- Skeletal muscle makes up 40% of total body mass and contains long, striated muscle fibers that are multinucleated. - Each muscle fiber contains numerous myofibrils composed of thin actin filaments and thick myosin filaments that overlap to form dark A bands and light I bands. - The functional unit of skeletal muscle is the sarcomere, defined as the segment between two Z disks, which contains overlapping actin and myosin filaments that slide past each other during muscle contraction. - Excitation-contraction coupling involves an action potential triggering calcium release from the sarcoplasmic reticulum, allowing calcium to bind troponin and initiate the contraction of actin and myosin

1. Skeletal Muscle

2.  40% of total body is skeletal muscle and other 5 to 10 %
is smooth muscle and cardiac muscle.
 Skeletal Muscle is voluntary, having cross striations.
 Each Skeletal muscle fiber is a muscle cell having a
diameter of 10 to 80 micrometer and length ranging from
few millimeters to centimeters.
 The outer membrane of muscle fiber is sarcolemma.
 There are also cell organelles and nuclei
(multinucleated)

4.  In a single muscle fiber there are large number of
myofibrils and these are present along the long axis of
muscle fiber.
 Diameter of each myofibril is 1 to 2 micrometer.
 Each myofibril is composed of two types of myofilaments
i.e. Thin or Actin filaments and Thick or Myosin
filaments.
 The thickness of actin filament is 15 angstrom unit and
its length is about 1 micron.
 The thickness of myosin filament is 100 to 120 angstrom
unit and its length is about 1.5 to 1.6 micrometer.
 These two types of filaments alternate or interdigitate
and partially overlap to form alternate dark and light
bands.
 In each myofibril there are about 3000 actin and around

5.  Dark bands are called A bands
 Light bands are called I bands.
 Dark bands consists of Myosin filaments and portion of actin
overlapping the myosin filaments.
 Light bands consists of only actin filaments.
 A band is anisotropic which means it has got unequal
refractive index to polarized light.
 I band is isotropic which means having uniform refractive
index to polarized light.
 In the centre of A band there is a lighter band called H zone.
H is for Hensen which means light, where there is no actin
overlapping the myosin.
 In the centre of H zone there is a line called M line (for
'mittelscheibe' or middle disc).
 In the centre of I band there is Z Disk. This gives attachment
to actin filaments on both sides.
 Z is for Zwischenscheibe which means disk or in between.

7.  The cross striatioins are due to alternate dark and light
bands.
 Z disk of all the muscle fibers are at the same level or
are continuous.
 There are small projections from the surface of the
myosin filaments and these projections are called cross
bridges.
 These cross bridges are absent in an area of 0.2 micron
in the centre.
 The portion of these myofibrils between two successive
z disks is called sarcomere.
 Sarcomere is the functional unit of the skeletal muscle.
 Sarcomere consists of 1 A Band in centre and portion of
I bands at both ends.
 At rest length of the sarcomere is between 2 to 2.5

8.  Skeletal Muscle consists of 80 % water and 20 %
proteins.
 In the Skeletal muscle there are three types of proteins.
 Stromal proteins i.e. collagen and elastin
 Cellular proteins i.e. albumin and globulin.
 Special Proteins i.e. Myoglobin and contractile proteins.

9. MYOGLOBIN
 It is a heme containing conjugated protein having
high affinity for oxygen.
 It can bind oxygen at very low PO2.
 P50 values of myoglobin is 6 mm of Hg.
 Myoglobin acts as a reservoir of oxygen in
muscle.

10. CONTRACTILE PROTEINS
 These proteins are involved in the contraction
mechanism. These include:
 Myosin
 Actin
 Tropomyosin
 Troponin
 Actinin
 Tyietin
 M proteins.

11. Myosin
 It is present in myosin filament.
 Each myosin filament has 200 molecules of Myosin.
 Each myosin molecule has got molecular weight of 480,000.
 Each molecule is composed of 6 polypeptide chains. 2 heavy
and 4 light chains.
 Each heavy chain has got a molecular weight of 200,000.
 The two heavy chains are wrapped together to form a double
helix.
 At the end each heavy chain is folded on itself to form the head
portion.

14. Myosin
 In the head portion there are 4 light chains each having a
molecular weight of 20,000
 Each myosin molecule has three parts i.e head, arm and tail or
body.
 The tail portion of myosin molecules are inside the myosin
filaments while the head and arms are directed outwards and
these project out of the surface of myosin molecule and the
head and the arm forms a cross bridge.
 In the central portion of myosin filament there will be only the tail
or body portion. So in the central portion there is no cross
bridge.

