Zoology MCQs for NEET — Practice Questions with Answers

(b) : Patella is a sesamoid bone. Sesamoid bones are formed by ossification of a tendon where the tendon moves over a bony surface.

Ans: (a)

• 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’ M 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.

(a): Each muscle fibre has many parallely arranged myofibrils. Each myofibril contains many serially arranged units called sarcomere which are the functional units. Each sarcomere has central 'A' band made of thick myosin filaments on either side of it marked by 'Z' lines. 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.

(d): A stretch reflex is the contraction of a muscle in response to a passive stretching of a muscle spindle within a muscle. This reflex enables the body to correct any unintended changes in body posture in a timely manner without waiting for input from the cerebral cortex as well as keeps appropriate tone in muscles. A well known example of stretch reflex is patellar reflex or knee jerk reflex, i.e., sudden kicking movement of the lower leg in response to a sharp tap on patellar tendon.

(c) : Gerdy fibres are the superficial transverse ligament of the palm.      

In old age, stiffness of joints is due to the decrease in synovial fluid. This fluid is responsible for lubrication in joints.

Oxygen content reduction makes the glycolyse intensity increased due to increase of ADP concentration in cell.

The light bands contain actin and is called I-band or Isotropic band.

(page-304)

The dark band called ‘A’ or Anisotropic band contains myosin.

(page-305)

Two filaments of tropomyosin also run close to the ‘F’ actins throughout its length. In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments.

(page-306)