Sarcomere | Definition, Structure, & Sliding Filament Theory

By | August 10, 2019


Sarcomere Definition

The sarcolemma or sarcomere is an excitable membrane cell and shares many properties with the cell membrane of the neuronal structure. The function of sarcolemma is to connect the basement membrane which wrapped all connective tissues. The sarcolemma also has an extracellular matrix containing various polysaccharides, which permit the cell to anchor into tissues for helping to support muscle fiber cells. Sarcolemma depolarization has transmitted the interior of the muscle fiber cells though this system. In meat processing, a muscle cell which separates the sarcoplasm from the extracellular surroundings is known as sarcolemma. In muscle fiber cells, a complex structure consisting of multiple layers is sarcolemma.


What is Sarcolemma?

The sarcolemma is the thin clear sheath which wrapped the fibers of skeleton muscles. It is a cell membrane which covered striated muscles cells. The sarcolemma is also known as myolemma. The sarcolemma is such as a plasma membrane but specialized in muscle fiber cells.

Sarcomere Structure:

Each sarcolemma or sarcomere is identical to biochemical composition to Plasmalemma, (that is another word for cell membrane). After observing under the microscope, a stacked pattern organized with a varied length of muscle fiber cells is seen. The bundle of filament arranged parallel to another is myofibril strands. The structure of muscle cells is tunnel-like extensions from the sarcolemma, pass through muscle fiber cells. Due to this, they said to be transverse. These extensions of the sarcomere or sarcolemma are called transverse tubules. When a muscle is our body contracts, it seems that this happens by following the sliding filament theory. This theory tells that muscle contracts in the body when filaments permitted to slide against each other. This interaction has the ability to produce the contractile force. The sarcomere structure is so crucial in this theory because a muscle needed to physically shorten. So, there is a need arise for a unit which can repay for the length or shortening of a flexing muscle.


Sarcomere Contraction:

Muscle is specialized in contractile the tissue that is a distinguishing character of animals. Muscle length or shorten support to animal for movement from the dexterity of octopus tentacles. When a sarcomere shortens, some regions also shorten but another stay in the same length. A sarcomere describes as the distance between two Z discs or Z lines. When a muscle contracts in our body the distance reduces between the Z discs. The central region of the A zone (H zone), contains only thick filaments (myosin), and became short during contraction. H zone becomes smaller by increasing actin overlap and myosin filaments, due to this muscles shorten.

Sarcomere Diagram


Sarcomere Anatomy:

Anatomical is said to be the term of microanatomy. The sarcomere is the basic unit function with muscle fiber cells. This is a distinguishing unit in some types of muscle tissue. Due to the striated nature of both skeletal muscle and cardiac muscle is observed by microscope slides.


Myofibril is a very fine contractile muscle fiber cells. It is groups of fibers which extend in a parallel way along with the length of striated muscle fiber cells. The thick and thin myofilaments make the myofibrils, which help to give its striped appearance of the muscle. The thick filaments consist of myosin, and the thin filaments are predominantly actin, with two muscle proteins (tropomyosin and troponin). Interaction between actin and myosin is caused by muscular contraction, as they both temporarily tie with each other and then released.

Thin Filament:

Thin filaments are made up of three proteins. These proteins are actin, troponin, and tropomyosin. Actin protein is the main component of the thin filament. Each actin protein has a tie ability for a myosin cross bridge. Troponin is the complex protein out of three. It occurs about every 40 nm on the filament. Troponin molecule has bind ability for Ca++. Tropomyosin is about 40 nm in length, with rod-shaped protein component. Thin filaments are attached with the Z discs of the striated muscle fibers.

Sarcomere Model:

The functional sarcomere model describes briefly the sliding filament theory of skeletal muscle contraction in an understandable manner. Configuration with including complete sarcomere, and function of thin filaments (having three proteins; actin, troponin, and tropomyosin) and thick filaments. The M-line and Z-line are identifiable.



Sliding Filament Theory Steps:

The sliding filament theory describes the force production and changing in length when a muscle fiber contracts. It takes into account the binding, movement, and releasing of proteins; actin and myosin, within the muscle cell to do contraction. This theory uses a series of steps to illustrate how the interaction of proteins (i.e. filaments) occurs to produce muscle contraction. This theory has some series of steps to perform their function.


  • Step 1:

First, in this theory muscle is relaxed. The actin has to be brought close together for the contraction of the muscle. The myosin has cross bridges to get actin close, which pull them, closed each other. But actin has proteins  (troponin and tropomyosin) which stops the cross-bridge from pulling them together. To dispense the tropomyosin and troponin, calcium comes along with and breaks them off. Which sanctions the cross-bridge to pull the actin together to each other, cause of muscle contraction.

  • Step 2:

In step 2, the calcium binds to troponin in the presence of a high amount of calcium, which causes to change its shape and movement of tropomyosin from the active site of the actin, and then myosin can attach itself to the actin to forming a cross bridge.

  • Step 3:

In step 3, energy releases to the breakdown of adenosine triphosphate (ATP), which permit the myosin to pull the actin inwards. It occurs in the whole length of every myofibril in the muscle.

  • Step 4:

In this step, when an ATP molecule binds to the head of the myosin, the actin detaches the myosin and the cross bride is broken. Myosin can reattach to an actin-binding site after ATP molecule is broken.

  • Step 5:

This is the last step of sliding filament theory. In this step, muscular contracts as long as there are enough amounts of ATP and calcium ion stores. When the nerve impulse stops the calcium, it pumped back to the sarcoplasmic reticulum. Then the actin returns to the resting position and permits the muscle to lengthen and relax the muscle.