Introduction to Muscles
Muscles are the organs that create movement. Humans have more than 630 active muscles in the body. There are three main kinds of muscles: smooth muscle, cardiac muscle and skeletal muscle. Smooth muscle is located inside blood vessels and internal organs. It is responsible for involuntary actions such as vessel constriction and digestion. Cardiac muscle is found only in the heart. It contracts to keep the heart's pumping blood and has a specialized structure to enable the flow of electrical signals. Skeletal muscles are the type responsible for voluntary movement. When speaking of muscles, they comprise the tissue most commonly thought of and make up your biceps, quadriceps, latissimus dorsi, among muscle groups.
Structure of Skeletal Muscle
Each skeletal muscle is an organ that contains muscle tissue, connective tissue, nerves and blood vessels. The individual cells of the muscle are called muscle fibers. They are a cylindrical-shaped cell with the capability of contraction. Muscle fibers are grouped into bundles that are surrounded by connective tissue. Different kinds of fibrous connective tissue surround each muscle fiber and fiber bundle as well as the entire muscle. This connective tissue is also continuous with your tendons at the ends of your muscles, which attach them to your bones. This is so that when any one muscle cell develops tension, it is transferred throughout the entire muscle and transferred to the tendon, which pulls on your bones and causes movement.
Activation of Skeletal Muscle
Your muscles are activated by motor neurons. One motor neuron innervates several muscle fibers due to its numerous nerve branches. Electrochemical signals from your brain are transferred through your spinal column to the motor neurons, which cause the release of a chemical called acetylcholine. Acetylcholine initiates muscular contractions.
Physiology
The system of muscles enables the skeleton to move. To accomplish this, your muscles contract and pull on your bones, which act as levers. Contractions often cause movement around joints, which act as pivot points in order to provide a mechanical advantage and enable the bones and muscles to transfer a greater amount of force. Muscles also often work in pairs to create opposing movements such as hip flexion and extension. Virtually all body movements involve the action of more than one muscle. This includes the prime movers, or agonists, which are the muscles directly involved in movement. There are secondary movers known as synergists that assist the movement, and there are antagonists, which are muscles that can slow down or stop the movement. The antagonists also serve to assist in joint stabilization and braking the limb toward the end of the movement.
References
- Essentials of Strength Training and Conditioning; Thomas R. Baechle, EdD; 2000
- Sam Houston State University
- Honors University of Maryland
