Muscles are specialized tissues within the body that serve their purpose primarily by contracting. This is accomplished when muscle tissue employs the proteins actin and myosin, which slide past each other to create movement. Highly-developed skeletal muscles are also associated with strength, and can be toned and improved through regular exercise or weight training.
Skeletal Muscles and Movement
All muscles in the body work by contracting to create movement. When we think of the muscular system, our first thoughts go primarily to the skeletal muscles we use to move our limbs. By working in pairs, muscles contract and relax in tandem to raise and lower limbs or to bend them at joints. When you bend your leg, for example, you contract your hamstring and relax your quadriceps. Relaxing the hamstring allows you to straighten your leg.
Sphincter Muscles’ Roles
Sphincter muscles are ringlike muscles located at openings within the body. The sphincter ani externus, for example, keeps the anal opening closed when you are not excreting. The sphincter urethrae aids you in controlling urination. The sphincter pylori, around the opening between the stomach and small intestine, holds food in the stomach until it is mixed with gastric juices. A sphincter within the eye also works by expanding and contracting, causing the pupil to expand under low light and contract when exposed to bright light.
Smooth Muscle Regulation of Organs and Fluids
Several organs rely on smooth muscles to function. These muscles control the flow of fluid through hollow organs. These include the bladder and the gastrointestinal tract. Smooth muscles also aid in contracting the uterus, and in moving fluid through blood vessels. Smooth muscles generally contract much more slowly than skeletal muscles, although with as much force as an equivalent mass of skeletal muscle, and their movements are often involuntary.
Abdominal Protection and Support
The muscular system provides some protection to the skeleton, but the area where muscles play the greatest primary protective role is the abdominal viscera. Here, in the space between the ribs and the hips, firm abdominal muscles absorb impacts and spare the delicate organs within from trauma. The muscles in the abdomen also provide some protection to the spine, and they aid the spine in providing support to the upper body.