Muscle contraction - class-XI

• Acetylcholine is the neurotransmitter used at the neuromuscular junction, it is the chemical that motor neurons of the nervous system release in order to activate muscles when acetylcholine is released when the neural signal reaches motor end plate. 
• Repeated activation of muscle induces processes resulting in decreased performance (fatigue) as well as enhanced performance (potentiation).  Lactic acid is formed and accumulated in the muscle under conditions of high energy demand, rapid fluctuations of the energy requirement and insufficient supply of O2. Hence,  Repeated activation of the muscle can lead to lactic acid accumulation.

During muscle contraction, the A - band with a length of 1.85 micrometers does not undergo any contraction. However, the I-bands and the H-zone shorten. This causes the Z lines to come closer together. So, the correct answer is 'The width of A-band remains constant'.

• In the hurdle race, a person requires energy production faster than our bodies can adequately deliver oxygen. 
• In those cases, the working muscles generate energy anaerobically. This energy comes from glucose through a process called glycolysis, in which glucose is broken down or metabolized into a substance called pyruvate through a series of steps.
•  When oxygen is limited, the body temporarily converts pyruvate into a substance called lactate, which allows glucose breakdown and thus energy production to continue.
•  The working muscle cells can continue this type of anaerobic energy production at high rates for one to three minutes, during which lactate can accumulate to high levels.

• 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.
• Hence the ion responsible for masking active sites for myosin for cross-bridge activity during muscle contraction is calcium.
• So, the correct answer is 'Calcium'

• The endoplasmic reticulum, i.e., sarcoplasmic reticulum of the muscle fibres is the storehouse of calcium ions.
• A neural signal reaching this junction releases a neurotransmitter (Acetylcholine) which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm from the sarcoplasmic reticulum and initiate the mechanism of muscle contraction.
• Hence sarcoplasmic reticulum of muscle fibres is the storehouse of calcium.
• So, the correct answer is 'Calcium'.

•  Active sites for myosin 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. The myosin, releasing the ADP and P1 goes back to its relaxed state. Hence immediate source of energy for muscle contraction is ATP.
  
• So, the correct answer is 'ATP'.

Both smooth muscle and cardiac muscle utilise sarcoplasmic and extracellular fluid as source of calcium ion, which is in contrast to skeletal muscle which derive it from sarcoplasm only.

• A sarcomere is the basic unit of muscle tissue in both cardiac and skeletal muscle. Myofibril activity is required for muscle contraction on the molecular level. 
• When ATP binds to myosin, it separates from the actin of the myofibril, which causes a 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 then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site.

In response to over-cooling, 

I-band consists of only thin filaments. A-band consists of both the thin and thick filaments. H-zone is the band of thick filament only. For skeletal muscle contraction following events occur:

Blood vessels narrowing, hairs standing up and shivering are all because of contraction of muscles. While reduced sweating is due to non-activation of sweat glands from the hypothalamus.

SER is associated with muscle contraction by release and uptake of Ca ions because SER functions as the packaging structure which stores the Ca ions until its release.

Troponin is attached protein tropomyosin and lies within the groove between actin filaments in the muscle tissue. The calcium attaches the troponin which changes the shape exposing binding sites for myosin on the actin filament. So the correct answer is 1.

Isometric is directly defined as ‘same length’ whereby ‘iso’ means the same and ‘metric’ means ‘length’ when referring to muscles. During isometric contractions, the muscle itself does not change in length while the tension never exceeds the load that must be carried. This means that while the muscle itself does not shorten, the tension will never exceed the opposing force.

During muscle contraction, the globular head of a myosin filament binds to the exposed active binding site on an actin filament, forming a cross bridge that facilitates sliding.

Mechanism of muscle contraction is best explained by the sliding filament theory, which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments. The actin filament slide over myosin filament thus reduces the length of the sarcomere and contracts the muscle fibre.

• When a nerve impulse reaches the neuromuscular junction, the calcium ions are released from the sarcoplasmic reticulum. Then the calcium ions trigger muscle contraction cycle by binding to the protein complex troponin, exposing the active-binding sites on the action. and this enables the myosin molecule heads to “grab and swivel” their way along the thin filament. This is the driving force of muscle contraction.
• Magnesium ions also play a role in regulating muscle contractions. The magnesium acts as a natural calcium blocker to help muscles relax. 
  
• Hence Chemical ions responsible for muscle contraction are Ca++ and Mg++ ions.
• So, the correct answer is 'Ca++ and Mg++ ions'.

Answer is- A

Cardiac muscles have mixture of properties of both  striated and unstriated muscles. .These muscle continue  rhythmic contraction throughout life under the control  of A.N.S.

• 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. Hence Size of A-bands remains the same, the Size of H-zone becomes smaller and Size of I-bands decreases.
• So, the correct answer is 'All the above'

Acetylcholine release from synaptic vesicles and travels across synaptic cleft and binds with protein receptors in sarcolemma. This causes the sarcolemma to become more permeable to sodium ions which causes them enter. Now sarcolemma is elctro positive inside and negative outside. This new potential difference is called action potential and sarcolemma now is called as depolarised. So the correct answer is 'Sarcolemma becomes permeable Na+ ions'.

• Active sites for myosin(Thick filament) are present on actin which are masked by troponin-C in resting state. when  Ca++ level increases it leads to the binding of calcium with a subunit of troponin i.e Troponin-C 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 center of ‘A’ band.
•  The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. Hence the ‘I’ band and 'H' band get reduced, whereas the ‘A’ bands retain the length.

