Muscular System

Background

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.