Sliding filament theorySliding filament theory AF AF Huxley and R Niedergerke, Huxley and R Niedergerke,

19541954Tropomyosin and troponin regulate the Tropomyosin and troponin regulate the

interaction between actin and myosin interaction between actin and myosin proteins of thick and thin filamentsproteins of thick and thin filaments

During contraction, cross-bridges attach During contraction, cross-bridges attach between actin and myosinbetween actin and myosin

Two filaments slide over each other when Two filaments slide over each other when energy is provided by the hydrolysis of energy is provided by the hydrolysis of ATPATP

Neuromuscular Bases of Neuromuscular Bases of ContractionContraction

Skeletal muscle contracts only after Skeletal muscle contracts only after stimulation from a motor neuronstimulation from a motor neuron

Normally, each motor neuron branches Normally, each motor neuron branches several times and stimulates a few to several times and stimulates a few to several hundred muscle fibersseveral hundred muscle fibers

Motor UnitMotor Unit

Motor neuron, (cell, etc.) Motor neuron, (cell, etc.)

Muscle fibers it innervatesMuscle fibers it innervates

Site: neuromuscular junction, Site: neuromuscular junction, motor end plate, myoneural motor end plate, myoneural

junctionjunction

Contraction beginsContraction begins

Initiation of the action potential by the Initiation of the action potential by the motor neuron motor neuron

Transmission of the AP across the motor Transmission of the AP across the motor end plate to the muscle fiberend plate to the muscle fiber

when AP reaches when AP reaches neuromuscular junctionneuromuscular junction

~ 200-300 vesicles of acetycholine (ACH, ~ 200-300 vesicles of acetycholine (ACH, neurotransmitter) are released into the gap neurotransmitter) are released into the gap between the motor neuron and the motor between the motor neuron and the motor end plate (cleft)end plate (cleft)

ACH diffused across the gap ACH diffused across the gap and reacts with receptor and reacts with receptor

molecules in the sarcolemmamolecules in the sarcolemma

Reaction causes an increase in Reaction causes an increase in permeability to sodium ions, resulting in permeability to sodium ions, resulting in depolarization of the sarcolemma or end-depolarization of the sarcolemma or end-plate potential plate potential

If end-plate potential is large enough to If end-plate potential is large enough to exceed a threshold (depending on skeletal exceed a threshold (depending on skeletal muscle type), the nerve impulse will be muscle type), the nerve impulse will be successfully transformed into a muscle successfully transformed into a muscle impulseimpulse

The impulse travels in all directions over The impulse travels in all directions over the muscle membrane when being the muscle membrane when being transmittedtransmitted

Deep into the fiber through the transverse Deep into the fiber through the transverse tubules (t-tubules)tubules (t-tubules)

ACH is released from the vesicles ACH is released from the vesicles and diffuses across the cleftand diffuses across the cleft

binds with receptor, increasing the binds with receptor, increasing the permeability of the sarcolemma to sodium permeability of the sarcolemma to sodium ionsions

depolarizing the sarcolemma (end-plate depolarizing the sarcolemma (end-plate potential)potential)

Impulse travels down t-tubules, all over Impulse travels down t-tubules, all over membrane to transverse tubulesmembrane to transverse tubules

As the AP is transmitted throughout the As the AP is transmitted throughout the fiber, the membranes of the cisterane in fiber, the membranes of the cisterane in the SR become more permeable to Cathe SR become more permeable to Ca++++ and the stored ions diffuse into the and the stored ions diffuse into the sarcoplasm of the fiber sarcoplasm of the fiber

Once the CaOnce the Ca++++ concentration is high concentration is high enough, (100X increase, 10enough, (100X increase, 10-5-5 M), the Ca M), the Ca++++ binds with the TnC molecule binds with the TnC molecule

Binding of CaBinding of Ca++++ to the TnC causes a to the TnC causes a positional change of the Tn, which also positional change of the Tn, which also effects the positioning of the tropomyosin, effects the positioning of the tropomyosin, moving it deeper into the groove between moving it deeper into the groove between the two actin strandsthe two actin strands

