Comprehensive Study of Skeletal Muscle Contraction and Its Processes

This assignment provides a detailed analysis of skeletal muscle contraction, starting with the different types of contractile and regulatory proteins in skeletal muscle, including myosin, actin, tropomyosin, and troponin, along with their molecular structures. It then describes the sarcomere, the functional unit of muscle contraction, detailing its structure and components such as the Z-line, A-band, I-band, H-zone, and M-line. The assignment further explains the triad, composed of the T-tubule and sarcoplasmic reticulum, and its function in excitation-contraction coupling. It elaborates on the excitation-contraction coupling process, the sliding filament theory of contraction, and the mechanism that keeps the muscle relaxed, covering the roles of acetylcholine, calcium ions, and ATP in these processes. Overall, the assignment offers a comprehensive understanding of the biological processes underlying muscle contraction and relaxation.

Mechanism of muscle contraction
1. Name of the different types of contractile proteins and regulatory
proteins in skeletal muscle with the molecular structure.
The contractile proteins involved in the skeletal muscle include:
Myosin: is composed of 2 heavy chains and 4 light chains. The 2 α-
helices are coiled forming a rod portion lying parallel to the myosin
myofilament and laterally forming two heads at a hinge region where they
split. The four light chains are split into two for each head. The heads
contain the enzyme ATP-ase used for the breakdown of ATP to power the
contraction of skeletal muscle.
Actin: is composed of 2 strands of actin
fibers including G actin monomers. They
both form a double-helix that attach to
either end of the sarcomere spanning the
length of the myofilament. They contain
myosin binding sites.

The regulatory proteins involved in the skeletal muscle include:
a. Tropomyosin: This regulatory protein of skeletal muscles consists of 2
strands that run diametrically opposite alongside coiled around the actin
filaments. The molecule is rod-shaped and approximately 40 nm in length.
Each tropomyosin molecule comes in contact with 7 units of actin starting
from the amino to carboxyl terminal. Tropomyosin contains 284 amino acid
helix with at least 40 known isoforms.
b. Troponin: consist of trimeric complex proteins including Troponin I, C
and T. Each has a specific function:
• Troponin I: consists of 209 amino acids in its protein chain. The
difference between this troponin molecule and the other is the N-terminal
extension of 26 amino acids. This extension allows for its interaction with
the other troponin molecules as well as its main function, bind to actin
and hold the actin-tropomyosin complex in place.
• Troponin C: consists of 161 amino acids in its protein chain. The protein
consists of 2 domains, one being the structural carboxyl terminal and the
other the regulatory amino terminal forming 2 domains. These 2 domains
are linked via a flexible linker, with each domain containing calcium-
binding double helix loop motifs. This allows for the main function of
Troponin C which is binding to the calcium released by the sarcoplasmic
reticulum for muscle contraction.

• Troponin T: consist of 262 amino acids. The amino-terminal consists of
negatively charged 59 residues and positively charged residues at the
carboxyl. The shape is formed as a rod located in the groove of the actin
filament. The amino-terminal is the one that interacts with tropomyosin,
which is the main function of the Troponin T.
2. Name the functional unit of contraction in skeletal muscle and giv
its structure with a diagram.
The functional unit of contraction is the sarcomere which is about 2
micrometers in length. The actin filaments are attached to each end of the
sarcomere to the points of the Z-discs consisting of the myosin filaments
in the middle of the sarcomere. The sarcomere consists of zones and
bands and lines:
• Z-line: attachment sites for actin. Move inwards during the contraction
of the muscles.
• A-band: spans the length of the myosin contained in the center of the
sarcomere. Also known as the think filaments of the sarcomere.
• I-band: Stretches from one end of the myosin to the beginning of the
other myosin in the next sarcomere. Contains the Z-line in the center.
• H-zone: the tip of one end of the actin to the other end. Does not exist
when the muscle has fully contracted.
• M-line: center of the A-band that serves as an attachment site for
myosin.

3. Components of ‘TRIAD’ and its functions
The triad is composed of the central T-tubule sandwiched between the
ends of the sarcoplasmic reticulum known as the terminal cisterna. When
an action potential is received within the skeletal muscle, the transmission
passes through the T-tubules, activating dihydropyridine receptors,
allowing for the release of calcium from the sarcoplasmic reticulum into the
sarcoplasm through a process called excitation-contraction coupling.
These junctions are located between the A and I bands.
4. Explain the following mechanisms.
a. Excitation-contraction coupling
b. Sliding filament theory of contraction
c. Mechanism that keeps the muscle relaxed
a. Excitation-contraction coupling: this refers to the excitation of the
neuron by an action potential and the contraction refers to the interaction
between actin and myosin. This mechanism consists of:
i. End plate potential: Acetylcholine is released from the motor neuron
and binds to the acetylcholine receptors on the membrane of the
muscle. This causes the membrane end plate to become depolarized,
causing an end plate potential.
ii. The end plate potential causes the action potential to travel down the
sarcolemma membrane by the wave of depolarization.
iii. This wave of action potential passes down the T-tubules activating
dihydropyridine receptors which are T-tubule voltage sensor that is
physically linked to calcium receptors in the terminal cisterna
iv. This causes an influx of calcium ions into the sarcoplasm where the
calcium binds to Troponin C located on the actin filaments.

v. The binding of calcium causes the rotation of the tropomyosin
exposing the myosin binding actin active sites. The cross bridge
between myosin and actin forms causing the muscle contraction.
vi. This continues until the contraction stops and all the calcium is
pumped back into the sarcoplasmic reticulum.
b. Sliding filament theory of contraction: this theory refers to the actual
mechanism of the contraction of the muscle where the sarcomere slides
and shortens causing the muscle to contract. The contraction of the
muscle occurs due to the sliding mechanism of the actin molecules
caused by the stroke of myosin. The mechanism consists of:
i. While ATP is bound to myosin, the molecule is at rest and in the resting
position. This is known as the “rigor” position
ii. ATP-ase then hydrolyzes the ATP to ADP + Pi causing the back
movement of the head preparing the head for binding. This movement
is referred to as “cocking” position
iii. When the Pi molecule leaves, it creates an exposed binding site in the
myosin head which binds to the exposed actin active site on the actin
filaments
iv. Release of ADP from the myosin heads causes the power stroke
motion to occur moving the actin filaments inwards moving the Z-discs
closer to the center after the power stroke has been completed,
another molecule of ATP binds to myosin bringing it back to the
relaxed state and the process continues again
c. The mechanism that keeps muscle relaxed: muscle relaxation is
when the sliding mechanism of the filaments stops. This can be due to a
number of reasons including energy or nervous system fatigue, lack of
voluntary nervous system control and lack of sensory nervous system
information. In all regards the following steps take place:
i. Motor neuron stops releasing acetylcholine causing the membrane to
repolarize
ii. Due to the lack of stimulation, the voltage-gated T-tubule close causing
the calcium channels to also close
iii. ATP driven pumps move calcium ions out of the sarcoplasm, back into
the sarcoplasmic reticulum
iv. Removal of the calcium causes the release of the ion from Troponin C.
Re-shielding the active sites of actin causing the muscle to relax.