This file is all about Skeletal Muscle contraction with reference to skeletal muscle Fibers, its structure, contraction, role of Ca++ in Contraction and types of Contraction.

2. There are three main types…
1-Slow Twitch Fibres 2-Fast Twitch Fibres 3-Very
Fast Twitch Fibres
These types produce different degrees of force due to slight
differences in the characteristics of how they work.
A- SLOW TWITCH FIBRES:
They contract slowly but can contract repeatedly over long
periods.
They have a good blood supply. Hence they are "RED
FIBRES ".
Suited to endurance activity using the aerobic energy
system which relies on Oxygen from the blood for the
supply of energy.
They are smaller and develop less force than fast twitch
SKELETAL MUSCLE:
TYPES OF SKELETAL MUSCLE
FIBRES:

3. B- FAST TWITCH FIBRES:
They have fast contraction speed and can use
aerobic(Oxygen dependant) energy sources as well as
anaerobic(Oxygen Indepedent) energy sources. They
are WHITE FIBRES. As they are less reliant on
Oxygen supplied by blood for energy and therefore
Fatigue Faster than slow twitch fibres.
Suited to speed, strength and power type activities.
Such as:
moderately heavy weight training and fast running
events such as 400 meters.
C- VERY FAST TWITCH FIBRES:
They contract extremely rapidly. They create very
forceful muscle contractions and fatigue quickly. They
are also " WHITE FIBRES " but unlike fast fibres.
They can only use anaerobic energy sources.
They also suited to speed, strength and power type

4. Q : What determines how many muscle fibre type an
individual has?
Ans: 1-Genetics 2-Hormone levels within
blood
3-Training undertaken
GENETICS: It is based on parent’s genes genetically
programmed to have certain percentage of each fibre.
The average person is born with around 60% fast twitch
and 40% slow twitch fibres(some persons may have larger
amounts of these fibres and therefore be more sui
TRAINING UNDERTAKEN: The ability to change the
fibre type is a common area of debate amongst exercise
physiologists. There is no evidence as yet to show that
fibre type can b changed.
However there is evidence to show that fibres adapt to the
type of training they are exposed to.
For Example:
If a person with predominantly slow twitch ′endurance ′

5. CHARACTERISTICS ARE SUMMARISED IN THE FOLLOWING TABLE
CHARACTERISTICS TYPE I TYPE II A TYPE II B
CONTRACTION SPEED
(mili seconds)
SLOW(90-140)
ms
FAST(50-100)
ms
VERY FAST(40-90)
ms
SIZE OF MOTOR
NEUTRONS(BIGGER
MOTOR NEUTRONS
ALLOW NERVE
IMPULSES TO
OPERATE MORE
QUICKLY)
SMALL LARGE VERY LARGE
RESISTANCE TO
FATIGUE
HIGH INTERMEDIATE LOW
ACTIVITY USED FOR ENDURANCE
TYPE ACTIVITIES
SHORT(<2 min)
HIGH INTENSITY
ACTIVITIES
VERY SHORT(1-30
sec) MAXIMAL
INTENSITY
ACTIVITIES
FORCE PRODUCTION LOW HIGH VERY HIGH

6. CHARACTERISTICS TYPE I TYPE II A TYPE II B
EFFICIENCY HIGH MEDIUM LOW
NO. OF MITOCHONDRIA
(IN MUSCLE FIBRES
PRODUCE ENERGY
AEROBICALLY)
HIGH MEDIUM LOW
CAPILLARY
DENSITY(TRANSPORT OF
OXYGEN AND NUTRIENT
TO MUSCLE AND
REMOVE WASTE
PRODUCTS)
HIGH INTERMEDIATE LOW
OXIDATIVE CAPACITY
(THE CAPACITY TO USE
OXYGEN FOR THE
PRODUCTION OF
ENERGY)
HIGH INTERMEDIATE LOW

