MUSCLE CONTRACTION

 

James L. Heckman, Ph.D.                                                               OT Physiology, 1999

 

Read pages 308 ‑ 323

I.                     Skeletal muscles are a collection of individual muscle fibers.

A.      The entire muscle is surrounded by a connective tissue sheath called the epimysium.

B.      Another connective tissue sheath, called the perimysium, divides the muscle into groups of muscle fibers called fascicles.

C.      Individual muscle fibers are surrounded by a thin connective tissue sheath called the endomysium.

D.      All the connective tissues are connected to each other so that when a muscle fiber contracts its force is transmitted directly to the tendon.

II.                   When a skeletal muscle is stimulated, it is able to generate force and shorten.

E.      A single stimulus, resulting in the generation of a single action potential, produces a rise and fall in tension called a twitch.

1.                  If an action potential is generated by a stimulus, the resulting twitch is an all‑or‑none response.

2.                  The strength of the muscle contraction can be increased by:

a)                 Increasing the number of muscle fibers stimulated.

b)                 Increasing the frequency of stimulation.

c)                  Altering the length of the muscle fiber so it is closer to its optimal contractile length.

F.      If the stimulus is repeated so that a train of action potentials is produced, the twitches summate.

3.                  The higher the frequency of stimulation, the greater the summation.

 

 

 

Figure 1

4.                  If the frequency is high enough, the tension rises smoothly to a maximum contraction called a tetanic contraction.

G.     The contractile proteins of the muscle fiber are connected to the bones by a series of structures called collectively the series elastic component (SEC) of the muscle.

5.                  When the muscle contracts (shortens) the SEC is stretched.

6.                  The SEC must be stretched before force can be transmitted to the bones.

7.                  The SEC can not be stretched all the way out during a single twitch.

a)                 The amount of force transmitted to the bone depends on how much the SEC is stretched.

b)                 The SEC is stretched more during summation than it is during a single twitch.

c)                  The SEC is stretched all the way out during a tetanic contraction.

III.                  Muscle fibers are divided into functional units called motor units.

H.      A motor unit consists of all the muscle fibers innervated by a single alpha motoneuron.

1.                  Each muscle fiber is innervated by a single branch of the alpha motoneuron.

2.                  When an action potential is generated by an alpha motoneuron, all of the muscle fibers it innervates are activated.

I.         The central nervous system controls the force of a muscle contraction by varying:

3.                  The number of alpha motoneurons activated and

4.                  The frequency of discharge in each of the alpha motoneurons.

IV.               Muscle contractions are called isometric or isotonic.

V.                 A.. In an isotonic contraction the force generated by the muscle is just equal (or slightly greater) than the load on the muscle.

1.                  As a result, the muscle shortens.

2.                  The contraction is called an isotonic contraction because the force (tension) produced by the muscle during the shortening remains constant.

J.       In an isometric contraction, the force generated by the muscle is less than the load on the muscle.

3.                  As a result, the muscle does not shorten.

4.                  The contraction is called an isometric contrac­tion because the overall length of the muscle remains constant.

VI.               V.. The structure of muscle fibers has been analyzed using electron and light microscopy.

K.      Individual muscle fibers extend from one end of the muscle to the other and are continuous with the tendons on both ends.

L.       The muscle fibers are divided longitudinally into subunits called myofibrils.

1.                  The myofibrils are about 1 micron in diameter.

2.                  Myofibrils are separated from each other by a system of longitudinal tubules.

 

 

 

Figure 2

M.     The myofibrils are divided into function units called sarcomeres.

VII.              Sarcomeres are bounded on each end by a perpendicularly oriented band of protein called the Z‑line.

VIII.            The contractile proteins within a sarcomere are organized into two sets of filaments.

a)                 The thick filaments (about 1.5 microns long) are in the center of the sarcomere.

b)                 The thin filaments (about 1 micron long) are attached to the Z‑lines on both ends of the sarcomere.

c)                  The thin filaments slide over the thick filaments when the length of the muscle fiber changes.

2.                  The arrangement of the filaments and Z‑lines gives skeletal (and cardiac) muscle its striated appearance. 

a)                 The thick filaments in the center of the sarcomere form a dark region called the A‑band.

