Skeletal Muscle Contraction & Relaxation -Mechanism

SKELETAL MUSCLE CONTRACTION & RELAXATION -MECHANISM

Q. 1 Intermediate filaments (IFs) help link adjacent sarcomeres together in skeletal muscle. Which of the following proteins is an intermediate protein used diagnostically in pathologic cases to indicate muscle origin?

 A Actin     
 B Desmin
 C Actinin
 D Clathrin
Q. 1 Intermediate filaments (IFs) help link adjacent sarcomeres together in skeletal muscle. Which of the following proteins is an intermediate protein used diagnostically in pathologic cases to indicate muscle origin?

 A Actin     
 B Desmin
 C Actinin
 D Clathrin
Ans. B

Explanation:

Desmin is the intermediate filament (IF) protein that polymerizes to form the desniin IF of skeletal muscle. It is found in smooth, skeletal, and cardiac muscle. The desmin IF links desmosomes of cardiac muscle, thus serving to stabilize cell junctions. Actin is the protein of microfilaments-the smallest diameter filament of the cytoskeleton. Actinin is an actin-associated protein (an actin-bundling protein) that serves to cross4ink actin bundles and anchor actin filaments to the cell membrane. Clathrin forms a basket or cage around endocytotic vesicles. Vimentin is a widely distributed IF protein present in many cells of mesodermal origin.


Q. 2 Contractile unit of muscle is? 
 A

Sarcomere

 B

Sarcolemma

 C

Myofibril

 D

Sarcotubular System

Q. 2 Contractile unit of muscle is? 
 A

Sarcomere

 B

Sarcolemma

 C

Myofibril

 D

Sarcotubular System

Ans. A

Explanation:

Sarcomere REF: Guyton’s physiology 22nd edition page 73

The area between two adjacent Z lines is called a sarcomere. The orderly arrangement of actin, myosin, and related proteins that produce this pattern. The thick filaments, which are about twice the diameter of the thin filaments, are made up of myosin; the thin filaments are made up of actin, tropomyosin, and troponin. The thick filaments are lined up to form the A bands, whereas the array of thin filaments forms the less dense I bands. The lighter H bands in the center of the A bands are the regions where, when the muscle is relaxed, the thin filaments do not overlap the thick filaments. The Z lines transect the fibrils and connect to the thin filaments. If a transverse section through the A band s examined under the electron microscope, each thick filament is seen to be surrounded by six thin filaments in a regular hexagonal pattern.


Q. 3

Which of the following protein prevents contraction by covering binding sites on actin tereby preventing its interaction with myosin?

 A

Thymosin

 B

Troponin

 C

Calmodulin

 D

Tropomyosin

Q. 3

Which of the following protein prevents contraction by covering binding sites on actin tereby preventing its interaction with myosin?

 A

Thymosin

 B

Troponin

 C

Calmodulin

 D

Tropomyosin

Ans. D

Explanation:

Tropomyosin molecules lie on top of the active sites of the actin strands, thereby preventing the  attraction between the actin and myosin filaments.

Ref: Review of Medical Physiology, 23rd Edition By Wiiliam F Ganong; Guyton and Hall Medical Physiology, 10th Edition, Page 71.


Q. 4

Which of the following is the function of Tropomyosin?

 A

It slides over myosin

 B

It causes release of calcium

 C

It helps in the fusion of actin and myosin

 D

It covers myosin and prevents attachments of actin and myosin

Q. 4

Which of the following is the function of Tropomyosin?

 A

It slides over myosin

 B

It causes release of calcium

 C

It helps in the fusion of actin and myosin

 D

It covers myosin and prevents attachments of actin and myosin

Ans. D

Explanation:

Tropomyosin molecules lie on top of the active sites of the actin strands, thereby preventing the attraction between the actin and myosin filaments.

Ref: Ganong’s Review of Medical Physiology, 23rd Edition, Chapter 5


Q. 5

Actin filament is not present in which of the band zones?

 A

H band

 B

I band

 C

M band

 D

Z band

Q. 5

Actin filament is not present in which of the band zones?

 A

H band

 B

I band

 C

M band

 D

Z band

Ans. A

Explanation:

The myosin and actin filaments overlap in peripheral regions of the A band, whereas a middle region (called the H zone) contains only myosin. H-band is the zone of the thick filaments that is not superimposed by the thin filaments.


