1) Muscle cells convert chemical energy from ATP into mechanical force through a sliding filament mechanism involving actin and myosin filaments. 2) Calcium ions released from the sarcoplasmic reticulum activate muscle contraction by causing troponin to move tropomyosin and expose active binding sites on actin for myosin cross-bridges. 3) Myosin cross-bridge binding, ATP hydrolysis, and the power stroke cause actin filaments to slide inward, shortening the sarcomere and contracting the muscle fiber.

1. Muscle Physiology
Chapter 6

2. • Muscle cells are highly specialized for the conversion of
chemical energy to mechanical energy.
• Specifically, muscle cells use the energy in adenosine
triphosphate (ATP) to generate force or do work
• The three basic types of muscle are skeletal muscle,
cardiac muscle, and smooth muscle
• 40 percent of the body is skeletal muscle
• 10 percent is smooth and cardiac muscle

3. • I. TYPES OF MUSCLE TISSUES:
• A. Skeletal Muscles
– composed of skeletal muscle cells, made up of fasicles (bundles)
– has cross striations in LM
– voluntary control (except for some: Proximal 3rd of esophagus)
– comprised all named voluntary muscles in the body
– most originate and/or insert in bone
• B. Cardiac Muscles
– composed of Cardiac Muscle Cells (striated)
– involuntary
– limited to Heart (Myocardium) and Large Blood Vessels attached to heart
– only one or two (mononucleated or binucleated)
– has intercalated discs
• C. Smooth Muscles
– composed of smooth muscle cells (not striated)
– involuntary
– forms muscular component of visceral organs

4. • Skeletal muscle
– is made up of individual muscle fibers that are the “building
blocks” of the muscular system
– Most skeletal muscles begin and end in tendons, and the
muscle fibers are arranged in parallel between the tendinous
ends, so that the force of contraction of the units is additive
– all skeletal muscles are composed of numerous fibers ranging
from 10 to 80 micrometers in diameter

5. Organization of Skeletal Muscle
• Myofilaments  Sarcomeres  Myofibrils  Muscle Cells fibers  Muscle
Bundles  Muscle (thick and thin)
- each muscle is composed of numerous cells called muscle fibers
- Each skeletal muscle fiber contains bundles of filaments, called myofibrils
- A myofibril can be subdivided longitudinally into sarcomeres
- Individual muscle fibers are then grouped together into fascicles
- The fascicles are joined together to form the muscle
myotendinous junction
is a specialized region of the tendon where the ends of the muscle fibers
interdigitate with the tendon for the transmission of the force of contraction of
the muscle to the tendon to effect movement of the skeleton

6. • *Endomysium
– surrounds muscle fibers
– consists of: Basal Lamina + Free Reticular Fibers + Connective Tissues
Elements
• *Perimysium
– connective tissue which envelops muscle bundles / fasicles
• *Epimysium
– -connective tissue which envelops muscles (ex. Gastrocnemius)

8. • A. Myofibrils (makes up a Muscle Fiber)
• smallest unit of contractile apparatus appreciated under LM
• Each myofibril is composed of about 1500 adjacent myosin
filaments and 3000 actin filaments, which are large
polymerized protein molecules that are responsible for the
actual muscle contraction
• have transverse striations of alternating light and dark bands
• -light and dark bands aligned with those of other Myofibrils
accounts for Striations

9. Myofibrils (filaments)
• it is organized to form the basic unit of contraction known as sarcomere
A. Thin filaments are composed of the following proteins:
1. Actin (A) : contains the active site where the myosin head binds
2. tropomyosin : covers the active site
3. troponin: regulatory protein seen only on striated ( Skeletal and cardiac )
muscles.it is compised of 3 subunits namely
i.troponin C: binds with calcium
ii. Troponin I : induces the tropomyosin to cover the actin active sites
iii. Troponin T : atttaches the troponin to the tropomyosin
B. Thick Filaments
are composed primarily of the myosin filaments

10. • the myosin and actin filaments partially interdigitate and
thus cause the myofibrils to have alternate light and dark
bands,
• The light bands
– contain only actin filaments
– are called I bands because
– are isotropic to polarized light.

