Tag: muscles and their structure

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'.
  

• Syncytial refers to a multinucleate condition where a single cell has more than one nucleus per cell i.e., multiple nuclei with a common cytoplasm.
• It is caused primarily due to multiple cell fusions of uninuclear cells.
• This condition usually arises when cells are infected with certain types of viruses (HIV, pneumoviruses etc.).
• In animals, this condition is seen in skeletal muscles (large skeletal muscle fibres are formed by the fusion of individual muscle cells).
• So, the correct answer is 'Syncytial'.

• 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'.

• 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