Skeletal Muscle Physiology

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Muscle cells convert chemical energy from X into mechanical energy (i.e. contraction) and heat.
Muscle accounts for roughly X% of the body weight of a typical individual.

Muscle cells convert chemical energy from ATP into mechanical energy (i.e. contraction) and heat.
Muscle accounts for roughly 50% of the body weight of a typical individual.

Unlike skeletal muscle fibers, individual cardiac muscle cells are electrically X. In addition, some cardiac muscle cells generate spontaneous rhythmic X (pacemaker activity).

Unlike skeletal muscle fibers, individual cardiac muscle cells are electrically coupled. In addition, some cardiac muscle cells generate spontaneous rhythmic action potentials (pacemaker activity).

T/F - In many cases, smooth muscle cells display spontaneous activity.

true

Molecular Basis of Skeletal Muscle Contraction (sliding filament mechanism):
Once x has been released from the sarcoplasmic reticulum (SR), it acts as an intracellular 'messenger' that turns on the contractile machinery of the sarcomeres.

Molecular Basis of Skeletal Muscle Contraction (sliding filament mechanism):
Once Ca2+ has been released from the sarcoplasmic reticulum (SR), it acts as an intracellular 'messenger' that turns on the contractile machinery of the sarcomeres.

The globular heads of the myosin molecules (with the binding sites) are called X. Individual thick filaments are about X microns long. Recall that the thick filaments correspond to the X band of the sarcomere.

The globular heads of the myosin molecules (with the binding sites) are called cross-bridges. Individual thick filaments are about 1.6 microns long. Recall that the thick filaments correspond to the A band of the sarcomere.

Each actin molecule has a binding site for the attachment of a X cross-bridge.

Each actin molecule has a binding site for the attachment of a myosin cross-bridge.

In a relaxed muscle fiber the actin x are not available for binding with myosin. This is because of the position of x and x. In the absence of Ca2+, x covers the binding sites of actin which otherwise could attach to the myosin cross-bridges

in a relaxed muscle fiber the actin binding sites are not available for binding with myosin. This is because of the position of tropomyosin and troponin. In the absence of Ca2+, tropomyosin covers the binding sites of actin which otherwise could attach to the myosin cross-bridges

In the absence of x, tropomyosin covers the binding sites of actin which otherwise could attach to the myosin cross-bridges
x is responsible for
stabilizing tropomyosin in
this blocking position. Each
troponin molecule consists of 3 globular subunits. One of these subunits binds to x, the second subunit attaches to x, and the third subunit has a binding site for x.

In the absence of Ca2+, tropomyosin covers the binding sites of actin which otherwise could attach to the myosin cross-bridgesTroponin is responsible for
stabilizing tropomyosin in
this blocking position. Each
troponin molecule consists of 3 globular subunits. One of these subunits binds to tropomyosin, the second subunit attaches to actin, and the third subunit has a binding site for Ca2+.

***Sliding Filament mechanism of muscle contraction
1. When Ca2+ is released from the SR, it binds to x molecules of the thin filaments and causes a conformational change, which moves the troponin-tropomyosin complex away from its blocking position. This exposes the actin binding sites and allows the
x crossbridges of the thick
filaments to bind to actin of the
thin filaments.

2. After a myosin cross-
bridge binds to actin, it undergoes
what is called a x. This
is a bending inward of the
crossbridge, which serves to pull
the thin filament x (i.e.,
toward the center of the
sarcomere).

3. The cross-bridge then detaches from the x filament and the myosin cross-bridge returns to its original conformation.
4. Then the x cross-bridge attaches to another actin molecule; however, because of the movement of the thin filament relative to the thick filament, this is a different (more distal) actin molecule.
5. This process repeats itself throughout the contraction. Of course, many cross-bridges and actin molecules are involved. Since the thin filaments are anchored to the Z lines, this action draws the Z lines closer together and shortens the sarcomere. Since the entire muscle fiber is excited in a very short time relative to the duration of a contraction, the net effect is that the muscle shortens.

1. When Ca2+ is released from the SR, it binds to troponin molecules of the thin filaments and causes a conformational change, which moves the troponin-tropomyosin complex away from its blocking position. This exposes the actin binding sites and allows the
myosin crossbridges of the thick
filaments to bind to actin of the
thin filaments.
2. After a myosin cross-
bridge binds to actin, it undergoes
what is called a powerstroke. This
is a bending inward of the
crossbridge, which serves to pull
the thin filament inward (i.e.,
toward the center of the
sarcomere).
3. The cross-bridge then detaches from the thin filament and the myosin cross-bridge returns to its original conformation.
4. Then the myosin cross-bridge attaches to another actin molecule; however, because of the movement of the thin filament relative to the thick filament, this is a different (more distal) actin molecule.
5. This process repeats itself throughout the contraction. Of course, many cross-bridges and actin molecules are involved. Since the thin filaments are anchored to the Z lines, this action draws the Z lines closer together and shortens the sarcomere. Since the entire muscle fiber is excited in a very short time relative to the duration of a contraction, the net effect is that the muscle shortens.

Remember that the myosin cross-bridge has two binding sites, i.e., an x site and an x binding site. Once ATP (actually x complex) is bound to the myosin crossbridge ATPase site, ATP will be split on the cross-bridge before the cross-bridge attaches to actin. When ATP is split by the myosin ATPase, the energy generated is stored in the myosin cross-bridge; ADP and Pi (the breakdown products of ATP) remain attached to the cross-bridge while it is in this high-energy state. In this state the myosin crossbridge can be thought of as "cocked", ready to perform work when it is able to bind to actin of the thin filament.

Remember that the myosin cross-bridge has two binding sites, i.e., an ATPase site and an actin binding site. Once ATP (actually ATP-Mg2+ complex) is bound to the myosin crossbridge ATPase site, ATP will be split on the cross-bridge before the cross-bridge attaches to actin. When ATP is split by the myosin ATPase, the energy generated is stored in the myosin cross-bridge; ADP and Pi (the breakdown products of ATP) remain attached to the cross-bridge while it is in this high-energy state. In this state the myosin crossbridge can be thought of as "cocked", ready to perform work when it is able to bind to actin of the thin filament.

If Ca2+ is not present in the myoplasm at a sufficient concentration the myosin will remain in this state because binding to actin cannot occur.

This is the resting or relaxed state of the muscle fiber. High-energy cocked state, not yet bound to actin, waiting for Ca2+ to come in and tell troponin to move tropomyosin out.