TnCTnC

Two different isoformsTwo different isoforms

One specific to fast muscleOne specific to fast muscle

One specific to slow muscleOne specific to slow muscle

Fast contain two binding sites for CaFast contain two binding sites for Ca++++

Site I and site IISite I and site II

Slow have only one binding site Slow have only one binding site

both sites must be filled to trigger both sites must be filled to trigger contractioncontraction

Conformational ChangeConformational Changethere is a conformational change with binding there is a conformational change with binding that exposes a hydrophobic cavity (the TnI that exposes a hydrophobic cavity (the TnI binding site). binding site).

Alters the interaction between TnI and TnCAlters the interaction between TnI and TnC

Instead of TnI binding to actin, it preferentially Instead of TnI binding to actin, it preferentially switches to binding domain on TnC, allowing switches to binding domain on TnC, allowing actin and myosin to interact.actin and myosin to interact.

Slow and Cardiac MuscleSlow and Cardiac MuscleSlow skeletal muscle has no site ISlow skeletal muscle has no site ISlow and cardiac muscle are activated by Slow and cardiac muscle are activated by one, not two calcium ions by the TnC one, not two calcium ions by the TnC isoforms subunit.isoforms subunit.Therefore, contraction frequency, power Therefore, contraction frequency, power output, and strength are typically down output, and strength are typically down regulated regulated all of the characteristics of these subunits and all of the characteristics of these subunits and their role in contraction are not yet clear their role in contraction are not yet clear (e.g., Tn complex may attach the tropomyosin (e.g., Tn complex may attach the tropomyosin to the actin) do know that each subunit plays to the actin) do know that each subunit plays a role in the contractile processa role in the contractile process

Actin binding sites Actin binding sites

Uncovered, actin and myosin can interactUncovered, actin and myosin can interact

ATPase activity of myosin head ATPase activity of myosin head immediately hydrolyzes ATPimmediately hydrolyzes ATP

Conformation of the head is that it extends Conformation of the head is that it extends perpendicularly towards the actin filament perpendicularly towards the actin filament at this timeat this time

The products of the hydrolysis (ADP and Pi) The products of the hydrolysis (ADP and Pi) remain bound to the head, which is now remain bound to the head, which is now “energized” with the energy released from the “energized” with the energy released from the reactionreaction

Myosin head now interacts with the binding sites Myosin head now interacts with the binding sites on the actin filamenton the actin filament

When actin and myosin bind, forming an When actin and myosin bind, forming an actomyosin complex, the stored energy is actomyosin complex, the stored energy is releasedreleased

This release of energy alters the This release of energy alters the position of the myosin head and position of the myosin head and produces force through the cross-bridge produces force through the cross-bridge movementmovement

the head tilts toward the arm of the the head tilts toward the arm of the cross bridge, providing the power stroke cross bridge, providing the power stroke for pulling the actinfor pulling the actin

the energy activating the power stroke the energy activating the power stroke is the energy already stored, like a is the energy already stored, like a cocked spring, by the conformational cocked spring, by the conformational change in the head when the ATP change in the head when the ATP molecule cleavedmolecule cleaved

when the head attaches to the active when the head attaches to the active site, there are changes in the site, there are changes in the intramolecular forces between the head intramolecular forces between the head and the arm of the cross-bridgeand the arm of the cross-bridge

this alignment of forces causes the this alignment of forces causes the head to tilt toward the arm and to drag head to tilt toward the arm and to drag the actin filament along with itthe actin filament along with it

this tilt of the head is called the power this tilt of the head is called the power stroke, and causes the release of ADP stroke, and causes the release of ADP and Piand Pi

immediately after tilting, the head immediately after tilting, the head automatically breaks away from the automatically breaks away from the active site, binding an ATPactive site, binding an ATP

