Ch. 6 - Contraction of Skeletal Muscle
Summation occurs in two ways:
(1) by increasing the number of motor units contracting simultaneously, which is called multiple fiber summation (2) by increasing the frequency of contraction, which is called frequency summation and can lead to tetanization.
Fast Fibers (Type II, White Muscle)
(1) fast fibers are large for great strength of contraction (2) an extensive sarcoplasmic reticulum is present for rapid release of calcium ions to initiate contraction (3) Large amounts of glycolytic enzymes are present for rapid release of energy by the glycolytic process.
Most of the energy required for muscle contraction is used to actuate the walk-along mechanism by which the cross-bridges pull the actin filaments, but small amounts are required for
(1) pumping calcium ions from the sarcoplasm into the sarcoplasmic reticulum after the contraction is over (2) pumping sodium and potassium ions through the muscle fiber membrane to maintain an appropriate ionic environment for propagation of muscle fiber action potentials
Slow Fibers (Type 1, Red Muscle)
(1) slow fibers are smaller than fast fibers (2) slow fibers are also innervated by smaller nerve fibers (3) compared with fast fibers, slow fibers have a more extensive blood vessel system and more capillaries to supply extra amount of oxygen
Fast Fibers (Type II, White Muscle)
(4) Fast fibers have a less extensive blood supply than do slow fibers because oxidative metabolism is of secondary importance. (5) Fast fibers have fewer mitochondria than do slow fibers, also because oxidative metabolism is of secondary importance. A deficit of red myoglobin in fast muscle gives it the name white muscle.
Slow Fibers (Type 1, Red Muscle)
(4) slow fibers have greatly increased numbers of mitochondria to support high levels of oxidative metabolism (5) slow fibers contain large amounts of myoglobin, and iron-containing protein similar to hemoglobin in red blood cells. Myoglobin combines with oxygen and stores it until needed, which also greatly speeds oxygen transport to the mitochondria. The myoglobin gives the slow muscle a reddish appearance and hence the name red muscle.
Size Principle
Allows the gradations of muscle force during weak contraction to occur in small steps, whereas the steps become progressively greater when large amounts of force are required.
Spaces between the myofibrils are filled with intracellular fluid called sarcoplasm, containing large quantities of potassium, magnesium, and phosphate, plus multiple protein enzymes.
Also present are tremendous numbers of mitochondria that lie parallel to the myofibrils. These mitochondria supply the contracting myofibrils with large amounts of energy in the form of adenosine triphosphate (ATP) formed by the mitochondria.
Troponin and Its role in contraction
Attached intermittently along the sides of the tropomyosin molecules are additional protein molecules called troponin.
When the muscle is at its normal resting length, which is at sarcomere length of about 2 micrometers, it contracts upon activation with the approximate maximum force of contraction.
However, the increase in tension that occurs during contraction, called active tension, decreases as the muscle is stretched beyond its normal length.
All muscle hypertrophy results from an increase in the number of actin and myosin filaments in each muscle fiber, causing enlargement of the individual muscle fibers; this condition is called simply fiber hypertrophy
Hypertrophy occurs to a much greater extent when the muscle is loaded during the contractile process. Only a few strong contractions each day are required to cause significant hypertrophy within 6 to 10 weeks.
In the relaxed state, the ends of the actin filaments extending from two successive Z disks barely overlap one another.
In the contracted state, these actin filaments have been pulled inward among the myosin filaments, so their ends overlap one another to their maximum extent.
the strength of contraction increases to a plateau, a phenomenon called the staircase effect, or treppe
It is believed to be caused primarily by increasing calcium ions in the cytosol because of the release of more and more ions from the sarcoplasmic reticulum with each successive muscle action potential and failure of the sarcoplasm to recapture the ions immediately
When a muscle contracts, work is performed and energy is required.
Large amounts of ATP are cleaved to form ADP during the concentration process, and the greater the amount of work performed by the muscle, the greater the amount of ATP that is cleaved.
The final source of energy is oxidative metabolism, which mean combining oxygen with the end products of glycolysis and with various other cellular foodstuffs to liberate ATP.
More than 95% of all energy used by the muscles for sustained, long-term contraction is derived from oxidative metabolism.
Prolonged and strong contraction of a muscle leads to the well-known state of muscle fatigue.
Muscle fatigue increases in almost direct proportion to the rate of depletion of muscle glycogen
The second important source of energy, which is used to reconstitute both ATP & phosphocreatine, is "glycolysis" of glycogen previously stored in the muscle cells.
