Skeletal Muscles, Fibers, Myofibrils and Myosin, Actin Filaments

Muscles are the motor units of the body. This means it makes different body parts move and allows substances to move within the hollow cavities of the body. There are broadly three types of muscles in the human body – skeletal muscle is attached to the bones (voluntary control), smooth muscle within the walls of different organs in the body (involuntary control) and cardiac muscle which is responsible for heart contraction (involuntary). Irrespective of the type of muscle, it all has the same effect – it produces force and therefore motion.

Parts of Skeletal Muscles

A simple breakdown of the muscle structure is that each muscle is made up of a collection of muscle fibers (muscles cells. Each of these cells is made up of smaller structures known as myofibrils that are in turn composed of microfilaments, actin and myosin.

Muscle Fibers

The largest unit of muscles are the muscle fibers (muscle cells or myocytes). There are several hundred to thousand fibers that make up a single muscle, each of which extends the entire length of the muscle. The end of each muscle fiber connects to a tendon fiber which collectively make up the tendons of the muscle. Muscle fibers are thin and measure between 10 to 80 micrometers in diameter. The fiber is surrounded by a thin membrane known as the sarcolemma which  is innervated by one or more nerve endings..  Within the fiber is the sarcoplasm containing large numbers of mitochondria and sarcoplasmic reticulum.

Myofibrils

Each muscle fiber is made of several hundred to thousand of myofibrils. These myofibrils are a combination of two protein microfilaments known as myosin and actin. Other proteins also make up these long tubular myofibrils. The main structural component of the myofribrils, namely the microfilaments, is both the thicker myosin and thinner actin filaments. The interlocking pattern of these microfilaments gives myofibrils an alternating light and dark pattern.

Amongst the myofibrils that make up the muscle fiber is the sarcoplasm, a fluid that is essentially the cytoplasm of the muscle fiber (cell). It contains large amounts of different electrolytes which are needed by the myofibrils for the process of muscle contraction. Alongside the myofibrils are numerous mitochondria which provides the energy for muscle contraction.

Actin and Myosin

The light bands (I bands) are actin filaments while the dark bands (A bands) are where the actin and myosin filaments overlap.  Actin filaments extend from both sides the Z disc (Z lines)  to partially interdigitate with myosin filaments. Z discs itself are a type of protein band different to actin and myosin. These Z-discs extend across successive myofibrils thereby attaching it to each myofibril along the entire muscle fiber.

The space between two successive Z-discs is called a sarcomere. It can to some extent be seen as the smallest functional unit of the muscle. The length of the sarcomere drastically decreases when a muscle fiber contracts. Titin proteins are dense molecules that holds the actin and myosin filaments in place between each other. It is a type of springy protein that allows the entire contractile unit to function. These lines are the reason why skeletal muscle has a striated appearance.

Muscle Contraction

The entire process of muscle contraction is a complex biochemical process. However, it can be simply described as a sliding filament mechanism. Actin microfilaments slide between the myosin filaments thereby pulling the entire muscle fiber from either end and making it shorter. In other words, the muscle is in a state of contraction.

This is initiated by an impulse from the nerves. The process is as follows :

• When the impulse reaches the nerve endings, it secretes the neurotransmitter known as acetylcholine.
• Channels at the part of the muscle fiber membrane where the acetylcholine is released opens and allows sodium to enter.
• This initiates an action potential along the membrane of the muscle fiber.
• Depolarization of the membrane allows for the action potential to extend into the fiber.
• This causes the sarcoplasmic reticulum to release large quantities of calcium ions stored within it.
• It is these calcium ions that trigger the attractive forces between the actin and myosin. However, since these filaments lie alongside each other but are held in its respective position, the attractive forces causes actin filaments to slide among the myosin filaments.
• The calcium ions are almost immediately pumped back into the sarcoplasmic reticulum and the entire process ceases. Therefore muscle contraction ends within a fraction of a second.

The process of muscle contraction requires significant amounts of energy and this is provided by the large number of mitochondria lying alongside the myofibrils.