the head returns to the perpendicular the head returns to the perpendicular positionpositiononce head is detached, a new molecule of once head is detached, a new molecule of ATP is hydrolyzed by the myosin ATPase, ATP is hydrolyzed by the myosin ATPase, energizing the head again so cycle can energizing the head again so cycle can repeatrepeatIn this new position, it binds with a new In this new position, it binds with a new actin binding siteactin binding site

heads of cross-bridges bend back and heads of cross-bridges bend back and forth, step by step, walking along the forth, step by step, walking along the actin filaments toward the center of the actin filaments toward the center of the myosinmyosineach myosin acts independent of each each myosin acts independent of each other, each attaching and pulling in a other, each attaching and pulling in a continuous but random cyclecontinuous but random cyclethe greater the number of cross-bridges the greater the number of cross-bridges in contact with actin at any given time, in contact with actin at any given time, the greater the theoretical force of the greater the theoretical force of contractioncontraction

because of the orientation of the actin and because of the orientation of the actin and myosin, the actin filaments move towards each myosin, the actin filaments move towards each other, the Z lines move closer together and the other, the Z lines move closer together and the H zone disappearsH zone disappearsthis process will continue until the Z membrane this process will continue until the Z membrane is pulled against the myosin filament or until the is pulled against the myosin filament or until the load on the muscle becomes too great for further load on the muscle becomes too great for further pulling to occur (assuming muscle stimulation is pulling to occur (assuming muscle stimulation is still occurring)still occurring)

Single Contraction Cycle Single Contraction Cycle

Contraction cycle of myosin cross-bridges Contraction cycle of myosin cross-bridges of a muscle shortens a muscle by 1% of its of a muscle shortens a muscle by 1% of its resting lengthresting length

consequently, the contraction cycle must consequently, the contraction cycle must be repeated over and over to significantly be repeated over and over to significantly shorten the whole muscleshorten the whole muscle

Fenn EffectFenn Effect

When a new ATP attaches to a myosin When a new ATP attaches to a myosin head, the cross-bridge can detach from head, the cross-bridge can detach from the actin and the greater amount of work the actin and the greater amount of work performed by the muscle, the greater performed by the muscle, the greater amount of ATP which is cleavedamount of ATP which is cleaved

ACHACHat same time contraction is occurring, ACH that at same time contraction is occurring, ACH that stimulated the contraction is being rapidly stimulated the contraction is being rapidly decomposed by the action of cholinesterase decomposed by the action of cholinesterase (enzyme present at the myoneural junction (enzyme present at the myoneural junction within the membranes of the motor end plate)within the membranes of the motor end plate)

rapid removal of ACH insures that a single nerve rapid removal of ACH insures that a single nerve impulse will not cause a continued stimulation of impulse will not cause a continued stimulation of the muslcethe muslce

Impulse Duration Impulse Duration

Usual duration of an impulse to skeletal Usual duration of an impulse to skeletal muscle is about 20 millisecondsmuscle is about 20 milliseconds

in order for contraction to continue, there in order for contraction to continue, there must be continual stimulation of the must be continual stimulation of the muscle fibermuscle fiber

the signal to stop contraction is the the signal to stop contraction is the absence of a nerve impulse at the junctionabsence of a nerve impulse at the junction

AP stopsAP stops

continually active calcium pump located continually active calcium pump located in the walls of the SR pumps the in the walls of the SR pumps the calcium ions out of the sarcoplasm and calcium ions out of the sarcoplasm and back into the SR via the fenestrated back into the SR via the fenestrated collar, and then the calcium diffuses collar, and then the calcium diffuses back into the cisternaeback into the cisternae

this lowers the concentration of calcium, this lowers the concentration of calcium, removing it from the TnC, the removing it from the TnC, the Tn/tropomyosin complex returns to its Tn/tropomyosin complex returns to its original conformation and the active original conformation and the active sites are coveredsites are covered

fiber returns to its relaxed positionfiber returns to its relaxed position