Chapter 20 Muscle Physiology Flashcards
Skeletal Muscle Organization
- Skeletal muscle is arranged in a hierarchial fashion
- Muscle
- Fascial
- Muscle fiber (cell)
- Myofibril
- Sarcomeres (from Z disk – Z disk)
- Myofilaments (thick/myosin; thin/actin)
Sarcomere EM
- A band- Thick and Thin
- H zone- Thick (center of A)
- M line- middle of H
- I band- portion of the sarcomere between 2 A bands
- Z disc- sarcomere border
Sarcomere Filament Geometric Relationship
Thick and thin filaments have a regular geometric arrangement with a hexagonal pattern of thin filaments around a single thick filament.
Thin Filament Structure
The primary component of a thin filament is filamentous actin (F-actin); a protein made up of globular subunits (G-actin)
2 other proteins: tropomyosin (filamentous) and troponin (globular) are also associated with actin in the thin filaments
Thick filaments are made up of the protein myosin which has 6 subunits: 2 alpha-helical tails, and 2 heads associated with 2 myosin light chains
Sliding Filament Theory of Muscle Contraction
Muscle contraction occurs when thick filaments and thin filaments slide over each other, reducing the overall length of the sarcomere
T-Tubule and Sarcoplasmic Reticulum Relationship
Excitation-Contraction Coupling
- An action potential from a motor neuron causes release of the neurotransmitter acetylcholine which causes an excitatory postsynaptic potential that reaches threshold and is conducted as an action potential throughout the muscle fiber
- The time between action potential arrival and actual contraction of the muscle is called the latent period
Excitation-Contraction Coupling: The Role of Intracellular Ca++
The graph on the right shows the relationship between the action potential (red), Ca2+ levels (green) and contractile force (blue)
Ca2+ and Regulation of Muscle Contraction
Isolated muscle preparations require calcium to produce tension/force
Ca2+ and ATP and Muscle Contraction
Both Ca2+ and ATP are necessary for muscle contraction
For muscle relaxation ATP is required in the absence of Ca2+
Intracellular Ca2+ Level Control
Ca2+ levels within the sarcoplasm are tightly regulated by a “team” of proteins:
- Voltage-gated dihydropyridine receptor (on T-tubules)
- Ryanodine receptors ( on SR)
- Calsequestrin (SR)
- Ca2+ pumps (SR
Triad Receptors
Protein receptors in the T- tubule (dihydropyridine receptor on the left) interact with SR protein receptors (ryanodine receptor) resulting in the release of Ca2+ from within the SR to become available for cross-bridge formation
Steps Leading to Ca2+ Release from SR
- At rest, Ca2+ is sequestered within the SR lumen (green)
- An AP conducted down the T-tubule causes the ryanodine Ca2+ channel to open in the SR via the voltage-gated dihydropyridine receptor
- Ca2+ pumps in the SR resequester intracellular Ca2+
Ca2+, Troponin, and Tropomyosin in Muscle Contraction Regulation
Recall the two proteins troponin and tropomyosin which are associated with F-actin
Normally the actin’s myosin-binding site is blocked by tropomyosin
Ca2+ dislodges the troponin, which changes the tropomyosin configuration exposing the myosin-binding site for cross-bridging to occur
Ca2+ and Muscle Contraction Regulation
The Ratchet Effect
Contraction-Relaxation Cycle Summary
- Plasma membrane depolarization
- AP conduction into middle of cell via T-tubule
- Voltage-sensitive dihydropyridine receptor changes conformation resulting in the opening of ryanodine receptors in the SR
- Ca2+ released into myoplasm from SR
- Ca2+ binds to troponin complex causing a conformational change exposing myosin-binding sites on the actin molecule
- Myosin head forms a “cross-bridge” with actin via a conformational change called the “power stroke” which pulls the actin filament towards the center of the sarcomere
- ATP hydrolysis at the myosin head allows the myosin molecule to “recock”; several cross-bridges are then formed, broken and reformed
- Ca2+ pumps in the SR resequester Ca2+ preventing the formation of new cross-bridges
Muscle Contraction Mechanics