11. – The dark bands
• contain myosin filaments
• as well as the ends of the actin filaments where they overlap the
myosin
• are called A bands
• are anisotropic to polarized light.
•
cross-bridges-
the small projections from the sides of the myosin filaments
It is the interaction between these cross-bridges and the actin
filaments that causes contraction.

12. sarcomere
• The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z
disks
• repeating units which make up Myofibrils
• -Z-Line to Z-line
• -smallest repetitive subunit of contractile apparatus
• -myofibril consists of numerous sacromeres arranges from end to end
• When the muscle fiber is contracted, 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.
• at this length the muscle is capable of generating its greatest force of contraction.

13. • Differences in the refractive indexes of the various parts
of the muscle fiber are responsible for the characteristic
cross-striations seen in skeletal muscle when viewed
under the microscope.
• The parts of the cross-striations are frequently
• identified by letters A, H, I, M, and Z

14. • 1. Z-Line
– Zwischenscheiben Line;
– Z-band; Z-disc
– dark line which bisects I-Band
• 2. Filaments:
– a. Thick
• occupy the A-Band
• span the region of the A-Band
• midpoints attached at the M-Line
– b. Thin
• run between and parallel to thick filaments
• more numerous but finer and shorter (1um vs. 1.5-1.6um)
• one end is attached to Z line

15. • 3. I-Band
– -portions of thin filaments that DON’T overlap thick filaments
• 4. A-Band
– thick filaments + overlapped thin filaments
– cross sections show thick filament surrounded by 6 thin filaments
• 5. H-Band
– Henle’s Band; Heller Band; Hell Band; Henson’s Band
– portions of thick filament not overlapped by thin
– lighter zone in center of A-Band
• 6. M-Line –
– Mittelscheibe Line
– -region where lateral connections are made between adjacent filaments -thin, dark band
which bisects the H-Band

16. • Myofilaments (Thick and Thin Filaments):
**Contain Four Proteins
• Actin
• Troponin
• Tropomyosin
• Myosin

17. • Thin Filament= Actin + Tropomyosin + Troponin
• Thick Filament = Myosin

18. • Structure of Thin Filament:
– polymers made up of two chains of actin that form a
• long double helix
• F-Actin
– backbone of the actin filament
– principal protein component
– two strands of globular (G-Actin) Molecules
– anchored by proteins (A-Actinin & Desmin) to Z-Line to keep aligned
– has binding sites for Myosin
• Tropomyosin and Troponin
• arranged on both sides of Actin Filament
• regulate muscle contraction

19. • Structure of Thick Filaments (Mainly Myosin)
– myosin-II
– with two globular heads and a long tail.
– The heads of the myosin molecules form cross-bridges with
actin.
– contains heavy chains and light chains,
– its heads are made up of the light chains and the amino
terminal portions of the heavy chains.

20. • has two parts: Tail + Head
– Tail -parts of the 2 heavy chains that twist
– Head
• remaining parts of one heavy chain + two light chains has the binding
site for actin
• comprises 60% of total proteins in Myofibrils (actin = 20%)
• much bigger and heavier than actin
• composed of 6 polypeptide chains: two heavy + 4 light
chains

21. Titin
• Filamentous Molecules of a protein that Keeps the Myosin and Actin
Filaments in Place
• Each titin molecule has a molecular weight of about 3 million, which
makes it one of the largest protein molecules in the body.
• Also, because it is filamentous, it is very springy.
• act as a framework that holds the myosin and actin filaments in place so
that the contractile machinery of the sarcomere will work.
• One end of the titin molecule is elastic and is attached to the Z disk,
acting as a spring and changing length as the sarcomere contracts and
relaxes.
• The other part of the titin molecule tethers it to the myosin thick filament.
• act as a template for initial formation of portions of the contractile
filaments of the sarcomere, especially the myosin filaments.

