MUSCLE
DR. AYISHA QURESHI
ASSISTANT PROFESSOR
MBBS, MPhil
MUSCLE
(1) purposeful movement of
the whole body or parts of
the body (such as walking or
waving your hand),
(2) manipulation of external
objects (such as driving a car
or moving a piece of
furniture),
(3) propulsion of contents
through various hollow
internal organs (such as
circulation of blood or
movement of a meal through
the digestive tract), and
(4) emptying the contents of
certain organs to the external
environment (such as
urination or giving birth).
MUSCLE:
Chemical energy
↓Muscle
Mechanical energy
Muscle forms about 50% of
the total body weight:
 40% skeletal muscle
 10% smooth & cardiac
muscle
Simply put, Muscles perform
the following functions:
 They contract…
 They generate heat
 They generate motion
 They generate force
 They provide support
TYPES of MUSCLE
(According to appearance or movement)
Muscle
Skeletal Muscle
Smooth Muscle
Cardiac Muscle
(Striated)
(Smooth)
(Striated)
(Voluntary)
(Involuntary)
(Involuntary)
Types of Muscle
SKELETAL MUSCLE:
Characteristics of
Skeletal Muscles:
 Attach to the
bone
 Move
appendages
 Support the
body
 Antagonistic
pairs: Flexors
& extensors
SKELETAL MUSCLE ANATOMY:
SKELETAL MUSCLE CELL STRUCTURE
A single skeletal muscle cell is also called a MUSCLE FIBER b/c of its greater
length than width.
•
•
•
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LENGTH: upto 75,000 µm or 2.5 feet.
DIAMETER: from 10 to 100 micrometers.
SHAPE: elongated & cylindrical.
OUTER MEMBRANE: called sarcolemma.
Nucleus & Organelles: present. Mitochondria, microsomes & ER
What is the chemical composition of the muscle?
 Proteins (20%) (either as enzymes or for muscle Cont.)
 Lactic Acid (in muscle that has undergone fatigue)
 ATP, ADP
 Myoglobin (stores O2 & gives colour to the muscle)
Skeletal Muscle Organization
Whole Muscle (an organ)
↓
Muscle Fiber (a single cell)
↓
Myofibrils (a specialized structure)
↓
Thick & Thin filaments
↓
Myosin & Actin (protein molecules)
Skeletal Muscle Organization
A single muscle fiber
LAYERS COVERING
A MUSCLE:
The skeletal
muscle has the
following layers
covering it:
• Epimysium
• Perimysium
• Endomysium
PROTEINS OF MUSCLE:
ACTIN & THIN
FILAMENTS
G-actin is the monomer
which will form the thin
filament. It is a protein
with a molecular weight of
43,000. It has a prominent
site for cross-linkage with
myosin.
G-actin
↓
F-actin
(6-7 nm long polymerized
G-actin, double stranded
in structure)
↓
Thin filaments
Regulatory Proteins of the Muscles
TROPOMYOSIN
TROPONIN
• Rod-like protein
• Mol. Weight: 70,000
• 2 chains: alpha & beta
chains
• Under resting conditions, it
covers the site for myosin
attachment on F-actin
molecule.
• Forms part of Thin filaments
• Globular protein complex
made of 3 polypeptides
• Forms part of thin filaments
Binds to Ca2+
Inhibitory in function
Attached to Tropomyosin
THIN FILAMENTS:
•
•
•
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Length: 1 µm
Diameter: 5-8 nm
No. of G-Actin mol: 300-400
Other Proteins:
- Nebulin: provides elasticity to the
sarcomere.
- Titin: is the largest known protein in the
body. It connects the Z-line to the M-line in the
sarcomere & contributes to the contraction of
skeletal muscle.
MYOSIN & THICK
FILAMENTS:
Thick filaments consist of 2
symmetrical halves that
are mirror images of each
other.
• Chief constituent is
MYOSIN, with a mol.
weight of 480,000.
• Its molecule has 2 ends,
a globular end having 2
heads & a rod-like tail.
• It has 6 peptide chains:
- 2 identical heavy
chains (200,000 each)
- 4 light chains ( 20,000
each)
Binding sites on Myosin molecule:
The myosin molecule has 2 binding sites:
1. Binding site for ACTIN
2. ATPase sit e
A SARCOMERE:
• A myofibril displays alternating dark & light
bands.
Myofibril
Dark bands
Light bands
(A bands)
(I bands)
Anisotropic
Isotropic
Thick & thin filaments
Thin filaments only
A sarcomere model:
A SARCOMERE
The area between 2 consecutive Z discs/ lines is called A
Sarcomere. It is the functional unit of a muscle.
It has a length of 2.3 µm.
It has the following important features:
• Z-disc
• M-line
• I-band
• A-band
• H-zone
• Titin
• Nebulin
Sarcomere: Organization of Fibers
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•
•
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Z disks
I band
A band
H Zone
M line
Titin
Nebulin
Figure 12-5: The two- and three-dimensional organization of a sarcomere
• Z-disc: are dense thin membranes made up of special
lattice-like proteins present transversely.
• Dark or A-band: Thick filaments present overlapped by
the thin filaments at the ends only.
• Light or I band: area present b/w the ends of the 2
thick filaments. It consists of thin filaments only.
• H-Zone: The lighter area in the middle of the A-band,
where the thin filaments do not reach. It consists of
thick filaments only.
• M-Line: A line that extends vertically down the middle
of the A-band in the center of the H-zone.
• Pseudo H-zone: M-line+ H-zone.
THE SARCOTUBULAR SYSTEM
Sarcotubular System
The sarcoplasm of the myofibril is filled with a
system of membranes, vesicles and tubules
which are collectively termed as The
Sarcotubular system.
It is made up of:
T-Tubules
Sarcoplasmic
Reticulum
SARCOTUBULAR SYSTEM
Sarcoplasmic Reticulum
(SR)
Transverse System of Tubules
(T-Tubules)
• It is a fine network of
interconnected
compartments
which run in the longitudinal axis
of a myofibril embedded in the I
and A bands, & surround them.
• They are surrounded by the
sarcoplasm & are NOT connected
to the outside of the cell.
• At their both ends they show
dilated ends called as Terminal
cisterns or sacs.
• They contain a protein called as
Calsequestrin, which binds and
holds CALCIUM.
• It is a system of tubules that runs
transverse to the long axis of the
muscle.
• They enter the myofibrils at the
junction b/w the A and I bands.
• The T-tubules open onto the
sarcolemma. It is an invagination
of the cell membrane & thus
communicates with the ECF.
• It functions to rapidly transmit
the AP from the sarcolemma to
all the myofibrils.
THE TRIAD
• The cisterns of the SR & the central portion of the Ttubules give rise to a characteristic pattern called the
TRIAD.
• Each TRIAD consists of 2 terminal sacs of SR & 1 central
t-tubule.
• There is no physical communication between each
component of the triad.
• In the triad, the cisterns of the SR have the Ryanodine
receptors which are complimentary to the
Dihydropyridine receptors on the t-tubule. They are
both involved in excitation-contraction coupling.