Structure of skeletal muscle - class-XI

Multinucleate cells are those cells which have more than one nucleus per cell. Skeletal muscles or striated muscles are multinucleated cells. However, smooth muscles or non-striated muscles and cardiac muscles are mononucleated.  
Thus, the correct answer is 'Striated muscle.'

• 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.
  
• Active sites for myosin 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.
• Hence on stimulation of skeletal muscle, calcium is immediately made available for binding to troponin from sarcoplasmic reticulum.
• So, the correct answer is 'Sacroplasmic reticulum'.

Smooth muscles are indeed non-striated, while the other statements contain inaccuracies (e.g., cardiac fibers are branched but typically have a single nucleus, and skeletal muscles are always striated).

The cytoplasm in the muscle fibre is called as sarcoplasm. Sarcolemma is the sheath or cell membrane present around the striated muscle fibre. The cytoplasm in the nerve cells is called as neuroplasm. The cytoplasm in the nucleus of the cell is called as necleoplasm. 

The all or none law states that the strength of the stimuli will not decide the reaction of the muscle fibre. f the stimulus is more than a certain threshold they will fire.  Either there will be a full response or there will be no response at all.

Muscle hypertrophy, or the increase in the thickness of muscle fibers, occurs primarily in response to resistance exercise.

Sarcomere is the smallest contractile unit of striated muscle fibre. Sarcomere occurs as repeating units along the length of a myofibril, occupying the region between Z lines of the myofibril. 

• Each muscle fibre(muscle cell) is lined by the plasma membrane called sarcolemma enclosing the sarcoplasm(cytoplasm of muscle fibre). 
• The characteristic feature of the muscle fibre(muscle cell) is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils.
• Each myofibril is made up of two kinds of proteins. They are actin and myosin.
• The function of the myofibril is to perform muscle contraction via the sliding-filament model. Hence Contractile fibrils of muscles are called myofibrils.
• So, the correct answer is 'Myofibrils'.

The area of contact between the ends of a large myelinated nerve fibre and a fibre of the skeletal muscle is called the motor end plate junction. It is also called as a neuromuscular junction or myoneural junction.
Thus, the correct answer is the option (A), 'Neuron and striated muscle'.

• The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. The light bands contain actin(thin filament) and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin(thick filament).
•  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 portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere Sarcomere is considered as the functional unit of contraction in a muscle fibre.
  
• So, the correct answer is 'Portion of a myofibril between two successive Z-lines'.
  

• Skeletal muscle fiber is made up of two types of filaments. They are thick filament (myosin) and thin filament (actin). The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils.
  
• The A band in skeletal muscle is also called as anisotropic band.
• The anisotropic band is made of both actin filaments and myosin filaments.
  
• So, the correct answer is 'Both A and B'.

Somatic nerves are part of the peripheral nervous system and are responsible for the voluntary control of skeletal muscles. Autonomic, sympathetic, and parasympathetic nerves control involuntary functions like heart rate and digestion.

• Myofibril is made up of two types of filaments. They are thin filament(actin) and thick filament(myosin).
• The light bands contain actin and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin. The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils.
  
•  In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. This central part of thick filament, not overlapped by thin filaments is called the ‘H’ zone.
  
• Hence H-zone in skeletal muscle fibre is due to Central gap between actin filaments extending through myosin filaments in A-band.
• So, the correct answer is 'Central gap between actin filaments extending through myosin filaments in A-band'.

• The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. The light bands contain actin(thin filament) and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin(thick filament). 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 portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere 
  
• 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 ‘I’ bands get reduced, whereas the ‘A’ bands retain the length.
•  The central part of thick filament, not overlapped by thick filament is the H-zone.
  
• So, the correct answer is '1,2 and 4

• 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. thus myosin forms cross bridges.
• 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 cross bridges of sarcomere in skeletal muscle are made up of myosin.
• So, the correct answer is 'Myosin'.

Intercalated discs support synchronized contraction of cardiac tissues. They occur at the Z line of the sarcomere and can be visualized easily when observing a longitudinal section of the tissue. Three types of cell junction make up an intercalated disc.

There are cardiac, skeletal and smooth muscle cells of which only skeletal or voluntary muscle cells are multinucleated. Such cells are syncytial. Skeletal muscle cells arise in the embryo as uninucleated entities. Later the cells fuse to give rise to the multinucleated muscle fibre.

Each half of pectoral and pelvic girdle consist of clavicle & scapula, tetany is a condition of rapid spasms in muscle due to low calcium ion in body fluid.