Sarcomere

Recommended video: Skeletal muscle tissue [12:25]

This type of tissue is found in skeletal muscles and is responsible for the voluntary movements of bones.

The sarcomere is the main contractile unit of muscle fiber in the skeletal muscle. Each sarcomere is composed of protein filaments (myofilaments) that include mainly the thick filaments called myosin, and thin filaments called actin. The bundles of myofilaments are called myofibrils. 

The structure of the sarcomere is traditionally described with dark and light bands visible under the microscope. This banding pattern in sarcomeres is due mainly to the arrangement of thick and thin myofilaments in each unit. These markings include:

• A bands (or anisotropic bands) - dark bands that contain whole thick filaments (myosin). 
• I bands (or isotropic bands) - light bands that contain only the thin filaments (actin) and are located between the two thick filaments. 
• Z disc - is an area that traverses the I bands and marks the point of the connection between the two neighboring actin filaments. That said, the sarcomere can also be described as the structure between the two Z discs. 
• M line - marks the middle of the sarcomere and contains the protein called myomesin.
• H zone - is the area between the M line and Z disc. The H zone contains only myosin.

The main function of the myofibrils is to produce a muscular contraction in which the filaments slide over each other. The contraction is induced at the neuromuscular junction via the neurotransmitter called acetylcholine. The myofibrils are functionally tightly associated with two regulatory proteins called troponin and tropomyosin which help in the regulation of muscle contraction. In addition, for muscle contraction the presence of calcium ions in the cell is crucial. The concentration of calcium within muscle cells is controlled by the unique form of endoplasmic reticulum called the sarcoplasmic reticulum.