Sarcomere
Original Editor - Lucinda hampton
Top Contributors - Swati Singh, Lucinda hampton, Kim Jackson and Vidya Acharya
Introduction
A sarcomere is the basic contractile unit of a myocyte (muscle fiber). A sarcomere comprises two main protein filaments (thin actin and thick myosin filaments) which are the active structures responsible for muscular contraction. The widely accepted theory describing muscular contraction is called the sliding filament theory, which proposes that the active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere[1].
Sliding Filament Theory
The sliding theory was proposed by scientists, who through the use of high-resolution microscopes visualized the actin and myosin filaments within a sarcomere. They could see the length of the sarcomere when relaxed and its shortening as it contracted, and were able to give names to particular zones. It explains the formation of the actomyosin complex responsible for muscular contraction. Other names of the theory are the walk-along theory and ratchet theory.
- A bands (or anisotropic bands): It is also called the dark band and contains the whole thick filament (myosin) as well as the end of actin filaments.
- I bands (or isotropic bands): it is called the light band that contains only the thin filament (actin). The thin filament lies between the two thick filaments.
- Z disc: it is the area where two actin filaments connect and transverse the I bands. Similarly, sarcomere can also be described as the structure between the two z discs.
- M line: The M line contains the protein called myomesin and it marks the centre of the sarcomere.
- H zone: is the area between the M line and Z disc and contains only the myosin[2].
This video explains the structure of Sarcomere-sarcomere bands and actin and myosin arrangement.
Changes in sarcomere during muscle contraction
- Length of sarcomere= decreases.
- Length of I band= decreases
- H zone= decreases or disappears ( if forceful contraction)
- Length of A band= constant[4]
Sliding Filaments
In essence, during contraction, a sarcomere shortens like a collapsing telescope, as the actin filaments at each end of a central myosin filament slide toward the myosin's center, as described below.
The movements of myosin appear like a molecular dance, with the myosin reaching forward to bind to the actin, contracting, and then releasing actin, before it reaches forward again to bind actin in a new cycle. This is the so-called myosin-actin cycling. As the myosin S1 segment binds and releases actin, it forms what are called cross bridges, which extend from the thick myosin filaments to the thin actin filaments. The contraction of myosin's S1 region is called the power stroke, which requires the hydrolysis of ATP (Adenosine triphosphate), which breaks a high-energy phosphate bond to release energy, resulting in force generation and shortening of an individual sarcomere. Because sarcomeres are joined end to end throughout an entire muscle fiber, their simultaneous contraction shortens the entire muscle[5].[1]
The mitochondria that are present in large numbers in skeletal muscle produce the much-needed ATP.
In sarcomeres, the contractile actin and myosin filaments are integrated in paracrystalline order by the action of accessory cytoskeleton proteins, forming what is often termed the sarcomeric cytoskeleton[6].
The video explains the mechanism of sliding filaments responsible for contraction.
Role of titin, calcium ions, and Adenosine triphosphate (ATP)
Titin is one of the abundant protein molecules in the body. It is filamentous, springy and provides elasticity by helping to hold the framework made by the adjacent placement of actin and myosin filaments.So that the mechanism of contraction can work out properly. It has two ends, one is springy which is attached to the Z line and another one is anchored to the myosin. In other words. it connects the M line and the Z line.
The energy required for the filament sliding is obtained by the ATP, which hydrolyzes into ADP (adenosine diphosphate) and inorganic phosphate. The myosin head serves as a catalytic enzyme.
Calcium ions play the most crucial role. The generation of action potentials triggers the release of calcium ions, which subsequently bind to troponin C. This binding induces a conformational change in the molecule, leading to the structural rearrangement that exposes the myosin-binding site on F-actin. This exposure occurs by tropomyosin displacement, allowing for myosin binding for contraction.[8]
Myosin and Actin filaments
Myosin is one of the three major classes of molecular motor proteins: myosin, dynein, and kinesin.
These are thick filaments with a diameter of 115Å and 1.5 μ.components of the myosin molecule.
- Tail portion- consists of two heavy chains that form a helical structure by twisting around each other.
- Head portion- at the end of the helical structure, chains turn away in opposite directions forming a rounded head. Each head has two sites, one for the ATP attachment & another for the Actin filament. It is not found in the middle part of the myosin filament, i.e. in the "H " zone.
Actin is the most abundant protein in most eukaryotic cells and has a pivotal role in muscle contraction as well as in cell movements. It is the essential building block of the microfilament system. [9]The actin molecule is called F-actin, formed by the polymerization of G-actin. The actin molecules in the filament are also arranged in the pattern of a double helical structure, with a active site.[4]. These filaments contain two additional types of protein named TROPOMYOSIN & TROPONIN.
TROPOMYOSIN- Each molecule has a molecular weight of around 70,000. In the resting condition, the tropomyosin (Tm) covers the active binding site of the actin filament, inhibiting muscular contraction. Tm plays a central role in regulating the actin-myosin interaction by controlling the access of myosin heads to actin filaments, allowing for precise control over muscle contraction and relaxation.[9]
TROPONIN-It has 3 sub-units, with distinctive roles:
- TROPONIN I- bound to F-actin.
- TROPONIN T- bound to the tropomyosin.
- TROPONIN C- bound to calcium ions.[4]
References
- ↑ 1.0 1.1 Mansfield PJ, Neumann DA. Essentials of Kinesiology for the Physical Therapist Assistant e-book. Elsevier Health Sciences; 2018 Oct 23.Available:https://www.sciencedirect.com/book/9780323544986/essentials-of-kinesiology-for-the-physical-therapist-assistant (accessed 10.7.2022)
- ↑ The biology notes Sarcomere Available:https://thebiologynotes.com/sarcomere/ (accessed 10.7.2022)
- ↑ Larry Keeley Sarcomere Structure. Available from https://www.youtube.com/watch?v=P1zD_MpTo0MAccessed 20/7/22
- ↑ 4.0 4.1 4.2 Sembulingam K, Sembulingam P. Essentials of MEDICAL PHYSIOLOGY.sixth edition.New Delhi.Jaypee Brothers Medical Publishers(P) Ltd.2012
- ↑ Krans JL. The sliding filament theory of muscle contraction. Nature Education. 2010;3(9):66. Available:https://www.nature.com/scitable/topicpage/the-sliding-filament-theory-of-muscle-contraction-14567666/ (accessed 11.7.2022)
- ↑ Gautel M, Djinović-Carugo K. The sarcomeric cytoskeleton: from molecules to motion. Journal of Experimental Biology. 2016 Jan;219(2):135-45. Available:https://journals.biologists.com/jeb/article/219/2/135/33486/The-sarcomeric-cytoskeleton-from-molecules-to (accessed 11.7.2022)
- ↑ HEATHER FIT,The sliding theory of contraction available from https://www.youtube.com/watch?v=U-yQywkad8E
- ↑ 1. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. S.L.: Elsevier - Health Science; 2021.
- ↑ 9.0 9.1 1. Barua B, Winkelmann DA, White HD, Hitchcock-DeGregori SE. Regulation of actin-myosin interaction by conserved periodic sites of tropomyosin. Proceedings of the National Academy of Sciences. 2012 Oct 22;109(45):18425–30.