Muscle Contraction and Mechanism of Muscle Contraction: Functions

What Is Muscle Contraction?

Muscle contraction is the event of tension creation in muscle fibre due to actin and myosin filament slide resulting in either shortening or lengthening of the muscle. The process has significant implications for many functions of physiology: it enables voluntary movements, such as posture and heat generation, and involuntary actions like the heart beating and interior organ functions, including digestion. There are three types of muscle tissue in the human body: skeletal muscles that provide for body movements, cardiac muscle found in the heart organ, which brings about rhythmic contractions to pump blood, and lastly, the smooth muscles found in the walls of internal organs that control such involuntary movements as peristalsis in the digestive tract.

Anatomy Of Muscles

The anatomy of muscles is discussed below-

Muscle Fibres

Skeletal muscle fibers are rather long, cylindrical cells that bear several nuclei at the periphery. They are covered with a plasma membrane called the sarcolemma. The sarcolemma envelops the sarcoplasm and within it, there are many myofibrils.

Myofibrils

These are rod-like structures lying parallel to each other in the muscle fibre and composed of repeated units known as sarcomeres. These consist of the contractile proteins, actin and myosin.

Sarcomeres

Sarcomere is rightly viewed as a portion of the muscle fibre existing between two successive and running Z-lines. Comprising overlapping thin and thick filaments, their interaction with actin and myosin, respectively, gives rise to the contraction and subsequent relaxation of muscles.

Mechanism Of Muscle Contraction

The mechanism of muscle contraction is discussed below-

Neuromuscular Junction

The neuromuscular junction is, literally speaking, a chemical synapse between the motor neuron and the skeletal muscle fibre. Derivation from the neuron axon ending, crossing the synaptic cleft, comes to rest at the motor end plate within the muscle fibre.

When the action potential reaches the axon terminal, acetylcholine is released into the synaptic cleft. Sometime later, ACh will bind to the receptors present on the motor end plate and finally depolarize it, which generates an action potential into the muscle fibre.

Excitation-Contraction Coupling

Role of calcium ions

The action potential travels down the sarcolemma and into the T-tubules, and then the Ca²⁺ is released from the Sarcoplasmic Reticulum into the Sarcoplasm.

Role of troponin and tropomyosin

This Ca²⁺ now combines with Troponin wherein, on a conformational change, the Tropomyosin exposes the myosin-binding site on Actin hence facilitating cross-bridge formation

Steps involved in excitation-contraction coupling

• Action Potential reaches the T-tubules

• Release of Ca²⁺ from the Sarcoplasmic Reticulum.

• Ca²⁺ binds with troponin; this use causes a change in the position of the tropomyosin.

• Myosin-binding sites on the actin are exposed, and contraction is initiated.

Cross-Bridge Cycle

Binding of myosin to actin

When the myosin-binding sites on the actin filaments become available, energized myosin heads bind to these exposed sites, forming cross-bridges.

Power stroke

The myosin head pivots to return to its previous position, taking the attached actin filament towards the mid-sarcomere; it releases ADP and phosphate.

Detachment of myosin

A newly bound ATP into the myosin head releases the myosin head from the actin filament.

Re-cocking of the myosin head

Hydrolysis replenishes the energy of the myosin head so that it is cocked again and hence at the beginning of another cycle.

Energy For Muscle Contraction

The energy for muscle contraction is discussed below-

Role Of ATP In Muscle Contraction

It provides energy for the cross-bridge cycle of muscle contraction. The binding of ATP to myosin heads allows the myosin heads to detach from actin after the completion of a power stroke. Hydrolysis re-energizes the myosin heads so that they are ready to go through another cycle of binding and pulling.

Sources Of ATP

Creatine phosphate

This high-energy compound gives its phosphate group directly back to ADP with great velocity, thereby regenerating a certain amount of ATP. This is an abbreviated form but instantaneous energy for contraction, sufficient activity to last about 10 seconds.

Glycolysis

Because this pathway is anaerobic, it degrades glucose to pyruvate directly by yielding 2 ATP molecules per glucose molecule. The energy yield from glycolysis caters to fleeting, very intense bursts of activities but results in the accumulation of lactic acid and thus muscle fatigue.

Aerobic respiration

This process takes place inside the mitochondria. Oxygen is used in completely oxidizing glucose, fatty acids, and amino acids into carbon dioxide and water. In this process, a quite significant amount of ATP is generated. Aerobic Respiration is effective and, by continuously supplying ATP can last for a longer period with moderate-intensity activities.

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