• Muscle is a  bundle of fibrous tissue in a body that has the ability to contract, producing movement in or maintaining the position of parts of the body.
• The total number of muscles in the human body comes in the range of 640-850.
• Involuntary muscular contraction is called muscle spasm. Spasms of skeletal muscles are most common and are often due to overuse and muscle fatigue, dehydration, and electrolyte abnormalities.
• So, the correct answer is 'Muscle spasm'.

• Each myofibril has alternate dark and light bands on it. The light bands contain actin and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin.
  
•  In the centre of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line. The thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called’ line.
•  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 actions are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction.
• Hence the events occurring in muscular contraction are  H-zone disappears, I-band reduces in width, The width of A band is unaffected and M-line and Z-line come closer.
• So, the correct answer is 'i, iii, iv, v

• When an impulse reaches the muscle fibres of a motor unit, it stimulates a reaction in each sarcomere between the actin and myosin filaments. 
• This reaction results in the start of a contraction and the sliding filament theory. And the Contraction period for a skeletal muscle is 4 sec.

• Potassium is an important mineral that functions as an electrolyte. 
• It helps regulate fluid balance, nerve signals and muscle contractions and nervous excitability. 
• Increased extracellular potassium levels result in depolarization of the membrane potentials of cells due to the increase in the equilibrium potential of potassium.
•  This depolarization opens some voltage-gated sodium channels but also increases the inactivation at the same time. Hence, Muscular and nervous excitability is reduced by K+. 

• The muscle contraction cycle is triggered by calcium ions binding to the protein complex troponin, exposing the active-binding sites on the actin.
  
• ATP then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site.

'ATP' is the source of energy for muscle contraction.

Muscle contraction:

' Ca${++}$ and Na${++}$ ' help in muscle contraction. So, the correct answer is (c).

During muscle contraction, actin and myosin form actomyosin, phosphate reserve comes from phosphocreatine and chemical energy is changed into mechanical energy. So, the correct answer is (d).

• When an electrical signal passes down the motor nerve to the muscle it causes the depolarisation of the muscle membrane.
• This triggers the release of calcium ions into the muscle interior where they bind to troponin.
• This causes the tropomyosin to shift from the face of the actin filament to which myosin heads needs to bind to produce contraction.
• Thus, the active sites on actin filaments are exposed.
  

In smooth muscle, contraction is regulated by the binding of calcium to calmodulin, which then activates myosin light chain kinase.

When a weak signal is sent by the CNS to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger. This is known as multiple fibre summations.

Actomyosin is a contractile complex of actin and myosin, that together with ATP, is active during muscular contraction.
Disruption of the myosin-actin interaction requires ATP, while the myosin molecule can hydrolyze ATP into ADP and Pi.
ATP disconnects actin from myosin, after which it is hydrolyzed by the myosin molecule to produce the energy required for muscle contraction.

• The resting stage is also called interphase. The interval in the cell cycle between two cell divisions when the individuals cannot be distinguished, the interphase goes through the resting stage. 
• A subunit of troponin called tropomyosin is masked during the resting stage binds myosin on the actin filaments.

Fastest-reacting muscle in the body is the Orbicularis oculi: the muscle that encircles the eye and closes the eyelid. It contracts in less than 0.01 second.

Cardiac muscles are the special types of muscles that show striations but they are not under the will of the organism. These are also called involuntary striated muscles because of this reason. They have rich blood supply as they have to perform the contraction regularly for the heart. They show the property of contraction even when they are isolated from the body temporarily. They are supplied with both the central and autonomic nervous system that regulate the heartbeat.

When skeletal muscle fibers are treated with glycerol, soluble proteins like myoglobin and various glycolytic enzymes leak out, while the structural proteins (actin and myosin) remain in the sarcomeres.

• Acetylcholine (ACh) is a neurotransmitter released by motor neurons that binds to receptors in the motor endplate when the neural signal reaches the motor endplate. Neurotransmitter release occurs when an action potential travels down the motor neuron's axon, resulting in altered permeability of the synaptic terminal membrane and an influx of calcium. Hence statement I is correct.
• Repeated activation of the muscles can cause the anaerobic breakdown of glucose which leads to the formation of lactic acid and its accumulation. Accumulation of lactic acid can irritate muscles and cause discomfort and soreness. Hence Statement IV is correct.
  
• So, the correct answer is 'I and IV alone are correct'.

The contraction of skeletal muscle includes ultrastructural and biochemical events. Ultrastructural events lead with stimuli excitation of T-system followed by cross-bridge formation (myosin and actin filaments involved) and the cross-bridge breakage. The biochemical events explain that stimuli lead to neurotransmitter secretion, excitation of T-system and release of Ca$^{2+}$.

• Myofibril activity is required for muscle contraction on the molecular level. 
  
• When ATP binds to myosin, it separates from the actin of the myofibril, which causes a contraction.
• Creatine phosphokinase helps in the regeneration of ATP.
• H zone disappears and Z zone moves towards each other.
• The muscle contraction cycle is triggered by calcium ions binding to the protein complex troponin, exposing the active-binding sites on the actin.
• ATP then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site.

Most of the muscle actions of the body are voluntary except for the pumping of heart and the reflex action. These muscle receive a signal through the motor nerves , send by the brain or the CNS.

Repeated activation leads to lactic acid accumulation (A). The myosin head binds ATP (B) and actin (C). The H-zone (D) is the central part of the thick filament.

When calcium is pumped back into the sarcoplasmic reticulum breaking the link between actin and myosin. Actin and myosin return to their unbound state causing the muscle to relax.

The myosin filament has meromyicin and it has two important parts, a globular head and a tail. The globular head has the site for ATPase enzyme and has binding sites for ATP and active sites for actin.