 

 

 

Figure 3

b)                 The light region between the A‑bands of two neighboring sarcomeres is called the I‑band.  The I‑band contains thin filaments and is bisected by the Z‑line.

c)                  The thin filaments partially overlap the thick filaments.  As a result, the periphery of the  A‑band is darker than the center.  The lighter, central region of the A‑band, is called the H‑band.

IX.               During contraction the thick and thin filaments slide across each other.

N.      The thick filaments are composed primarily of the protein, myosin.

1.                  Myosin is a large filamentary protein divided into a tail and head region.

2.                  The tail region is embedded within the thick filament.

3.                  The head region, called a cross bridge, projects out of the thick filament towards the thin filament.

 

 

Figure 4

4.                  When the muscle contracts the cross bridges attach to the thin filaments, bend towards the center of the sarcomere, and cause the thin filaments to slide over the thick filaments.

O.     The thin filaments are composed of three proteins: actin, tropomyosin, and troponin.(see figure 5)

5.                  Actin is a small globular protein.

a)                 Two chains of individual actin molecules are intertwined to form the backbone of the thin filament.

b)                 When the cross bridges attach to the thin filaments they bind to the actin molecules.

 

 

 

Figure 5

6.                  Tropomyosin is a filamentary protein that lies along the groove between the two actin chains.

P.      When the muscle is at rest, the tropomyosin chain prevents the cross bridges from binding to actin.

Q.     When the muscle is activated, the tropomyosin moves from its resting position and allows the cross bridges to bind with actin.

X.                 Troponin is a globular protein attached to both actin and tropomyosin.  It moves the tropomyosin from its resting position when the muscle is activated.

R.      During each cross bridge cycle, the myosin cross bridges attach to actin, bend towards the center of the sarcomere, detach, and return to their original perpendicular position.

1.                  One molecule of ATP is used each time the cross bridge cycles.

2.                  The ATP molecule and the ATPase which removes the phosphate from the ATP are both bound to the cross bridge.

 

 

Figure 6

3.                  When the cross bridge binds to actin, the ATPase is activated, and ATP is converted to ADP.

4.                  The energy liberated from ATP when it is split into ADP and phosphate is used to bend the cross bridge.

5.                  After the cross bridge bends and the ADP dissoci­ates from it, a new molecule of ATP becomes attached.

6.                  The cross bridge detaches from the actin after the new molecule of ATP binds to it.

S.      The sliding filament theory of muscle contraction can be used to explain the length‑tension relationship.

7.                  The force of muscle contraction varies with the resting length of the muscle.

 

 

 

Figure 7

8.                  The muscle is able to generate the greatest amount of tension when it is at its optimal length (Lo).

a)                 The optimal length of a muscle fiber is the length at which all the cross bridges on the thick filament are opposite, or overlap, actin molecules on the thin filaments.

b)                 Muscles are usually at their optimal length when relaxed.

9.                  Increasing or decreasing the resting length of the muscle reduces the number of cross bridges that can bind to actin and thus reduces the amount of tension the muscle can generate.

XI.               Calcium plays an essential role in the regulation of muscle contraction.

T.       When skeletal muscle is relaxed, tropomyosin prevents the myosin cross bridges from binding to actin.

1.                  When troponin is activated, it moves tropomyosin out of the way, allowing myosin and actin to interact.

2.                  Troponin is activated when calcium binds to it.

U.      Under resting conditions, the intracellular concentration of calcium (10-7 M) is too low for it to bind to troponin.

3.                  However, when an action potential is produced on the skeletal muscle membrane, the intracellular concentration of calcium rises.

4.                  The intracellular concentration (10‑5 M) is high enough to activate all of the troponin in the muscle fiber.

5.                  As a result, tropomyosin moves out of the way and cross bridge cycling begins.

6.                  Cross bridge cycling continues until the calcium is removed from the intracellular fluid.

XII.              The resting concentration of calcium is kept low by a system of longitudinal tubules called the sarcoplasmic reticulum (SR).

V.      Calcium pumps (see figure 6‑15) that are similar to the Na‑K pump but transport calcium instead of sodium and potassium are located on the SR membrane and remove calcium from the intracellular fluid.

W.    The calcium is stored in the terminal cisternae of the SR.

X.       The terminal cisternae come in close contact with a set of membrane invaginations called the transverse tubules.

Y.      The depolarization of the transverse tubules by an action potential causes calcium to be released from the terminal cisternae.

 

 

 

Figure 8