Exercise Physiology: Muscle structure and the mechanism of muscular contraction (The sliding filament theory of muscular contraction)

1. Muscle structure and the mechanism of muscular contraction
BY
Mr. Prabhjot Singh
Assistant Professor
Department of Physical Education & Sports

2. What is Muscle
A muscle is a collection of muscle tissues that contract together to
generate force. A muscle is made up of fibers of muscle cells
surrounded by protective tissue, which are bundled together with
many more fibers, all of which are surrounded by a thick protective
tissue. A muscle contracts and shortens in response to ATP, exerting
force on the objects to which it is attached. Muscles are classified
into several types, each of which affects a different part of the body.

3. Structure of Muscle
A muscle is made up of several muscle tissues that are bundled
together and surrounded by epimysium, a tough connective
tissue that resembles cartilage. The epimysium protects
fascicles, which are bundles of nerve cells that run in long
fibers. The perimysium, which surrounds these fascicles, acts as
a protective layer. This layer facilitates the flow of nerves and
blood to the individual fibers. The endomysium, a protective
layer, is then wrapped around each fiber. A muscle is organized
in a basic pattern of bundled fibers separated by protective
layers, as shown in the image below.

4. Sarcolemma
Basement membrane and endomysial connective tissue surround
the sarcolemma, which is the plasma membrane of the muscle cell.
The layer of sarcolemma fuses with tendon fibers at one end of the
muscle fiber, and these tendon fibers insert into the bone. As a
result, the tendon connects each individual muscle fiber to the
bone.
Sarcoplasm
The cytoplasm of a muscle fibre is known as sarcoplasm. It's a
water solution that contains ATP and phosphagens, as well as
enzymes, intermediates, and product molecules involved in a
variety of metabolic reactions.

5. Myofibrils
Myofibrils are protein filament bundles that contain the
cardiomyocyte's contractile elements, or the machinery or motor
that drives contraction and relaxation.
There are two types of filaments in myofibrils: thin filaments
and thick filaments. Thick filaments are made up of strands of
the protein myosin coiled together, whereas thin filaments are
made up of strands of the protein actin and a regulatory protein.
Sarcomeres are functional units made up of thin and thick
filaments that form partially overlapping layers.

6. Contt.
The myofibril appears to have dark and light bands due to the
way the myofilaments are arranged, giving the muscles a
striated appearance. The dark bands are known as A bands,
and they are made up of thick and thin filaments. The H-zone,
which contains only thick filaments, and the M-line, which
contains enzymes involved in energy metabolism, are located
in the center of the A band. Between the A bands are the light
bands, also known as I bands, which are regions with only
thin filaments. The I bands are centred on the Z line, a disc
made up of the protein –actin in that anchors the thin actin
filaments and serves as a sarcomere subunit boundary.

7. The sliding filament theory of muscular contraction
H.E.Huxley hypothesis (1969). The theory explains how
muscles contract mechanically and chemically. During
contraction, the actin filaments slide over the myosin filaments,
not changing their length.
This theory's mechanical, physiological, and biomechanical
processes can be broken down into five stages. The following is
a list of them:
•Rest
•Excitation-Coupling
•Contraction
•Recharging
•Relaxation

9. Rest: At rest, most of the calcium required for muscle
contraction is stored in the sarcoplasmic reticulum, so very
few myosin cross-bridges are bound to actin. In this state,
the muscles are resting because no tension is accumulated
in the muscles. In this state, the muscles are resting because
no tension is accumulated in the muscles.
Excitation-Coupling: Nerve impulses reach the
neuromuscular junction and cause the release of a
chemical called acetylcholine. Calcium (Ca +) is then
released from the sarcoplasmic reticulum. This separates
the troponin-tropomisin from actin. This means that
myosin can attach to actin-a cross-bridge is formed.

11. Contraction: ATP is split (ADP + Pi) and energy is
released. Myosin draws actin (ZLine / Zdiscs get closer to
each other) and sarcomere shortens (for example, muscle
shortens).
Recharging: Myosin then separates from actin and breaks
the cross-bridge while ATP recombines to the myosin
head. The whole process repeats.
Relaxation: When a nerve impulse is no longer present,
calcium returns to the sarcoplasmic reticulum and actin
returns to its resting position, causing the muscle to
lengthen and relax.

12. Thank You