23. orientation of the sarcomere

24. composition during contraction
sarcomere
I band thin narrows
A band thick and thin constant
H band thick narrows

25. Sarcoplasm
• Is the Intracellular Fluid between Myofibrils.
• 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.

26. Sarcoplasmic Reticulum
• Is a Specialized Endoplasmic Reticulum of Skeletal
Muscle.
• this reticulum has a special organization that is extremely
important in regulating calcium storage, release, and
reuptake and therefore muscle contraction
• The rapidly contracting types of muscle fibers have
especially extensive sarcoplasmic reticula.

27. GENERAL MECHANISM OF MUSCLE CONTRACTION
• The initiation and execution of muscle contraction occur in the following
sequential steps.
• 1. An action potential travels along a motor nerve to its endings on
muscle fibers.
• 2. At each ending, the nerve secretes a small amount of the
neurotransmitter substance acetylcholine.
• 3. The acetylcholine acts on a local area of the muscle fiber
membrane to open “acetylcholinegated” cation channels through
protein molecules floating in the membrane.
• 4. Opening of the 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 opening
of voltage-gated sodium channels, which initiates an action potential at
the membrane.

28. • 5. The action potential travels along the muscle fiber membrane in the
same way that action potentials travel along nerve fiber membranes.
• 6. The action potential depolarizes the muscle membrane, and much of
the action potential electricity flows through the center of the muscle
fiber. Here it causes the sarcoplasmic reticulum to release large
quantities of calcium ions that have been stored within this reticulum.
• 7. The calcium ions initiate attractive forces between the actin and
myosin filaments, causing them to slide alongside each other, which is
the contractile process.
• 8. After a fraction of a second, the 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

29. MOLECULAR MECHANISM OF MUSCLE
CONTRACTION
• Muscle Contraction Occurs by a Sliding Filament Mechanism.
• 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 Z disks have been pulled by the actin filaments up to the ends
of the myosin filaments.
• Thus, muscle contraction occurs by a sliding filament
mechanism.

30. what causes the actin filaments to slide inward among the
myosin filaments?
• 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 1. action potential travels along the muscle fiber --->
• causes the 2. sarcoplasmic reticulum to release large quantities of
calcium ions --->3. The calcium ions activate the forces between the
myosin and actin filaments, and 4. contraction 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.

31. Adenosine Triphosphatase Activity of the Myosin Head.
• myosin head is essential for muscle contraction
• it 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.

32. • Before contraction begins, the heads of the
crossbridges bind with ATP. The ATPase activity of the
myosin head immediately cleaves the ATP but leaves
the cleavage products, ADP plus phosphate ion, bound
to the head.
• When the troponin-tropomyosin complex binds with
calcium ions, active sites on the actin filament are
uncovered and the myosin heads then bind with these
sites

33. Inhibition of the Actin Filament by the TroponinTropomyosin Complex
– the active sites on the normal actin filament of the relaxed
muscle are inhibited or physically covered by the troponin-
tropomyosin complex.
– Consequently, the sites cannot attach to the heads of the
myosin filaments to cause contraction.
– Before contraction can take place, the inhibitory effect of the
troponin-tropomyosin complex must itself be inhibited.

34. Activation of the Actin Filament by Calcium Ions
• presence of large amounts of calcium ions, inhibits the the inhibitory effect of the troponin-
tropomyosin on the actin filaments
• The mechanism : Unknown
– When calcium ions combine with troponin C, each molecule of which can bind strongly
with up to four calcium ions, the troponin complex supposedly undergoes a conformational
change that in some way tugs on the tropomyosin molecule and moves it deeper into the
groove between the two actin strands.
– This action “uncovers” the active sites of the actin, thus allowing these active sites to
attract the myosin cross-bridge heads and cause contraction to proceed.
– Although this mechanism is hypothetical, it does emphasize that the normal relation
between the troponin-tropomyosin complex and actin is altered by calcium ions, producing
a new condition that leads to contraction.

