>2. Muscle: Structure and Function

Skeletal muscle-- one of the three major muscle types: cardiac, skeletal, and smooth-- is striated muscle tissue under voluntary control of the somatic nervous system.

<Basic Structure
A muscle cell, or a myocyte, is the basic building block of striated muscle tissue. They are composed of myofibrils, bundles of sarcomeres-- long fibrous protein filaments-- which slide past each other to contract or relax during intracellular chemical reactions. Sarcomeres are linked end to end creating long chains, and banded together to form myofibrils. These myofibrils are further bound together to form muscle fiber.

<Non-Muscular Structures
The fibers and muscle are surrounded by connective tissue called fascia, which attaches, stabilizes, and encloses the muscle, and reduces friction of muscular force.

Tendons attach the muscles to bone, and transmit muscular force. This transmitted muscular force produces torque at the joints, which we use to produce intended movement.

<Mechanism
The mechanism behind a muscle cell contraction involves the triggering of a chemical reaction within the cell that leads to a shortening of muscle fiber. For intracellular reactions to occur properly, certain conditions must be met-- such as proper PH range, substrate availability, and electrolyte balance.

By definition, electrolytes are a substance that produces a conductive solution when dissolved in a polar solvent (like water). In physiology, electrolytes are critical for regulating nerve and muscle function. The most important electrolytes for physical activity are sodium- the main electrolyte in extracellular fluid- and potassium, the main intracellular electrolyte.