Muscles are one of the most dynamic forms of tissue in the body. They can grow---an act called hypertrophy---in response to trauma brought on by physical effort and repeated muscle contractions. The purpose of muscles is simply to allow the organs in the body, especially the bones, to move.
Features
Muscles are made up of long strands called muscle fibers. Within these fibers are smaller strands known as myofibrils, which consist of thick and thin filaments. The thick filaments, with an input of energy, attach themselves to the thin filaments and pull them inward. This is known as a muscle contraction. The sarcomere is the basic unit of contraction. It consists of a band of filaments along the myofibril. There are 8,000 sarcomeres per myofibril and 2,500 myofibrils per fiber. When muscle hypertrophy occurs, new myofibrils are added to the fibers.
Organelles
Muscles include a number of important organelle structures such as the mitochondria, which provide the energy for the muscle to fuel its activity, and the neuromuscular junction. This latter structure is actually a synapse between the terminal point of the motor neuron and the muscle fiber. Here is where the electrical signal is carried from the nervous system to the muscles to facilitate muscle contractions.
Types
Muscles come in three types. Cardiac muscles are found in the heart and help circulate blood throughout the body. They pump about 5 liters of blood every minute. Smooth muscles are located in the circulatory, lymphatic, digestive and reproductive systems. Their automatic contraction reduces the size of the organ. Finally, skeletal muscles attach directly to the skeleton and allow bones to move. They can be broken down into type I, or slow-twitch fibers, and type II, or fast-twitch fibers. Slow-twitch fibers are rich in mitochondria to battle fatigue. Fast-twitch fibers quickly burn carbohydrate stores for energy to fuel fast bursts of activity.
Anatomy
Most skeletal muscles attaches to one bone at an origin point and to a second bone at an insertion point. These points where the bone and muscle meet are tendons. As the muscle contracts, the insertion is pulled toward the origin. Multiple muscles work together at the same time. One muscle, the extensor, must contract at the same time as a second muscle, the flexor, to flex or bend the joint. Together, these muscles are called an antagonistic pair.
Considerations
There are approximately 650 muscles in the human body, making up half your total weight. Each one has a somewhat unique appearance. For example, the fibers of the pectoral muscles are arranged like fans across the chest. The rectus abdominus in the abdominal area is crossed by three fibrous bands that give the muscle the familiar "washboard" look.
