The muscle system is responsible for the locomotion of the human body. That is not its only function. It can also aid the body's flexibility. The muscular system contains three different types of muscle. The tissues contract to produce movement of the body. There is a total of 650 skeletal muscles of the human body and most have an identical twin on the opposite side of the body. Many muscles can be deep inside the body or part of a superficial layer. Muscles can compose up to 50% of body mass depending on fitness level. The muscle system truly demonstrates that the body is a living machine. It has components and runs on a particular type of energy. The skeletal ( sometimes referred to as voluntary) muscle are the ones that respond to control and command from the brain. This means the muscle has the ability to contract or lengthen. This is done by chemical messengers that are produced by the brain. Skeletal muscles are attached to bones by a network of tendons. When viewed under a microscope they have a stripped appearance. The term used to describe this appearance is striated. This tissue contains numerous cell nuclei and be found around the periphery of the cell. This is the basic structure of skeletal muscle. The tendons act like an anchor for the muscle tissue. Without the critical role of the tendon nothing would keep the skeletal muscle in place. Another type of muscle is known as involuntary muscle (also referred to as visceral or smooth muscle). These muscles will perform actions without conscious control. Theses muscles are in certain locations of the human body. They are found in the intestines, stomach, the walls of arteries and even the veins. The muscle itself is shaped like spindles. Structurally, they narrow at both ends and have one nucleus in the cell center. The reason these muscle have an involuntary function is due to the functions of particular organ systems. Arteries require dilating and constricting action to keep the circulatory system running. The brain making conscious decisions to pump blood would be inefficient. There is an element of partial control of certain functions, but the involuntary muscles help share the work load. The third major muscle classification is cardiac muscle. This muscle can be both striated and smooth. The main function is to make the heart beat in an involuntary manner. The fibers are branched out. This allows for the transmission of electric messages. These messages give instructions for the muscles to produce heart beats. This is a summery of muscle cell types. The muscle cells have different roles to play, but have one goal. That is the movement of the human body. There are numerous muscles that are on the chest, arms, legs, back, and thorax of the human body . Muscles do not work in an isolated manner, but in pairs. Muscles during contraction can pull. This means the muscle of joint will move in one direction and another muscle will move in an alternate direction. This concept of muscular contraction can be seen when a person is lifting a weight. The muscles of the arm will work in pairs. Muscle cells have the ability to release heat and energy by means of burning glucose and fatty acid. The energy will be used to produce movement of actin and myosin filaments. It requires assistance from the nervous system and the motor impulse ( from a motor nerve) will induce the release of calcium. Calcium will fill the muscles allowing for myofibril contraction . Actin and myosin slide past one another, which shortens the muscle cell. When they slide apart again, muscle will go into relaxation. The Basic Structure of the Muscular System. During movement the muscles can switch between contraction and relaxation. Calcium is not only important for bones, but the function of the muscles. Both men and women have the same muscles. Men on average have more total muscle mass due to factors in endocrinology. Males produce more testosterone, which allows for a more powerful bone structure and higher level of muscle mass. Women's higher estrogen levels allow for their muscles to be more fatigue resistant. The difference in strength is not solely testosterone levels, but the amount of fast twitch and slow twitch muscle fiber. Fast twitch muscle fiber generates more explosive power, while slow twitch muscle fiber has more endurance. There is no difference in male and female muscle at the cellular level. There is a difference in the distribution of type I and and type II muscle fibers. Muscular size can be increased through exercise. This can work for both men and women, but males would attain higher levels of muscular hypertrophy. How this works is that micro trauma causes tears in the muscle tissue. The tissue will reform much thicker to prevent injury. Like other organ systems, the muscular system has methods of repair and function. The muscle cell itself is a fascinating unit. The myofibre rises out of many muscle cells. Many nuclei are outside and a protective sheath covers the tissue. The sheath contains the ability to insulate myofibre, which allows for selective muscle contraction. The strength and extent of the muscle contraction can be controlled by electric impulse. Connective tissue sheaths will merge to form tendons. One myofibre can contain a multitude of myofibrils. Myofibrils are contractile proteins that are cylinder shaped and act like pistons in an engine. The myofilaments are critical proteins which collaborate with myosin and actin. Myofilaments can be thick or thin and overlap one another. They are striated in appearance. Proteins are essential to the muscles and other organ system of the body. Muscles of the face. Most of the body's muscles are skeletal muscles. This means it has further classifications for anatomical study. The skeletal muscles can be classified according to shape. They can be divided into three major zones. The first is the middle belly, followed by a sinewy origin. This will connect to a bone at another end. This could also connect to a ligament or another structure. The list of both superficial and deep muscles. The abdominal wall consists of aponeusis and the linea alba. This large part of connective tissue The aponeusis is similar to a tendon. Both the aponeusis and linea alba merge together to form one structure. Muscle is in a sense an organic armor. The topmost layer consists of the external oblique muscle. The fibers continue downwards and forwards starting from the edges of the fifth to twelfth ribs. The fibers will reach the iliac crest and linea alba. This muscle is responsible for muscle contraction of the rib cage downward. There is a middle layer, which is part of the thorax of the body. The internal oblique is present in this section, which has fibers run to the iliac crest to the inguinal ligament reaching the linea alba. The muscular contraction in this area allows the ribcage to rotate ( pulls it down to the hip and lower back). When a person breaths this allows the ribcage to go down. Muscles that are found in the legs. The lowest layer contains the transverse abdominal. The fibers of this muscle run horizontally across the abdomen. They start at the inguinal ligament to the iliac crest, to the six coastal cartilages and secure themselves in the midline linea alba. The most important task of this muscle section is to stabilize the thorax and including the pelvic as well as abdominal organs. There also is the function of compressing intestinal organs. The muscles that reside in the arm. The skeletal muscles have unique classifications. Flat muscles are ones that form large sheets or covering like structures. Fusiform muscles tend to have fibers which will run parallel to one another. They will merge to form a tendon and have the ability to contract over long distances. The pennate muscles are stronger, due to denser fibers. They are shaped like feathers and can be found in the legs , shins, and shoulders. The two headed parallel muscle can be attached to different places allowing for stability. Orbicular muscles act as sphincters opening and closing certain areas. Triangular muscles also act as stabilizers, but also contribute to adding to the maximum force of contraction. Straight muscles also contribute to stabilization of the muscular system. Three headed parallel muscles attach from different sides acting as a connector to other units. The two bellied parallel muscle has two muscular sections divided by a sinew section. The quadrate muscle is a flat for sided shape that acts a a covering. The muscular system is a web of tissues, structures interlocking, with tendons, and ligaments working together to produce movement. References Brewer, Sarah. The Human Body A Visual Guide To Anatomy. London: Quercus, 2012 Svarney-Barnes, Patricia. Science Reference Desk. New York: Macmillan, 1995. Arellano Doctor, Vanessa. "Is There a Difference Between Female and Male Muscles?" LIVESTRONG.COM. LIVESTRONG.COM, 12 Sept. 2015. Web. 27 Sept. 2015. .