28. Excitation Contraction Coupling in Skeletal Muscle

The force produced during an isometric contraction depends on the number of fibers activated.

All of the muscle fibers innervated by an alpha motoneuron contract each time the alpha motoneuron generates an action potential. The central nervous system varies the amount of force produced by a muscle by varying the number of alpha motoneurons that are active (recruitment). Often, the smaller fibers are recruited first, then larger fibers. *small fibers are fatigue resistant* During a sustained tension (less than maximum tension), units cycle through periods of activity and inactivity: allows motor units to rest in rotation.

Calcium pumps (SERCA)

An active transport system which utilizes ATP directly. Ca2+ pumps are located on the longitudinal portion of the SR membrane. They remove calcium from the sarcoplasm.

*Explain why the amount of shortening during an isotonic contraction varies with afterload* The length tension relationship limits the amount of shortening that can occur during an isotonic contraction

At the beginning of an isotonic contraction, the maximum force the muscle is capable of producing is greater than the afterload. As the muscle shortens, the maximum force it can produce varies. The muscle continues to shorten until the maximum force that the muscle can produce equals the afterload. The greater the afterload, the less the muscle can shorten (and the greater the length of the muscle when shortening stops).

*Explain why the force produced by repetitive stimulation of a muscle fiber is greater than the force produced by a single twitch*. The force produced during an isometric contraction depends on the frequency of action potentials generated by the muscle

Because the duration of a twitch is greater than the duration of an action potential, the contractions produced by successive action potentials can *summate*. Although the calcium released by a single action potential activates all of the contractile protein, the calcium is removed from the myoplasm before the contractile force reaches its full potential. If a second action potential is initiated before all of the calcium is removed from the myoplasm, cross bridge cycling continues for a longer period of time and the force of contraction is increased. The greater the frequency of action potentials, the greater the summation of contractile force.

Troponin T (TnT)

Binds the troponin complex to tropomyosin and actin.

Troponin C (TnC)

Contains binding sites for calcium.

*Describe the MECHANICAL steps in the crossbridge cycle and explain how the crossbridge cycle results in shortening of the muscle.* The sliding filament theory

Each cross bridge cycle consists of four steps. 1. Binding of the cross bridge with the actin molecules on the thin filament. 2. Binding of the cross bridge towards the center of the skeletal muscle fiber. 3. Detachment of the cross bridge from the actin molecule. 4. Return of the cross bridge to its original position.

One thing I would like you to remember: 2.0 m and 2.2 m.

Greatest contractile force! Optimal sarcomere length. The maximum number of cross bridges can bind to the thin filament.

Which regions of the sarcomere shrink during contraction?

H Band I Band

An isotonic contraction consists of three phases

It contracts isometrically until the force developed is equal to the afterload. It shortens isotonically at a constant velocity until it shortens as much as it is capable of shortening. It contracts isometrically until it begins to relax.

Troponin I (TnI)

Keeps the troponin complex tied to actin and inhibits the interaction between the myosin cross bridge and actin.

Tetanus

Occurs when the frequency of action potentials is high enough to prevent any relaxation of contraction between action potentials and the force.

Isotonic contraction

Prior to shortening, the muscle develops the force necessary to lift the afterload. The greater the afterload, the greater the number of myosin molecules that must be bound to actin. The greater the number of myosin cross bridges involved in the contraction, the slower the velocity of shortening.

Contractility related to calcium levels in skeletal versus cardiac muscle:

Skeletal muscle is *all or none* based on level on calcium Cardiac muscle is gradation (More calcium = stronger contraction)

In an environment with no sodium, what will happen to: Skeletal Muscle Cell? Cardiac Muscle Cell?

Skeletal muscle will still contract -- no calcium channel Cardiac muscle will no longer contract -- has a calcium channel

*Explain why the force developed by a skeletal muscle fiber varies with its length*. Isometric contraction

The force produced by a muscle depends on the number of myosin cross bridges that bind to actin during the contraction. The number of cross bridges that bind to actin is related to the overlap between the thick and thin filaments.