Rapid enzymatic break down of the glycogen to pyruvic acid and lactic acid liberates energy that used to convert ADP to ATP; the ATP can then be used directly to energize additional muscle contraction and also to re-form the stores of phosphocreatine.
Even when muscles are at rest, a certain amount of tautness usually remains, which is called muscle tone
Skeletal muscle tone results entirely from a low rate of nerve impulses coming from the spinal cord. these nerve impulses, in turn, are controlled partly by signals transmitted from the brain to the appropriate spinal cord anterior motoneurons and partly by signals that originate in muscle spindles located in the muscle itself
Rate of synthesis of muscle contractile proteins is far greater when hypertrophy is developing, leading also to progressively greater numbers of both actin and myosin filaments in the myofibrils.
Some of the myofibrils themselves have been observed to split within hypertrophying muscle to form new myofibrils, but the importance of this process in usual muscle hypertrophy is still unknown
The first source of energy that is used to reconstitute the ATP is the substance phosphocreatine, which carries a high-energy phosphate bond similar to the bonds of ATP.
The high-energy phosphate bond of phosphocreatine has a slightly higher amount of free energy than of each bond of ATP.
All the muscle fibers innervated by a single nerve fiber are called a motor unit
The muscle fibers in each motor unit are not all bunched together in the muscle but overlap other motor units in micro bundles of 3 to 15 fibers.
When a muscle remains unused for many weeks, the rate of degradation of the contractile proteins is more rapid than the rate of replacement.
The pathway that appears to account for much of the protein degradation in a muscle undergoing atrophy is the ATP-dependent ubiquitin-proteasome pathway
The myosin filament is made up of 200 or more individual myosin molecules.
The protruding arms and heads together are called cross-bridges.
The calcium ions in turn activate the forces between the myosin and actin filaments, and concentration begins. However, energy is needed for the contractile process to proceed
This energy comes from high-energy bonds in the ATP molecule, which is degraded to adenosine diphosphate (ADP) to liberate the energy.
Under rare conditions of extreme muscle force generation, the actual number of muscle fibers has been observed to increased.
This increase in fiber number is called fiber hyperplasia.
Virtually all body movements are caused by simultaneous contraction of agonist and antagonist muscles on opposite sides of joints.
This process is called coactivation of the agonist and antagonist muscles, and it is controlled by the motor control centers of the brain and spinal cord.
When the frequency reaches a critical level, the successive contractions eventually become so rapid that they fuse together and the whole muscle contraction appears to be completely smooth and continuos.
This process is called tetanization
When some but not all nerve fibers to a muscle are destroyed, the remaining nerve fibers branch off to form new axons that then innervate many of the paralyzed muscle fibers.
This process results in large motor units called macromotor units, which can contain as many as five times the normal number of muscle fibers for each motoneuron coming from the spinal cord.
The myosin head functions as an adenosine triphosphatase (ATPase) enzyme.
This property allows the head to cleave ATP and use the energy derived from the ATP's high-energy phosphate bond to energize the contraction process.
Also in the sarcoplasm surrounding the myofibrils of each muscle fiber is an extensive reticulum, called the sarcoplasmic reticulum
This reticulum has a special organization that is extremely important in regulating calcium storage, release, and repute and therefore muscle contraction.
Several hours after death, all the muscles of the body of into a state of contracture called "rigor mortis"; that is, the muscles contract and become rigid, even without action potentials.
This rigidity results from loss of all the ATP, which is required to cause separation of the cross-bridges from the actin filaments during the relation process.
Another type of hypertrophy occurs when muscles are stretched to greater than normal length.
This stretching causes new sarcomeres to be added at the ends of the muscle fibers, where they attach to the tendons.
To perform work means that energy is transferred from the muscle to the external load to lift an object to a greater height or to overcome resistance to movement
W = L x D
In fact, new sarcomeres can be added as rapidly as several per minute in newly developing muscle.
When a muscle continually remains shortened to less than its normal length, sarcomeres at the ends of the muscle fibers can actually disappear.
Troponin I
has a strong affinity for actin
Lack of dystrophin or mutated forms of the protein cause muscle cell membrane destabilization, activation of multiple pathophysiological processes, including altered intracellular calcium handling, and impaired membrane repair after injury.
increase in membrane permeability to calcium, thus allowing extraceulluar calcium ions to enter the muscle fiber and to initiate changes in intracellular enzymes that ultimately lead to proteolysis and muscle fiber breakdown.
Calcium ions are pumped back into the sarcoplasmic reticulum by a Ca++ membrane pump and remain stored in the reticulum until a new muscle action potential comes along;
this removal of calcium ions from the myofibrils causes the muscle contraction to cease.
