Question
During muscular contraction, which of the following events occur? [NEET 2021]
I. ‘H’zone disappears
II. ‘A’ band widens
III. ‘I’ band reduces in width
IV. Myosine hydrolyses ATP, releasing the ADP and $\mathrm{Pi}$
V. Z-lines attached to actins are pulled inwards.
Choose the correct answer from the options given below.
(a) I, III, IV and V
(b) I, II, III and IV
(c) II, III, IV and V
(d) II, IV, V and I
Answer/Explanation
Ans. (a)
Statements I, III, IV, and V are correct and statement $I \mathrm{is}$ incorrect and can be corrected as During muscular
contraction by utilising energy from ATP hydrolysis the myosin head binds to the exposed active sites on actins to form a cross bridge.
This pulls the attached actin filaments towards the center of A band. This movement contricts ‘A’ band.
Question
Calcium is important in skeletal muscle contraction because it [NEET 2018]
(a) detaches the myosin head from the actin filament
(b) activates the myosin ATPase by binding to it
(c) binds to troponin to remove the masking of active sites on actin for myosin
(d) prevents the formation of bonds between the myosin cross bridges and the actin filament
Answer/Explanation
Ans. (c)
Calcium plays a key regulatory role in muscle contraction. $\mathrm{Ca}^{2+}$ ions bind to troponin and changes its shape and position. This in turn, alter the shape and position of tropomyosin and hence, the active sites on $\mathrm{F}$-actin are exposed. Due to this, myosin cross-bridges are able to bind to these active sites and muscle contraction occurs.
Question
Lack of relaxation between successive stimuli in sustained muscle contraction is known as [NEET 2016, Phase I]
(a) fatigue
(b) tetanus
(c) tonus
(d) spasm
Answer/Explanation
Ans. (b)
Sustained muscle contraction due to repeated stimulus is known as tetanus. This results due to muscle fatigue.
Question
Name the ion responsible for unmasking of active sites for myosin for cross-bridge activity during muscle contraction. [NEET 2016, Phase II]
(a) Calcium
(b) Magnesium
(c) Sodium
(d) Potassium
Answer/Explanation
Ans. (a)
$\mathrm{Ca}^{++}$ions bind with $\mathrm{Tp}_{\mathrm{c}}$ unit of troponin which is responsible for masking of active sites for myosin through tropomyosin. The event initiates cross-bridge activity during muscle contraction mechanism.
Question
Smooth muscles are [NEET 2016, Phase II]
(a) involuntary, fusiform, non-striated
(b) voluntary, multinucleate, cylindrical
(c) involuntary, cylindrical, striated
(d) voluntary, spindle-shaped, uninucleate
Answer/Explanation
Ans. (a)
Smooth muscles are involuntary, fusiform and non-striated. These muscles are located in the inner walls of hollow visceral organs of the body like alimentary canal, reproductive tract, etc.
They do not exhibit any striation and are smooth in appearance. Their activities are under autonomic and hormonal control and thus they are also known as involuntary muscles.
Hence, option (a) is correct.
Question
The $\mathrm{H}$-zone in the skeletal muscle fibre is due to [NEET 2013]
(a) the absence of myofibrils in the central portion of A-band
(b) the central gap between myosin filaments in the A-band
(c) the central gap between actin filaments extending through myosin filaments in the A-band
(d) extension of myosin filaments in the central portion of the $\mathrm{A}$-band
Answer/Explanation
Ans. (c)
$\mathrm{H}$-zone in skeletal muscle is due to the central gap between actin filaments extending through myosin filaments in the A-band. Alternate arrangement of dark and light bands gives the striated appearance to a skeletal muscle. At the centre of A-band, a comparatively less dark zone called $\mathrm{H}$-zone is present.In the centre of $\mathrm{H}$-zone, $\mathrm{M}$-line is present, formed by the threats that connect the myofilaments.