When Ca2+ is released from the SR, it will bind to x, resulting in the exposure of the actin binding site as already described. At this point the myosin crossbridge will bind to actin.

When myosin binds to actin the powerstroke (i.e., the inward bending of the cross- bridge) occurs. At the same time, x are released from the cross- bridge.
The myosin crossbridge remains attached to actin until x binds to the crossbridge. When this occurs, the crossbridge detaches from the actin of the thin filament.

The ATP is then split, producing the high-energy state of the x. This is where we started. The cycle then repeats itself as long as Ca2+ and ATP are available.

When Ca2+ is released from the SR, it will bind to troponin, resulting in the exposure of the actin binding site as already described. At this point the myosin crossbridge will bind to actin.
When myosin binds to actin the powerstroke (i.e., the inward bending of the cross- bridge) occurs. At the same time, ADP and Pi are released from the cross- bridge.
The myosin crossbridge remains attached to actin until a new molecule of ATP binds to the crossbridge. When this occurs, the crossbridge detaches from the actin of the thin filament.
The ATP is then split, producing the high-energy state of the myosin crossbridge. This is where we started. The cycle then repeats itself as long as Ca2+ and ATP are available.

***Explain the phenomenon of rigor mortis and how it involves muscle contraction mechanisms? What happens to ATP and Ca2+

Note that if fresh ATP is not available (as occurs after death), then following the (last) powerstroke the cross-bridge will not detach from actin. This is referred to as
the rigor complex, and is demonstrated by rigor mortis following death, when ATP
supplies have been exhausted, but intracellular Ca2+ remains high.

What is the rigor complex?

Note that if fresh ATP is not available (as occurs after death), then following the (last) powerstroke the cross-bridge will not detach from actin. This is referred to as
the rigor complex, and is demonstrated by rigor mortis following death, when ATP
supplies have been exhausted, but intracellular Ca2+ remains high.

Relaxation
 After an action potential has caused the Ca2+ release channels of the SR to open, they will (provided a second action potential does not arrive) close within a period of about x ms or so.

Relaxation
 After an action potential has caused the Ca2+ release channels of the SR to open, they will (provided a second action potential does not arrive) close within a period of about 10 ms or so.

The SR membrane contains a high density of x pump molecules, which use x to actively transport Ca2+ from the x into the SR. The pump is stimulated by
the increased concentration of Ca2+ in the cytoplasm when Ca2+ is released. Once the SR Ca2+ release channel is closed, cytoplasmic Ca2+ concentration falls to its low resting value
(< 0.1 μM) within a short period of time, typically about x-x ms.

The SR membrane contains a high density of Ca2+ATPase pump molecules, which use
ATP to actively transport Ca2+ from the cytoplasm into the SR. The pump is stimulated by
the increased concentration of Ca2+ in the cytoplasm when Ca2+ is released. Once the SR
2+ 2+
Ca release channel is closed, cytoplasmic Ca concentration falls to its low resting value
(< 0.1 μM) within a short period of time, typically about 50-100 ms.

In addition to the SR Ca2+ pump, this protein is also involved in relaxation, particularly in fast 2+
("white") fiber types. x rapidly binds free Ca in the myoplasm and thus contributes to relaxation. Over a short period of time this (temporarily) bound Ca2+ is once again pumped back into the SR. x is considerably more abundant in fast (white) fiber types than in slow (red) fibers and contributes to their more rapid relaxation.

In addition to the SR Ca2+ pump, parvalbumin is also involved in relaxation, particularly in fast 2+
Skeletal Muscle Physiology (Ramos-Franco)
("white") fiber types. Parvalbumin rapidly binds free Ca in the myoplasm and thus contributes to relaxation. Over a short period of time this (temporarily) bound Ca2+ is once again pumped back into the SR. Parvalbumin is considerably more abundant in fast (white) fiber types than in slow (red) fibers and contributes to their more rapid relaxation. Skeletal muscle fiber types will be discussed later.

What protein is considerably more abundant in fast (white) fiber types than in slow (Red) fiber types, and contributes to their more rapid relaxation

parvalbumin

What is the function of parvalbumin?

In addition to the SR Ca2+ pump, parvalbumin is also involved in relaxation, particularly in fast 2+
Skeletal Muscle Physiology (Ramos-Franco)
("white") fiber types. Parvalbumin rapidly binds free Ca in the myoplasm and thus contributes to relaxation. Over a short period of time this (temporarily) bound Ca2+ is once again pumped back into the SR. Parvalbumin is considerably more abundant in fast (white) fiber types than in slow (red) fibers and contributes to their more rapid relaxation.

As Ca2+ concentration falls in the space surrounding the contractile proteins the tension developed by the muscle fiber x (with a small delay).

As Ca2+ concentration falls in the space surrounding the contractile proteins the tension developed by the muscle fiber declines (with a small delay).

In total, the duration of tension in skeletal muscle resulting from a single action potential is typically about x-x ms. The duration depends on the type of muscle fiber. Note that this is a long time in comparison to the duration of the action potential itself, which is only about x ms.

In total, the duration of tension in skeletal muscle resulting from a single action potential is typically about 50-100 ms. The duration depends on the type of muscle fiber. Note that this is a long time in comparison to the duration of the action potential itself, which is only about 1-2 ms.

Acetylcholine (ACh) is the only transmitter used at neuromuscular junctions. It is synthesized from x and x, and is stored in vesicles in the axon terminal.

Acetylcholine (ACh) is the only transmitter used at neuromuscular junction junctions. It is synthesized from acetyl CoA and choline, and is stored in vesicles in the axon terminal.

Acetylcholine Receptors. On the motor end-plate are located several million ACh (x x) receptors. These receptors are x-gated, cation-selective (x,x,x) channels, which open upon binding of ACh. The amount of ACh that is normally released during an action potential will open approximately x ion channels in the motor end-plate.

Acetylcholine Receptors. On the motor end-plate are located several million ACh (nicotinic cholinergic) receptors. These receptors are ligand-gated, cation-selective (Na+, K+, Ca2+) channels, which open upon binding of ACh. The amount of ACh that is normally released during an action potential will open approximately 400,000 ion channels in the motor end-plate.

Neurotransmitter Degradation/Removal. Activation of ACh receptors terminates by ACh degradation and removal. Acetylcholinesterase in the post-synaptic membrane degrades ACh. x is taken back into the presynaptic motor nerve terminal for re-synthesis of ACh. Some other ACh molecules diffuse away from the cleft.

