adults (1/8000)
- etiology: expansion in the number of CTG repeats in the 3’ UTR of myotonin (protein kinase)
- mechanism: three hypotheses
- repeat expansion alters chromatin structure and transcription of nearby genes
- mutant DM transcript is trapped in the nucleus, resulting in decreased myotonin expression
- expanded DM transcript has a toxic gain of function that disrupts nuclear function (favored)
- E-C coupling myopathies
- malignant hyperthermia
- symptoms: uncontrolled prolonged contraction in response to halogenated anesthetics, depolarizing relaxants
- cause: mutations in the ryanodine (SR) Ca2+ channel, causing loss of intracellular Ca2+ regulation
- central core diseases
- symptoms: similar to malignant hyperthermia; demonstrates myofibrillar cores (histological abnormalities)
- cause: similar to malignant hyperthermia; cores derive from elevated intracellular [Ca2+]
muscular dystrophies
- introduction
- Duchenne muscular dystrophy (DMD): fatal, multisystemic disease with progressive myocyte necrosis
- mechanism: necrosis is associated with disruptions in sarcolemma, elevated [Ca2+]i
- distribution: 1/3,500 live-born males
- progression: most are wheelchair-bound by 12, dead by 20 secondary to cardiovascular failure
- Becker muscular dystrophy (BMD): similar, more mild neuroskeletal symptoms
- etiology: both DMD, BMD caused by defective gene (X-chromosome) that codes for dystrophin
- the DMD gene and dystrophin
- DMD gene
- history: one of the first disease-causing genes to be identified by positional cloning
- size: 2.5 million base pairs (largest gene in the human genome)
- clinical: absence (DMD), reduced abundance (DMD, BMD), or abnormal size (BMD) cause dystrophy
- dystrophin: gene product of the DMD gene
- size: Mr = 427,000
- structure: similar to structural proteins α-actinin and spectrin
- abundance: 0.002% of skeletal muscle (vs. 10% for actin, 25% for myosin)
- the dystrophin-glycoprotein complex
- costameres: subsarcolemmal protein assemblies that align with Z-lines of peripheral myofibers
- function: laterally transmit contractile forces from sarcomeres to CT, neighboring myocytes
- mechanism: physically attach force-generating sarcomeres to the sarcolemma
- structure: includes dystrophin
- dystrophin-glycoprotein complex: huge protein complex in skeletal muscle
- structure
- extracellular glycoprotein: α-dystroglycan
- transmembrane glycoproteins: β-dystroglycan, α-, β-, γ-, δ-sarcoglycan, sarcospan
- cytoplasmic proteins: synthrophin, dystrobrevin
- function: transmembrane linker between costameric cytoskeleton and the ECM, stabilizing the membrane
- all components are 90% reduced in abundance in dystrophin-deficient muscle
- numerous proteins coupling the sarcolemma and Z lines can, when defective, cause myopathy
therapeutic strategies for muscular dystrophies
- current
- no existing treatment can lengthen the lifespan
- no existing treatment can even significantly improve the quality of life of afflicted persons
- future
- stem cell therapy: cellular fusion in diseased tissues, allowing for rescue of defective cells
- concept: injection of totipotent, pluripotent stem cells carrying a normal gene copy
- problem: efficiency of delivery, immune rejection, infection
- gene therapy: deliver a normal copy of a defective gene to diseased tissues
- viral vectors
- concept: incorporation of dystrophin into modified viruses
- problem: limited DNA carrying capacity, transient expression, and immune reactions
- lipid carriers
- concept: cationic lipid complexation with DNA, facilitating translocation across the plasma membrane
- problem: low transfer efficiency, lack of tissue specificity
- direct transfer
- concept: direct uptake, expression of naked plasmid DNA
- problem: low transfer efficiency
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