Question
Which one of the following is the correct pairing of a body part and the kind of muscle tissue that moves it? [CBSE AIPMT 2009]
(a) Heart wall – Involuntary unstriated muscle
(b) Biceps of – Smooth muscle fibres upper arm
(c) Abdominal – Smooth muscle wall
(d) Iris $\quad-$ Involuntary smooth muscle
Answer/Explanation
Ans. (d)
Smooth muscles are plain, non-striated, involuntary or unstriped muscles due to the absence of striations. These occur in the walls of hollow internal organs, in capsules of lymph glands, spleen, etc., in iris and ciliary body of eyes, skin dermis, penis and other accessory genitalia, etc.
Question
The contractile protein of skeletal muscle involving ATPase activity is [CBSE AIPMT 2006]
(a) myosin
(b) a-actinin
(c) troponin
(d) tropomyosin
Answer/Explanation
Ans. (a)
The kinesin, myosin and dynein proteins of skeletal muscle involve ATPase activity. This activity cause the contraction of skeletal muscles, propelling action of cilia and flagella and the intracellular transport of organelles.
Question
ATPase enzyme needed for muscle contraction is located in [CBSE AIPMT 2004]
(a) actinin
(b) troponin
(c) myosin
(d) actin
Answer/Explanation
Ans. (c)
During muscular contraction myosin cross bridges heads. Myosin is not only an actin binding protein, it is also an ATPase (an enzyme) which is activated by $\mathrm{Ca}^{2+}$ and $\mathrm{Mg}^{2+}$ ions. Hence, ATPase enzyme is located in myosin.
Question
Which statement is correct for muscle contraction? [CBSE AIPMT 2001]
(a) Length of $\mathrm{H}$-zone decrease
(b) Length of A-band remains constant
(c) Length of I-band increases
(d) Length of two Z-line increases
Answer/Explanation
Ans. (b)
When $\mathrm{Ca}^{+}$ions combine with troponin then in consequence muscle contraction will initiates.
During contraction, the Z-lines come closer together and the sarcomere becomes shorter. The length of A-band remains constant. I-bands shortens and $\mathrm{H}$-zone narrows.
Question
What is sarcomere? [CBSE AIPMT 2001]
(a) Part between two $\mathrm{H}$-lines
(b) Part between two A-lines
(c) Part between two I-bands
(d) Part between two Z-lines
Answer/Explanation
Ans. (d)
Huxley reported the fine structure of striated muscle fibre or myofibril. Each myofibril is made up of A-bands (dark band anisotropic) and I-bands = isotropic (light bands isotropic). These two bands are made up of myosin and actin filament respectively. Each I-band is divided into two equal halves by a thin, fibrous and transverse zig-zag partition, called Z-band or Z-disc or Krause’s membrane.
The part of the myofibril between two successive Z-lines functions as a contractile unit called sarcomere.
Question
The functional unit of contractile system in striated muscle is [CBSE AIPMT 1998]
(a) myofibril
(b) sarcomere
(c) Z-lines
(d) cross bridges
Answer/Explanation
Ans. (b)
Skeletal muscle is also called striated muscle because the regular arrangement of the myofilaments creates as repeating pattern of light and dark bands. Each repeating unit is a sarcomere, the basic functional unit of the muscle.
Question
Which of the following is the contractile protein of a muscle? [CBSE AIPMT 1998]
(a) Myosin
(b) Tropomyosin
(c) Actin
(d) Tubulin
Answer/Explanation
Ans. (a)
Myosin-II, a two-headed tailed variety of myosin is involved in muscle contraction.
Question
Which ion is essential for muscle contraction? [CBSE AIPMT 1994]
(a) $\mathrm{Na}$
(b) $\mathrm{K}$
(c) $\mathrm{Ca}$
(d) $\mathrm{Cl}$
Answer/Explanation
Ans. (c)
Calcium is an essential element required for the contraction of muscles. Release of calcium ions from sarcoplasmic reticulum trigger the muscle contraction process. In fact, calcium ions and ATP, actin and myosin interact forming actomyosin, which causes the muscles to contract.
Question
Intercostal muscles occur in [CBSE AIPMT 1988]
(a) abdomen
(b) thigh
(c) ribs
(d) diaphragm
Answer/Explanation
Ans. (c)
Intercostal muscles, i.e. internal intercostal and external intercostal are attached with the ribs, these are the main muscles for normal quite breathing. External intercostal muscle and diaphragm contract during inspiration and relax during expiration.