Muscles, Tendons, Ligaments and Cartilage Muscles pull on the joints, allowing us to move. They also help the body do such things as chewing food and then moving it through the digestive system. Even when we sit perfectly still, muscles throughout the body are constantly moving. Muscles help the heart beat, the chest rise and fall during breathing, and blood vessels regulate the pressure and flow of blood. When we smile and talk, muscles help us communicate, and when we exercise, they help us stay physically fit and healthy. Muscle: Muscle is the tissue of the body which primarily functions as a source of power. There are three types of muscle in the body. Muscle which is responsible for moving extremities and external areas of the body is called "skeletal muscle." Heart muscle is called "cardiac muscle." Muscle that is in the walls of arteries and bowel is called "smooth muscle." There are three types of muscles in the body. Cardiac muscle makes up the heart. Smooth muscle cells line the blood vessels, gastrointestinal tract, and certain organs. Skeletal muscles attach to the bones and are used for voluntarily movements of the body. Visceral Muscle Visceral muscle is found inside of organs like the stomach, intestines, and blood vessels. The weakest of all muscle tissues, visceral muscle makes organs contract to move substances through the organ. Because visceral muscle is controlled by the unconscious part of the brain, it is known as involuntary muscle—it cannot be directly controlled by the conscious mind. The term “smooth muscle” is often used to describe visceral muscle because it has a very smooth, uniform appearance when viewed under a microscope. This smooth appearance starkly contrasts with the banded appearance of cardiac and skeletal muscles. Cardiac Muscle Found only in the heart, cardiac muscle is responsible for pumping blood throughout the body. Cardiac muscle tissue cannot be controlled consciously, so it is an involuntary muscle. While hormones and signals from the brain adjust the rate of contraction, cardiac muscle stimulates itself to contract. The natural pacemaker of the heart is made of cardiac muscle tissue that stimulates other cardiac muscle cells to contract. Because of its self-stimulation, cardiac muscle is considered to be autorhythmic or intrinsically controlled. The cells of cardiac muscle tissue are striated—that is, they appear to have light and dark stripes when viewed under a light microscope. The arrangement of protein fibers inside of the cells causes these light and dark bands. Striations indicate that a muscle cell is very strong, unlike visceral muscles. Skeletal Muscle Skeletal muscle is the only voluntary muscle tissue in the human body—it is controlled consciously. Every physical action that a person consciously performs (e.g. speaking, walking, or writing) requires skeletal muscle. The function of skeletal muscle is to contract to move parts of the body closer to the bone that the muscle is attached to. Most skeletal muscles are attached to two bones across a joint, so the muscle serves to move parts of those bones closer to each other. Skeletal muscle cells form when many smaller progenitor cells lump themselves together to form long, straight, multinucleated fibers. Striated just like cardiac muscle, these skeletal muscle fibers are very strong. Skeletal muscle derives its name from the fact that these muscles always connect to the skeleton in at least one place. Most skeletal muscles are attached to two bones through tendons. Tendons are tough bands of dense regular connective tissue whose strong collagen fibers firmly attach muscles to bones. Tendons are under extreme stress when muscles pull on them, so they are very strong and are woven into the coverings of both muscles and bones. Muscles move by shortening their length, pulling on tendons, and moving bones closer to each other. One of the bones is pulled towards the other bone, which remains stationary. The place on the stationary bone that is connected via tendons to the muscle is called the origin. The place on the moving bone that is connected to the muscle via tendons is called the insertion. The belly of the muscle is the fleshy part of the muscle in between the tendons that does the actual contraction. A tendon is a fibrous connective tissue which attaches muscle to bone. Tendons may also attach muscles to structures such as the eyeball. A tendon serves to move the bone or structure. A ligament is a fibrous connective tissue which attaches bone to bone, and usually serves to hold structures together and keep them stable. What is Cartilage? Cartilage is a tough but flexible tissue that is the main type of connective tissue in the body. Around 65–80% of cartilage is water, although that decreases in older people, and the rest is a gel-like substance called the ‘matrix’ that gives it its form and function. There are three main types of cartilage: • Hyaline, or articular cartilage, is found in the joints, septum of the nose (which separates the nostrils), and the trachea (air tube). • Elastic cartilage, which has elastic fibres that make the cartilage more flexible, is found in the ear, part of the nose and the trachea. • Fibrous cartilage occurs in special cartilage pads called menisci that help to disperse body weight and reduce friction, such as in the knee. In the joints, hyaline cartilage forms a very low friction, 2-4 mm thick layer that coats the bony surfaces. This allows the bones of the joint to glide over one another during movement and, ideally, last a lifetime. It also serves as a cushion and shock absorber in the joint. How Do Muscles Work? The movements your muscles make are coordinated and controlled by the brain and nervous system. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem. The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum (pronounced: ser-uhBEL-um). When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, causing them to contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa. The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain where and how the arm or leg is moving and what position it's in. This feedback results in smooth, coordinated motion. If you want to lift your arm, your brain sends a message to the muscles in your arm and you move it. When you run, the messages to the brain are more involved, because many muscles have to work in rhythm. Muscles move body parts by contracting and then relaxing. Muscles can pull bones, but they can't push them back to the original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. Then, when the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend at your elbow, the biceps contracts. Then the biceps relaxes and the triceps contracts to straighten the elbow. WHAT IS A MUSCLE SPASM? Muscle spasms occur when a skeletal muscle contracts and does not relax. Muscle spasms are forceful and involuntary. A sustained muscle spasm is called a muscle cramp. Leg muscles, especially the quadriceps (thigh), hamstrings (back of thigh), and gastrocnemius (calves), are most likely to cramp, but any skeletal muscle in the body can cramp. There are many potential causes of muscle cramps including physical exertion in hot weather, overexertion, dehydration, electrolyte imbalance, and physical deconditioning. Muscle cramps can range from being a mild nuisance to incapacitating and extremely painful. The cramped muscle may be visibly distorted or look knotted. Twitching may be evident. The area of a muscle cramp may be firm to the touch. Some muscle cramps last just a few seconds, while others can last 15 minutes or more.