Neurotransmitter Degradation/Removal. Activation of ACh receptors terminates by ACh degradation and removal. Acetylcholinesterase in the post-synaptic membrane degrades ACh. Choline is taken back into the presynaptic motor nerve terminal for re-synthesis of ACh. Some other ACh molecules diffuse away from the cleft.

Skeletal muscle action potential
 Surface membrane. An x is a special type of excitatory
Post-Synaptic Potential that only occurs at motor end-plates and is analogous to an EPSP (excitatory post-synaptic potential) at a neuron-neuron synapse. An EPP produces local x current flow at the motor end plate, and its magnitude depends on the x and x of ACh at the end plate. This initiates an action potential in the muscle membrane that is then propagated rapidly over the surface of the muscle fiber (sarcolemma). Similar to action potentials in neurons, Na+ current through voltage-gated Na+ channels generates the upstroke of the action potential in the muscle membrane, while K+ current generates repolarization of the muscle membrane.

Skeletal muscle action potential
 Surface membrane. An endplate potential (EPP) is a special type of excitatory
Post-Synaptic Potential that only occurs at motor end-plates and is analogous to an EPSP (excitatory post-synaptic potential) at a neuron-neuron synapse. An EPP produces local inward current flow at the motor end plate, and its magnitude depends on the amount and duration of ACh at the end plate. This initiates an action potential in the muscle membrane that is then propagated rapidly over the surface of the muscle fiber (sarcolemma). Similar to action potentials in neurons, Na+ current through voltage-gated Na+ channels generates the upstroke of the action potential in the muscle membrane, while K+ current generates repolarization of the muscle membrane.

The transverse tubular system (T system)

1. At the junction between the x band and the x band (i.e., twice for each sarcomere) the surface membrane or x invaginates into the cell to form a mesh of tubules which run perpendicular to the long axis of the fiber. These planar meshes of tubules run between myofibrils and
penetrate to the center of the fiber. This is the transverse tubular system (often simply called the T system). See FIGURE 9. The T system membrane is continuous with the x.
Because the T system is continuous with the surface membrane, an action potential propagating along the surface of the fiber will also x, and thus to the center of the fiber.

1. At the junction between the A band and the I band (i.e., twice for each sarcomere) the surface membrane or sarcolemma invaginates into the cell to form a mesh of tubules which run perpendicular to the long axis of the fiber. These planar meshes of tubules run between myofibrils and
penetrate to the center of the fiber. This is the transverse tubular system (often simply called the T system). See FIGURE 9. The T system membrane is continuous with the sarcolemma.
Because the T system is continuous with the surface membrane, an action potential propagating along the surface of the fiber will also propagate into the T tubular membrane, and thus to the center of the fiber.

The process by which electrical excitation of the muscle fiber leads to contraction of the muscle fiber is known as x.

The process by which electrical excitation of the muscle fiber leads to contraction of the muscle fiber is known as excitation-contraction (EC) coupling.

Sarcoplasmic Reticulum
The sarcoplasmic
reticulum (SR) is a
modified endoplasmic
reticulum that is greatly
elaborated in skeletal
muscle. The SR forms an
interconnected network
of tubules that surround
all of the x (FIGURES 10 & 11).

Near the plane of each network of T tubules the SR enlarges into structures called terminal cisternae or x. These x come in very close proximity to the T tubules. Each T tubule is associated with # adjacent lateral sacs (triad).
The interior of the SR is separated from the muscle cytoplasm by the SR membrane. The SR is also separated from the T system by a small gap. The SR membrane contains a very high density of x pumps that use ATP to actively pump x into the interior of the SR. In a resting muscle, the cytoplasmic Ca2+ concentration is less than 0.1 μM because almost all Ca2+ is stored within the x.

Sarcoplasmic Reticulum
The sarcoplasmic
reticulum (SR) is a
modified endoplasmic
reticulum that is greatly
elaborated in skeletal
muscle. The SR forms an
interconnected network
of tubules that surround
all of the myofibrils (FIGURES 10 & 11). Near the plane of each network of T tubules the SR enlarges into structures called terminal cisternae or lateral sacs. These lateral sacs come in very close proximity to the T tubules. Each T tubule is associated with two adjacent lateral sacs.
The interior of the SR is separated from the muscle cytoplasm by the SR membrane. The SR is also separated from the T system by a small gap. The SR membrane contains a very high density of Ca2+ pumps that use ATP to actively pump Ca2+ into the interior of the SR. In a resting muscle, the cytoplasmic Ca2+ concentration is less than 0.1 μM because almost all Ca2+ is stored within the SR.

T-SR junction. When an action potential occurs in the T tubule membrane it triggers x release from the SR. This Ca2+ is necessary for muscle contraction as will be described below. Although the precise molecular mechanism(s) by which an action potential in the T system causes the SR of skeletal muscle to release Ca2+ is not yet completely understood, many details of this mechanism have been described.

T-SR junction. When an action potential occurs in the T tubule membrane it triggers Ca2+ release from the SR. This Ca2+ is necessary for muscle contraction as will be described below. Although the precise molecular mechanism(s) by which an action potential in the T system causes the SR of skeletal muscle to release Ca2+ is not yet completely understood, many details of this mechanism have been described.

The region in which the T tubule membrane comes into close proximity with the lateral sacs of the SR is called the x.

In this region the T tubule membrane contains numerous protein molecules called x receptors. The molecules are related to voltage-sensitive Ca2+ channels, but (although they do pass a tiny amount of Ca2+ current) their main role is not the transmembrane movement of ions.

Instead, they constitute the x sensors of EC coupling.
The membrane of the lateral sacs of the SR contains protein molecules called x, which align with the DHP receptors of the adjacent T membrane. In fact, these x are the Ca2+ release channels of the SR membrane

The region in which the T tubule membrane comes into close proximity with the lateral sacs of the SR is called the T-SR junction.

In this region the T tubule membrane contains numerous protein molecules called dihydropyridine (DHP) receptors. The molecules are related to voltage-sensitive Ca2+ channels, but (although they do pass a tiny amount of Ca2+ current) their main role is not the transmembrane movement of ions.

Instead, they constitute the voltage sensors of EC coupling.
The membrane of the lateral sacs of the SR contains protein molecules called ryanodine receptors (RyR), which align with the DHP receptors of the adjacent T membrane. In fact, these RyR are the Ca2+ release channels of the SR membrane

T/F - A major role of the T-tubule membrane dihydropyridine (DHP) receptors is the transmembrane movement of ions.