*Describe the process of excitation-contraction coupling.* Excitation-Contraction (EC) coupling in Skeletal Muscle

The four steps of EC coupling 1. Generation of the muscle action potential and depolarization of the T-tubule 2. The release of calcium from the sarcoplasmic reticulum 3. The binding of calcium to troponin and the initiation of the cross bridge cycle 4. The re-accumulation of calcium within the SR

afterload

The load lifted by the muscle when it shortens

*Describe the mechanical properties of isometric and isotonic contractions.* Isotonic

The muscle develops force and shortens. The load lifted by the muscle when it shortens is called the afterload. The velocity of shortening during an isotonic contraction is inversely proportional to the afterload. The amount of shortening that occurs during an isotonic contraction is inversely proportional to the afterload.

The force produced during an isometric contraction depends on:

The preload (i.e., the overlap between thick and thin filaments). The number of muscle fibers activated. The frequency of action potentials generated by the muscle.

The sliding filament theory

The sliding filament theory is used to explain the contraction of skeletal muscle fibers. When skeletal muscle contracts, the thin filaments slide over the thick filaments. The movement of the thin filaments over the thick filaments is produced by the cross bridge cycle.

*Describe the mechanical properties of isometric and isotonic contractions.* Isometric

The total length of the muscle does not change. The initial length of the muscle prior to contraction is called the preload.

*"I want you to remember about extracellular calcium, and calcium channels in skeletal muscles."*

There are NO calcium channels in skeletal muscle Therefore, extracellular calcium plays a very small role in skeletal muscle contraction.

- Latent Period

This is the period of time from the action potential to the onset of contraction. The time delay is due to the excitation-contraction coupling.

- Contraction Phase

This is the time that tension is developing due to the cross-bridge cycling.

- Relaxation Phase

This is the time that the tension is decreasing (i.e., relaxing) and is longer than the contraction phase. This is due to the amount of time it takes to get all the Ca2+ sequestered.

Troponin is composed of three proteins:

Troponin C (TnC) = calcium Troponin T (TnT) = tropomyosin and actin Troponin I (TnI) = inhibitory

Which to properties are inversely proportional to the afterload in an isotonic contraction?

Velocity and Amount of shortening

*Explain why the force developed by a skeletal muscle fiber varies with its length*. The relationship between the preload and the force of contraction is called the length-tension relationship

*Force generated is directly related to the number of cross-bridges created in the sacromere* The maximum number of cross bridges can bind to the thin filament when the initial sarcomere length is between 2.0 m and 2.2 m. *As the length increases* from 2.2 m to 3.5 m, the overlap between the thick and thin filaments decreases, the number of myosin cross bridges that bind to actin decreases and the force developed by the muscle decreases. *As the length decreases* from 2.0 m to 1.5 m, the thin filaments interfere with each other, decreasing the amount of force that can be produced. The amount of Ca2+ that binds to troponin decreases at short resting lengths.

dihydropyridine receptor

*Not a calcium channel* A special receptor that acts as a voltage sensor in the T-tubule. Depolarization of the cell will change the conformation of this receptor. The receptor will open. It can be inhibited by dihydropyridine

preload

*The load you put on a muscle to make it stretch*. The initial length of the muscle prior to contraction

*Explain why the force developed by a skeletal muscle fiber varies with its length*. Passive Length-Tension Relationship

*related to titin--prevents muscle from overstretching* Stretching the muscle requires relatively little force until the preload reaches 3.2 µm. The resistance to stretch depends on the elastic properties of titin and other proteins in parallel with the thick and thin filaments. The increased stiffness at high preloads prevents the muscle from being over stretched.

