Sarcotubular system, Excitation contraction coupling, Molecular theory of muscle contraction

Sarcotubular system
Excitation contraction coupling
Molecular theory of muscle contraction

1. U

Sarcotubular system
Excitation contraction coupling

Molecular theory of muscle contraction

DR.CHARUSHILA, ASSISTANT PROFESSOR, PHYSIOLOGYZMCH,
30/03/2021 pe ine

2. Sarcotubular system

System of membranous structures
in form of vesicles and tubules

in sarcoplasm of muscle fiber.

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3. Structures in Sarcotubular System

T-Tubules

L (longitudinal)Tubules / SR

These are narrow tubules
formed by invagination of
sarcolemma.

These are closed tubules
that run in long axis of the
muscle fiber, form SR

These tubules penetrate all
way from one side of
muscle fiber to another side

These tubules form a closed
tubular system around each
myofibril

Because of their origin from
sarcolemma, T tubules
open to exterior of muscle.
Therefore, ECF runs
through their lumen.

Do not open to exterior like
T tubules

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4. L Tubule

* Correspond to ER of other cells.

+ At regular intervals, throughout length of myofibrils, L-tubules dilate & form pair of lateral
sacs -terminal cisternae.

+ Each pair of terminal cisternae is in close contact with T tubule

+ Triad = T tubule along with cisternae on either side

+ In human skeletal muscle, triads are situated at junction between ‘A’ band and ‘I’ band.
* Ca ions are stored in L tubule € amount of calcium ions is more in cisternae.

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5. Functions Of Sarcotubular System

Function of T-Tubules

+ Responsible for rapid transmission of impulse in form of AP from sarcolemma to
myofibrils.

Function of L-Tubules

+ Ltubules store large quantity of calcium ions.

+ When AP reaches cisternae of L tubule, Ca ions are released into sarcoplasm.

* The process by which Ca ions cause contraction of muscle is called excitation contraction
coupling.ag

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6. Molecular Basis Of Muscular Contraction

* Molecular mechanism is responsible for formation of actinomyosin complex that
results in muscular contraction.

1. Excitation-contraction coupling.

7. + DHPR -
Voltage gated Ca channel in T tubule Aband Iband

Sarcomere

+ RyR -
Ligand gated Ca channels in L tubule/SR

Transverse tubule
Longitudinal
sarcoplasmic reticulum

Terminal cistem
Ryanodine recapror Triad

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8. STEPS : Excitation contraction coupling
When AP reaches tip of T-tubule

It activates voltage-gated channels called DHP receptors (located in T-tubule membrane )

Activated DHP receptors in turn trigger opening of Ca2+ release channels located on
terminal cisterns, ryanodine receptor (RYR).

This is possible because lateral cisterns are located very close to tips of T-tubules and
protein channels of cisterns (DHP and RYR) are mechanically interlocked.

Thus, when the DHP is activated by the depolarization of T-tubules, it undergoes
conformational changes which result in the opening of RYR

9. STEPS : Excitation contraction coupling

Due to opening of calcium release channels (RYR), calcium ions diffuse into the cytoplasm.

10. Excitation-contraction coupling in
the muscle, showing (t) an action
potential that causes release of
calcium ions from the sarcoplas-
mic reticulum and then (2) re-
uptake of the calcium ions by a
calcium pump.

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Actin filaments

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‘Myosin flaments

11. b. Myoplasmic
Y ice

80 120
Time (msec)

Koeppen A Stanton: Berne and Levy Physiology, Gth Edition.
Copyright © 2008 by Mosby, an imprint of Elsevier, Inc. All rights reserved

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30/03/2021 De

u

12. Sliding Mechanism / Ratchet theory/ walk along theory.

FE] al
Zline Relaxed Zine Contracted
à

Z line Thick Thin
filament filament 8

M line

cos

Source: Barrett KE, Barman SM, Boitano S, Brooks Hi Ganong's Raview of Medical
Physiology, 29rd Edition httpi//www. sccessmedicine.com

Copyright © The McGraw-Hill Companies, Inc, All rights reserved.

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13. Central
bare zone

Globular heads (S-1)
with light chains
Proteolytic cleavage
at hinge regions

Gross-bridge
extending out
from thick
filament

Insoluble tail (LMM)
(buried in thick filament) >

B
ESSByHane SS SOOR by Mosby: EEE ne Au

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RL DAHOD GUJARAT =

14. Troponin

* It is formed by three subunits:

1. Troponin |, which is attached to F actin

2. Troponin T, which is attached to tropomyosin
3. Troponin C, which is attached to calcium ions.

Tropomyosin F-Actin Troponin -—-
| |

AOC T>
. pes

See = tht tee

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ARE DAHOD GUJARAT 4

15. Sliding Mechanism/ Ratchet theory / walk along theory.
A. F. Huxley and H. E. Huxley in 1954
Process of contraction

+ Initiation

* Attachment of myosin head to actin - ATP break
* Power stroke- Release of ADP & IP

* Detachment - New ATP

+ Reactivation- ATP break

16. Steps of cross-bridge cycling

Tropomyosin

17. Steps of cross-bridge cycling

Tropomyosin

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USHILA, ASSISTANT PROFES

F.

Actin

18. Tropomyosin

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19. Z line Myosin binding site
Fan Myosin head
H zone

Stage!

| Stage u

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ij = DAHOD, GIARAT

20. * Formation of the actin=myosin=ADP.Pi complex

21. filament.
New ATP

* The release of ADP and Pi allows a fresh
ATP molecule to bind to myosin head.

* The myosin-ATP complex has a low
affinity for actin, and therefore,

22. 5. Reactivation of myosin head. ATP break TEST 33
* Freshly bound ATP molecule splits again and myosin head is reactivated for

next cycle to begin.

+ Energized head extends perpendicularly towards actin filament and gets attached
to new active site for repeating cycle.

* Thus with each cross-bridge cycle, there is movement of actin filament
towards centre of myosin to small degree.