Chapter 6: Contraction of Skeletal Muscle Flashcards
Percent composition of the body that is skeletal muscle
40%
Percent composition of the body that is smooth and cardiac muscle
10%
The diameter of skeletal muscle fibers
10-80 micrometers
Except for about 2% of the muscle fibers, each fiber is usually innervated by only one nerve ending, located near the _________ of the fiber.
A. Middle
B. Tendinous End
C. Muscle Spindle
A. Middle
The thin membrane enclosing a Skeletal Muscle Fiber
Sarcolemma
What makes up the outer coat of the sarcolemma?
Thin layer of Polysaccharide material that contains numerous thin collagen fibrils
At each end of the muscle fiber, the polysaccharide layer of the sarcolemma fuses with a _________ fiber.
tendon fiber
The tendon fibers, in turn, collect into bundles to form the muscle tendons that then connect the muscles to the bones.
Each myofibril is composed of about how many adjacent myosin and actin filaments?
1500 myosin (thick filaments) 3000 actin (thin filaments)
What causes the alternating light and dark bands of the myofibrils?
interdigitation of the myosin and actin filaments
The light bands contain only actin filaments and are called:
I bands because they are isotropic to polarized light
The dark bands contain myosin filaments, as well as the ends of the actin filaments, where they overlap the myosin, and are called:
A bands because they are anisotropic to polarized light.
These are the projections from the sides of the myosin filaments:
cross-bridges
Which parts of the myofibrils interact to cause contraction
It is the interaction between these cross-bridges
and the actin filaments that causes contraction
The ends of the actin filaments are attached to a ______ from which these filaments extend in both directions to interdigitate with the myosin filaments
Z disk
At which length of the sarcomere is the muscle capable of generating its greatest force of contraction?
2 micrometers
The length of the sarcomere is about 2 micrometers. At this length, the actin filaments completely overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another. As discussed later, at this length, the muscle is capable of generating its greatest force of contraction.
This part of the myofibril is composed of filamentous proteins different from the actin and myosin filaments, passes crosswise across the myofibril and also crosswise from myofibril to myofibril, attaching the myofibrils to one another all the way across the muscle fiber.
Z disk
The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z disks is called a:
sarcomere
What is the length of the sarcomere when the muscle is contracted
2 micrometers
Which filamentous molecules keep the myofilaments in place?
Titin Filamentous Molecules Keep the Myosin and Actin Filaments in Place
This protein forms the filamentous molecules that maintain the side-by-side relationship between myosin and actin filaments
Titin
Each titin molecule has a molecular weight of about ________, which makes it one of the largest protein molecules in the body.
3 million
These springy molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomeres will work
Titin
To which portion of the sarcomere does the elastic end of titin attach to?
Z disk
To which portions of the sarcomere do the ends of the titin molecule attach to?
Myosin thick filament Z disk (elastic end)
This molecule may also act as a template for the initial formation of portions of the contractile filaments of the sarcomere, especially the myosin filaments
Titin
What is the intracellular fluid between myofibrils?
Sarcoplasm
The intracellular fluid between myofibrils contains large quantities of which ions?
Potassium, Magnesium and Phosphate
plus multiple protein enzymes
Which organelles lie parallel to the myofibrils providing large amounts of energy during contraction?
Mitochondria
Also present in the sarcoplasm 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.
This specialized organelle surrounding myofibrils of each muscle fiber is extremely important in regulating calcium storage, release, and reuptake; hence, these are especially extensive in rapidly contracting types of muscle fibers
Sarcoplasmic Reticulum
In the general mechanism of muscle contraction, which channels are opened as Acetylcholine acts on a local area of the muscle fiber membrane?
acetylcholine-gated CATION channels
Upon opening of acetylcholine-gated channels during muscle contraction, what ions diffuse to the interior of the muscle fiber membrane causing local depolarization prior to action potential?
Sodium ions
The opening of acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This action causes a local depolarization that in turn leads to the opening of voltage-gated sodium channels, which initiates an action potential at the membrane.
Which channels of the sarcolemma open and generate action potential during muscle contraction?
Voltage-gated sodium channels
What molecular mechanism initiated by calcium ions is theoretically known as the process of muscle contraction?
Sliding Filament Mechanism
What forces cause the actin filaments to slide inward among the myosin filaments?
This action is caused by forces generated by interaction of the cross-bridges from the myosin filaments with the actin filaments.
Where does the energy needed for the process of muscle contraction come from?
