Physiology of Muscle

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Smooth Muscle: Walls of hollow organs (vascular, GI, bladder). Sheets that respond as a functional syncytium. Compress cavities
Cardiac: Heart only. Branching fibers that respond as a functional syncytium. Compress heart chambers.
Skeletal (or striated): Limb movement, postural. Linear force to move tissue masses. True syncytium
True syncytium.: Fused embryonic myocytes form myotubes that develop into multinucleate fibers.
Muscle: Group of fibers that form contractile mass
Fiber: single muscle cell (syncytium)
Fibril: bundle of actin and myosin filaments that form an interacting unit (many of these in one fiber)
Filament -: individual units of contractile protein, i.e. actin filaments and myosin filaments (many of these in one fibril)
Each myosin attaches to ___ actins: 6
Action potentials reach the cell interior and SR via the _________: t-tubules
T-tubules contact the SR near the _____ where ___ easily accesses the _____________ on actin: z-disk / Ca2+ / troponin-tropomyosin complexes
1. Z to Z is 2. A is 3. I is 4. Blue is 5. Red is: 1. Sarcomere
2. Anisotropic Actin + Myosin
3. Isotropic Actin
4. Actin
5. Myosin
Contractile Mechanism: Myosin binds ATP and releases binding to actin
ATP hydrolysis extends S1 head
Myosin binds actin
Release of Pi allows return of S1 head to original position (â€œpullsâ€ actin)
Entry of new ATP displaces ADP and releases myosin-actin bond
Myosin is now free to extend again with ATP hydrolysis
Contractions are regulated by controlling the availability of ______: Ca2+
In the presence of Ca2+ myosin can: access actin and promote contraction
Ca2+ is sequestered in the SR by __________: active transport
Action potentials on the sarcolemma initiate ______: Ca2+ release from the SR through dihydropyridine-ryanodine receptor complexes
Re-uptake of Ca2+ by the SR _______: restores the Tp blockade of myosin binding sites
The filaments return to their original positions (extended sarcomeres) by: passive recoil from load-bearing
Tropomyosin (Tp) is attached to actin by _______ that ________: Troponin complexes (Tn-T, Tn-C, Tn-I)

position Tp to block the binding site for myosin.
_____ binding to Tn-C shifts the complex so that Tp no longer obstructs the myosin binding site on actin. Myosin is free to bind actin.: Ca2+
Myosin S1 heads have opposite orientations on each end of the filament. When myosin exerts force on the actin filaments,: opposing actin strands are pulled closer together, thereby shortening the sarcomeres, the fibril, and the fiber.
Skeletal muscle fibers require ___________ in order to elicit a contraction: neural stimulation
If neurons are lost to mechanical trauma or pathology the attendant muscles will: atrophy and cease to function
Each myofiber must be innervated T/F: T
One neuron can innervate only one fiber T/F: F -- multiple fibers
One neuron and all the fibers it innervates is termed a _______: â€œmotor unitâ€
All of the fibers in a motor unit are stimulated as a group T/F: T
Motor Unit =: neuron and the myofibers it innervates
Variable numbers of fibers/spinal motor neuron determine what: level of control
3-6 fibers/neuron =: fine control (eye)
120-165 fibers/neuron =: power (leg muscle)
Action potentials in the neuron are conveyed to the muscle via the ______ where action potentials are initiated in the _______: motor end-plate

