The human body is a beautiful thing. The manner in which a complex set of systems incorporate different processes with biochemical reactions to complete a task is unbelievable. For example, have you ever taken a moment to think about what goes on behind a muscle movement? Let’s analyze it further…
Muscle tissue is made up of many sarcomeres, which are the smallest contractile units of a muscle. These sarcomeres are made up of even smaller myofilaments, such as myosin and actin. A thick filament contains a lot of myosin heads and tails and a thin filament contains actin. This is where the basic component of muscle movement occurs in the body.
The process behind the movement of these myofilaments is fairly complex. It is a cycle that begins with an energy source constantly present in our bodies known as adenosine triphosphate (ATP). As the name suggests, ATP contains three phosphates. The myosin heads from the thick filament take off a phosphate from ATP, transforming it into adenosine diphosphate (ADP), which contains only two phosphates. From this, the myosin head becomes reoriented and acquires more energy. Then, the myosin heads bind to the actin of a thin filament. By binding, they are now referred to as a “cross-bridge”. Afterwards, this cross-bridge opens and the ADP is released. Finally, the myosin heads and actin will detach from each other. These events all lead to one contraction of the skeletal muscle.
Calcium is also an important factor in muscle movement. Whenever calcium is ionized it becomes Ca2+, and is utilized in the contraction and relaxation of muscles. Ionized calcium is found in the sarcoplasmic reticulum of skeletal muscle, which stores and releases the calcium into the muscle cell. Once there is calcium in the muscle cell, contraction of the muscle begins. The membranes of muscle cells have calcium pumps that allow the calcium to be quickly brought back to the sarcoplasmic reticulum. When there is a decreased amount of calcium in the muscle cell, the myosin heads are covered, which causes the muscles to relax. The calcium works in cooperation with the myofilaments of the muscle tissue to coordinate muscle contraction and relaxation.
The nervous system also contributes to muscle movement as there are many nerve impulses that are involved and communicate directly with muscles. To begin with, the nerve impulse that comes at the synaptic end bulbs stimulates the voltage gate channels to open. Calcium then flows inward because there is a higher concentration of it outside of the cells. As calcium comes in, acetylcholine (a neurotransmitter) is released. It is diffused across the synaptic cleft between the motor neuron and motor end plate. Whenever acetylcholine binds to the receptors in the motor end plate, an ion channel opens. This allows small positively charged ions, such as sodium ions, to flow across the membrane. This flow of sodium ions causes the inside of the muscle fiber to be more positively charged, and this creates an action potential. This action potential causes the stored ionized calcium to be released, causing muscle contraction. The release of acetylcholine is kept in check by releasing an enzyme called acetylcholinesterase, which breaks down acetylcholine.
Describing these processes has hopefully left you in awe just as I was when I first discovered it. The human body is worthy of appreciation for all of its beautiful complexities and functions. Just as Julien Offroy de la Mettrie once said: “The human body is machine which winds its own springs”.