Muscles move by shortening their length, pulling on tendons, and moving bones closer to each other. One of the bones is pulled towards the other bone, which remains stationary. The place on the stationary bone that is connected via tendons to the muscle is called the origin. The place on the moving bone that is connected to the muscle via tendons is called the insertion. The belly of the muscle is the fleshy part of the muscle in between the tendons that does the actual contraction.
Organisation of skeletal muscle
A muscle is composed of many muscle fibres. The individual muscles are separated from each other and held in place by a covering called the fascia. This fascia also forms tendons and aponeuroses that connect muscles to bones and muscles to muscles. An aponeurosis is a flat sheet or ribbon of tendon-like material that anchors a muscle or connects it with the part that the muscle moves.
A muscle also contains three different layers of connective tissue:
Epimysium – outermost layer, surrounds entire muscle
Perimysium – separated and surrounds the Fascicles (bundles) of muscle fibres
Endomysium – surrounds each individual muscle fibre
Skeletal muscle fibres contain numerous nuclei and mitochondria.
The muscle fibre membrane is called the sarcolemma and the cytoplasm is called the Sarcoplasm. Within the sarcoplasm are many parallel fibres known as myofibrils.
Each myofibril is made of many protein filaments called Myofilaments. There are two types:
• MYOSIN – thick filaments
• ACTIN – thin filaments
Thick filaments: Thick filaments are made of many bonded units of the protein myosin. Myosin is the protein that causes muscles to contract.
Thin filaments: Thin filaments are made of three proteins:
Actin- Actin forms a helical structure that makes up the bulk of the thin filament mass. Actin contains myosin-binding sites that allow myosin to connect to and move actin during muscle contraction.
Tropomyosin- Tropomyosin is a long protein fibre that wraps around actin and covers the myosin binding sites on actin.
Troponin- Bound very tightly to tropomyosin, troponin moves tropomyosin away from myosin binding sites during muscle contraction.
Structure of a Sarcomere (functional unit of a muscle)
Actin and myosin filaments are arranged in an overlapping pattern of light (“I” bands) and dark (“A” bands).
In the middle of each “I” band is a line called a “Z” line.
The section of a myofibril from one Z-line to the next Z-line is the sarcomere.
The arrangement of these sarcomeres next to each other produces the striations of the skeletal muscle fibres.
Skeletal Muscle Function
Muscles undergo contraction, but if muscles are not attached to bones, this would not occur. Hence, as muscles shorten during a contraction, they pull upon the bones to which they are attached. There needs to be a counteracting force to balance the movement, since there is a pull on one side of the bone. Therefore, muscles are arranges such that the pull of one muscle is counteracted by the pull of other muscles or muscle groups.
There are about 640 muscles in the human body.
Skeletal muscles are named, based on many different factors, including their location, origin and insertion, number of origins, shape, size, direction, and function.
A typical skeletal muscle consists of two ends:
i) The origin is one end of the muscle that remains fixed during its contraction.
ii) The insertion is the other end which moves during its contraction. In the limb muscles, the origin is usually proximal to insertion.
The following lessons will focus on the main muscles that move the limbs, as well as the major muscles of the face and neck, chest, pelvic floor and abdominal wall. The origin, insertion and action of the muscle will be described.