Jimmy Kimber
LECOM – Seton Hill OMS1
Physiology Chapter 6: Contraction of Skeletal Muscle
Learning Objectives
1. What is the sarcolemma?
Sarcolemma is the cell membrane of the muscle fiber. It consists of a true cell membrane
(plasma membrane) and an outer coat made up of a thin layer of polysaccharide that contains
collagen fibrils. At the end of each fiber, the sarcolemma fuses with a tendon fiber.
2. Define myofibril
Myofibrils are composed of adjacent myosin and actin filaments. Groups of myofibrils bound
together for a muscle fiber. The myofibril is divided up into sarcomeres by locations lying
between two successive Z-discs.
3. Use figure 6-1 to describe composition of the sarcomere
Sarcomere has regions:
Isotropic (I) Bands – light bands containing only actin filaments
Anisotropic (A) Bands – dark bands where actin and myosin overlap
Z – Disc – filamentous proteins from which filaments extend to interdigitate with myosin
Jimmy Kimber
LECOM – Seton Hill OMS1
4. What keeps myosin and actin filaments in place?
A very large number of filamentous molecules of the heavy protein titin keeps myosin and actin
filaments in place. Titin is very springy, which helps it to act as a framework to hold myosin and
actin in place so that the contractile machinery of the sarcomere will work.
5. What are components of sarcoplasm?
Sarcoplasm contains: potassium, magnesium, phosphate, protein enzymes, and MANY
MITOCHONDRIA – for energy. Also, there will be a sarcoplasmic reticulum that is more
developed in rapidly contracting types of muscle fibers.
6. Describe the steps of muscle contraction
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Action potential travels along motor nerve
Nerve secretes acetylcholine into the motor end plate
Acetylcholine opens multiple acetylcholine-gated channels
Channels allow large quantities of sodium ions to diffuse to interior = action potential
Action potential travels along muscle fiber membrane
Potential depolarizes the muscle membrane and electricity flows through the center of the
fiber, causing release of Ca2+ from sarcoplasmic reticulum
Ca2+ initiates attractive forces between actin and myosin
Ca2+ is pumped back into sarcoplasmic reticulum to end contraction
7. What is the reason for the sliding filament mechanism?
The sliding filament mechanism says that actin filaments slide inward along the myosin
filaments to cause muscle contraction. It occurs because of forces generated by cross-bridges
between the actin and myosin. Under resting conditions, the forces are inactive. But, during an
action potential, the influx of calcium allows for activation. Energy is needed for the reaction,
which comes from ATP.
8. Describe the structure of the myosin filament
The myosin filament is made of multiple myosin molecules. Each molecule is composed of 6
polypeptide chains: 2 heavy chains and 4 light chains. The heavy chains are wrapped spirally to
form a double helix, called the tail. At each end of the chain is a globular peptide called the
head. The 4 light chains are part of the myosin head (2 per head), and they control its
movement during contraction.
The myosin filament has 200 or more myosin molecules bundled together at their central
portions to form a body with many heads hanging outward. The body hangs to the side with the
head, providing an arm with two flexible hinges.
9. What is the role of ATPase on the myosin head?
The myosin head itself functions as an ATPase enzyme which allows it to cleave ATP and use the
energy from the ATP’s high-energy phosphate bond to energize the contraction process.
Jimmy Kimber
LECOM – Seton Hill OMS1
10. Describe the actin filament
The actin backbone is made of a double stranded F-actin protein molecule wound in a helix.
This is made by polymerized G-actin molecules, each attached to one molecule of ADP (which
provides the active site for cross-bridge interaction of the myosin filament). The actin filament
also contains tropomyosin, laid over the active sites on the actin strands, as well as troponin
molecule complexes bound to the side of the tropomyosin.
11. What is tropomyosin, troponin?
Tropomyosin is a protein that wraps spirally around the sides of the F-actin helix. It lies on top
of the active sites so that attraction can’t occur between actin and myosin.
Troponins are protein molecules that lie along the sides of the tropomyosin complexes. It
attaches the tropomyosin to the actin, and with its affinity for calcium, initiates the contraction
process:
- Troponin I has affinity for actin
- Troponin T has affinity for tropomyosin
- Troponin C has affinity for Calcium
12. Why is the tropomyosin-troponin complex inhibitory? How is it inhibited?
A pure actin filament without troponin-tropomyosin will bind instantly with the heads of a
myosin molecule. So, it is believed that the active sites of actin in relaxed muscle are physically
covered by this complex.
To uncover it, calcium ions combine with troponin C, which causes a conformational change that
tugs on the tropomyosin molecule and moves it deeper into the groove between the two actin
strands.
13. What is the “walk-along” (ratchet) theory?
The heads of 2 cross bridges attach to an active site, and this causes profound changes in the
intermolecular forces between the head and arm of its cross bridge. The new alignment causes
the head to tilt toward the arm and drag actin along with it – the power stroke. Then, then head
breaks away and returns to its original position to repeat the maneuver.
14. What is the Fenn Effect?
The greater the amount of work that is performed by the muscle, the greater the amount of ATP
that is cleaved.
