NEET Practice Questions (MCQs) with Answers & Solutions

(d) : Prostaglandin is any of a group of organic compounds derived from essential fatty acids and causing a range of physiological effects in animals. Prostaglandins have been detected in most body tissues. They act at very low concentrations to cause the contraction of smooth muscle; natural and synthetic prostaglandin are used to induce abortion or labor in humans and domestic animals. Two prostaglandin derivatives have antagonistic effects on blood circulation : thromboxane A₂ causes blood clotting, while prostacyclin causes blood vessels to dilate. Prostaglandins are also involved in inflammation, being released from affected tissues.

(c): A muscle fibre would contract only when it recieves stimulation of certain intensity called threshold stimulus. Response of a muscle fibre to a stimulus is not proportional to its intensity. It is absent when the intensity is subliminal. Muscle fibre contracts to the maximum whether the stimulus has threshold value or supraliminal value.

(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.

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