Muscle is a soft contractile connective tissue.They are primarily responsible for maintaining and changing posture, locomotion, as well as movement of internal organs such as the contraction of heart and the movement of food through the digestive system via peristalsis. In short muscle perform many useful function and help us in doing everything in day to day life.
It’s originates from the mesodermal layer of embryonic germ cells and process is known as myogenesis.
The term muscle is derived from the latin word Muculus meaning “little mouse” perhaps because of the shape of certain muscles or because contracting muscles look like mice moving under the skin.
There are more than 650 muscles in human body, and they are divided into three different types; skeletal, smooth, cardiac.
STRUCTURE OF SKELETAL MUSCLES
Skeletal muscle , also called
- voluntary muscles,
- striped muscles,
- striated muscles and
- somatic muscles they form 40% to 50% of body mass.
Skeletal muscles are attached to bones by tendons, and they produce all the movements of body parts in relation to each other. They are multi-nucleated and are arranged parallel to one another.
- LARGEST MUSCLES IN HUMAN BODY- The gluteus maximus located in the buttock.
- LONGEST MUSCLES IN HUMAN BODY- Sartorius
- SMALLEST MUSCLE IN HUMAN BODY- Stapedius
- STRONGEST MUSCLE IN HUMAN BODY-Masseter in the jaw.
- SHORTEST MUSCLE IN HUMAN BODY- Styloglossus, Tensor Tympani.
Muscle mass or muscle tissue is made up of a large number of individual muscle cells or Myocytes. Muscle cells are commonly called muscle fibers because these cells are long and cylindrical in appearance.
- Each muscle mass separated from the neighboring tissue by a thick fibrous tissue layer called fascia.
- Beneath the fascia, muscle is covered by a connective tissue sheath called Epimysium.
- Group of muscle fibers or muscle cells called Bundles or fasiculi.
- Connective tissue sheath that covers each fasciculus is called perimysium.
- And covering of each muscle fiber is called endomysium.
Muscle fiber is cylindrical in shape.
- length of the muscle fiber- varies between 1c.m. and 4c.m, depending upon the length of muscles.
- The diameter of the muscle fiber- varies from 10μ- 110μ.
Covering of each muscle fiber is called endomysium. beneath the endomysium , plasma membrane present it is called sarcolemma, cytoplasm of the muscle is known as sarcoplasm.
Structure embedded within the sarcoplasm are-
- Golgi apparatus
- Sarcoplasmic reticulum
- Glycogen droplets etc..
Myofibril are the fine parallel filament present in the sarcoplasm of the muscle cell.
- Diameter of myofibril- 0.2 to 2μ.
- Length of myofibril- 1c.m. to 4c.m.
Microscopic structure of a myofibril- each myofibrils consists of a number of two alternating band, the two bands are-
- Light band or “I” band or isotropic band .
- Dark band or “A” band or anisotropic band.
Difference between A band and I band
|‘ A’ BAND||‘ I ‘ BAND|
|1) It has wide H- zone .||1) It has thin Z-line .|
|2) It gives dark appearance and hence also called dark band.||2)It gives light appearance , hence also called light band .|
|3) It contain myosin filaments and part of actin filaments.||3)It contain part of actin filaments.|
|4) Its length remain unchanged during muscle contraction.||4)It shortens during muscle contraction.|
The portion of myofibril in between two Z-lines is called sarcomere.
SARCOMERE- A sarcomere is the basic functional unit of striated muscles, which is to say, it’s the basic building block of most muscle cells. it is also called the basic contractile unit of the muscles.
The sarcomeres are what give skeletal and cardiac muscles their striated appearance, which was first described by Leeuwenhoek.
- A sarcomere is defined as the segment between two neighbouring Z-lines (or Z-discs, or Z bodies). In electron micrographs of cross-striated muscle, the Z-line appears as a series of dark lines. They act as an anchoring point of the actin filaments.
- Surrounding the Z-line is the region of the I-band (for isotropic). I-band is the zone of thin filaments that is not superimposed by thick filaments (myosin).
- Following the I-band is the A-band (for anisotropic). An A-band contains the entire length of a single thick filament.
- Within the A-band is a paler region called the H-zone. H-band is the zone of the thick filaments that is not superimposed by the thin filaments (actin).
- Within the H-zone is a thin M-line
Electron microscopic studies reveal that the sarcomere consists of thread like structures called myofilaments.
1) Actin filaments-
- Actin filament are thin filament.
- Diameter- 20Å
- length- 1μ
- This filament extend from either side of the Z lines, run across I band and enter into A band upto H zone.
2) Myosin filaments-
- Myosin filaments are thick filament
- diamtere of 115Å
- length of 1.5μ.
- these filaments are situated in ‘A’ band.
Myofibrils made up of myofilaments-