1. CHAPTER 12
• Structure and action of skeletal muscle
• Mechanisms of contraction
• Neural control of skeletal muscle contraction
• Cardiac and smooth muscle
2. Types of Muscle in the Body
Muscle is involved in all movements of parts of our body. Muscle create
movements by converting chemical energy to mechanical energy.
- Skeletal muscle:
muscle mainly attached to the skeleton and used for movement.
“muscle of locomotion and exercise”
- Smooth muscle:
muscle mainly lining internal organs of the body, such as blood vessels, gut,
urinary bladder, etc.
“muscle of regulatory movements, closure & opening, constriction &
dilation”
- Cardiac muscle:
muscle of the heart. Its contraction and relaxation causes pumping action of the
heart.
6. Skeletal Muscle Structure
- A single muscle cell is called muscle fiber.
- Each muscle fiber contains several hundreds to several thousands of structures
called myofibrils.
- Diameter of a muscle fiber ranges from 10-80 µm, but the length may extend the
entire length of the muscle (e.g the thigh muscle fiber is about 35 cm long).
- Plasma membrane of muscle fiber is called sarcolemma.
- Each muscle fiber is ultimately attached to a bone via a hard connective tissue
called tendon.
- Cytoplasm of muscle fiber is called sarcoplsam.
- The endoplasmic reticulum is called sarcoplasmic reticulum (storage site for
calcium).
- Transverse tubules are structures that:
- pass laterally through the muscle fiber,
- Transmit nerve signals closer to individual myofibrils,
- Provide access of oxygen and glucose to inner parts of muscle fiber.
8. Skeletal Muscle Contraction
• Contraction (shortening in length) of a muscle fiber is due to sliding of
thick and thin filaments against each other at opposite directions. This is
called sliding filament theory.
• The size of filament remains the same but they slide against each other
resulting in shortening of sarcomere.
• The sliding occurs by formation of cross bridges between the filaments.
• During the shortening of the sarcomere the A band remain the same
length while the H and I bands shorten causing the Z discs to come
closer to each other.
14. Excitation-Contraction Coupling
• When a nerve impulse is received by a muscle fiber this leads to its contraction.
The sequences which lead to contraction are as follow:
1- Nerve impulse causes the release of Ach from axon terminal into the
neuromuscular junction.
2- The ACh binds with a receptor on the muscle fiber and generates action
potential on the muscle fiber.
3- The action potential spreads over the sarcolemma as well as the T tubuls.
4- Depolarization of the T tubuls cause release of calcium from the
sarcoplasmic reticulum into the sarcoplasm.
5- Calcium binds to troponin which leads to muscle contraction.
As long as the level of calcium in sarcoplasm is above 1 µM the contraction
will continue. For the contraction to stop (relaxation) the calcium in the
Sarcoplasm is actively removed back into sarcoplasmic reticulum and stored
there for next contraction.
16. Motor Unit
Each somatic motor neuron with all its muscle
fibers that it innervates is called motor unit.
17. Properties of Skeletal Muscle
• Skeletal muscle can be studied in vitro: e.g Gastronemius muscle of frog.
• This muscle can be stimulated with electric shock and contraction can be
recorded by force transducer. Using this preparation, the followings can be
studied:
• Muscle twitch : applying a single electric shock causes the muscle to contract
and relax.
• Summation : if a second electric shock immediately follows the first, another
muscle twitch will be produced which will ride on the first one.
• Tetanus : high frequency stimulation of the muscle causes continuous
contractions which fuse together.
• Series-Elastic Component : connective tissue and tendons show elasticity
(resist distension). This helps in muscle relaxation.
18.
19. Types of Skeletal Muscle Contractions
Isotonic contraction:
a type of contraction that results in muscle
shortening. In this type of contraction the
tension on the muscle fibers remain
constant.
iso = same, tonic = strength
Isometric contraction:
when a muscle remains the same length
during contraction while the tension
changes.
iso = same, metric = length
20. Smooth Muscle
• The arrangement of the actin and myosin filaments is different.
Therefore showing no striation.
• Fibers are smaller than skeletal muscle fibers.
• Same chemical substances (actin and myosin) cause contraction.
Sliding filament mechanism applies here.
• Smooth muscle has slower rate of contraction and relaxation than
skeletal and cardiac muscle.
• Less energy is required for smooth muscle contractions.
• Smooth muscle can contract without action potential, using local
factors.
• Level of cytoplasmic Ca++ is crucial for contraction, however the Ca ++
mainly comes from outside the cell (has less developed SR).
21. Cardiac Muscle
• Like skeletal muscle the cardiac muscle is striated.
• Is contracted by the sliding filament mechanism.
• Unlike skeletal muscle, the cardiac muscle cells are interconnected by
gap junctions.
• The cells are joined electrically, this is called myocardium (single
functional unit).
• Contractility (ability to contract) is increased by epinephrine and
stretch.
• Can produce action potential automatically.