False - The region in which the T tubule membrane comes into close proximity with the lateral sacs of the SR is called the T-SR junction.

In this region the T tubule membrane contains numerous protein molecules called dihydropyridine (DHP) receptors. The molecules are related to voltage-sensitive Ca2+ channels, but (although they do pass a tiny amount of Ca2+ current) their main role is not the transmembrane movement of ions.
Instead, they constitute the voltage sensors of EC coupling.
The membrane of the lateral sacs of the SR contains protein molecules called ryanodine receptors (RyR), which align with the DHP receptors of the adjacent T membrane. In fact, these RyR are the Ca2+ release channels of the SR membrane

The Ca2+ release channels of the SR membrane

ryanodine receptors

The membrane of the x of the SR contains protein molecules called ryanodine receptors (RyR), which x with the DHP receptors of the adjacent T membrane. In fact, these RyR are the x release channels of the SR membrane (FIGURE 11)

The membrane of the lateral sacs of the SR contains protein molecules called ryanodine receptors (RyR), which align with the DHP receptors of the adjacent T membrane. In fact, these RyR are the Ca2+ release channels of the SR membrane (FIGURE 11)

Precisely how the DHP receptors of the T membrane cause the RyRs of the SR membrane to open (and thereby release Ca2+ from the SR into the cytoplasm) is not completely understood. However, there is considerable evidence that the mechanism may be X. If this is correct, then depolarization of the T tubule membrane by an action potential would cause this to happen.

Precisely how the DHP receptors of the T membrane cause the RyRs of the SR membrane to open (and thereby release Ca2+ from the SR into the cytoplasm) is not completely understood. However, there is considerable evidence that the mechanism may be mechanical coupling. If this is correct, then depolarization of the T tubule membrane by an action potential would cause a conformational change in the DHP receptor (voltage sensor), which in turn causes the opening of the Ca2+ release channels (ryanodine receptors), allowing Ca2+ stored in the SR compartment to be released into the cytoplasm. FIGURES 11 & 12 are cartoons of this process.

Note that the structural arrangement of the T system, SR, and myofibrils assures that Ca2+ does not have to diffuse more than about x to reach any point in the contractile machinery of the muscle fiber.

Note that the structural arrangement of the T system, SR, and myofibrils assures that Ca2+ does not have to diffuse more than about 1 micron to reach any point in the contractile machinery of the muscle fiber.

*****Key features of skeletal muscle excitation and contraction - from endplate potential to Ca2+ removal from cytoplasm by SR Ca2+ pump.

11 steps

1. Endplate potential
2. Surface membrane action potential
3. T system action potential
4. Ca2+ release from the SR (opening of the SR Ca2+ release channel)
5. Ca2+ binds to troponin
6. Troponin-tropomyosin unblock actin binding site
7. Energized myosin crossbridges bind to actin (ATP has already been split, ADP + Pi still attached)
8. Powerstroke (bending of crossbridge, ADP + Pi released)
9. Detachment of crossbridge (requires binding of ATP)
10. Energizing of crossbridge (ATP split)
11. Relaxation -> removal of Ca2+ from cytoplasm by SR Ca2+ pump (Cytoplasmic Ca2+ falls after closing of SR Ca2+ release channel)

Dystrophin and the dystrophin-glycoprotein complex (DGC).

1. Dystrophin is a member of a x complex, i.e., the dystrophin-glycoprotein complex. The importance of these proteins to heart muscle will be described in the Heart lectures. However, these same proteins (or isoforms of them) are equally important in skeletal muscle. Dystrophin binds to the x cytoskeleton (non-sarcomeric actin) at one end and to elements of the dystrophin-gylcoprotein complex embedded in the x at the other end. The basic molecular arrangement of the dystrophin-glycoprotein complex is shown in FIGURE 13. Dystrophin and the DGC form an important linkage between the x cytoskeleton, the membrane itself and the x matrix. A vital function of dystrophin and the DGC appears to be the x of the periphery of the muscle cell during contraction.
Disruptions of the dystrophin-glycoprotein complex apparently render muscle fibers more susceptible to x and thus lead to muscle wasting. It is now well established that primary defects in dystrophin and the dystrophin-glycoprotein complex are responsible for several forms of muscular dystrophy and cardiomyopathy (see Clinical Correlations below).

The cytoskeleton of muscle cells consists of a variety of different proteins forming structures whose primary function is to link or anchor structural components within the cell; other proteins serve to attach this internal cytoskeleton to the extracellular matrix and to stabilize the membrane. The cytoskeleton in effect forms a scaffolding surrounding and organizing the contractile proteins into their precise myofibrillar geometry. There are both transverse and longitudinal cytoskeletal elements involved in the arrangement of muscle fibers and both extramyofibrillar and intramyofibrillar domains of the cytoskeleton have been identified. Other roles of the cytoskeleton include the anchoring of membrane proteins in specific locations - for example the ACh receptors at the neuromuscular junction.
Dystrophin and the dystrophin-glycoprotein complex (DGC). Dystrophin is a member of a multi-protein complex, i.e., the dystrophin-glycoprotein complex. The importance of these proteins to heart muscle will be described in the Heart lectures. However, these same proteins (or isoforms of them) are equally important in skeletal muscle. Dystrophin binds to the subsarcolemmal actin cytoskeleton (non-sarcomeric actin) at one end and to elements of the dystrophin-gylcoprotein complex embedded in the surface membrane at the other end. The basic molecular arrangement of the dystrophin-glycoprotein complex is shown in FIGURE 13. Dystrophin and the DGC form an important linkage between the actin membrane cytoskeleton, the membrane itself and the extracellular matrix. A vital function of dystrophin and the DGC appears to be the stabilization of the periphery of the muscle cell during contraction.
Disruptions of the dystrophin-glycoprotein complex apparently render muscle fibers more susceptible to necrosis and thus lead to muscle wasting. It is now well established that primary defects in dystrophin and the dystrophin-glycoprotein complex are responsible for several forms of muscular dystrophy and cardiomyopathy (see Clinical Correlations below).

The vital function of dystrophin and the dystrophin-glycoprotein complex

A vital function of dystrophin and the DGC appears to be the stabilization of the periphery of the muscle cell during contraction.