7. MYOGLOBIN CONTENT
(A PIGMENT THAT
BINDS TO OXYGEN
GIVING THE FIBRE A
RED COLOUR)
HIGH MEDIUM LOW
GLYCOLYTIC CAPACITY
(TO STORE AND
BRAKEDOWN GLYCOGEN
FOR USE AS A HIGH
INTENSITY ENERGY
SOURCE)
LOW HIGH HIGH
ATPASE LEVELS
(IT IS AN ENZYME THAT
CONTROLS THE
BRAKEDOWN AND
SYNTHESIS OF ATP FOR
ENERGY)
LOW INTERMEDIATE HIGH

8. Connective tissue sheaths of skeletal muscle: epimysium, perimysium,
and endomysium.
Bone
Perimysium
Endomysium
(between individual
muscle fibers)
Muscle fiber
Fascicle
(wrapped by perimysium)
Epimysium
Tendon
Blood vessel
Fascicle
FIBRE

9. Muscle mass:
Muscle mass or muscle tissues are made up of a
large number of individual muscle cells.
The muscle cells are commonly called muscle fibers
because they are long and cylinder in appearance.
Fascia:
Muscle mass is
separated from the
neighboring tissues by a
thick fibrous tissue layer
known as fascia.
Epimysium:
Beneath the fascia
muscle is covered with a
connective tissue sheath

10. Fasciculi:
In the muscle the muscle fibers are arranged in
various groups called bundles or fasciculi.
Perimysium:
Connective tissue sheath that covers each
fasciculus is called perimysium.
Endomysium:
Each muscle fiber is covered by a connective
tissue layer called the endomysium.

11. Muscle fiber:
Each muscle cell or muscle fiber is cylindrical in
shape.
Average length of the fiber is 3cm. It varies
between 1cm to 4 cm, depending upon the length
of the muscle. The diameter of the muscle fiber
varies from 10µ to 100µ.
Each muscle fiber is enclosed by a cell membrane
called plasma membrane, that lies beneath the
endomysium called sarcolema.
Cytoplasm of the muscle is known as sarcoplasm.
The sarcolema consist of true cell membrane
called plasma membrane.
Structures embedded with in the sarcoplasm are;
1. Nuclie
2. Myofibril

12. Myofibrils:
Each muscle fiber itself contains cylindrical
organelles known as Myofibrils.
Myofibrils run through the entire length of the
muscle fiber, the myofibrils appear like small
distinct dots within the sarcoplasm.
In some muscle fibers, some of the myofibrils are
arranged in groups called cohnheim’s areas or
fields.
Surrounding the myofibrils there is a network of
tubules and channels called the Sarcoplasmic
reticulum.

14. Muscle contraction:
Is caused by interactions of thick and thin
filaments.
Structure of protein molecules determine its
interactions.

15. Each myofibril contains myofilaments:
Thick filaments:
A band contain thick filament(primarily composed of
myosin) is called as A band which:
Move closer together and Do not Shorten.
H band shorten:
Contain only myosin and Shorten during
contraction.
Thin filaments:
I band contains thin filaments(primarily composed
of actin) is called as I band which:
Decrease in length and it is a Distance between A
bands of successive Sarcomeres.
Center of each I band is Z disc.
Muscle Contracts because of sliding of thin

17. Sarcomere:
Z disc to Z disc.
M lines:
Produced by protein filaments in a sarcomere.
Anchor myosin during contraction.
Titin:
Elastic protein that runs through the myosin from M
line to Z disc.
Contributes to elastic recoil of muscle.

18. Microscopic anatomy of a skeletal
muscle fiber.
I band I bandA band
Sarcomere
H zone
Thin (actin)
filament
Thick (myosin)
filament
Z disc Z disc
M line
(c)Small part of one myofibril enlarged to show the myofilaments
responsible for the banding pattern. Each sarcomere extends from
one Z disc to the next.