35. • Interaction of One Myosin Filament, Two Actin Filaments,
and Calcium Ions to Cause Contraction
• Interaction of the “Activated” Actin Filament and the
Myosin Cross-Bridges—The “Walk-Along” Theory of
Contraction
• As soon as the actin filament 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
filament, and this, in some way, causes contraction to
occur

36. STEPS IN CROSS BRIDGING
• A. Influx of Calcium triggers exposure of the binding sites on Actin
• B. Binding of Actin to Myosin (energized cross bridge: Myosin-ADP-Pi binds to actin)
• C. Powerstroke of cross bridge that cause the Sliding Filaments (ADP-Pi released from Myosin-Actin)
• D. Binding of ATP which results to Cross Bridge disconnection from Actin
• E. Hydrolysis of ATP which leads to reenergizing and repositioning of the cross bridge
• F. Transport of Calcium back to the Sarcoplasmic Reticulum
Note:Cross Bridges are not bound nor disconnected at the same time Length of Myofilaments do not
change; only H-zone shortens

37. overview of the cross bridge cycling
• step 1. attachment of the head
to the active sites (BINDING)
• step 2. Power stroke (head
bending from 90 to 45 degrees)
• Step 3. Disengagement
(release
• Step 4. Returned to the cocked
position (head extends from 45
to 90 degrees)

38. • The cross-bridge cycling
mechanism just
described is called the
sliding filament theory
because the myosin
cross-bridge is pulling the
actin thin filament toward
the center of the
sarcomere, which results
in an apparent “sliding” of
the thin filament past the
thick filament

39. walk-along mechanism
• the heads of two cross-bridges attaching to and disengaging
from active sites of an actin filament
• When a head attaches to an active site, this attachment
simultaneously causes profound changes in the intramolecular
forces between the head and arm of its cross-bridge.
• The new alignment of forces causes the head to tilt toward the
arm and to drag the actin filament along with it.
• This tilt of the head is called the power stroke
• .Immediately after tilting, the head then automatically breaks
away from the active site.
• the head returns to its extended direction.
• In this position, it combines with a new active site farther down
along the actin filament;
• the head then tilts again to cause a new power stroke
the greater the number of cross-bridges in contact with the actin
• filament at any given time, the greater the force of
contraction.

40. Fenn Effect
• When a muscle contracts, work is performed and energy
is required.
• Large amounts of ATP are cleaved to form ADP during the
contraction process, and the greater the amount of work
performed by the muscle, the greater the amount of ATP
that is cleaved;
• this phenomenon is called the Fenn effect.

41. • The concentration of ATP in the muscle fiber, about
• 4 millimolar, is sufficient to maintain full contraction
• for only 1 to 2 seconds at most. The ATP is split to
• form ADP, which transfers energy from the ATP molecule
to the contracting machinery of the muscle fiber.
• the ADP is rephosphorylated to form new ATP within
another fraction of a second, which allows the muscle to
continue its contraction.

42. There are three sources of the energy for this rephosphorylation
• 1. 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 that of each ATP bond
phosphocreatine is instantly cleaved, and its released energy causes
bonding of a new phosphate ion to ADP to reconstitute the ATP.
– the total amount of phosphocreatine in the muscle fiber is also small—
only about five times as great as the ATP.
– the combined energy of both the stored ATP and the phosphocreatine
in the muscle is capable of causing maximal muscle contraction for
only 5 to 8 seconds.

43. • 2.“glycolysis” of glycogen previously stored in the muscle cells
• . Rapid enzymatic breakdown of the glycogen to pyruvic acid and lactic
acid liberates energy that is 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.
• importance of this glycolysis
• 1.the glycolytic reactions can occur even in the absence of oxygen,
muscle contraction can be sustained for many seconds and sometimes
up to more than a minute, even when oxygen delivery from the blood is
not available.
2. the rate of formation of ATP by the glycolytic process is about 2.5 times
as rapid as ATP formation in response to cellular foodstuffs reacting with
oxygen.

