There are three types of muscles found in the human body: **smooth muscle**, **skeletal muscle**, and **cardiac muscle**. **Smooth muscle** is characterized as having involuntary, non-striated, uninucleate, and unbranched cells. **Skeletal muscle**, on the other hand, is described as voluntary, striated, multinucleated, and unbranched. Lastly, the **cardiac muscle** is the only muscle that forms our heart. it is composed of involuntary, striated, branched cells, and intercalated discs that connect one cardiac muscle to another.

Both smooth and cardiac muscles are designed well to facilitate the involuntary contraction and movement of the internal organs including the function of the heart to pump blood. On the other hand, skeletal muscles regulate voluntary movements which can be consciously controlled by ourselves.

Muscle contraction happens when the muscle proteins slide past each other to produce a movement. During contraction, a cross-bridge is created between the myosin (thick) filaments and the actin (thin) filaments. Once the cross-bridge is formed, it changes shape and starts to pull the actin filaments. This results in the actin filaments sliding toward the sarcomere, then it detaches from the cross-bridge.

Proper and regular exercise is vital in the maintenance of muscle strength and flexibility, and tone. Regular exercise keeps muscles to stay firm, and it enables them to increase their size and strength.

There are various exercises that we can do to strengthen our muscles.

1. Aerobic exercises – helps our body to utilize energy efficiently. It also helps us train our hearts and lungs, thus, increasing our physical endurance.

2. Resistance exercises – increases the size and strength of our muscle tissues. This type of exercise also helps in reducing body fat.

Muscular system is responsible for posture, locomotion, and movement of other organ systems.

Acetylcholine plays an important role in muscle contraction. It helps trigger an impulse in the muscle fibers. This impulse produces calcium ions that cause an interaction between the thick and thin filaments.

The answer is no. A contracting muscle will not respond to a second stimulus unless the first contraction has stopped. A muscle will only stop contraction once the production of a neurotransmitter called acetylcholine stops and an enzyme destroys the remaining acetylcholine. Once the contraction has stopped, it is now ready to receive another stimulus from the motor neurons.