Becker muscular dystrophy
very similar to, but less severe than, Duchenne muscular dystrophy, later onset & longer survival
The increase of the total mass of a muscle is called muscle hypertrophy.
When the total mass decreases, the process is called muscle atrophy.
ADP molecules are believed to be the active sites on the actin filaments with which the cross-bridges of the myosin filaments interact to cause muscle contraction.
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Actin filaments are composed of actin, tropomyosin, and troponin.
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All the muscles of the body are continually being remodeled to match the functions that are required for them.
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Along with the increasing size of myofibrils, the enzyme systems that provide energy also increases.
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Before contraction can take place, the inhibitory effect of the troponin-tropomyosin complex must itself be inhibited
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If the muscle contracts slowly or without any movement, small amounts of maintenance heat are released during contraction, even though little or no work is performed, thereby decreasing the conversion efficiency to as little as zero.
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In the final stage of denervation atrophy, most of the muscle fibers are destroyed and replaced by fibrous and fatty tissue.
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In the presence of large amounts of calcium ions, the inhibitory effect of the troponin-tropomyosin on the actin filaments is itself inhibited.
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In the resting state, the tropomyosin molecules lie on top of the active sites of the actin strands so that attraction cannot occur between the actin and myosin filaments to cause contraction.
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It is the interaction between cross-bridges and the actin filaments that causes contraction.
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Muscle contraction is said to be isometric when the muscle does not shorten during contraction and isotonic when it does shorten but the tension on the muscle remains constant throughout the contraction.
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Muscles operate by applying tension to their points of insertion into bones, and the bones in turn form various types of lever systems
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Summation means the adding together of individual twitch contractions to increase the intensity of overall muscle contraction.
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The bond between the head of the cross-bridge and the active site of the actin filament causes a conformational change in the head, prompting the head to tilt toward the arm of the cross-bridge and providing the power stroke for pulling the actin filament.
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The decreasing velocity of contraction with load is caused by the fact that a load on a contracting muscle is a reverse force that oppose the contractile force caused by muscle contraction.
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The fibrous tissue that replaces the muscle fibers during denervation atrophy also has a tendency to continue shortening for many months, which is called contracture.
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The greater the number of cross-bridges in contact with the actin filament at any given time, the greater the force of the contraction .
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The muscular dystrophies include several inherited disorders that cause progressive weakness and degeneration of muscle fibers, which are replaced by fatty tissue and collagen.
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The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z disks is called a sarcomere.
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The side-by-side relationship between the myosin and actin filaments is maintained by a large number of filamentous molecules of a protein called titin
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The whole muscle has a large amount of connective tissue in it; in addition, the sarcomeres in different parts of the muscle of not always contract the same amount.
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Ubiquitin is a regulatory protein that basically labels which cells will be targeted for proteosomal degradation.
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When a muscle loses its nerve supply, it no longer receives the contractile signals that are required to maintain normal muscle size. Therefore, atrophy begins almost immediately.
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as the sarcomere shortens and the actin filaments beings to overlap the myosin filaments, the tension increases progressively until the sarcomere length decreases to about 2.2 micrometers.
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As soon as the actin filaments is activated by the calcium ions, the heads of the cross-bridges from the myosin filaments become attracted to the active sites of the actin filaments, and this, in some way, causes contraction to occur.
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As the strength of the signal increases, larger and larger motor units begin to be excited as well.
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Every muscle of the body is composed of a mixture of so-called fast and slow muscle fibers, with still other fibers gradated between these two extremes.
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Muscle contraction occurs by a sliding filament mechanism.
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Tetany occurs because enough calcium ions are maintained in the muscle sarcoplasm, even between action potentials, so that full contractile state is sustained without allowing any relaxation between the action potentials .
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What causes the actin filaments to slide inward among the myosin filaments?
Caused by forces generated by interaction of the cross-bridges from the myosin filaments with the actin filaments. Under resting conditions, these forces are inactive, but when an action potential travel along the muscle fiber, this causes the sarcoplamsmic reticulum to release large quantities of calcium ions that rapidly surround the myofibrils.
The concentration of ATP in the muscle fiber is sufficient to main full contraction for only 1 to 2 seconds to most.
The ATP is split to form ADP, which transfers energy from the ATP molecule to the contracting machinery of the muscle fiber.
Duchenne muscular dystrophy (DMD)
affects only males because it is transmitted as an X-linked recessive trait and is caused by a mutation of the gene that encodes for a protein called dystrophin, which links actin to proteins in the muscle cell membrane
Troponin C
binds calcium
Troponin T
binds to tropomyosin