The force exerted on an object by a contracting muscle is known as x, while the force on the muscle exerted by the weight of an object is the x. Whether or not force generation leads to fiber shortening depends upon the relative x of the tension and load.

The force exerted on an object by a contracting muscle is known as muscle tension, while the force on the muscle exerted by the weight of an object is the load. Whether or not force generation leads to fiber shortening depends upon the relative magnitudes of the tension and load.

What are muscle tension and load?

The force exerted on an object by a contracting muscle is known as muscle tension, while the force on the muscle exerted by the weight of an object is the load. Whether or not force generation leads to fiber shortening depends upon the relative magnitudes of the tension and load.

In an x contraction, the tension of the muscle remains constant while the muscle length changes (shortens). Such a contraction occurs when a muscle x, causing a load to be moved. This is a xcentric contraction. Muscle tension is greater/lower than the opposing load.

In an isotonic contraction, the tension of the muscle remains constant while the muscle length changes (shortens). Such a contraction occurs when a muscle shortens, causing a load to be moved. This is a concentric contraction. Muscle tension is greater than the opposing load.

In an x contraction the muscle is prevented from shortening. Therefore, tension is developed with the muscle at a constant length. Muscle tension is x to the opposing load.

In an isometric contraction the muscle is prevented from shortening. Therefore, tension is developed with the muscle at a constant length. Muscle tension is equal to the opposing load.

T/F - Some muscle activity in the body can be categorized as either isometric or isotonic. However, during many movements muscles may shift between isotonic and isometric contractions.
In addition, muscles are not limited to pure isotonic or isometric contractions. In many situations muscle length and tension both vary throughout the range of motion.

TRUE

In x contraction the load pulls the muscle to a longer length in spite of the opposing force being produced by the cross bridges. This is an xcentric contraction. The lengthening of the muscle fibers is not an active process produced by the contractile proteins, but is a consequence of the x being applied to the muscle. Muscle tension is x than the opposing load. For example, when the knees extensor muscles in your thighs are used to lower you to a seat from a standing position, the muscle despite being activated does not shorten, but actually lengthens

In lengthening contraction the load pulls the muscle to a longer length in spite of the opposing force being produced by the cross bridges. This is an eccentric contraction. The lengthening of the muscle fibers is not an active process produced by the contractile proteins, but is a consequence of the external forces being applied to the muscle. Muscle tension is less than the opposing load. For example, when the knees extensor muscles in your thighs are used to lower you to a seat from a standing position, the muscle despite being activated does not shorten, but actually lengthens

when the knees extensor muscles in your thighs are used to lower you to a seat from a standing position, the muscle despite being activated does not shorten, but actually lengthens - Is this a concentric or eccentric contraction.

when the knees extensor muscles in your thighs are used to lower you to a seat from a standing position, the muscle despite being activated does not shorten, but actually lengthens - this is eccentric contraction.

1.7.2 Length-Tension Relationship
The active length-tension relationship for a skeletal muscle fiber is shown in FIGURE 15A.
This figure shows the relative tension developed as a function of muscle length (relative to usual resting length). The resting length of a skeletal muscle fiber in the body is normally near its optimal length, i.e., the length at which maximum x is developed. The amount of tension that can be developed x as the muscle fiber's length is increased or decreased relative to this optimal length. Note that active tension falls to zero for lengths less than about x% of the resting length or more than about x% of the resting length. However, note that skeletal muscle fibers normally only operate in a range from about x% to x% of their resting length.
This figure also plots the passive tension developed when the fiber is stretched and the total tension (sum of passive and active tension).
The length-tension relationship can be understood in terms of the sliding filament mechanism of contraction.
The small range of lengths where tension development is maximum corresponds to the situation in which the x is optimal.
As the fiber length increases beyond the optimum length, developed tension x essentially linearly. This is because as the fiber is stretched the x are pulled out from the thick filaments and the region of overlap decreases. Thus, fewer x binding sites are accessible to bind with the cross-bridges of the thick filaments. With sufficient stretching of the fiber, there is no longer any overlap of the thick and thin filaments and no active x can be produced.
As the fiber length is decreased below the optimal length, tension is also reduced. There are several reasons for this decrease:
 As initial length decreases, thin filaments from the opposite sides of the sarcomere begin to x. This results in a reduction of the number of actin binding sites that are exposed for cross-bridge binding. Note that if a cross-bridge from one side of a sarcomere binds to actin from a thin filament from the opposite side of the sarcomere, it will not x.
 With further shortening, the ends of the thick filaments bump into the x lines.
 Finally, it is also believed that when the muscle shortens to less than about 80% of its
optimal length x is reduced.
The length-tension relationship can also be plotted as tension versus sarcomere length as
shown in FIGURE 15B. Note that only active tension is shown.

1.7.2 Length-Tension Relationship
The active length-tension relationship for a skeletal muscle fiber is shown in FIGURE 15A.
This figure shows the relative tension developed as a function of muscle length (relative to usual resting length). The resting length of a skeletal muscle fiber in the body is normally near its optimal length, i.e., the length at which maximum tension is developed. The amount of tension that can be developed declines as the muscle fiber's length is increased or decreased relative to this optimal length. Note that active tension falls to zero for lengths less than about 60% of the resting length or more than about 170% of the resting length. However, note that skeletal muscle fibers normally only operate in a range from about 70% to 130% of their resting length.
This figure also plots the passive tension developed when the fiber is stretched and the total tension (sum of passive and active tension).
The length-tension relationship can be understood in terms of the sliding filament mechanism of contraction.
The small range of lengths where tension development is maximum corresponds to the situation in which the overlap of the thick and thin filaments is optimal.
As the fiber length increases beyond the optimum length, developed tension falls essentially linearly. This is because as the fiber is stretched the thin filaments are pulled out from the thick filaments and the region of overlap decreases. Thus, fewer actin binding sites are accessible to bind with the cross-bridges of the thick filaments. With sufficient stretching of the fiber, there is no longer any overlap of the thick and thin filaments and no active tension can be produced.
As the fiber length is decreased below the optimal length, tension is also reduced. There are several reasons for this decrease:
 As initial length decreases, thin filaments from the opposite sides of the sarcomere begin to overlap. This results in a reduction of the number of actin binding sites that are exposed for cross-bridge binding. Note that if a cross-bridge from one side of a sarcomere binds to actin from a thin filament from the opposite side of the sarcomere, it will not produce tension.
 With further shortening, the ends of the thick filaments bump into the Z lines.
 Finally, it is also believed that when the muscle shortens to less than about 80% of its
optimal length Ca2+ release from the SR is reduced.
The length-tension relationship can also be plotted as tension versus sarcomere length as
shown in FIGURE 15B. Note that only active tension is shown.