High-energy bonds in the ATP
Molecular weight of each myosin molecule
480,000
2 heavy chains (200,000 each) = 400,000
+
4 light chains (20,000 each) = 80,000
How many polypeptide chains are there in a myosin molecule?
6 polypeptide chains
How many heavy and light chains does a myosin molecule have?
2 heavy chains and 4 light chains
What is the molecular weight of 1 myosin heavy chain?
200,000
What is the molecular weight of 1 myosin light chain?
20,000
Which chains form the tail of a myosin molecule?
2 heavy chains wrapping spirally around each other to form a double helix
One end of each of the heavy chains is folded bilaterally into a globular polypeptide structure called a:
myosin head
How many free heads are there at one end of the double helix myosin molecule?
2 heads
How many light chains are there in one head of a myosin heavy chain?
2 light chains to each head
At least how many myosin molecule are there in a myosin filament
at least 200
The tails of the myosin molecules form which part of the thick filament?
Body
The central portion of one of these filaments is shown in Figure 6-6B, displaying the tails of the myosin molecules bundled together to form the body of the filament, while many heads of the molecules hang outward to the sides of the body.
The part of the myosin molecule that hangs to the side along with the head is known as the:
arm
Which parts of the myosin molecule, together, form the cross-bridges?
arm and head
These are the 2 points of flexibility in each cross-bridge
hinges:
one where the arm leaves the body of the myosin filament and the other where the head attaches to the arm
Hinged portion of the myosin participate in the contraction process?
Hinged head
**The hinged arms allow the heads either to be extended far outward from the body of the myosin filament or brought close to the body. The hinged heads, in turn, participate in the contraction process, as discussed in the following sections.
The total length of each myosin filament is uniform, almost exactly _____ micrometers
1.6 micrometers
Because the hinged arms extend away from the center, there are no cross-bridge heads in the center of the myosin filament for a distance of about _____ micrometer
0.2 micrometer
To ensure that the cross-bridges extend in all directions around the filament, the myosin filament is twisted. By how much degree is each successive pair of cross-bridge displaced from the previous pair?
120 degrees
Which part of the myosin functions as an enzyme?
Myosin head:
Another feature of the myosin head that is essential for muscle contraction is that it functions as an adenosine triphosphatase (ATPase) enzyme
Which protein molecule forms the backbone of the actin filament?
F-actin protein molecule
**The backbone of the actin filament is a double-stranded F-actin protein molecule. The two strands are wound in a helix in the same manner as the myosin molecule.
Which protein molecule polymerizes to form a strand of F-actin?
G-actin molecule
The molecular weight of a G-actin molecule
42,000
These 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
ADP molecules
**Attached to each one of the G-actin molecules is one molecule of ADP.
How many active sites are there on the overall actin filament about every 2.7 nanometers?
1
The active sites on the two F-actin strands of the double helix are staggered, giving one active site on the overall actin filament about every 2.7 nanometers.
The approximate length of an actin filament:
1 micrometer
**Myosin filament is 1.6 micrometers
The molecular weight of a tropomyosin
70,000
The length of a tropomyosin
40 nanometers
These molecules wrap around the sides of the F-actin helix lying on top of the active sites of the actin strands
Tropomyosin
These protein molecules are complexes of three loosely bound protein subunits, attached intermittently along the sides of the tropomyosin molecules
Troponin
The subunits of the troponin molecules have strong affinity with which specific structures?
Troponin I = Actin
Troponin T = Tropomyosin
Troponin C = Calcium
Apart from ATP, which ions need to be present for the pure actin filament (without the troponin-tropomyosin complex) to bind instantly and strongly with the heads of the myosin molecules?
Magnesium
The active sites on the normal actin filament of the relaxed muscle are inhibited or physically covered by which regulatory complex?
Troponin-tropomyosin complex
How many calcium ions can each molecule of Troponin C bind?
4 calcium ions
This theory hypothesizes that initiation of muscle contraction is by the interaction of the Activated Actin Filament and the Myosin Cross-Bridges.
Ratchet Theory
or
Walk-along theory of contraction
As the myosin head attaches to an active site, profound changes in the intramolecular forces between the head and arm of the cross-bridge occur. This changes the alignment of forces causing the head to tilt toward the arm, known as _________, dragging the actin filament along with it
power stroke
True or False:
- Each of the cross-bridges is believed to operate independently of all the others, with each attaching and pulling in a continuous repeated cycle.