19. Sliding filament theory:
Definition:
when a muscle contracts , the thin filaments slide
past the thick filaments, and the sarcomere
shortens. This process comprised of several steps
is called SLIDING FILAMENT THEORY. It is also
called WALK ALONG THEORY or the RACHET
THEORY.
Sliding of filaments is produced by the actions of
cross bridges.
Cross bridges:
Are part of the myosin proteins that extends out
towards actin. It form arms that terminate in heads.
Each myosin head contain an ATP binding site.

21. Contraction:
Myosin binding site splits ATP to ADP and Pi. ADP
and Pi remain bound to myosin until myosin heads
attach to actin.
Pi is released causing the power stroke to occur.
Power stroke pulls actin towards the center of the A
band. ADP is released when myosin binds to a fresh
ATP at the end of the power stroke. Release of ADP
upon binding to another ATP, causes the cross bridge
bond to break. Cross bridge detach, ready to bind
again.
Regulation of Contraction:
Regulation of cross bridge attachment to actin is due
to:
Tropomyosin:
Lies with in grove between double row of G-actin. In
relaxed muscle state Tropomyosin blocks binding sites

23. Role of Ca in Muscle Contraction:
Muscle Relaxation:
[Ca] in sarcoplasm lowers when tropomyosin blocks
attachment. Which prevents muscle contraction.
Ca is pumped back into the SR in the terminal
cisternae.
Na diffusion produces end-plate
potential(depolarization) + ions are attracted to
negative plasma membrane.
If depolarization sufficient, threshold occurs,
producing Action potential. This action potentials
travel down sarcolemma and T tubules.
Sarcoplasmic reticulum, terminal cisternae releases
Ca from chemical release channels. Ca is also
released through a Ca induced Ca release.
Ca attaches to troponin and Tropomyosin-troponin

24. Action Potential:
It is generated by increase in sodium ions in
sarcolemma.
That travels along the T tubules and leads to
excitation-contraction coupling.
Excitation-contraction coupling:
Action potential reaches a triad. Where releasing
calcium occur
This Triggering contraction requires myosin heads to be
in cocked position which is loaded by ATP energyConditions for skeletal muscle contraction:
There must be a neural stimulus. There must be
calcium in the muscle cell. ATP must be available for
energy.
So a few things can stop the contraction such as:
Energy system fatigue. Nervous system fatigue.
Voluntary nervous system control. Sensory nervous

25. Types of contraction:
1. Isotonic contraction 2. Isometric contraction
3. Eccentric contraction
In order for a muscle fiber to shorten they must
generate a force greater than the opposing forces
that act to prevent movement of that muscle
insertion.
Isotonic contraction:
Force of contraction remains constant through out
the shortening process.
Velocity of muscle shortening decreases as load
increases.
Isometric contraction:
Length of muscle fibers remain constant if the
number of muscle fibers activated is too few to

27. Upper neuron control of skeletal Muscle:
Cerebellum:
Receives sensory input from muscle spindles, golgi
tendon organs and areas of cerebral cortex devoted to
vision hearing and equilibrium. All output from
cerebellum is inhibitory
Disorders of Muscle Contraction:
Muscle Atrophy: It is characterized by weakening and
shrinking of a muscle. It may be caused by
immobilization and loss of neural stimulation.
Muscle Hypertrophy: It is characterized by
Enlargement of a muscle. Which leads to more
capillaries and more mitochondria.
It is caused by Strenous exercise as well as Steroid
hormones.
Tetany: Sustained contraction of a muscle
Result of a rapid succession of nerve impulse

28. A motor unit consists of a motor
neuron and all the muscle fibers it
innervates.
Spinal cord
Motor neuron
cell body
Muscle
Branching axon
to motor unit
Nerve
Motor
unit 1
Motor
unit 2
Muscle
fibers
Motor neuron
axon
Axon terminals at
neuromuscular junctions
Branching axon
terminals form
neuromuscular
junctions, one per
muscle fiber (photo-
micrograph 330x).
(b)
(a)
Motor Unit