By opening this webpage, you used at least four muscles (six if you used a smartphone). These were all skeletal muscles, one of the three muscle types found throughout the body that make up the muscular system.
The other two types are cardiac and smooth muscles. Cardiac muscles comprise the heart (and check out a 3D animation), and smooth muscles line blood vessels and several hollow visceral organs, such as the intestines, where they help move materials through the body.
Muscle Types
The difference between muscle types comes down to the organization of proteins that enable them to contract.
In cardiac and skeletal muscles, these proteins—actin and myosin—make up structures called sarcomeres. Sarcomeres are organized in repeating stripes, creating a pattern that further classifies these two muscle types as striated muscles.
Smooth muscle lacks these repeating stripes, giving rise to its name and appearance. Its circular or lengthwise lining along blood vessels and organ walls allows it to shrink the walls' diameter and length, pushing content—like blood or food—through the passage (see comparison here).
Movement
The point of all muscles is to enable movement, though the purpose and manner of that movement differ between muscle types.
Muscle movement is driven by the contraction of muscle cells or fibers. Picking up a book from your bedside table requires coordinated contraction (like in the biceps) and lack of contraction (like in the triceps).
According to the "sliding filament theory," striated muscles move via the lengthening and shortening of sarcomere bundles known as myofibrils as myosin pulls itself along actin. (This theory may shift in the future as exercise science is constantly evolving).
Smooth muscle movement arises from the lack of sarcomeres. This muscle type's eye-shaped cells also contract via the interaction of myosin and actin, but the interlacing structure of these protein filaments allows their contraction to pull at the muscle from all sides. (See peristalsis).
Neuroscience
Only skeletal muscles—which make up most of the muscular system—can be consciously controlled, like when engaging in a squat or picking up your niece. This is done via the somatic nervous system, also known as the voluntary nervous system.
The other two muscle groups—smooth and cardiac—are involuntary, meaning they cannot be consciously controlled. This includes the behavior of internal organs and involuntary movements and reflexes, such as sneezing.
All movements, including breathing and pumping blood, require communication between muscles and the brain. This is made possible via parts of the peripheral nervous system. This includes nerves that lie outside of and report to the central nervous system, which is housed entirely in the brain and spinal cord. In some cases, such as defecation, these two nervous systems can work together.
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The muscular system is responsible for movement by and throughout the body. Smooth muscle contractions push food and blood through the digestive tract and blood vessels, respectively. Skeletal muscles are responsible for voluntary movement or lack thereof by the body. Cardiac muscle controls the beating of the heart, which is the pump that powers the circulatory system responsible for distributing energy and nutrients to cells across the body.
The force that pushes a barbell far over one's head during a clean-and-jerk starts deep within skeletal muscle cells, but the order to move the muscle starts in the brain. When we want to move a muscle, a signal travels from the brain and into the muscle cells in question, sparking a chemical reaction that causes the muscle's smallest contractile units—sarcomeres—to shorten and forces the muscle to contract.
Muscle memory is less about remembering how to ride a bike and more about what's happening in muscle cells. Increasing the cells' size, say by working out, leads to an increase in nuclei, which increases the production of mitochondria. When your fitness regimen falls to the wayside and you decide to re-enter the gym, these gains-based nuclei are already present to quicken muscle cells' adaptation to exercise.
Scientists observed zebrafish embryos as they grew and isolated the several-second-long initial heartbeats to better understand this crucial moment in every heart's life. The zebrafish's heart starts beating 20 hours into their existence. Each heart cell activates electrically at random, without rhythm. As these cells mature, they become more sensitive to electrical charges; as they all fire, a rhythm is established and a pulse is born.
These prosthetics are plugged directly into the skeletal system using titanium implants like those used in dentistry. Electrodes are implanted in the muscles and nerves around the residual limb, which may require reconfiguration of muscle movement and contraction to restore full range of motion. Neural signals related to commands like "close your hand" are then translated into code and trained into a tiny computer in the limb.
A study examining the muscle function of older adults who have at least one adverse childhood experience (ACE) suggests that trauma digs itself into muscle cells. Participants who reported childhood trauma were found to have lower maximum levels of cellular energy than people who reported fewer or no ACEs. Such compromised mitochondrial function "doesn't bode well," as it relates to a variety of health and age-related outcomes.
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