- The greater the number of cross-bridges in contact with the actin filament at any given time, the greater the force of contraction
- True
2. True
What is the phenomenon wherein the greater amount of work performed by the muscle, the greater the ATP that is cleaved to ADP?
Fenn effect
Fenn effect - the increased liberation of heat in a stimulated muscle when it is allowed to do mechanical work.
What is the conformation of the myosin head once it cleaves the ATP to ADP and phosphate?
Extended PERPENDICULAR toward the actin filament but not yet attached to the actin
What causes the detachment of the myosin head from the actin?
Binding of new ATP at the site of release of the ADP
How much tension is developed by the activated muscle when the actin filament has pulled all the way out to the end of the myosin filament, with no actin-myosin overlap?
Zero
At what length of the sarcomere is the actin filament assumed to have already overlapped all the cross-bridges of the myosin filament but has not yet reached the center of the myosin filament?
2.2 micrometers
At what length of the sarcomere do the 2 Z disks abut the ends of the myosin filaments?
1.6 micrometers
Known as the tension that occurs during muscle contraction
active tension
A skeletal muscle contracts rapidly when it contracts against no load to a state of full contraction in about ___ second for the average muscle
0.1 second
When the load has been increased to equal the maximum force that the muscle can exert, the velocity of contraction becomes:
zero = no contraction results, despite activation of the muscle fiber
**When loads are applied, the velocity of contraction decreases progressively as the load increases
This is a reverse force that opposes the contractile force caused by muscle contraction
Load
**This decreasing velocity of contraction with load occurs because a load on a contracting muscle is a reverse force that opposes the contractile force caused by muscle contraction. Therefore, the net force that is available to cause the velocity of shortening is correspondingly reduced.
Which events of muscle contraction requires energy for the process to proceed?
Most: Trigger of the walk-along mechanism
Small amounts:
(1) Pumping calcium ions from the sarcoplasm into the SR once contraction is over
(2) Pumping sodium and potassium ions through the muscle fiber membrane to maintain an appropriate ionic environment for the propagation of muscle fiber action potentials
What is the normal concentration of ATP in the muscle fiber that is sufficient to maintain full contraction for 1-2 seconds?
4 millimolar
What is the first source of energy that is used to reconstitute the ATP for muscle contraction?
Phosphocreatine
Between ATP and Phosphocreatine, which contains a higher energy phosphate bond?
Phosphocreatine
Harper:
PC = -10.3 kcal/mol
ATP (to ADP) = -7.3 kcal/mol
ATP (to AMP) = 7.7 kcal/mol
The combined energy of both the stored ATP and Phosphocreatine in the muscle is capable of causing maximal muscle contraction how many seconds? (range)
5-8 seconds
What are the 3 sources of energy for muscle contraction?
Phosphocreatine
Glycolysis
Oxidative Metabolism
The rate of ATP formation by glycolysis is about ___ times as rapid as ATP formation in response to cellular foodstuffs reacting with oxygen
2.5 times as rapid
Maximum duration in which Glycolysis can sustain muscle contraction
1 minute
Where does the muscle derive 95% of all the energy it uses for sustained long-term contraction?
Oxidative Metabolism
For extremely long-term maximal muscle activity (over a period of many hours), the greatest proportion of energy comes from which foodstuff consumed?
fats
In muscle activity periods of 2-4 hours, as much as one half of the energy can come from which foodstuff?
carbohydrates (glycogen)
The __________ of an engine or a motor is calculated as the percentage of energy input that is converted into work instead of heat.
efficiency
What is the amount (in percent) of the input energy to muscle that can be converted into work? (aka the normal efficiency of a muscle)
25%
the rest become heat
Only what percentage of the energy in ATP can actually be converted into work?
40-45%
Maximum efficiency can be realized only when the muscle contracts at a ____ velocity
moderate or 30% of maximum
What decreases the efficiency to zero during a slow muscle contraction or those without movement?
Maintenance heat
**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.
What reduces the efficiency during very rapid muscle contraction?
Viscous Friction within the muscle
**If contraction is too rapid, much of the energy is used to overcome viscous friction within the muscle itself, and this too reduces the efficiency of contraction
When the muscle does not shorten, the muscle contraction is said to be:
Isometric
When the muscle shortens but the tension on the muscle remains constant throughout, the contraction is said to be:
isotonic
On which does isotonic contraction depend?
Load against which the muscle contracts, and the inertia of the load
Which system of muscle contraction is often used when comparing the functional characteristics of different muscle types?