Force-Velocity or Load-Velocity Relationship:
1. During an x contraction, the larger the load (which determines the tension or force developed), the lower the velocity at which the muscle shortens. Maximum force is developed when the load cannot be moved at all; in this case the muscle does not shorten and the contraction is x. Maximum x is achieved when the muscle works against no external load.

velocity is achieved when the muscle works against no external load.

Mechanisms That Vary the Tension Developed by Skeletal Muscle
The strength of contractions of a whole skeletal muscle can be varied by:
1. changing the x developed by individual fibers within the muscle, and b) changing the x of fibers contracting.

Mechanisms That Vary the Tension Developed by Skeletal Muscle The strength of contractions of a whole skeletal muscle can be varied by:
a) changing the tension developed by individual fibers within the muscle, and b) changing the number of fibers contracting.

Two main ways to change the strength of contractions of the whole skeletal muscle

Mechanisms That Vary the Tension Developed by Skeletal Muscle The strength of contractions of a whole skeletal muscle can be varied by:
a) changing the tension developed by individual fibers within the muscle, and b) changing the number of fibers contracting.

The length of the muscle fiber at the onset of contraction
changes the amount of x that is developed by the fiber

tension

The length of the muscle fiber at the onset of contraction
changes the amount of tension that is developed by the fiber.

The x can also change the amount of tension developed by
individual muscle fibers. Recall that the muscle action potential lasts only about 1-2 ms, while the duration of the resulting twitch is typically about 100 ms. Note that the term twitch refers to the contraction of a muscle fiber resulting from a single action potential. The refractory period goes on for a brief time (a few ms) after the end of the action potential, but
a second action potential can occur long before the end of the period of force development that results from the first action potential.

If a second action potential occurs before the muscle fiber has completely relaxed, there is a x of twitches, with the maximum tension development being x than the tension from a single twitch. See FIGURE 17. When the muscle fiber is stimulated so rapidly that it does not have a chance to relax at all between action potentials, a maximal sustained contraction results; this is referred to as x.

As you have already seen, the length of the muscle fiber at the onset of contraction
changes the amount of tension that is developed by the fiber.
The frequency of stimulation can also change the amount of tension developed by
individual muscle fibers. Recall that the muscle action potential lasts only about 1-2 ms, while the duration of the resulting twitch is typically about 100 ms. Note that the term twitch refers to the contraction of a muscle fiber resulting from a single action potential. The refractory period goes on for a brief time (a few ms) after the end of the action potential, but
a second action potential can occur long before the end of the period of force development that results from the first action potential.
If a second action potential occurs before the muscle fiber has completely relaxed, there is a summation of twitches, with the maximum tension development being larger than the tension from a single twitch. See FIGURE 17. When the muscle fiber is stimulated so rapidly that it does not have a chance to relax at all between action potentials, a maximal sustained contraction results; this is referred to as tetanus.

What is tetanus?

When the muscle fiber is stimulated so rapidly that it does not have a chance to relax at all between action potentials, a maximal sustained contraction results; this is referred to as tetanus.

Name the 5 major factors responsible for influencing the amount of tension developed by single muscle fibers

1. The length of the muscle fiber at the onset of contraction
2. Frequency of stimulation
3. Fiber diameter
4. Fiber type
5. Extent of fatigue

*Also, the number of fibers contracting within a whole skeletal muscle also affects the tension developed - but this isn't specific to single fibers.

The number of fibers contracting within a whole skeletal muscle also affects the x developed:
Evidently, the more individual muscle fibers that contract within a whole muscle, the x the tension that can be developed. It is also obvious that larger diameter whole muscles (with more fibers) can produce more force than smaller whole muscles. The number of fibers contracting depends on the x of the muscle.

The number of fibers contracting within a whole skeletal muscle also affects the tension developed:
Evidently, the more individual muscle fibers that contract within a whole muscle, the larger the tension that can be developed. It is also obvious that larger diameter whole muscles (with more fibers) can produce more force than smaller whole muscles. The number of fibers contracting depends on the innervation of the muscle.

Whole skeletal muscles are innervated by a x number of motor neurons. However, each motor neuron innervates x muscle fibers. The group of muscle fibers innervated by a single motor neuron is referred to as a x. An action potential in a given x will normally cause all of the muscle fibers in its motor unit to contract simultaneously. The number of xper motor unit and the number of x recruited will clearly influence the strength of whole muscle contraction.

The motor unit: Whole skeletal muscles are innervated by a large number of motor neurons. However, each motor neuron innervates many muscle fibers. The group of muscle fibers innervated by a single motor neuron is referred to as a motor unit. An action potential in a given motor neuron will normally cause all of the muscle fibers in its motor unit to contract simultaneously. The number of fibers per motor unit and the number of motor units recruited will clearly influence the strength of whole muscle contraction.

What is a motor unit?

The group of muscle fibers innervated by a single motor neuron is referred to as a motor unit

Note that the individual fibers of a single motor unit, ARE/ARE NOT grouped together in one part of the muscle; instead, they are dispersed throughout the whole skeletal muscle.

ARE NOT

T/F the individual fibers of a single motor unit are usually grouped together in one part of the muscle

false - the individual fibers of a single motor unit ARE NOT grouped together in one part of the muscle; instead, they are dispersed throughout the whole skeletal muscle.

Both the number of fibers per motor unit and the number of motor units per muscle vary considerably from one muscle to another. Muscles that have been designed to produce relatively coarsely controlled but powerful movements (e.g., the muscles of the legs) have motor units with x numbers of individual fibers. On the other hand, muscles that produce very precise and delicate movements (e.g., the muscles of the hand and the muscles that control the movement of the eyes) have been designed with x muscle fibers per motor unit.

Number of muscle fibers per motor unit: Both the number of fibers per motor unit and the number of motor units per muscle vary considerably from one muscle to another. Muscles that have been designed to produce relatively coarsely controlled but powerful movements (e.g., the muscles of the legs) have motor units with large numbers of individual fibers, up to roughly 2000 per motor unit. On the other hand, muscles that produce very precise and delicate movements (e.g., the muscles of the hand and the muscles that control the movement of the eyes) have been designed with far fewer muscle fibers per motor unit (a few dozen individual fibers per motor unit).