44. • 3. oxidative metabolism
– which means combining oxygen with the end products of glycolysis
and with various other cellular foodstuffs to liberate ATP.
– More than 95 percent of all energy used by the muscles for
sustained, long-term contraction is derived
– foodstuffs that are consumed:
– carbohydrates - 1/2 of energy, up to 2-4 hrs
– fats- greatest proportion of energy comes fom fats, for extremely
long term muscle activity ( over period of many hours
– and protein.

45. energy sources
direct
phosphorylation
Glycolysis oxidative
phosphorylation
speed in generating
ATP
Rapid rapid slow
duration it can supply
ATP
seconds minutes indefinite
utilizes oxygen no no yes

46. MOTOR UNIT
• Motor units are generally composed of only one type of
muscle fiber
• it is the functional contractile unit of a muscle
• it is composed of
• a. anterior horn cell
• b. axon (motor nerve)
• c. Muscle fibers ( innervated by the anterior horn cells)
• a muscle organ (i..e biceps) is composed of several motor
units of different types

48. types of motor unit
Small (type 1) large (type 2)
muscle fibers few (type 1) many (type 2B
fatigue resistant yes no
axons small; slow conducting large; fast conducting
excitability
function
more
recruited first; frequently active
regualr activity
less
recruited only during forceful
contraction
during emergency

49. Skeletal Muscle Types
• 1. White Muscle Fibers (Fast Twitch Glycolytic)
– -uses Glycolysis to synthesize ATP because of the absence of
Oxygen (no myoglobin)
– -fast synthesis of ATP
– -used by Sprinters
• 2. Red Muscle Fibers (Slow Twitch Oxidative)
– -uses Oxudative Phosphrylation (high oxygen because of
Myoglobin)
– -used by Marathon runners because they are able to get
oxygen:
– more time to breathe

50. SKELETAL MUSCLE FIBERS
• 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. slow fibers have a more extensive blood vessel system and more
capillaries to supply extra amounts of oxygen.
• 4. Slow fibers have greatly increased numbers of mitochondria to
support high levels of oxidative metabolism.
• 5. 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 and hence the name red muscle.

51. 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.
• 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 secondary.
• A deficit of red myoglobin in fast muscle gives it the name
white muscle.

52. TYPE 1 SLOW
OXIDATIVE
TYPE 2A
FAST OXIDATIVE
TYPE 2B
FAST GLYCOLTIC
MYOSINE
ISOENZYME
SLOW FAST FAST
CONTRACTION
VELOCITY
SLOW FAST FAST
SARCOPLASMIC
RETICULUM
PUMPING CAPACITY
MODERATE HIGH HIGH
MAIN SOURCE OF
ATP
OXIDATIVE
PHOSPHORYLATION
BOTH GLYCOLYSSIS
FATIGABLE NO NO YES
GLYCOLYTIC
CAPACITY
MODERATE HIGH HIGH
OXIDATIVE CAPACITY:
MYOGLOBIN;DENSITY
HIGH HIGH LOW
OTHER NAME RED RED WHITE
DIAMETER MODERATE SMALL LARGE

54. Length-Tension Relationship
• When muscles contract, they generate force (often measured as tension or
stress) and decrease in length.
• In examination of the biophysical properties of muscle, one of these parameters
is usually held constant, and the other is measured after an experimental
maneuver.
isometric contraction
one in which muscle length is held constant, and the force generated during the
contraction is then measured.
isotonic contraction is one in which the force (or tone) is held constant, and the
change in length of the muscle is then measured.

55. • When a muscle at rest is stretched, it resists stretch by a
force that increases slowly at first and then more rapidly
as the extent of stretch increases.
• This purely passive property is due to the elasticity of the
muscle tissue.

