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Most behavior and many physiological actions of animals, such as beating of the heart and moving of food through the digestive tract, depend on muscle contraction. Wherever tissues contract, muscle cells are responsible. As introduced in Key Concept 39.1 and shown in Figure 39.2, there are three types of vertebrate muscle:
Skeletal muscle is responsible for all voluntary movements, such as running or playing a piano. It is also involved in many involuntary actions, including breathing, shivering, and maintaining posture.
Cardiac muscle is responsible for the beating of the heart.
Smooth muscle creates movement in many hollow internal organs, such as the digestive system, bladder, and blood vessels, and is under the control of the autonomic (involuntary) nervous system.
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When skeletal muscle contracts, the sarcomeres shorten and the band pattern changes.
Muscle contraction is due to repeated cross bridge forming, breaking, and reforming between the actin and myosin filaments causing them to slide past each other.
Action potentials arriving at a neuromuscular junction trigger action potentials in the muscle cell membrane, resulting in release of Ca2+ from the sarcoplasmic reticulum.
Cardiac muscle has similarities and differences with skeletal muscle that are important for their respective functions.
Skeletal and smooth muscle occur in all animal phyla, but cardiac muscle is unique to vertebrates. All three muscle types share the same underlying molecular mechanisms for generating force. We will use vertebrate skeletal muscle as our primary example. Later in this section we will discuss the differences in cardiac and smooth muscle that adapt them to their particular functions, and at the end of Key Concept 47.2 we will describe some interesting specializations of insect muscle.