Sliding Filament Theory: Sarcome and Muscle Contraction

Ans: Sliding Filament Theory is the theory that explains the process of muscle contraction during which the thin filaments slide over the thick filaments. This interaction further results in the shortening of the myofibril.

• A sarcomere's actin and myosin myofilament organization plays a crucial role in the process of muscle contraction.
• Muscle contraction is considered to cause actin and myosin filaments to move past one another.
• The sliding filament model of muscle contraction is similar to interlocking fingers in that drawing them together reduces the distance.

Ans: ATP releases myosin from the actin filaments. When contraction occurs in the muscle, the myosin attaches to the actin filaments. ATP is attached to the myosin head. During the contraction, the ATP releases the myosin from the actin molecule and results in muscle relaxation.

Ans: Active cross bridges are produced when myosin heads connect like hooks to adjacent actin filaments. The actin-myosin filament crossbridge functions as an enzyme. Myosin ATpase is an enzyme that converts stored ATP into ADP and inorganic phosphate, releasing energy.

• This freed energy is utilised to propel the myosin head toward the actin strand.
• Myosin head tilts and pulls actin filament along, causing myosin and actin filament to glide against each other.
• A sarcomere's opposing ends of actin myofilament migrate toward each other, resulting in muscular contraction.

The cross bridge detaches after sliding, and the actin and myosin filaments return to their original positions. The active cross bridge forms and reforms for 50-100 times in a second utilising ATP.

• As a result, muscle fibre has a large number of mitochondria.
• Sarcomeres can contract by 30-60% of their length during muscular contraction.

Ans: Calcium and ATP are cofactors (nonprotein components of enzymes) necessary for muscle cell contraction and ATP provides the energy. Troponin and tropomyosin, two proteins that control muscle contraction by preventing myosin from attaching to filamentous actin, require calcium.

• Tropomyosin prevents myosin from binding to actin in a resting sarcomere.
• It must spin around actin filaments to reveal myosin-binding sites before myosin can bind it.

In 1994, William Lehman and his colleagues revealed how tropomyosin rotates by examining the structure of actin and myosin in calcium-rich versus low-calcium solutions.

• They discovered that the presence of calcium is required for the contraction mechanism by comparing the actions of troponin and tropomyosin under these two circumstances.
• Troponin (the smaller protein) changes tropomyosin's location and pushes it away from the myosin-binding sites on actin, thus unblocking the binding site.
• Myosin binds to actin to initiate cross-bridge cycling once the myosin-binding sites are accessible and sufficient ATP is available.

Ans: When the nerve signal is no longer present to contract, calcium is reintroduced into the sarcoplasmic reticulum through active transport pumps. In muscle cells, the sarcoplasmic reticulum is specialised for calcium storage, ensuring that there is enough calcium available for contraction.

• There is no free calcium in the sarcoplasm to bind to troponin, tropomyosin slips back across the active sites of Actin.

With no binding sites accessible, the myosin heads are unable to form crossbridges and stay extended, waiting for the next nerve signal to contract, revealing the binding sites once more.

• The sarcomere recovers to its normal length passively, and the muscle relaxes.
• When a muscle is completely tensed, it shrinks by around 30% of its resting length.
• When the contraction is over, the muscle returns to its full resting length.
• Resting length is restored passively by other elastic proteins in the sarcomere that function like rubber bands and assist actin in recoiling, or by contracting an opposing muscle.

Ans: Hugh Huxley and Jean Hanson proposed the sliding filament model of muscle contraction in 1954. Hugh Huxley's pioneering work drove the model's formulation and subsequent confirmation.

• Huxley pioneered the use of x-ray diffraction to investigate contraction in living striated muscle, using the paracrystalline lattice to eventually comprehend contraction in terms of single molecules.
• Progress necessitated the creation of apparatus with ever-increasing spatial and temporal resolution, which fueled the development of synchrotron facilities.
• These are now utilised for the majority of protein crystallography and muscle research.
• Huxley's early work merged electron microscopy and biochemistry to study and interpret changes in x-ray patterns.

He enhanced electron microscopy methods such as thin sections, and negative staining. These methods had allowed him to answer significant issues about the structure and organisation of thick and thin filaments in muscle, as well as the interaction of myosin with actin and its control.

• Huxley demonstrated that myosin's ATPase domain creates the cross bridges of thick filaments that bind actin, and he proposed that myosin moves in distinct steps on actin. 

Ans: Few factors that can stop the muscles from contracting are: 

• Fatigue of the energy system: Because there is no more ATP in the muscle cell, it can no longer contract.
• Nervous system fatigue: The nervous system is unable to generate impulses in adequate quantities or at a fast enough rate to maintain the stimulation and produce calcium release.
• Voluntary nervous system control: When the brain instructs the nerve that causes the muscle to contract to cease transmitting that signal, no more calcium ions enter the muscle cell and the contraction ends.
• Sensory nervous system information: For example, a sensory neuron (nerve that detects stimuli such as pain or the weight of something) provides feedback to the brain indicating that a muscle is injured while attempting to lift a heavy weight, and thus the impulse to that muscle telling it to contract is stopped.

Ans: A lack of strength and control over a muscle or set of muscles in a specific region of the body is referred to as paralysis. The majority of the time, this is not related to an issue with the muscles.

• It is most likely the result of a malfunction anywhere along the network of nerve cells that connects the body part to your brain and back.
• These nerve cells provide messages to your muscles, causing them to move.

Trigger signals sent from the brain govern muscle action. When any element of the relay system is destroyed, such as the brain, spinal cord, nerves, or the junction between the nerve and the muscle, the impulses do not reach the muscles, resulting in paralysis.

• The relay system can be destroyed in a variety of ways.
• Paralysis can be caused by a birth abnormality such as spina bifida, which happens when the brain, spinal cord, and/or the covering that protects them do not develop properly. 

Ans: Most individuals become paralysed as a consequence of an accident or a medical condition that alters how their muscles and nerves work. The following are the most prevalent causes of paralysis:

• Stroke
• Injury to the spinal cord
• Injuries to the head
• Multiple sclerosis (MS)

Some of the other factors are:

• Cerebral palsy
• Guillain-Barré syndrome
• Neuropathy of the periphery
• Toxins/poisons
• Lou Gehrig's disease (ALS)

Ans: Muscle contraction is initiated by a signal transmitted by motor neurons in the central nervous system. The connection between a motor neuron and a sarcolemma is known as the neuromuscular junction.

• When a neuronal signal reaches this junction, the sarcolemma generates an action potential and releases acetylcholine.
• This causes the sarcoplasm to release calcium ions as it travels along the muscle fiber.
• After that, calcium attaches itself to actin filament troponin, exposing myosin's active sites.
• Using the energy from the hydrolysis of ATP, myosin attaches itself to the exposed active site on actin. 
• Contraction happens as a result of the Z lines connected to them being pulled as well. 

Ans: The functional unit of striated muscle is called a sarcomere. This indicates that it is the simplest component of our skeletal muscle. The muscle type that starts all of our voluntary movements is called skeletal muscle. This is the primary function of the sarcomere. Sarcomeres can contract collectively to start broad, sweeping movements. These microscopic units' special structure enables them to synchronize the contractions of our muscles.