56. • If the muscle is stimulated to contract at these various lengths, a
different relationship is obtained.
• contractile force increases as muscle length is increased up to a
point (designated LO to indicate optimal length).
• As the muscle is stretched beyond LO, contractile force
decreases.
• This length-tension curve is consistent with the sliding filament
theory, described previously
• At a very long sarcomere length (3.7 µm), actin filaments no
longer overlap with myosin filaments, and so there is no
contraction

57. As muscle length is decreased toward LO,
the amount of overlap increases, and contractile force
progressively increases.
As sarcomere length decreases below 2 µm, the thin
filaments collide in the middle of the sarcomere,
the actin-myosin interaction is disturbed, and hence
contractile force decreases.

58. • For construction of the length-tension curves, muscles
were maintained at a given length, and then contractile
force was measured (i.e., isometric contraction).

60. 1. Length–tension relationship
• measures tension developed during isometric
contractions when the muscle is set to fixed lengths
(preload).
• a. Passive tension is the tension developed by stretching
the muscle to different lengths.
• b. Total tension is the tension developed when the muscle
is stimulated to contract at different lengths.
• c. Active tension is the difference between total tension
and passive tension.

61. • ■ Active tension
– represents the active force developed from contraction of the muscle.
– It can be explained by the cross-bridge cycle model.
• ■ Active tension is proportional to the number of cross-bridges
formed.
• Tension will be maximum when there is maximum overlap of thick
and thin filaments.
• When the muscle is stretched to greater lengths, the number of
cross-bridges is reduced because there is less overlap.
• When muscle length is decreased, the thin filaments collide and
tension is reduced.

62. • Muscle length and Active tension
• the greater the muscle length from equilibrium till resting
length, the greater the active tension
• the greater the muscle length from resting length and
beyond, the lesser is the active tension

63. Effect of Muscle
Length on Force of
Contraction in the
Whole Intact Muscle.
when the muscle is at its normal resting
length, which is at a 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—

64. Force–velocity relationship
• ■ measures the velocity of shortening of isotonic
contractions when the muscle is challenged with different
afterloads (the load against which the muscle must
contract).
• ■ The velocity of shortening decreases as the afterload
increases.

65. Force-Velocity Relationship
• The velocity at which a muscle shortens is strongly
dependent on the amount of force that the muscle must
develop.
• In the absence of any load, the shortening velocity of the
muscle is maximal (denoted as V0).
• V0 corresponds to the maximal cycling rate of the cross-
bridges (i.e., it is proportional to the maximal rate of
energy turnover [ATPase activity] by myosin).

66. • Thus V0 for fast-twitch muscle is higher than that for slow-twitch
muscle.
• Increasing the load decreases the velocity of muscle shortening until,
at maximal load, the muscle cannot lift the load and hence cannot
shorten (zero velocity).
• Further increases in load result in stretching the muscle (negative
velocity).
• The maximal isometric tension (i.e., force at which shortening velocity
is zero) is proportional to the number of active cross-bridges between
actin and myosin, and it is usually greater for fast-twitch motor units
(because of the larger diameter of fast-twitch muscle fibers and
greater number of muscle fibers in a typical fast-twitch motor unit).

67. • the power-stress curve reflects the rate of work done at
each load and shows that the maximal rate of work was
done at a submaximal load (namely, when the force of
contraction was approximately 30% of the maximal tetanic
tension)

69. ENERGETICS OF MUSCLE CONTRACTION WORK
OUTPUT DURING MUSCLE CONTRACTION
• work
– is defined by the following equation: W = LxD
– in which W is the work output, L is the load, and D is the
distance of movement against the load
– When a muscle contracts against a load, it performs work. 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.

