Skeletal Muscle Contraction
Dr Asma Jabeen
Learning objectives
At the end of this lecture the students should be able to:
a. Identify the internal structure of skeletal muscle fiber.
b. Describe mechanism of action of actin and myosin during muscle
contraction.
c. Identify the role of calcium in muscle contraction.
d. Explain mechanism of contraction in different muscle fibers.
e. Explain the Sources of energy for muscle contraction.
f. Distinguish between fast versus slow muscle fibers.
g. Identify clinical application (Muscle fatigue, denervation and
electromyography).
Skeletal Muscle
Skeletal muscles attach to skeleton.
 Contraction moves bones, allowing the body
to perform a variety of motor activities

Homeostatic functions:
Acquiring, chewing and swallowing food
 Essential for breathing
 To move the body away from harm
 Heat generating muscle contractions

Physiologic Anatomy of
Skeletal Muscle
Cellular Organization
Muscle fibers
• single cells
• multinucleated
• Surrounded by the sarcolemma
• Outer coat: thin layerof polysaccharide material with numerous
thin collagens
Myofibrils
• contractile elements
• surrounded by the
sarcoplasm
Cellular organelles - lie between myofibrils (mitochondria,
sarcoplasmic reticulum etc.)
Molecular Organization
E
F
Myosin Filament
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




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Multiple myosin molecules, mol wt 480,000
Each molecule composed of six polypeptide chains
Two heavy, mol wt 200,000, Four light chains..
mol wt 20,000
Tails of molecules bundled together to form BODY
of the filament
Many heads hang outward along with part of the
body called arm forming the CROSS BRIDGES
Two HINGES taking part in actual contraction
process
ATPase activity of myosin head
Myosin Filament
The Actin Filament
− the I band filament
− tethered at one end at the Z disc
− 1 mm long: v. uniform nebulin forms guide
for synthesiT
F-actin
• double-stranded helix
• composed of polymerized G-actin
• ADP bound to each G-actin
(active sites)
• myosin heads bind to active sites
Tropomyosin
• covers active sites
• prevents interaction
with myosin
Troponin
• I - binds actin
• T - binds tropomyosin
• C - binds Ca2+
Structure of Actin and Myosin
Interaction of one myosin filament, two
Actin filaments and calcium ions to
cause contraction
A pure actin filament binds strongly and instantly with
myosin heads. Binding is prevented by presence of
toponin- tropomyosin complex. Before contraction, the
Inhibitory effect should be inhibited
Role of calcium ions
Calcium binding with troponin C , causes
a conformational change in troponin complex that tugs
on the tropomyosin molecule and moves it deeper into
Groove between the two actin strands
Steps in skeletal muscle contraction
(excitation contraction coupling)
Action potential along sarcolemma
II. AP down to T-tubule
III. Calcium released from terminal cisternae SR into myoplasm
IV. Increased intracellular calcium
V. Binding of calcium to troponin C
VI. Displacement of tropomyosin exposing myosin binding site
on actin
VII. Thus CROSS BRIDGE form between actin and myosin
VIII. Conformational change in myosin
IX. Shortening of sarcomere and contraction of the muscle
fibre
I.
Walk along Theory of contraction
OR
Ratchet theory of contraction
Walk along Theory of contraction
The heads of the cross bridges bend back and forth
and step by step walk along the actin filament, pulling
the ends of the two successive actin filaments towards
the center of the myosin filament
Greater the number of cross bridges in contact with
the actin filament at any given time, the greater is the
force of contraction.

The attachment of head to the active site of
actin causes profound changes in intramolecular
forces between head and arm of its cross bridge.
 The new alignment of forces causes the head to tilt
towards the arm and to drag the actin filament
 This tilt of the head is called “POWER STROKE”

Tilting causes detachment of head, it extends back
and attaches to another active site
Actin and Myosin Interaction
SLIDING FIALMENT THEORY
Role of calcium and ATP in skeletal muscle
contraction
Contraction of skeletal muscle requires increase in
intracellular calcium
 Calcium-binding protein (calsequestrin) helps hold
calcium ions in SR (Ca+2 concentration 10,000 times
higher than in cytosol)
 ATP is required for the release of myosin head from
actin.
 After death since ATP is no longer available the
skeletal muscles of the body remains in sustained
contracted state as known as rigor mortis.