Isometric system
**This is because isometric system records changes in force of muscle contraction independently of load inertia
What is the duration of isometric contraction of ocular muscles?
less than 1/50 second
What is the duration of isometric muscle contraction of the gastrocnemius muscle?
about 1/15 second
Which system of muscle contraction occurs when the force of the muscle contraction is greater than the load?
Isotonic contraction
Which system of muscle contraction occurs when the load is greater than the force of muscle contraction?
Isometric contraction
Which organelle is found in great numbers in Type I muscle fibers?
Mitochondria
for high levels of oxidative metabolism
The anterior tibialis is composed mainly of which type of muscle fibers?
fast muscle fibers
The soleus muscle is composed mainly of which type of muscle fibers?
slow muscle fibers
Which iron-containing protein is responsible for the reddish appearance of Type I muscle fibers?
Myoglobin
**Slow fibers contain large amounts of myoglobin, an 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—hence, the name red muscle.
All the muscle fibers innervated by a single nerve fiber are called a:
Motor unit
Which organelle is found to be quite extensive in Type II muscle fibers?
Sarcoplasmic Reticulum
Which enzymes are present in large amounts in fast muscle fibers?
Glycolytic enzymes
for rapid release of energy by the glycolytic process
This consists of a motor neuron and the group of skeletal muscle fibers it innervates
Motor unit
Which muscles have more nerve fibers for fewer muscle fibers?
Small muscles
Since they that react rapidly and are required to have a more exact control
What is the average number of muscle fibers in a motor unit? (range)
80-100 muscle fibers
The muscle fibers in each motor unit are not all bunched together in the muscle but overlap other motor units in microbundles of how many muscle fibers?
3 to 15 fibers
This means adding together of individual twitch contraction to increase the intensity of overall muscle contractions
Summation
*Force Summation: Muscle contraction of different force
What are the two ways of summation in muscle fibers?
Multiple fiber summation
and
Frequency summation
Which type of Force summation can lead to tetanization?
Frequency summation
This type of summation occurs by increasing the number of motor units contracting simultaneously
Multiple fiber summation
The phenomenon in which small motor units are recruited first, followed by the large motor units as the strength of the signal increase
Size principle
Why are small motor units recruited first?
This is because smaller motor units are innervated by small motor neurons from the spinal cord which are more excitable than the larger ones
Which type of summation has the important feature for the provision of smooth contraction even at low frequencies of nerve signals?
Multiple fiber summation
**Another important feature of multiple fiber summation is that the different motor units are driven asynchronously by the spinal cord; as a result, contraction alternates among motor units one after the other, thus providing smooth contraction, even at low frequencies of nerve signals.
The phenomenon that 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.
Size principle
The process wherein the successive contractions eventually become so rapid that they fuse together, and the whole muscle contraction appears to be completely smooth and continuous
Tetanization
This is the state of sustained full contraction (without relaxation) that occurs due to the maintained calcium ions in the sarcoplasm, even between action potentials.
Tetany
What is the average maximum strength of tetanic contraction of a muscle operating at normal muscle length?
Between 3 and 4 kg/cm2 of muscle, or 50 pounds/inch2
How much tension can be generated in the patellar tendon if the quadriceps muscle has 16 sq inches of muscle belly?
800 pounds
16 x 50 lbs/sq in
This is the phenomenon of changes in muscle strength at the Onset of Contraction increasing to a plateau after several twitches later.
Staircase effect or Treppe phenomenon
This refers to the idea that an increase in heart rate increases the force of contraction generated by the myocardial cells with each heartbeat despite accounting for all other influences.
Bowditch effect is also known as the Treppe phenomenon, staircase phenomenon, or frequency-dependent activation
This phenomenon resulting to increased strength of contraction 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.
Staircase effect or Treppe phenomenon
The tautness of muscles that remain even when muscles are at rest
muscle tone
What is the stimulus for muscle tone?
low rate of nerve impulses 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.
Studies in athletes have shown that muscle fatigue increases in almost direct proportion to the rate of depletion of:
Muscle glycogen
This state occurs after the prolonged strong contraction of muscles, and is attributed mainly from the inability of the contractile and metabolic processes of the muscle fibers to continue supplying the same work output.
Muscle Fatigue
**However, experiments have also shown that transmission of the nerve signal through the neuromuscular junction, discussed in Chapter 7, can diminish at least a small amount after intense prolonged muscle activity, thus further diminishing muscle contraction
Interruption of _____ through a contracting muscle lead to almost complete muscle fatigue within 1-2 minutes
blood flow
because of the loss of nutrient supply, especially the loss of oxygen.