Q. 6

Name the structure that is NOT involved in excitation-contraction coupling in striated muscle:

 A

Microtubules

 B

Sarcolemma

 C

Sarcoplasmic Reticulum

 D

Motor end plate

Q. 6

Name the structure that is NOT involved in excitation-contraction coupling in striated muscle:

 A

Microtubules

 B

Sarcolemma

 C

Sarcoplasmic Reticulum

 D

Motor end plate

Ans. A

Explanation:

Microtubules play no role in excitation-contraction coupling. Action potentials are propagated by the sarcolemma as an essential step in excitation-contraction coupling. Ca++ release from the sarcoplasmic reticulum is necessary for excitation-contraction coupling. The motor end plate is the site of neuromuscular transmission, the first stage of excitation-contraction coupling. T-tubules provide an electrical link whereby action potentials trigger Ca++ release from the sarcoplasmic reticulum.


Q. 7

Thin filaments in skeletal muscles are mainly composed of which of the following?

 A

Myosin

 B

Actin

 C

Tropomyosin

 D

Dystrophin

Q. 7

Thin filaments in skeletal muscles are mainly composed of which of the following?

 A

Myosin

 B

Actin

 C

Tropomyosin

 D

Dystrophin

Ans. B

Explanation:

Thin filaments are polymers made up of two chains of actin that forms a long double helix. Tropomyosin molecules are long filaments located in the groove between the two chains in the actin. Each thin filament contains 300–400 actin molecules and 40–60 tropomyosin molecules. 

 
  • Troponin molecules are small globular units located at intervals along the tropomyosin molecules. There are 3 troponin subunits. 
  • Troponin T binds the troponin components to tropomyosin.
  • Troponin I inhibits the interaction of myosin with actin.
  • Troponin C contains the binding sites for the Ca2+ that helps to initiate contraction.
  • Thick filaments have twice the diameter of thin filaments and are made up of myosin.
Ref: Ganong’s Review of Medical Physiology, 24e, chapter 5

Q. 8

Sarcomere is the area between two:

 A

A bands

 B

I bands

 C

Z lines

 D

H bands

Q. 8

Sarcomere is the area between two:

 A

A bands

 B

I bands

 C

Z lines

 D

H bands

Ans. C

Explanation:

Differences in the refractive indexes of the various parts of the muscle fiber are responsible for the characteristic cross-striations seen in skeletal muscle when viewed under the microscope. The parts of the cross-striations are frequently identified by letters. The light I band is divided by the dark Z line, and the dark A band has the lighter H band in its center. A transverse M line is seen in the middle of the H band, and this line plus the narrow light areas on either side of it are sometimes called the pseudo-H zone. The area between two adjacent Z lines is called a sarcomere.            
         
Ref:
Ganong’s Review of Medical Physiology 23rd edition, Chapter 5.

 


Q. 9

Which of the following will take place during skeletal muscle contraction?

 A

A band shorten

 B

Both H and I band shorten

 C

Both A and I band shorten

 D

Both A and H band shorten

Q. 9

Which of the following will take place during skeletal muscle contraction?

 A

A band shorten

 B

Both H and I band shorten

 C

Both A and I band shorten

 D

Both A and H band shorten

Ans. B

Explanation:

During contraction, the I band decreases in size as thin filaments penetrate the A band. The H band—the part of the A band with only thick filaments—diminishes in width as the thin filaments completely overlap the thick filaments. A net result is that each sarcomere, and consequently the whole cell (fiber), is greatly shortened.

Also know:
  • The light I band is divided by the dark Z line, and the dark A band has the lighter H band in its center. A transverse M line is seen in the middle of the H band, and this line plus the narrow light areas on either side of it are sometimes called the pseudo-H zone. The area between two adjacent Z lines is called a sarcomere.
  • The thick filaments, which are about twice the diameter of the thin filaments, are made up of myosin; the thin filaments are made up of actin, tropomyosin, and troponin. 
  • The lighter H bands in the center of the A bands are the regions where, when the muscle is relaxed, the thin filaments do not overlap the thick filaments.
Ref: Mescher A.L. (2010). Chapter 10. Muscle Tissue. In A.L. Mescher (Ed), Junqueira’s Basic Histology: Text & Atlas, 12e.

Q. 10

The major cation directly involved in the interaction of actin and myosin in skeletal muscle is:

 A

Ca++

 B

Na +

 C

K+

 D

Mg++

Q. 10

The major cation directly involved in the interaction of actin and myosin in skeletal muscle is:

 A

Ca++

 B

Na +

 C

K+

 D

Mg++

Ans. A

Explanation:

The major cation directly involved in the interaction of actin and myosin in skeletal muscle is calcium (Ca++). Calcium binds with troponin to cause tropomyosin to uncover the active sites on the actin filaments so that the myosin heads can attach to form cross bridges. Na+ and K+ are important in the mechanism of the action potential.
 
Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 5. Excitable Tissue: Muscle. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds),Ganong’s Review of Medical Physiology, 24e. 


Q. 11

When a muscle is not contracting, actin and myosin are prevented from reacting by:

 A

Troponin-tropomyosin

 B

Phosphocreatine

 C

Heavy meromyosin

 D

Acetylcholinesterase

Q. 11

When a muscle is not contracting, actin and myosin are prevented from reacting by:

 A

Troponin-tropomyosin

 B

Phosphocreatine

 C

Heavy meromyosin

 D

Acetylcholinesterase

Ans. A

Explanation:

When a muscle is not contracting, actin and myosin are prevented from reacting by troponin-tropomyosin. Tropomyosin covers the active sites on actin filaments which prevents myosin heads from attachment. Troponin must bind with Ca++ in order to cause the tropomyosin to move off the active sites. Phosphocreatine has a high energy phosphate bond which is used to reconstitute ATP. Heavy meromyosin refers to the heavy chains in the myosin filament. Acetylcholinesterase and acetylcholine are at the neuromuscular junctions and do not directly affect actin and myosin crossbridge formation.
 
Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 5. Excitable Tissue: Muscle. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds),Ganong’s Review of Medical Physiology, 24e.

 


Q. 12

The energy for muscle contraction is supplied by the enzymatic hydrolysis of ATP by ATPase present in:

 A

Heavy meromyosin

 B

Light meromyosin

 C

Tropomyosin

 D

Troponin

Q. 12

The energy for muscle contraction is supplied by the enzymatic hydrolysis of ATP by ATPase present in:

 A

Heavy meromyosin

 B

Light meromyosin

 C

Tropomyosin

 D

Troponin

Ans. A

Explanation:

The energy for muscle contraction is supplied by the enzymatic hydrolysis of ATP by ATPase present in heavy meromyosin. Two strands of heavy meromyosin are found in each myosin head – one of which is folded over as a protein mass. Light meromyosin is also found in myosin but is not involved in splitting ATP. Troponin and tropomyosin are located on the actin filament.
 
Ref: Murray R.K. (2011). Chapter 49. Muscle & the Cytoskeleton. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper’s Illustrated Biochemistry, 29e.

Q. 13

All of the following proteins are involved during contraction of muscles, EXCEPT:

 A

Myosin

 B

Actin

 C

Myoglobin

 D

Troponin

Q. 13

All of the following proteins are involved during contraction of muscles, EXCEPT:

 A

Myosin

 B

Actin

 C

Myoglobin

 D

Troponin

Ans. C

Explanation:

The contractile mechanism in skeletal muscle largely depends on the proteins,
  • Myosin-II
  • Actin
  • Tropomyosin
  • Troponin
Troponin is made up of three subunits: troponin I, troponin T, and troponin C.
 
Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 5. Excitable Tissue: Muscle. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong’s Review of Medical Physiology, 24e.

Q. 14

Which of the following events happens when a nerve impulse arrives at the Neuromuscular Junction:

 A

Release of Calcium from the Sarcoplasmic Reticulum

 B

Acetylcholine binds to receptors on the postsynaptic sarcolemmal membrane

 C

Acetylcholine binds to receptors on the postsynaptic sarcolemmal membrane

 D

Efflux of calcium through synaptic vesicles into the synaptic cleft

Q. 14

Which of the following events happens when a nerve impulse arrives at the Neuromuscular Junction:

 A

Release of Calcium from the Sarcoplasmic Reticulum

 B

Acetylcholine binds to receptors on the postsynaptic sarcolemmal membrane

 C

Acetylcholine binds to receptors on the postsynaptic sarcolemmal membrane

 D

Efflux of calcium through synaptic vesicles into the synaptic cleft

Ans. B

Explanation:

B i.e. Acetylcholine binds to receptors on the postsynaptic sarcolemmal membrane

–                       Action potential conducted along the nerve fiber enhances the endocytic release of ACh (acetyl choline) from vesicles (packets) into the neuromuscular cleft by enhancing the permeability of presynaptic membrane to Ca++ ions (lit Ca++ influx not efflux through voltage gated channels).