sarcolemma
Difference in electrical potential across membrane due to ________: unequal ion distribution
Electrogenic ______ antiporter establishes concentration gradients: Na+/K+
_______ â€œLeak channelsâ€ allow diffusion of positive charges out of cell, leaves net negative charge inside of plasma membrane (-70mv): K+-specific
ion gradients and charge gradient ____ low inside; ____ high inside: Na+/ K+
Action Potential Threshold: Stimulus of sufficient amplitude opens voltage-gated Na+ channels
Action Potential: ____ influx along charge and concentration gradient causes depolarization and overshoot (Em=+50mv): Na+
Action Potential: Na+ channels inactivate ________: intrinsically (not voltage-dependent)
Action Potential: Voltage-gated ___ channels also open, but conductance is slower (1/10) than for Na+ (slower response to charge): K+
Action Potential: ____ efflux along charge and concentration gradient causes repolarization and hyperpolarization (Em=-80mv): K+
Action Potential: Resting Membrane Potential restoration: Voltage-gated K+ channels close -- Resting Membrane Potential restored as Na+ diffuses and is diluted in cytoplasm
All-or-None response: Once initiated, ion flux is self-propagating and undergoes a complete cycle
Electrical Activity and Contraction: 1. Change in membrane potential (action potential) moves along sarcolemma to _______: T-tubules
Electrical Activity and Contraction: 2. Ion flux: Na+ into cell causes membrane depolarization activating voltage-gated dihydropyridine receptors
Electrical Activity and Contraction: 3. ______ contact and activate (change conformation of ______ on SR allowing ____ to diffuse out to cytoplasm: Dihydropyridine receptors ryanodine receptors
Ca2+
Electrical Activity and Contraction: 4. _____ efflux repolarizes membrane which inactivates ______________ and allows ___uptake into SR by active transport.: K+
dihydropyridine receptor Ca2+
Impulse transmission between the T-tubule and SR occurs via: the Dihydropyridine and Ryanodine Receptors
_____________ precedes (and is complete before) the generation of tension.: Action potential
â€œLatent Periodâ€ between action potential and development of tension is when: the Ca2+ release mechanisms are being activated and Ca2+ is binding to Tp-Tn complex.
Refractory Period: Muscle cannot respond to a second stimulus
Absolute Refractory Period: A secondary stimulus of any strength is unable to elicit a contraction because the Na+ channels are inactivated (intrinsically) and unresponsive to charge
Relative Refractory Period â€“: Suprathreshold amplitude stimulus can induce a secondary contraction after some of the Na+ channels are reactivated and responsive to voltage. (persistance of K+ conductance attenuates the response)
Twitch: Ca2+ is released
initiates contraction
transported back into SR
Insufficient time for full force of contractile element to be transferred across the series elastic element
Low tension generated
Isotonic: Iso=equal tonic=force
Isometric: Iso=equal metric=length
Treppe: â€œStaircaseâ€ appearance of tension
Increasing tension on subsequent stimulations as stimulus frequency increases
Ca2+ accumulation in cytoplasm (insufficient time for active transport into SR)
Myosin binding site remain open longer
More tension generated with increasing Ca2+ availability
Rapid contractions transfer more tension across series elastic elements (sustained stretch)
Tetanus (Tetany): Maximal tension at highest stimulus frequency
Limited by refractory period
Ca2+ accumulation in cytoplasm (insufficient time for active transport into SR)
Myosin binding sites remain open
Full tension generated by all contractile elements
Rapid contractions transfer maximal tension across series elastic elements (sustained stretch)
Resting Length-Tension Curve: Tension generated is directly proportional to the number of Myosin S1 heads exerting force
The degree of overlap between myosin and actin determines how many heads will move over a distance to generate the force
More stretch provides more distance for heads to move
Limited by the number of heads in contact with actin to start the contraction
Force is determined by _______________: the distance traveled by S1 heads
Resting length of muscle determines _____________: actin-myosin overlap and the force that can be generated during contraction
Force-Velocity Curve:: Shorter sarcomeres have fewer S1 heads
Distance along actin to complete a full contraction is shorter
Time for full contraction is less (higher shortening velocity)
Total force generated is less
Fibers with short sarcomeres contract at higher velocity but lower tension
Fibers with long sarcomeres have more S1 heads â€œpullingâ€ simultaneously to generate greater tension, but the distance along actin is greater and takes longer for a full contraction
Fibers with long sarcomeres contract at lower velocity but generate greater tension
What determines the contractile characteristics of a muscle?: -Skeletal muscles are generally composed of mixed fiber types
-The proportion of each fiber type determines the contractile properties of that muscle
-Fiber type is determined by innervation
If the plasma membrane produces action potentials the contraction will be an _______ twitch: all-or-none
If the plasma membrane produces a graded depolarization the contraction will be ______: graded
The chemical nature of the myosin heads determines the rate of cross-bridge detachment from actin and, thus, _________: maximal contractile velocity
________ on the SR determines how long Ca2+ will remain available following stimulation: Density of Calcium pumps
____________ determine maximal rate of oxidative ATP synthesis and, therefore, fatigue resistance: Number of mitochondria and capillary density
Type I Muscle Fibers Force Production= Intramuscular ATP Stores= Intramuscular PC Stores= Contraction Speed= Endurance= Glycogen Stores= Aerobic Enzyme Activity= Capillary Density=: Force Production=low
Intramuscular ATP Stores=low
Intramuscular PC Stores=low
Contraction Speed=slow
Endurance=high
Glycogen Stores=no difference
Aerobic Enzyme Activity=high
Capillary Density=high
Type II Muscle Fibers Force Production= Intramuscular ATP Stores= Intramuscular PC Stores= Contraction Speed= Endurance= Glycogen Stores= Aerobic Enzyme Activity= Capillary Density=: Force Production=high
Intramuscular ATP Stores=high
Intramuscular PC Stores=high
Contraction Speed=fast
Endurance=low
Glycogen Stores=no difference
Aerobic Enzyme Activity=low
Capillary Density=low
Tonic fiber types: contract very slowly, do not produce action potentials, no twitch contractions (extraocular muscles, intrafusal fibers)
Phasic Type I fiber types: slow-twitch oxidative, APs, slow Ca2+ kinetics, many mitochondria, highly vascularized, myoglobin (red), fatigue-resistant (postural)
Phasic Type IIa fiber types: â€“ fast-twitch oxidative, APs, fast Ca2+ kinetics, many mitochondria, highly vascularized, myoglobin (red), fatigue-resistant (rapid repetitive movements)
Phasic Type IIb fiber types: fast-twitch glycolytic, APs, fast Ca2+ kinetics, few mitochondria, little myoglobin (white), fatigue easily (rapid non-repetitive movements)

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