T/F - Both the number of fibers per motor unit and the number of motor units per muscle vary considerably from one muscle to another.

True

Motor unit recruitment: The number of motor units that are involved in a particular whole-muscle contraction depends on the x of the contraction. Stronger contraction needed/desired → more motor units recruited →stronger the resulting contraction.

Motor unit recruitment: The number of motor units that are involved in a particular whole-muscle contraction depends on the strength of the contraction. Stronger contraction needed/desired → more motor units recruited →stronger the resulting contraction.

In most situations, only a portion of muscle motor units will be needed for a (sub-maximal) whole muscle contraction. In this situation during a sustained contraction the body uses a scheme called x to avoid or delay fatigue. In this case motor unit activity is alternated, giving some motor units a chance to rest while other motor units take over. This sort of activity must be very finely coordinated so that sustained contractions remain x. Such asynchronous recruitment is very common in x muscles, but is also used during sustained sub-maximal contractions of other skeletal muscles. Obviously, this strategy cannot be used during a maximal contraction of a muscle when all motor units are simultaneously active.

In most situations, only a portion of muscle motor units will be needed for a (sub- maximal) whole muscle contraction. In this situation during a sustained contraction the body uses a scheme called asynchronous recruitment of motor units to avoid or delay fatigue. In this case motor unit activity is alternated, giving some motor units a chance to rest while other motor units take over. This sort of activity must be very finely coordinated so that sustained contractions remain smooth. Such asynchronous recruitment is very common in postural muscles, but is also used during sustained sub-maximal contractions of other skeletal muscles. Obviously, this strategy cannot be used during a maximal contraction of a muscle when all motor units are simultaneously active.

Describe asynchronous recruitment of motor units

In most situations, only a portion of muscle motor units will be needed for a (sub- maximal) whole muscle contraction. In this situation during a sustained contraction the body uses a scheme called asynchronous recruitment of motor units to avoid or delay fatigue. In this case motor unit activity is alternated, giving some motor units a chance to rest while other motor units take over. This sort of activity must be very finely coordinated so that sustained contractions remain smooth. Such asynchronous recruitment is very common in postural muscles, but is also used during sustained sub-maximal contractions of other skeletal muscles. Obviously, this strategy cannot be used during a maximal contraction of a muscle when all motor units are simultaneously active.

As will be discussed below, skeletal muscles usually consist of a mixture of different fiber types (slow and fast oxidative types and fast glycolytic). These fiber types differ metabolically and in their resistance to fatigue. During light to moderate activities, the x motor units are recruited first. If activity is very prolonged and/or very intense, the last motor units to be recruited are the x types.

As will be discussed below, skeletal muscles usually consist of a mixture of different fiber types (slow and fast oxidative types and fast glycolytic). These fiber types differ metabolically and in their resistance to fatigue. During light to moderate activities, the fatigue-resistant motor units are recruited first. If activity is very prolonged and/or very intense, the last motor units to be recruited are the fatigue-prone (fast glycolytic) types.

A). Diagram of a cross section through a muscle composed of three types of motor units (B). Tetanic muscle tension resulting from the successive recruitment of the three motor units. Note that motor unit 3, composed of fast-glycolytic fibers, produces the greatest rise in tension because it is composed of the largest-diameter fibers and contains the largest number of fibers per motor unit.

Explain why motor unit 3 in this picture produces the greatest rise in tension. What type of muscle fibers would you expect in this motor unit and why?

motor unit 3, composed of fast-glycolytic fibers, produces the greatest rise in tension because it is composed of the largest-diameter fibers and contains the largest number of fibers per motor unit.

Sources of Energy
Cellular processes must provide biochemical energy for the contractile mechanism,
primarily by re-synthesizing ATP from ADP. The metabolic pathway utilized to
supply ATP depends upon the type of x and conditions of x.
The most readily available pool of energy is the high-energy phosphate bond of x. The enzyme creatine phosphotransferase transfers the high-
energy phosphate of x to ADP, thus reforming ATP.

Sources of Energy
Cellular processes must provide biochemical energy for the contractile mechanism,
primarily by re-synthesizing ATP from ADP. The metabolic pathway utilized to
supply ATP depends upon the type of muscle and conditions of contraction.
The most readily available pool of energy is the high-energy phosphate bond of creatine phosphate. The enzyme creatine phosphotransferase transfers the high-
energy phosphate of phosphocreatine to ADP, thus reforming ATP.

The enzyme x transfers the high-
energy phosphate of phosphocreatine to ADP, thus reforming ATP.

The enzyme creatine phosphotransferase transfers the high-
energy phosphate of phosphocreatine to ADP, thus reforming ATP.

Glycogen is a far more abundant energy source in skeletal
muscle than creatine phosphate. Glycogen is degraded to x and then to lactate through anaerobic glycolysis to form # ATP molecules.
In the presence of oxygen, pyruvate instead enters the x. The products of the citric acid cycle are then made available for aerobic oxidative phosphorylation that forms # additional ATP molecules.

Glycogen is a far more abundant energy source in
muscle. Glycogen is degraded to pyruvate and then to lactate through anaerobic glycolysis to form 2 ATP molecules.
In the presence of oxygen pyruvate instead enters the citric acid cycle. The products of the citric acid cycle are then made available for aerobic oxidative phosphorylation that forms 36 additional ATP molecules.

Compared to creatine phosphate, this is a far more abundant energy source in muscle

Glycogen is a far more abundant energy source in
muscle. Glycogen is degraded to pyruvate and then to lactate through anaerobic glycolysis to form 2 ATP molecules.
In the presence of oxygen pyruvate instead enters the citric acid cycle. The products of the citric acid cycle are then made available for aerobic oxidative phosphorylation that forms 36 additional ATP molecules.

At the end of muscle activity, x and x stores will have decreased
and will need to be replenished. These processes require energy, and so a muscle continues to consume x at an increased rate (called x debt) for some time after its activity has stopped.
When a skeletal muscle is repeatedly stimulated, the maximal tension that the muscle can produce will eventually x. This decline in muscle tension is x. Additional characteristics of fatigue include decreased shortening x and a slower rate of x. The onset and rate of fatigue will depend upon the skeletal muscle x, and the x and x of the stimulation.

