The human body is a beautiful thing. I am not simply talking about its external features, such as physical appearance or size, but instead how a complex set of systems work so wonderfully together. The manner in which these systems incorporate different processes with biochemical reactions to complete a task is unbelievable. A specific function that the human body provides that I would like to discuss is voluntary muscle movement. We mainly view voluntary muscle movement on a macroscopic scale, and tend to forget the microscopic reactions that occur within the larger system. Every time I review all of the intricacies and synchronicities involved, I am in awe. It is almost as if I can feel the processes occur inside of my own body.
One of the first body systems that come to mind when muscle movement is discussed is, of course, the muscular system. 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 has a lot of myosin heads and tails. 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.
Another important factor to consider whenever examining muscle movement is calcium. The mention of calcium tend to elicit images of bones for most individuals, however, there are many other uses for it in the body. Whenever calcium is ionized, becoming Ca2+, it is used in muscle movement, for the contraction and relaxation of muscles. It works in tandem with the myofilaments, as well as other structures in the body. This ionized form of calcium is found in the sarcoplasmic reticulum of the skeletal, which stores and releases the calcium into the muscle cell. Once there is calcium in the muscle cell, contraction of the muscle begins. The membrane 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.
Another important contribution towards muscle movement is the nervous system. There are many nerve impulses that are involved and communicate directly with the muscle. 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, a type of neurotransmitter called acetylcholine is released. It is diffused across the synaptic cleft between the motor neuron and motor end plate. Whenever the neurotransmitter 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 Ca2+ to be released, causing muscle contraction. The release of acetylcholine is kept in check by releasing an enzyme called acetylcholinesterase, which breaks down acetylcholine.
There are many other components involved with voluntary muscle movement, such as bones, joints, etc. We have simply scratched the surface covering basic reactions to do with contraction and relaxation. Upon further review of the required steps, it appears as though these elements discussed build onto each other and take part in a symbiotic relationship. 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. It is just as Julien Offroy de la Mettrie once said: “The human body is machine which winds its own springs”.
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