Skeletal Muscle Fibre.
The single cell of a skeletal muscle is called “Skeletal Muscle Fiber”.
They are cylindrical in shape and their length varies as per length of the muscle.
They are held parallel to each other and show the presence of alternate dark and light bands due to arrangement of the muscle proteins.
The plasma membrane is called “Sarcolemma”.
The endoplasmic reticulum is called “Sarcoplasmic reticulum”.
The cytoplasm is called “Sarcoplasm”.
The muscle fiber is multinucleated i.e. it contains many nuclei.
The sarcoplasm contains many mitochondria for production of ATPs.
The sarcoplasm contains a special protein called “Myoglobin” which stores oxygen.
Myofibrils and Filaments.
The sarcoplasm of the skeletal muscle fiber contains several thread-like structures called “Myofibrils.”
The myofibrils are made up of smaller structures called “Filaments”.
The filaments are of two types,
Thin filaments: Made up of Actin, troponin-tropomyosin.
Thick filaments: Made up of Myosin.
Both filaments work in the process of muscle contraction.
Sarcomere.
The basic functional unit of the filament is called a “Sarcomere”.
They are placed in a rep[eating arrangement.
They contain perpendicular protein plates called “Z lines” that form the boundary of a Sarcomere.
Within the boundaries lie alternate thick and thin filaments.
M line proteins attach the thick filaments with each other.
The thin filaments are attached to the Z line proteins.
During the process of contraction the Thick filaments pull the Thin filaments towards the center of the sarcomere, causing the contraction of sarcomere and hence contracting the muscle itself.
Sliding filament mechanism of Muscle contraction.
The length of the skeletal muscle shortens as thick and thin filaments of the sarcomere slide over one another, causing contraction of the skeletal muscle.
The process is known as “Sliding filament mechanism”.
The thin filaments are made up of Actin, troponin, tropomyosin.
The thick filaments are made up of,
Myosin heads: projected towards thin filaments.
Myosin tails: forms the shaft of thick filament.
As action potential for muscle contraction is generated the sarcoplasmic reticulum releases stored calcium in the sarcoplasm.
The release calcium ions binds with the troponin and hence moves away the troponin-tropomyosin complex away opening the Myosin binding site on thin filaments.
The myosin heads now bind to the free binding site on the actin filaments and push them to the center of the sarcomere.
Once the site becomes free again the cycle continues.
The contraction cycle can be summarized in the following four steps.
ATP Hydrolysis.
Formation of cross bridges.
Power stroke.
Detachment of Myosin from Actin.
ATP Hydrolysis:
The Myosin head contains an ATP binding site an ATPase enzyme.
When ATP binds to the Myosin head the enzyme hydrolyses it to ADP giving the Myosin head energy.
The ADP and formed Phosphate group remain attached to the myosin head.
Formation of cross bridges:
The energized myosin head attaches to the myosin binding site on the actin filament.
This binding of myosin head to the actin during contraction is termed as “Cross Bridge Formation”.
Power stroke:
Once cross bridges are formed the power stroke occurs.
The cross bridges pull the actin filaments towards the center of the sarcomere this pulling is called powerstroke.
This results in the contraction of the sarcomere and ultimately the skeletal muscle.
Detachment of Myosin from Actin:
At the end of the powerstroke in the center of the sarcomere an ATP molecule attaches to the myosin head.
This causes detachment of the myosin from actin filament.
Commonly Asked Questions:
Write a short note on the structure of a skeletal muscle.
Write a short note on “Physiology of Muscle Contraction”.
Write a short note on the “Sliding Filament Mechanism of Muscle Contraction”.