–  Sequence of events when a nerve impulse arrives at neuromuscular junction is: Calcium influx in presynaptic neural membrane though voltage gated Ca++ channels -> Exocytosis of ACh from synaptic vesicles into synaptic cleft -> ACh binds to Nm receptors on post synaptic sarcolemmal membrane which are ligand gated Na+ channels -+ Na+ influx; development of EPP & current sink -> AP conducted in both directions and enter muscle fiber via T-tubules -> Ca++ released (efflux) from sarcoplasmic reticulum (ryanodine receptor channels) in response to voltage change sensed by DHPR on T tubules Binding of Ca++ to troponin C, uncovering myosin binding sites of actin l/t formation of cross linkage (bridging) that allows muscle contraction.

Nerve impulse reach NM junction -+ Increased Ca++ permeability of presynaptic neural membrane d/t opening of voltage gated Ca++ channels –> Influx of calcium from synaptic space to interior of presynaptic nerve ending —> Calcium ion attracts acetyl choline (ACh) vesicles and draws

/ fuse them to neural membrane adjacent to dense bars leading to a marked increase in exocytosis/ release of ACh in synaptic cleft?

ACh diffuses through synaptic space and binds to nicotinic cholinergic (Nm) receptors in the post synaptic motor end plate. (ACh receptors are concentrated at the top of subneural clefts = junctional folds) —> Binding of ACh to ACh receptors (which are ligand gated sodium channel) increases Na+ and K+ conductance of muscle cell membrane.

Influx of positively charged Na+ ions into the muscle fibre creates a local positive potential change inside the muscle fiber i.e. end plate potential (EPP) -4 Current sink created between depolarized end plate (with EPP) and adjacent muscle plasma membrane on (both sides) initiates an action potential (ie depolarizes the adjacent muscle membrane to its firing level) —> Action potentials are generated on either side of end plate and are conducted away from end plate in both directions along the muscle fiber -+ AP (depolarization) spreads to all parts & to interior of muscle fibers by way of T (transverse) tubules (communicate externally with ECF, contain ECF and are actually internal extension of cell membrane) -4 As AP reaches T-tubules, the voltage change is sensed by DHPR (dihydro pyridine receptors) on T tubule -+ DHPRs are physically linked to calcium release (ryanodine receptor) channels in the adjacent sarcoplasmic reticular (SR) cisternae. AP in T-tubule cause conformational change in voltage sensing DHP receptors, which unlocks (opens) the Ca’+ release channels in terminal cistern of SR and permitting Ca” to rapidly diffuse into the sarcoplasm (thick & thin filament). The calcium induced Ca” release quickly amplifies the release of Ca” -4 Binding of Ca– to troponin C, uncovering myosin binding sites on actin -4 Formation of cross linkage between actin and myosin and sliding of thick on thin filaments producing movement.


Q. 15

The band which disappears on muscular contraction is‑

 A

A

 B

H

 C

I

 D

M or CM

Q. 15

The band which disappears on muscular contraction is‑

 A

A

 B

H

 C

I

 D

M or CM

Ans. B

Explanation:

B i.e. H


Q. 16

In Muscle contraction all are true except

 A

A bond remains unchanged

 B

H zone disappears

 C

I band becomes wider

 D

Two Z lines come closer

Q. 16

In Muscle contraction all are true except

 A

A bond remains unchanged

 B

H zone disappears

 C

I band becomes wider

 D

Two Z lines come closer

Ans. C

Explanation:

C i.e. I band becomes wider


Q. 17

Which protein prevents contraction by covering binding sites on action and myosin :

 A

Troponin

 B

Calmodulin

 C

Thymosin

 D

Tropomyosin

Q. 17

Which protein prevents contraction by covering binding sites on action and myosin :

 A

Troponin

 B

Calmodulin

 C

Thymosin

 D

Tropomyosin

Ans. D

Explanation:

D i.e. Tropomyosin


Q. 18

Tropomyosin :

 A

Helps in the fusion of actin and myosin

 B

Covers myosin and prevents attachments of actin ad myosin

 C

Slides over myosin

 D

Causes Cat‘ release

Q. 18

Tropomyosin :

 A

Helps in the fusion of actin and myosin

 B

Covers myosin and prevents attachments of actin ad myosin

 C

Slides over myosin

 D

Causes Cat‘ release

Ans. B

Explanation:

B i.e. Covers myosin and prevents attachment of actin and myosin


Q. 19

Which of the following triggers muscle contraction

 A

Ca binding tropomyosin

 B

Ca binding troponin C

 C

ATP breakdown

 D

Ca binding troponin I

Q. 19

Which of the following triggers muscle contraction

 A

Ca binding tropomyosin

 B

Ca binding troponin C

 C

ATP breakdown

 D

Ca binding troponin I

Ans. B

Explanation:

B i.e. Calcium binding troponin C

  • In the resting state of skeletal muscle tropomyosin molecule lie on top of active sites of actin filamentsQ, so that attraction cannot occur between actin & myosin filaments to cause contraction.