The Phases of a Twitch Contraction

- Latent Period - Contraction Phase - Relaxation Phase

EC Coupling: Calcium Re-uptake and the end of the Cross Bridge Cycling

1. Ca2+ is pumped from the sarcoplasm into the SR against a concentration gradient. SERCA is an active transport system which utilizes ATP directly - Ca2+ diffuses to the terminal cisternae where it binds to calsequestrin. 2. The removal of calcium from the myoplasm causes calcium to dissociate from TnC. - TnI separates from TnC and rebinds to tropomyosin. - Tropomyosin returns to its position covering the myosin binding sites on actin. 3. Cross bridge cycling stops.

*Describe the CHEMICAL steps in the crossbridge cycle and explain how the crossbridge cycle results in shortening of the muscle.* EC coupling: Binding of Calcium to Troponin and Initiation of the Cross Bridge cycle.

1. Calcium initiates the cross bridge cycle when it binds to troponin C. 2. The binding of calcium to troponin C causes troponin I to separate from tropomyosin and actin 3. The movement of TnI allows tropomyosin to move away from the myosin binding sites on actin. 4. Myosin attaches to the exposed binding sites on actin and the cross bridge cycle begins.

EC coupling: Calcium release of from the sarcoplasmic reticulum

1. Depolarization of the T-tubule membrane causes Ca2+ channels on the SR to open. 2. The T-tubule contains a *dihydropyridine receptor*, which acts as a voltage sensor. 3. The SR membrane contains a calcium channel, called the ryanodine receptor. 4. The DHP receptor is coupled to the ryanodine receptor by a protein which blocks the Ca2+ release channel when the membrane is polarized. 5. Depolarization of the T-tubule causes the charged protein to move towards the DHP receptor, unblocking the Ca2+ release channel within the ryanodine receptor.

*Describe the load-velocity relationship of an isotonic contraction and how it varies with preload*. The velocity of shortening during an isotonic contraction is inversely proportional to the afterload.

1. Increasing the after load --Increases the latency between activation of the muscle and shortening. --Increases the force produced by the muscle during shortening. --Decreases the velocity of shortening. --Decreases the amount of shortening. 2. The velocity of shortening at any given afterload depends on the preload. 3. The maximum velocity of shortening occurs at zero afterload Vmax 4. Vmax is not affected by changes in initial muscle length or preload. 5. Vmax is determined by molecular properties of the contractile proteins (eg. The ATPase activity of myosin).

Calsequestrin

A binding storage protein in the terminal cisternae of the SR

Ryanodine receptor.

A calcium channel in the SR membrane that is coupled to the DHP receptor. It is blocked by a protein when the cell is polarized, and unblocked when the cell in unpolarized -- releasing calcium.

Tropomyosin

A long molecule covering seven actin molecules. In the resting state, tropomyosin covers the myosin binding sites on actin.

twitch contraction

The contractile force produced by a single action potential.

The increased force produced by summation and tetanus results from stretching out a series elastic element

The contractile proteins are connected to the bones by elastic structures called the series elastic component (SEC) The SEC stretches during a contraction. The greater the stretch of the SEC, the greater the force transferred to the bones. Stimulating the muscle fiber before the muscle relaxes (when the SEC is partially stretched) produces a greater stretch of the SEC and a greater force of contraction.

*Describe the CHEMICAL and MECHANICAL steps in the crossbridge cycle and explain how the crossbridge cycle results in shortening of the muscle.* ‪The Sliding Filament Theory of Muscle Contraction‬‬

The energy for each cross bridge cycle is derived from the hydrolysis of one ATP molecule. 1. ATPase is activated when the myosin molecule binds to actin. 2. The ATP is bound to myosin as a high-energy complex referred to as myosin ADPPi. 3. When the cross bridge bends, ADP and Pi are released from myosin. 4. The thin filament detaches from myosin when a new molecule of ATP binds to the myosin molecule.

Actin-myosin ATPase

The enzyme catalyzing the hydrolysis of ATP contained within the cross bridge that is activated when the myosin molecule binds to actin.