Muscle contractions without actual shortening are called isometric contractions
Muscle contractions with muscle shortening are called isotonic contractions
Contraction of Skeletal Muscle
- Motor unit
- Motor neuron entering muscle branches to many fibers
- 1 motor nerve/fiber
- many fibers/nerve
- Motor unit = motor neuron and all muscle fibers that it innervates
- Size of unit varies
- small - two muscle fibers/unit (larynx, eyes)
- large - hundreds, thousands/unit (biceps, gastrocnemius, lower back muscles)
- Fibers in motor unit spread throughout muscle for coverage
- Motor neuron entering muscle branches to many fibers
- Graded muscle responses
- Force of muscle contraction varies depending on need
- A twitch does not provide much force
- Contraction force can be altered 3 ways:
- Changing frequency of stimulation (i.e. tetanic contractions)
- Changing muscle length (length-tension relationships)
- Changing force of stimulation (increasing # of motor units contracting - recall that a single muscle fiber does not increase the size of its contraction with an increased stimulus (all-or-none principle)
Contractile States: Twitch/Tetanus
Under normal conditions, prolonged contraction can be achieved by increasing the frequency of APs arriving at the muscle fiber
Peak tension is developed during a tetanic contraction
The increased tension compared to a twitch is largely due to increased availability of intracellular Ca2+
Length-Tension Curve for Sarcomere Contraction
Maximal force (tension) is generated when there is an optimal amount of overlap between the thick and thin filaments.
Under normal conditions, skeletal muscle fibers are in the plateau portion of the above curve.
Length-Tension Relationship for the Muscle as a Whole
Like an individual sarcomere, the muscle as a whole also demonstrates an optimal length for producing maximal tension.
Sources of ATP for Muscle Contraction
The large amounts of ATP needed for myosin recocking and Ca2+ pumps can be produced anaerobically or aerobically
In addition to cellular ATP production, muscle fibers contain creatine phosphate which is a ready short-term source of phosphate groups for ATP formation
Muscle Fiber Types
- Different fiber types have evolved in vertebrates with different contractile and metabolic properties
- These adaptations allow for a large variety of activities from quick escapes, to prolonged low-intensity activities
- Tonic fibers: contract very slowly and do not product twitches; found in postural muscles of lower vertebrates and mammalian muscle spindles
- Slow-twitch (Type I) fibers: contract and fatigue slowly (red meat)
- Fast-twitch/oxidative (Type IIa) fibers: contract quickly and fatigue relatively slowly (not as slowly as Type I fibers)
- Fast-twitch/glycolytic (Type IIb) fibers: contract and fatigue rapidly (white meat)
Summary of Muscle Fiber Type Properties
Muscle Fiber Types
A particular muscle typically has a mix of fiber types as illustrated by this specially stained electron micrograph
Smooth Muscle
- Single-unit smooth muscles
- Gap junctions
- Walls of vertebrate visceral organs
- Myogenic – activation of a few fibers can generate contraction that moves in a wavelike manner
- Multi-unit smooth muscles
- No gap junctions
- Iris of eye, blood vessel muscles
- Neurogenic - Act independently and contract only when stimulated by neurons or hormones
- Smooth muscles contain actin and myosin, but they lack sarcomeric arrangement.
Regulation of Smooth Muscle Contraction
- Smooth muscles
- Lack troponin
- Have caldesmon
- binds to thin filaments
- prevents myosin:actin binding
- Caldesmon is removed by two different mechanisms:
- After calmodulin and calcium bind, they bind to caldesmon, freeing up the myosin binding sites on actin.
- Calcium-calmodulin works to phosphorylate myosin allowing it to interaction with actin
- Like skeletal muscle, all of these mechanisms act by allowing actin and myosin to form cross-bridges; intracellular Ca2+ levels are important for regulating contraction as with skeletal muscle.
Characteristics of Vertebrate Muscle Fiber Types