If we assume that a large biceps muscle has a cross-sectional area of 6 square inches, the maximum force of contraction would be about:
6 x 50 lbs/sq in = 300 pounds
The study of different types of muscles, lever systems, and their movements is called:
kinesiology
The process caused by simultaneous contraction of agonist and antagonist muscles on opposite side of joints
Coactivation of Agonist and Antagonist muscles
**Controlled by the motor control centers of the brain and spinal cord
What do you call the increase in the total mass of a muscle?
Muscle hypertrophy
What do you call the decrease in total mass of a muscle?
muscle atrophy
The condition wherein there is an increase in the number of actin and myosin filaments in each muscle fiber, causing enlargement of the individual muscle fiber
fiber hypertrophy
Significant hypertrophy can result within how many weeks?
6 to 10 weeks
Which component of the muscle fiber increases during hypertrophy?
actin and myosin filaments and enzyme systems that provide energy (especially glycolysis)
What is the pathway that appears to account for much of the protein degradation in a muscle undergoing atrophy?
ATP-dependent ubiquitin-proteasome pathway
These are large protein complexes that degrade damaged or unneeded proteins by a chemical reaction that break peptide bonds.
Proteasomes
**Proteasomes are large protein complexes that degrade damaged or unneeded proteins by proteolysis, a chemical reaction that breaks peptide bonds.
This is a regulatory protein that basically labels which cells will be targeted for proteosomal degradation
Ubiquitin
This condition or process causes new sarcomeres to be added at the ends of the muscle fibers, where they attach to the tendons increasing the length of the muscle fibers and causing hypertrophy
Stretching
What is the mechanism that cause an increase in the actual number of muscle fibers aka hyperplasia?
Linear splitting of previously enlarged fibers
**Fiber hyperplasia
Under rare conditions of extreme muscle force generation, the actual number of muscle fibers has been observed to increase (but only by a few percent), in addition to the fiber hypertrophy process. This increase in fiber number is called fiber hyperplasia. When it does occur, the mechanism is linear splitting of previously enlarged fibers.
During muscle denervation which causes rapid atrophy, how many months will one expect to see degenerative changes to appear in the muscle fibers?
2 months
If the nerve supply to the denervated muscle grows back rapidly, full return of function can occur in as little as 3 months but, from then onward, the capability of functional return becomes less and less, with no further return of function after ____ years.
1 to 2 years
Which type of tissue replace the muscle fibers in the final stage of denervation atrophy?
Fibrous and Fatty tissues
The process by which the fibrous tissue that replaced the muscle fibers during denervation atrophy continues to shorten for several months
Contracture
**The fibrous tissue that replaces the muscle fibers during denervation atrophy also has a tendency to continue shortening for many months, a process called contracture.
What do you call the large motor units formed by the remaining nerve fibers that branched off to form new axons that innervate many of the paralyzed muscle fibers?
Macromotor units
**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 formation of large motor units decreases the fineness of control one has over the muscles but allows the muscles to regain varying degrees of strength.
What causes the state of contracture and rigidity that occurs several hours after death, even without action potentials.
Loss of all ATP
ATP is required to cause separation of the cross-bridges. During death, the loss of ATP results to rigor mortis that remain until the muscle proteins deteriorate about 15-25 hours later.
These conditions include several inherited disorders that cause progressive weakness and degeneration of muscle fibers, which are replaced by Fatty tissue and Collagen
Muscular dystrophies
This is an X-linked recessive condition caused by the mutation of the gene that encodes for the protein that normally links the actin to proteins of the cell membrane or sarcolemma
Duchenne Muscular Dystrophy (DMD)
What is the protein lacking in DMD causing muscle cell membrane destabilization and activation of multiple pathophysiological processes including altered intracellular calcium handling and impaired membrane repair after injury?
Dystrophin
**Dystrophin and associated proteins form an interface between the intracellular contractile apparatus and extracellular connective matrix.
Abnormal dystrophin was found to increase the membrane permeability to:
calcium
**this allows extracellular calcium ions to enter the muscle fiber and initiate changes in intracellular enzymes that ultimately lead to proteolysis and muscle fiber breakdown.
The average age of DMD patients when they are usually confined to wheelchairs
12 years
The common cause of death of DMD patients
Respiratory failure
**usually the age of 30 years
This condition is cause by mutations of the gene that encodes dystrophin but has a later onset and longer survival
Becker Muscular Dystrophy