Mechanism of contraction: when Ca2+ are released during the contraction process, the troponin complex undergoes a confirmational change that in some way shifts the tropomyosin molecules into the groove between the two actin strands. This uncovers the active sites on actin thus allowing myosin to bind the actin & contraction proceed.

  • In resting skeletal muscle tropomyosin (a long filamentous protein) covers the active sites of actin filament where myosin head binds to acting. So that the attraction cannot occur between actin and myosin filaments to cause contraction.
  • Initiation of muscle contraction occurs, when Ca++ binds toroponin CQ. Binding causes lateral displacement of tropomyosin into the groove between two actin filaments. This uncovers active sites on actin thus allowing myosin head to bind the actin and contraction proceeds.

Q. 20

The difference between skeletal an smooth muscle contraction and relaxation is/are:

 A

Troponin (+ve)

 B

Myosin light chain kinase

 C

Ca+2

 D

Actin

Q. 20

The difference between skeletal an smooth muscle contraction and relaxation is/are:

 A

Troponin (+ve)

 B

Myosin light chain kinase

 C

Ca+2

 D

Actin

Ans. A

Explanation:

A i.e. Troponin (+ve)


Q. 21

Stiffening of body muscles after death due to ATP depletion: 

COMEDK 13

 A

Cadaveric spasm

 B

Cadaveric rigidity

 C

Hypostasis

 D

Gas stiffening

Q. 21

Stiffening of body muscles after death due to ATP depletion: 

COMEDK 13

 A

Cadaveric spasm

 B

Cadaveric rigidity

 C

Hypostasis

 D

Gas stiffening

Ans. B

Explanation:

Ans. Cadaveric rigidity


Q. 22

Acetylcholine acting on nicotinic receptors produces:

KCET 12

 A

Contraction of skeletal muscle

 B

Secretion of saliva

 C

Bradycardia

 D

Pupillary constriction

Q. 22

Acetylcholine acting on nicotinic receptors produces:

KCET 12

 A

Contraction of skeletal muscle

 B

Secretion of saliva

 C

Bradycardia

 D

Pupillary constriction

Ans. A

Explanation:

Ans. Contraction of skeletal muscle


Q. 23

All of the following are true about excitation contraction coupling except:

 A

Acetylcholine is released at the nerve terminal

 B

Calcium is pumped back into the sarcoplasmic reticulum during relaxation

 C

Calcium is released from sarcoplasmic reticulum during contraction

 D

Calcium binds to tropomyosin to initiate muscle contraction

Q. 23

All of the following are true about excitation contraction coupling except:

 A

Acetylcholine is released at the nerve terminal

 B

Calcium is pumped back into the sarcoplasmic reticulum during relaxation

 C

Calcium is released from sarcoplasmic reticulum during contraction

 D

Calcium binds to tropomyosin to initiate muscle contraction

Ans. D

Explanation:

Ans. d. Calcium binds to tropomyosin to initiate muscle contraction

Calcium binds to Troponin C, not to the tropomyosin, to initiate muscle contraction.

`The calcium released into the cytosol binds to Troponin C by the actin filaments, to allow cross-bridge cycling, producing force and, in some situations, motion.’


Q. 24

Myosin and actin filaments are kept in place by ‑

 A

Tropomyosin

 B

Troponin

 C

Actinin

 D

Titin

Q. 24

Myosin and actin filaments are kept in place by ‑

 A

Tropomyosin

 B

Troponin

 C

Actinin

 D

Titin

Ans. D

Explanation:

Ans. is ‘d’ i.e., Titin

  • The side-by-side relationship between the myosin and actin filaments is difficult to maintain.
  • This is achieved by a large number of filamentous molecules of a protein called titin.
  • Titin molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomere will work.