Sources of energy
1. Phosphocreatine: carrying a high energy
Phosphate bond. Immediately cleaved and energy released is used
to reconstitute ATP. Along with ATP provide energy for 5 to 8 sec.
2. Glycolysis : of glycogen previously stored in
muscle. Formation of pyruvic acid and lactic
acid liberates energy, used to convert ADP to
ATP
 ATP is then used directly for muscle contraction and also to
reform the stores of phosphocreatine
3.
Oxidative metabolism:
It is combining of oxygen with end products of glycolysis
and with various other cellular foodstuffs like CHO,
fats and proteins to liberate ATP.
Fast twitch muscles (=IIA or IIB)
e.g. lateral rectus muscle
II. Slow twitch muscle (=IA)
e.g. soleus muscle
 Fast fibres are used for short time period , their
activity is fast but they fatigue quickly
 Slow fibres are used for sustained activity like
maintenance of posture
 Most muscles contain a mixture of these three fiber
function
of
the
muscle.
types,
and
their
proportion
Types of skeletal muscle fibres
depends upon the
I.
Type I:Slown
oxidative
(red)
Type IIB: fast Type IIA :fast
glycolytic
oxidative
(white)
(red)
Myosin
isoenzyme
(ATPase rate)
slow
fast
Fast
Sarcoplasmic
reticulum
calcium
pumping
capacity
Moderate
High
High
Oxidative
capacity:
mitochondria
content,
caplillary
density,
myoglobin
high
Low
Very high
Glycolytic
activity
Moderate
High
High
Diameter
Moderate
Large
Small
CHARACTERIS SKELETAL
TICS
MUSCLES
CARDIAC
MUSCLE
SMOOTH
MUSCLE
STRUCTURE
STRIATED
STRIATED
NON STRIATED
SHAPE
CYLINDRICAL
CYLINDRICAL
SPINDLE SHAPED
SARCOTUBULAR
SYSTEM
PRESENT, T TUBULE
AT A-I JUNCTION
POORLY
DEVELOPED T
TUBULE AT Z LINE
PRESENT BUT NOT
CHARACTERISTIC
SARCOPLASMIC
RETICULUM
DEVELOPED
MORE DEVELOPED
POORLY
DEVELOPED
NERVE
SUPPLY
SOMATIC
NERVES
AUTONOMIC
NERVES
AUTONOMIC
NERVES
CONTROL
VOLUNTARY
INVOLUNTARY
INVOLUNTARY
EXCITATION
CONTRACTIO
N COUPLING
RAPID
MORE RAPID
VERY SLOW
I. Muscle fatigue
 Muscle fatigue is the
decline in ability of a muscle to
generate force.
 Fatigue is not the result of depletion of energy stores
rather it is due to accumulation of metabolic by
products.
 Accumulation of lactic acid decreases myoplasmic
pH and inhibits actin myosin interactions.
 Fast fibres are more sensitive to the effects of low
pH.
II.



Electromyography
Electromyography (EMG) is a technique for
evaluating and recording the electrical activity
produced by skeletal muscles.
EMG is performed using an instrument called an
electromyograph, to produce a record called an
electromyogram.
An electromyograph detects the electrical potential
generated by muscle cells when these cells are
electrically or neurologically activated.
Denervation
When motor nerve to a skeletal muscle is cut it causes
i. Disuse atrophy- ultimately fibrous muscle
ii. Flaccid paralysis
iii. Abnormal excitability of the muscle and its
increased sensitivity to circulating acetylcholine
III.
Thank You