Muscular System can be divided into three types:

• Skeletal Muscles

• Cardiac Muscles

• Smooth (non striated muscles)

Cardiac or Heart muscles are distinguished from the Skeletal Muscles as their muscle fibers are connected lateral to each other. The natural Pacemaker (Sinus Node) of Heart is comprised of Cardiac Muscle Tissues that stimulates the muscle cells of heart to contract. Cardiac muscles are innervated by Sinus Node (impulse generating tissue) of the Autonomic Nervous System (part of Peripheral Nervous System) for control. These muscles are responsible for mainly pumping the blood. Unlike Visceral Muscle these muscles are striated (have stripes of light and dark band). Striation of these muscles shows that they have arrangement of proteins inside cells making them very strong. Cardiac Muscles are connected together by X and Y shaped cells through special junctions known as Intercalated Disks. This Intercalated Disk is made due to the interlocking of two neighboring cells in a finger like projection providing a strong bond. These junctions of Intercalated Disks allow the lifetime function of Heart by resisting high blood pressure and the strain of blood pumping. This structure also helps in electrochemical signal spreading allowing the Heart to beat as a unit.

Smooth muscles being involuntary in functions are controlled through direct contact of Autonomic Nervous System. Smooth Muscles are also known as Visceral Muscles and found inside the organs like Stomach, Intestines and Blood Vessels. They are weakest of all the muscles. These muscles move substances by contracting the organs. They are named Smooth Muscles because of their smooth and unpaired view under a microscope, this smooth appearance is in contrast with Cardiac and Skeletal Muscles.

Skeletal Muscles are forms of Striated Muscle Tissues that are in control of Somatic Nervous System. These muscles are the only muscles of human body that can be controlled consciously. Skeletal Muscles cells are formed by the gathering of Progenitor Cells (Stem Cell with ability to differentiate into a specific type of cell) making long and straight multinucleated fibers. These muscles are striated just like Cardiac muscles and are very strong. These muscle are attached to the bones with the help of Collagen fibers called Tendons. The movement of these muscles is carried out by pulling on tendons and causing the bones to move closer to each other. Contraction is carried out by the muscle belly, a fleshy region between the tendons. The name skeletal Muscles is derived for these muscles because of the fact that they are connected to Skeleton in at least one place. There are almost 639 skeletal muscles in human body. Some of the major muscles are as follows.

• Gastrocnemius for flexion of Knee

• Tibialis Posterior for Inversion and flexion of Foot at Ankle

• Soleus for Plantar flexion like pressing the pedal of car

• Gluteus Maximus for rotation and extension of Hip

• Trapezius Muscle for retracting the Scapula

• Deltoid Muscle for shoulder flexion and extension

CONTROL

The point where the Motor Neuron attaches to a muscle forms a Neuromuscular Junction, which is the main point of muscle control. Acetylcholine (neurotransmitter for skeletal muscle contraction), when the action for muscle reaches the junction known as Synapse, is released from Axon of the Nerve Cell. Chemical messengers cross the Synapse and cause the electrical changes. These electrical changes are received in Muscle Cells. Impulse from the muscle cell cause the release of Calcium and brings a short muscle contraction known as Fasciculation (Muscle Twitch).

FUNCTION

The main and key role of the Muscular System is to help in the movement of body and body parts. Muscles are the only tissues of body causing contraction before moving other parts of body.

Another important function of this system is to provide posture and shape to body.  Muscles contract keeping themselves still and in place rather than causing movement. Some muscles, like the muscles of back, have the ability to give long term support to the body. These muscles are able to work for a long time without tiring.

One of its functions is the movement of substances inside the body. Cardiac and Visceral (smooth) muscles primary function is the flow of blood and food in the body parts.

Lastly, Muscular System generates body heat. High metabolism rate of muscle contracting results in production of great amount of waste heat. The overexerting of muscles results in excess body heat resulting in sweating.

CLINICAL SIGNIFICANCE

The problem at Neuromuscular Junction results in a very prolonged contraction known as Tetani.

The function loss at the junction can also result in Paralysis.

One of the conditions affecting the muscular system is Muscle Fatigue. The condition occurs when muscle runs out of energy in course of Aerobic and Anaerobic respiration resulting in tiring of muscle and its ability to contract.