15. Describe cross-bridge cycling
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Before contraction, ATP binds with the heads of the cross-bridges. ATPase activity cleaves it
into ADP and phosphate, which remain bound
Troponin-tropomyosin complex binds to calcium, and allows binding of myosin and actin
Binding of the head and active site causes a conformational change and provides the power
stroke for pulling the actin filament using the stored energy from ATP
Jimmy Kimber
LECOM – Seton Hill OMS1
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ADP and phosphate are released
A new ATP binds, causing detachment of the head from actin
Cleavage of ATP moves the arm back to its perpendicular position
16. Use figure 6-8 to describe the effect of filamentous overlap on tension development
Point D: actin filament has pulled all the way out to the end of the myosin filament, with no
actin-myosin overlap. The tension developed by the activated muscle is zero
Point B: two actin ends begin to overlap
Point C: actin overlaps all cross bridges with myosin but not at the center of the myosin filament
Point A: two Z discs abut the ends of the myosin filaments
17. How is muscle length (stretch) related to tension developed?
As you increase length, the tension increases up to a point, then decreases. When a muscle is in
its normal resting length (2 um) it contracts upon activation with the approximate maximum
force of contraction. However, the increase in tension that occurs during contraction (active
tension) decreases as the muscle is stretched beyond normal length.
Jimmy Kimber
LECOM – Seton Hill OMS1
18. What is the relation of the load applied to the velocity of contraction?
Load is inversely proportional to velocity. When the load is increased to the maximum force
that the muscle can exert, the velocity of contraction becomes zero and no contraction results,
despite activation of the muscle fiber.
19. What is work?
W = LxD , Load x Distance
20. Describe each of the 3 sources of energy for muscle contraction
Energy is needed for: walk-along mechanism, pumping calcium ions from sarcoplasm into
sarcoplasmic reticulum, and maintaining the ionic environment with Na/K-ATPase
Sources:
- Phosphocreatine: slightly higher free energy than ATP bonds, total amount is little, and
causes contraction for only 5-8 seconds
- Glycolysis of glycogen: anaerobic process; rate of formation is 2.5x faster than oxidative
metabolism, causes contraction for up to 1 minute
- Oxidative metabolism: over 95% of energy for sustained contraction, supplied by fats,
carbohydrates, proteins
21. Why is muscle contraction not very efficient? How can the efficiency be maximized?
Efficiency is the amount of energy input that can be converted to work, not heat.
In muscles, efficiency is <25% because ½ of the energy is lost during ATP formation. About 4045% of energy in ATP can be converted to work.
Jimmy Kimber
LECOM – Seton Hill OMS1
Efficiency can be maximized if the muscle contracts at a moderate velocity (approximately 30%
of Vmax). If too slow, maintenance heat is lost. If too fast, there is energy wasted in
overcoming friction.
22. Define isometric, isotonic
Isometric – muscle doesn’t shorten during contraction
Isotonic – muscle shortens, but tension remains constant
23. Differentiate between fast and slow muscle fibers
Fast muscle (white) : large, have great strength of contraction
- Extensive sarcoplasmic reticulum for rapid Ca2+ release
- Many glycolytic enzymes
- Less extensive blood supply; less O2 use
- Fewer mitochondria
Slow muscle (red): smaller fibers; innervated by smaller nerves
- More extensive vasculature
- Greatly increased mitochondria
- Many myoglobin (combines with O2 and stores it), has red appearance
24. What is the motor unit? How is its size related to function?
Motor unit – all the muscle fibers innervated by a single nerve fiber
Small muscles requiring fine control need many smaller units. Large muscles that do not require
fine control may have many muscle fibers per motor unit.
25. Describe the 2 types of force summation:
Multiple Fiber Summation: CNS sends a weak signal to contract a muscle, and the smaller motor
units are stimulated in preference to the larger ones. As the signal strength increases, the larger
units begin to be excited (size principle). This is important because it allows for gradation of
force. Different units are driven asynchronously; gives smooth contraction
Frequency Summation: the frequency of contraction allows one contraction to start before the
preceding once ends. So the strength of the second adds to the first.
26. Why does tetanization occur?
Tetanization: the frequency reaches a critical level where contractions are so rapid they fuse
together. Beyond this point, the frequency is so fast that calcium ions are maintained in the
muscle sarcoplasm even between action potentials so that full contractile state is maintained.
27. Describe the staircase effect
Also called Treppe – when a muscle contracts after a long period of rest, its initial strength is
much less than that of the strength 10-50 twitches later. The strength increases to a plateau. It
Jimmy Kimber
LECOM – Seton Hill OMS1
is caused by increasing calcium ions in the cytosol because of progressive release from the SR
with each action potential.
28. What maintains skeletal muscle tone?
Muscle tone is the tautness that remains when the muscles are at rest. It is the result of low
rate nerve impulses from the spinal cord.
29. What must you know to analyze the lever systems of the body?
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Point of muscle insertion
Distance from the fulcrum
Length of the lever arm
Position of the lever
30. Describe the types of muscle remodeling
Hypertrophy: total mass increases; increase in the number of actin and myosin filaments;
synthesis of contractile proteins is greater
Hyperplasia: extreme muscle force generation causes linear splitting of previously enlarged
fibers
Atrophy: result of denervation; causes destruction of muscle fiber and replacement by fibrous
fatty tissue.
31. Describe the effects of poliomyelitis
In polio, some but not all of the nerve fibers are destroyed. The remaining nerve fibers branch
off to form new axons, which causes large motor units (macromotor units). This decreases the
fineness of control but allows the muscles to regain some degree of strength.
32. What causes rigor mortis?
Rigor mortis is a state of contracture in which the muscles contract and become rigid. It results
from loss of ATP, which is required to cause separation of the cross-bridges from the actin
filaments. It lasts until the proteins deteriorate about 15-25 hours after death.