70. Efficiency of Muscle Contraction
• Efficiency = mechanical work/ ATP consumed
• skeletal muscle has 40-55% efficiency. The rest of the ATP used is loss through heat
• Maximum efficiency can be realized only when the muscle contracts at a moderate
velocity.
• If the muscle contracts slowly or without any movement --> small amounts of
maintenance heat are released during contraction, --> little or no work is performed
---> decreasing the conversion efficiency to as little as zero.
• if contraction is too rapid ---> large proportions of the energy are used to
overcome viscous friction within the muscle itself ---> reduces the efficiency of
contraction.
• Ordinarily, maximum efficiency is developed when the velocity of contraction is
about 30 percent of maximum.
•

71. CHARACTERISTICS OF WHOLE MUSCLE
CONTRACTION
Isotonic contraction occurs when the force of the
muscle contraction is greater than the load and the
tension on the muscle remains constant during the
contraction; when the muscle contracts, it shortens
and moves the load.
Isometric contraction occurs when the load
is greater than the force of the muscle
contraction; the muscle creates tension when it
contracts, but the overall length of the
muscle does not change.

72. type of contraction muscle tension muscle length work done
isotonic constant variable yes
isometric variable constant no

73. Force Summation
• Muscle Contractions of Different Force
• Summation - means the adding together of individual
twitch contractions to increase the intensity of overall
muscle contraction.
• Summation occurs in two ways:
• (1) multiple fiber summation - by increasing the number of
motor units contracting simultaneously
• (2) frequency summation - by increasing the frequency of
contraction, - can lead to tetanization

74. MECHANICS OF SKELETAL MUSCLE
CONTRACTION
Motor Unit –motor neuron and all the muscle cells it
stimulates
Recruitment –stimulus of additional motor units to increase
strength of contraction
Stimulus –any change in internal and external environment
that changes level of excitability

75. • Summation
• means the adding together of individual twitch contractions to increase
the intensity of overall muscle contraction
– 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

76. • Temporal Summation
• -increase muscle tension brought about by increase in frequency of
stimulation
• -2nd stimulus of the same intensity applied before completion of relaxation
• -2nd contraction + 1st contraction
• -2nd peak is higher because of additional influx of Calcium promotes a 2nd
contraction, which is
• added to the 1st contraction
• -at 100-110 ms time interval, the height of the 2nd curve = height of 1st curve
bec:
• a. Cross bridge cycling has stopped (no actvitiy to be summed) b. Calcium
Ions from 1st contraction are all transported back into cisternae

77. Multiple Fiber Summation
• a. Treppe (Staircase Effect) -increase in tension may
result from increased muscle warming efficiency of
enzyme
• -strength of contraction is increased but relaxation was
complete
• b. Temporal Summation
• -due to increase in tension may result from an increase
availability of intracellular Calcium
• -result in continual increase tension as one contraction
was added to previous

78. • c. Incomplete Tetanus (Un-fused Tetanus)
• -rapid shortened contraction
• -some degree of relaxation is visible after each contraction
• -w/ increase frequency of stimulation muscle exhibits shorter contraction
cycle
• d. Complete Tetanus (Fused Tetanus)
• -with rapid multiple stimulation, contraction fuse into smooth, continuous,
total
• contraction without evidence of any cyclical relaxation
• -abundant intracellular calcium provides continual availability of binding sites
• on action for cross bridge cycling

79. • e. Fatigue
• -with continued rapid stimulation, muscle is no longer able to sustain its level
• of tension but gradually elongates
• **Factors which cause Fatigue:
• 1. Build up of Acidic Compounds w/c affect protein functioning
• 2. Relative lack of ATP
• 3. Ionic Imbalances resulting from membrane activities
• 4. With rest and adequate blood supply, fatigue is corrected
• Interruption of blood flow through a contracting muscle leads to almost complete muscle fatigue within 1 or 2 minutes because of
the loss of nutrient supply, especially the loss of oxygen

80. Skeletal Muscle Tone
• Even when muscles are at rest, a certain amount of
tautness usually remains, which is called muscle tone.
• Because normal skeletal muscle fibers do not contract
without an action potential to stimulate the fibers, skeletal
muscle tone results entirely from a low rate of nerve
impulses coming from the spinal cord.

81. REMODELING OF MUSCLE TO MATCH FUNCTION
• Muscle Hypertrophy
– The increase of the total mass of a muscle
Muscle Atrophy
the total mass decreases
Virtually 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.

