Red andwhite muscle are subtypes of skeletal muscle, referring to the color of themuscle due to blood supply and also cellular amounts of myoglobin protein. Myoglobin binds oxygen very tightly, releasing the oxygen only when cell levelsof oxygen become very low. In this way myoglobin can be though of as a type ofcellular scuba tank. Fast twitchmuscle fibers are adapted for sprinting and strength, and also correspond towhite muscle. Slow twitch muscle fibers are adapted for endurance, andcorrespond to red muscle.
Smooth muscle is involuntary, being under thecontrol of the autonomic nervous system.
This muscle type is found in the wallsof hollow, internal, organs- such as blood vessels, gallbladder, intestine,stomach, and lung airways. Cardiac muscle is located in the wall of theheart, and like smooth muscle is under involuntary control. Cardiac muscletissue has special cellular properties that allow very fast communicationbetween the muscle cells, including intercalated discs, or gap junctions, thatconnect each cell together, allowing for very rapid transfer of ions betweenthe muscle cells. Neurotransmitter molecules interact with muscle cell membrane molecularreceptors at the neuromuscular junction, or synapse, eliciting a series ofcellular events leading to shortening and contraction of the muscle cells. Whenmany muscle cells contract, the organ or tissue containing the muscle cellsshortens, allowing movement to occur, such as limb motion, intestinal motility,or gallbladder contraction.