15. Myosin
 The myosin molecule is highly flexible at two points and
these points are called Hinges.
 One point is junction between head and arm and second
point is junction between arm and body.
 So at these hinges the molecule is flexible.
 There are two important sites on myosin heads i.e.
 Actin binding site
 Catalytic site
 The actin binding sites interacts with the active site on the
actin filaments.
 Catalytic site has got ATPase activity

17. Actin
 It forms the skeleton of the actin filaments.
 It consists of 2 F -Actin strands coiled together.
 Each F -Actin strand have a molecular weight of about 42,000.
 Each F -Actin strand is composed of G-actin molecule
 So we can say that G actin molecules polymerize to from F actin
strands.
 Attach to each G actin molecule is a molecule of ADP.
 Where this molecule is attached, the site is known as active site
of actin filament.
 Active sites on actin filaments are at a distance of 2.7 nm.

19. TROPOMYOSIN
 There are two tropomyosin strands which are wrapped
together
 Each strand consists of tropomyosin molecules
 At rest tropomyoin strands are in such a position that these
cover the active sites on actin filaments physically
 This prevents the interaction between actin and myosin
and thus prevent contraction
 During contraction these strands move to deeper positions
to unmask the active sites to start contraction.

21. TROPONIN
 It is a complex of small proteins attached to tropomyosin at a
distance of 40 nm.
 Troponins have molecular weight of 18000 to 35000.
 There are three types of troponins i.e. Troponin T, Troponin I
and Troponin C.
 Troponin T has affinity for tropomyosin. So through this the
complex is bound to tropomyosin.
 Troponin I has Affinity for Actin and the bound troponin I and
Actin keeps tropomyosin in such a position that at rest,
tropomyosin physically covers the active sites.
 Troponin C has got affinity for calcium Ions.

23.  ACTININ: It is a protein which attaches the actin filaments
with the Z disks.
 TITIN: It connects the Z disk with the M line.
 M PROTEIN: It is in the M line.

24. SarcoTubular System

25.  There is Sarcoplasmic reticulum in the Skeletal Muscle
fiber which has got specific type of arrangement called
sarcotubular system.
 It consists of longitudinal tubules and transverse
tubules.
 The longitudinal tubules have got dilated terminal ends
called terminal cisternae and these surround the T
Tubules
 T Tubules start from one side of the muscle fiber to the
other side.
 It opens into the ECF and also contains the ECF.

27.  If we take longitudinal section we see two terminal
cisternae and in the center a TTubule.
 This is called Triad.
 Sarcotubular system is important for storage, release
and reuptake of calcium ions.
 Calcium ions released from the Terminal Cisternae of
Sarcotubular system Initiates the mechanism of
Contraction.

28.  A continually active calcium pump located in the
walls of the sarcoplasmic reticulum pumps
calcium ions away from the myofibrils back into
the sarcoplasmic tubules.
 This pump can concentrate the calcium ions
about 10,000-fold inside the tubules.
 In addition, inside the reticulum is a protein called
calsequestrin that can bind up to 40 times more
calcium

29.  Depolarization of the muscle fiber results into its
contraction
 As a result of depolarization calcium ions are released
from the terminal cisternae of the sarcoplasmic
reticulum.
 These calcium ions bind with troponin C to initiate the
contraction mechanism
 These 2 process i.e. excitation and contraction are
coupled by calcium ions

30. Excitation Contraction Coupling

31. COUPLING
 The process of converting an electrical stimulus i.e.
Action potential to a mechanical response i.e.
muscular contraction is called as Excitation
contraction coupling.

33. COUPLING
 After the NMJ transmission AP travels along the
Sarcolemma.
 Then it enters deep into the muscle fiber through T
Tubules.
 Depolarization from here spreads to the membrane
of terminal cisternae.
 This causes the opening of voltage gated calcium
channels causing release of calcium form the
terminal cisternae into the sarcoplasm.
 This calcium then binds with troponin c to cause
contraction.

34. CHANGES in SARCOMERE WHICH
OCCUR DURING CONTRACTION

35.  Shortening of sarcomere
 The 2 Z Disks become closer
 H zone disappears or reduces
 Ends of actin filaments may touch or overlap each other
 I band decreases
 A band remains unchanged
 Size of the actin and myosin filaments remain
unchanged