At the end of muscle activity, creatine phosphate and glycogen stores will have decreased and will need to be replenished. These processes require energy, and so a muscle continues to consume oxygen at an increased rate (called oxygen debt) for some time after its activity has stopped.
When a skeletal muscle is repeatedly stimulated, the maximal tension that the muscle can produce will eventually decrease. This decline in muscle tension is muscle fatigue. Additional characteristics of fatigue include decreased shortening velocity and a slower rate of relaxation. The onset and rate of fatigue will depend upon the skeletal muscle fiber type, and the intensity and duration of the stimulation.

T/F - A muscle still continues to consume oxygen at an increased rate for some time after its activity has stopped.

true

what is oxygen debt?

muscle continues to consume oxygen at an increased rate (called oxygen debt) for some time after its activity has stopped.

All skeletal muscle fibers do not have the same mechanical and metabolic characteristics.
Different types of fibers can be identified on the basis of their maximal velocities of X (fast or slow), and the major biochemical pathway used to form X (oxidative phosphorylation or glycolysis).

All skeletal muscle fibers do not have the same mechanical and metabolic characteristics.
Different types of fibers can be identified on the basis of their maximal velocities of shortening (fast or slow), and the major biochemical pathway used to form ATP (oxidative phosphorylation or glycolysis).

Two major biochemical pathways muscles can use to form ATP

All skeletal muscle fibers do not have the same mechanical and metabolic characteristics.
Different types of fibers can be identified on the basis of their maximal velocities of shortening (fast or slow), and the major biochemical pathway used to form ATP (oxidative phosphorylation or glycolysis).

Most people have an average of about X% each of fast and slow fibers. However, this can vary between individuals. Those people genetically endowed with a higher percentage of the X fibers are good candidates for power and sprint activities. In contrast, those people genetically endowed with a greater proportion of x fibers are more likely to be successful in endurance activities, such as marathon running.

Most people have an average of about 50% each of fast and slow fibers. However, this can vary between individuals. Those people genetically endowed with a higher percentage of the fast-glycolytic fibers are good candidates for power and sprint activities. In contrast, those people genetically endowed with a greater proportion of slow-oxidative fibers are more likely to be successful in endurance activities, such as marathon running.

They are found in muscles specialized for maintaining low-intensity contractions for long periods of time without fatigue, such as the muscles of the back and legs that support the body's weight against the force of gravity.

slow oxidative type I

These fibers have high oxidative capacity, due to their numerous mitochondria and high capillary density, and consequently have low fatigability. They also contain myoglobin, which is responsible for their red color.

slow oxidative type I

They share characteristics with each of the other two types. They contract more rapidly than the slow-oxidative fibers and can maintain the contraction for a longer period of time than can the fast- glycolytic fibers. These fibers have medium oxidative capacity with moderate fatigability.

Fast-oxidative-glycolytic fibers type IIa

They are found in muscles that are adapted for performing high intensity contraction for short periods of time, such as arm muscles that are used to lift heavy objects. They have little myoglobin (and are therefore sometimes called white fibers). These fibers have low oxidative capacity, due to having few mitochondria with a low capillary density, and consequently have high fatigability.

Fast glycolytic fibers type IIb

***Big table of skeletal muscle fiber types.
Compare between slow oxidative (red), fast oxidative/glycolytic (red), and fast glycolytic (white) the
1. primary source of ATP
2. Mitochondria amount
3. Myoglobin content
4. Glycogen content
5. Rate of fatigue
6. Contraction velocity
7. fiber diameter
8. motor unit size
9. size of motor neuron innervating fiber.

T/F - The number of muscle fibers is determined prenatally, and remains essentially constant throughout adult life.

true

The number of muscle fibers is determined prenatally, and remains essentially x throughout adult life. The changes in muscle size result primarily from changes in the size of x.
The growth of muscle involves the addition of new x at the ends of the myofibrils (lengthening), and/or the formation of additional myofibrils within the cells (x) or adding new cells (x).

The number of muscle fibers is determined prenatally, and remains essentially constant throughout adult life. The changes in muscle size result primarily from changes in the size of individual muscle fibers.
The growth of muscle involves the addition of new sarcomeres at the ends of the myofibrils (lengthening), and/or the formation of additional myofibrils within the cells (hypertrophy) or adding new cells (hyperplasia).

Differentiate between hypertrophy and hyperplasia and lengthening

Hypertrophy - "fattening" - formation of additional myofibrils within the cells
Hyperplasia - "duplication" - formation of new cells
Lengthening - adding sarcomeres to ends of myofibrils

Diagram a muscle cell undergoing hypertrophy - hyperplasia - or lengthening

Growth in muscle cells may consist of adding new myofibrils (depicted as a series of model sarcomeres) within a cell (hypertrophy), formation of new cells (hyperplasia), or adding more sarcomeres in series as the muscle cells lengthen along with skeletal growth.

x

T/F Myofibrils are repeating subunits (about 2-3 microns in length) that are arranged end-to- end to form sarcomeres.

False

T/F The A band consists of thin filaments that do not overlap with thick filaments.

False

T/F The transverse tubular system (T system) is an intracellular organelle.

False

T/F - The sarcoplasmic reticulum (SR) is an intracellular organelle.

True

T/F Thick filaments are composed of myosin and thin filaments are primarily composed of actin.

True

T/F - Tropomyosin binds Ca2+ causing a conformational change that causes troponin to uncover binding sites on actin for crossbridge attachment.

False

T/F- Parvalbumin is more abundant in fast fiber types than in slow oxidative fibers.

True

T/F - The dystrophin-glycoprotein complex is a group of cytoskeletal proteins that occur
exclusively in the Z lines of skeletal and cardiac muscle.

False

T/F- The absence of dystrophin from skeletal muscle results in Duchenne muscular dystrophy.

True

T/F- A patient who has the same type of genetic defects found in Duchenne muscular dystrophy but associated with the encoding of cardiac muscle proteins will display a hypertrophic cardiomyopathy.

False

T/F Skeletal muscle normally (i.e., within the body) works at sarcomere lengths less than 2.0 microns.

False

T/F An isometric contraction is one in which the muscle is prevented from shortening.

True

T/F Normally the frequency of action potentials can affect the amount of tension developed by
skeletal muscle (i.e., skeletal muscle displays summation of tension).

True

T/F Motor units generally contain more skeletal muscle fibers in whole muscles designed to produce precise delicate movements than in whole muscles designed to produce coarse powerful movements.

False

T/F Slow oxidative fibers contain less myoglobin than fast glycolytic fibers

False

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