Muscle Biology

The internal movements are performed by the involuntary muscles. The involuntary muscles function on their own and cannot be controlled willingly. The cardiac muscles are the involuntary muscles which are responsible for the pumping of the heart. The heart pumps and supplies the oxygenated blood to the different tissues in the body. The smooth muscles are responsible for carrying out peristalsis in the tube like structures of the alimentary canal. This movement helps in propelling the food in the forward direction. Both these muscles are of the involuntary type. 

• The large muscle of the upper arm flexes the arm and powerfully twists the forearm, turning the palm upward.
  
• This muscle extends from the head of the head of the humerus over the elbow to the ulna bone to help flex the elbow, and also enables pronation of the forearm.

The muscles attached to the vocal cords can expand and contract, thus making them tight and loose. The voice quality changes depending on whether the vocal cords are tight or loose.

Stapedius is a muscle on the wall of the tympanic cavity of the middle ear. It is the smallest skeletal muscle in the human body. It acts reflexively in response to loud sounds by reducing excessive vibrations that may injure the internal ear. Thus the correct answer is option B.

• The Muscles work by contraction and relaxation this represents that muscles can pull bones, but they can't push them back to the original position.
• For this reason, muscles are often arranged in pairs that pull bones in opposite directions.
• Hence muscles attached to the bones move the body by pull movement only.
• So, the correct answer is 'Pull movement only'.

• High extracellular K+  leads to depolarization and ensuing inactivation of voltage-gated Na+ channels and loss of excitability in isolated muscles, exercise-induced loss of K+ is likely to reduce muscle excitability and thereby contribute to muscle fatigue. Hence, Muscular and nervous excitability is lowered by K.

•  Upon muscle contraction, the A-bands do not change their length, whereas the I-bands and the H-zone shorten.
•  This causes the Z lines to come closer together. The protein [tropomyosin] covers the myosin binding sites of the actin molecules in the muscle cell. Hence, A-Band remains constant  is true of muscle contraction
 So, the correct answer is 'A-Band remains constant'.

• Muscle contraction thus results from an interaction between the actin and myosin filaments that generates their movement relative to one another. 
• The molecular basis for this interaction is the binding of myosin to actin filaments, allowing myosin to function as a motor that drives filament sliding. Hence, The binding of troponin to calcium in muscle cells exposes the binding site of Myosin.

ATPase needed for muscle contraction is present over the myosin. Myosin binds to actin at a binding site. Myosin has another binding site for ATP at which enzymatic activity of myosin ATPase hydrolyzes ATP to ADP which releases an inorganic phosphate molecule and energy. The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position. The myosin head is then in a position for further movement possessing potential energy. The enzyme myosin ATPase catalyses the reaction in the presence of Ca$^{2+}$ and Mg$^{2+}$. So, the correct answer is 'Myosin'.

• Active sites for myosin(Thick filament) are present on actin which are masked by troponin in resting state. when  Ca++ level increases it leads to the binding of calcium with a subunit of troponin on actin(thin) filaments and thereby remove the masking of active sites for myosin.
• Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. 
• This pulls the attached actin filaments towards the centre of ‘A’ band.
•  The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction.
• magnesium acts as a natural calcium blocker to help muscles relax. Hence Mg ion is not involved in the muscular contraction but it is involved in muscle relaxation by blocking calcium ions.
• So, the correct answer is 'Mg'

Cross bridge cycling forms the basis for movements and force production in the muscle cells. The myosin heads are known as cross bridges because they can bind and move along actin filament. Sarcomere is a stretch of myofibril between two z lines where myosin is in center and actin at periphery, slightly overlapping myosin. Nerve impulse causes release of calcium ions from sarcoplasmic reticulum and binds to troponin which changes configuration of tropomyosin and exposes cross bridge heads on myosin allowing actin myosin binding. This causes contraction of muscle fibre. ATP is required for myosin to dissociate from actin. The calcium ions released from sarcoplasmic reticulum regulate when cross bridging cycle can occur. 

Calcium is the ion released into the sarcoplasma from sarcoplasmic reticulum during the polarisation.  Ca++ attaches to the Troponin-C. This brings a confirmational change in the tropomyosin. As a result unmasking of active sites on actin for myosin takes place. Cross bridges are formed between actin and myosin. This results in muscle contraction.

• The muscle contraction cycle is triggered by calcium ions binding to the protein complex troponin, exposing the active-binding sites on the actin. 
• ATP can then attach to myosin, which allows the cross-bridge cycle to start again; further muscle contraction can occur. 
• Cardiac myosin activators stimulate myosin ATPase, thereby increasing force generation. Hence, Calcium ions bring about muscle contraction
• through Activation of myosin ATP-ase.