Important muscle proteins

  1. Myosin :- Myosin is the protein that constitutes the thick filaments. Myosin of skeletal muscle is myosin-IL Myosin participates in the contractile mechanism and also acts as an ATPase.
  2. Actin :- Actin is the major protein of thin filament. It is the actin which slides over myosin during contraction.
  3. Tropomyosin : – It is the other protein of thin .filament. It covers the active sites (myosin binding sites) on actin. When Ca+2 concentration of cytoplasm (sarcoplasm) is raised, it uncovers the active sites of actin and allow the contraction So, the ‘cross-bridge cycling’ is switched off or on by the tropomyosin molecule which slides on the actin molecule to cover or uncover the active sites on it.

Q. 25

Troponin C mediated function is of which of the following?

 A

Dystrophin

 B

Calmodulin

 C

Actin

 D

Calcineurin

Q. 25

Troponin C mediated function is of which of the following?

 A

Dystrophin

 B

Calmodulin

 C

Actin

 D

Calcineurin

Ans. C

Explanation:

Ans. is `c’ i.e., Actin

Ca+ binds to troponin and this troponin – Ca+ complex cause lateral displacement of tropomysin.

As a result active sites of actin becomes uncovered and interact with myosin.

Important skeletal muscle proteins

1 .Myosin : – Myosin is the protein that constitutes the thick filaments. Myosin of skeletal muscle is myosin-IL Myosin participates in the contractile mechanism and also acts as an ATPase.

2.Actin : – Actin is the major protein of thin filament. It is the actin which slides over myosin during contraction.

3.Tropomyosin : – It is the other protein of thin filament. It covers the active sites (myosin binding sites) on actin. When Ca+2 concentration of cytoplasm (sarcoplasm) is raised, it uncovers the active sites of actin and allow the contraction So, the ‘cross-bridge cycling’ is switched off or on by the tropomyosin molecule which slides on the actin molecule to cover or uncover the active sites on it.

4. Troponin : – It is a protein also associated with thin filament. The regulatory action of Ca+2 on tropomysin is mediated by troponin, a Ca+2 binding protein. When the cytoplasmic (sarcoplasmic) Ca+2 concentration rises Ca+2 ions bind to troponin-C. The troponin – Ca+2 complex induces changes in Troponin-I and Troponin-T, which in turn brings about a shift of tropomyosin away from the active sites of actin. When the sarcoplasmic Ca+2 concentration falls, Ca+2 dissociates from troponin-C and tropomyosin slides back on the actin filament to cover the active sites. Thus, the first event during muscle contraction, after cytoplasmic rise in Ca+2 con­centration, is binding of troponin C to Ca+2 which trigers the further steps by sliding the tropomysin away from the active sites of actin.

There are additional structural proteins in skeletal muscles.

i) Actinin : It binds actin to Z-lines.

ii) Titin : It is the largest known protein and connects Z-lines to M-lines and is responsible for passive stiffness of muscle, by limiting range of motion of sarcomere in tension.

iii) Desmin : It adds structures to Z lines in part by binding the Z-lines to plasma membrane.


Q. 26

Immediate energy supply for muscle contraction ‑

 A

GTP

 B

ATP

 C

Creatine phosphate

 D

Fatty acid

Q. 26

Immediate energy supply for muscle contraction ‑

 A

GTP

 B

ATP

 C

Creatine phosphate

 D

Fatty acid

Ans. B

Explanation:

Ans. is ‘B’ i.e.,ATP

Source of energy for muscular activity

  • The immediate source of energy for all muscle contraction is ATP, followed immediately by creatine phosphate.
  • In sternous exercise ATP store is sufficient only for 1-2 seconds and creatine phosphate for another 5-7 seconds.
  • Thus, energy rich phosphagen stores (ATP and creatine phosphate) permit severe muscle contraction for 8-10
  • seconds only.
  • After this, energy is obtained from the metabolism of stored glycogen or from circulating glucose and free fatty acids, depending upon the availability of oxygen.
  • Energy source during in exercise can be summarized by : ‑
  • Short burst of intense activity (e.g., 100 meter sprint or weight lifting) : – All energy comes from ATP and creatine phosphate. Breakdown of these compound is an anaerobic processes.
  • Little longer intense exercise (e.g., 200 meter sprint or 100 meter swim) :- Besides ATP and creatine phosphate, glycogen is metabolised by anerobic glycolytic pathways to provide a ready source of energy. So, muscle work is anaerobic.
  • Longer duration exercise (e.g., jogging, marathan run) : – The muscle work is aerobic and energy comes
  • from aerobic utilization ofglucose and free fatty acids. More glucose is utilized at the initial stage, but as the exercise is prolonged, free fatty acids become the predominant fuel.


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