82. • 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.
• When a muscle remains unused for many weeks, the rate of
degradation of the contractile proteins is more rapid than the rate of
replacement. Therefore, muscle atrophy occurs.
ATP-dependent ubiquitin-proteasome pathway -
• The pathway that appears to account for much of the protein
degradation in a muscle undergoing atrophy

83. Hyperplasia of Muscle Fibers
• .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.

84. Muscle Denervation Causes Rapid Atrophy.
• When a muscle loses its nerve supply, it no longer
receives the contractile signals that are required to
maintain normal muscle size.
• atrophy begins almost immediately.
• After about 2 months, degenerative changes also begin
to appear in the muscle fibers.
• If the nerve supply to the muscle grows back rapidly, full
return of function can occur in as little as 3 months, but
from that time onward, the capability of functional return
becomes less and less, with no further return of function
after 1 to 2 years.

85. contracture
• The fibrous tissue that replaces the muscle fibers during
denervation atrophy also has a tendency to continue
shortening for many months
• one of the most important problems in the practice of
physical therapy is to keep atrophying muscles from
developing debilitating and disfiguring contractures.
• This goal is achieved by daily stretching of the muscles or
use of appliances that keep the muscles stretched during
the atrophying process

86. Excitation and Contraction of Smooth Muscle
• smooth muscle, which is composed of far smaller fibers
that are usually 1 to 5 micrometers in diameter and only
20 to 500 micrometers in length
• has thick and thin filaments that are not arranged in
sarcomeres; therefore, they appear homogeneous rather
than striated
• Functions
• a. motility (intestines)
• b. Storage (urinary bladder)
• c. conduit (airway or blood vessels)

87. CONTROL OF ACTIVITY
• A. Neural: Autonomic Nervous system
• B. Non Neural: Hormones or PAcemaker Cells
• TYPES of smooth muscle
• A. Based on Duration of Contraction
• 1. Primarily Phasic : contract intermittently or
rhythmically (stomach, intestines)
• 2. Primarily TOnic: contracts continuously (sphincter
muscles, blood vessels)

88. type based on coordination of COntraction
• 1. Multi-unit smooth muscle
• ■ is present in the iris, ciliary muscle of the lens, and vas
deferens. ■ behaves as separate motor units
• contract independently from each other
• has little or no electrical coupling between cells.
• ■ is densely innervated; contraction is controlled by
neural innervation (e.g., autonomic nervous system).

89. • 2. Unitary (single-unit) smooth muscle
• contract in coordinated fashion
• ■ is the most common type and is present in the uterus,
gastrointestinal tract, ureter, and bladder
• ■ is spontaneously active (exhibits slow waves) and
exhibits “pacemaker” activity which is modulated by
hormones and neurotransmitters.
• ■ has a high degree of electrical coupling between cells
and, therefore, permits coordinated contraction of the
organ (e.g., bladder).

90. • 3. Vascular smooth muscle ■ has properties of both
multi-unit and single-unit smooth muscle.

91. comparative physiology of the muscle types
skeletal cardiac smooth
usual mechanism for initiation of
contraction
acetylcholine stimulating the
nicotinc receptors
pacemaker potential -acetylcholine stimulating the
nicotinic receptors -synaptic
transmission
pacemaker potentials
-receptor ligand stimulation
dihydroperidine receptors remains close always open sometimes open in some muscle
participation of extracellular calcium never always sometimes
amount of calcium released by the
Sarcoplasmic Reticulum
constant variable variable
Ca ++ sensor troponin troponin calmodium
cross bridge cycling 4 steps 4 steps 4 steps (phasic)
6 steps (tonic)
trigger for relaxation low mycoplasmic calcium due to
the Ca-Mg ATPase pump of the
sarcoplasmic reticulum
Low mycoplasmic calcium due to
the action of the
Ca-Mg ATPase pump of the
sarplasmic reticulum
Plasma membrane's active Ca
pump
Plasma membrane's Ca-Na
exchange pump
Deactivation of the calcium
calmodulin complex
Activation of the myosin
phosphatase