Muscle Contraction – Class 11 | Chapter – 20 | Biology Short Notes Series PDF
Muscle Contraction:Muscle contraction is the tightening, shortening, or lengthening of muscles when you do some activity. It can happen when you hold or pick up something, or when you stretch or exercise with weights. Muscle contraction is often followed by muscle relaxation, when contracted muscles return to their normal state.
Muscle Contraction
Muscles serve several purposes in your body. Your muscles contract for any number of reasons, but they primarily do the following:
Offer stability to your joints and connective tissues – Your muscles lengthen and shorten, sometimes involuntarily, as your body needs them.
Produce heat to maintain your body temperature – Around 40% of your body’s temperature converts into muscle work. Shivering is your body’s response to feeling cold, and your skeletal muscles activate to warm your body.
Maintain posture – Muscles help you maintain a position like sitting or standing.
Purpose of Muscle Contraction
When a nerve impulse arrives at the neuromuscular junction, a substance called acetylcholine is released. Acetylcholine depolarizes the motor end plate, which spreads throughout the muscle through the transverse tubules, causing calcium (Ca+) to be released from the sarcoplasmic reticulum.
In the presence of high Ca+ concentrations, the Ca+ binds to Troponin. It alters its shape and thereby displace Tropomyosin from the Actin’s active site. Myosin filaments may now connect to Actin and create a cross-bridge.
The breakdown of ATP releases energy. This allows Myosin to pull Actin filaments inwards which further shorten the muscle. This happens all the way down the length of every myofibril in the muscle cell.
When an ATP molecule attaches to the Myosin head, the Myosin detaches from the Actin and the cross-bridge is broken. When the ATP is degraded, the Myosin head can reattach to an Actin binding site farther down the Actin filament and repeat the ‘power stroke.’ The ratchet mechanism refers to the repetitive tugging of Actin over myosin.
This mechanism of muscle contraction can continue indefinitely as long as there are sufficient ATP and Ca+ reserves. When the impulse is terminated, Ca+ is pushed back to the Sarcoplasmic Reticulum, and Actin returns to its resting position, causing the muscle to extend and relax.