Muscle is one of the four principal tissue types; it produces movement by pulling on the dense connective tissue which forms the tendons and periosteum. In the human body there are four different types of muscle: 1. Smooth muscle, also known as involuntary, or nonstriated muscle, is responsible for the generation of the tunica muscularis, or muscle coat, of the internal organs and blood vessels and makes up the stroma of several visceral organs. The contraction of smooth muscle cells is involuntary, as opposed to the contraction of striated muscle; it is slower and requires less energy but can be maintained for longer periods of time. The sarcoplasm (cytoplasm of muscle cells) of smooth muscle cells is rich in myofbrils, which are not arranged in sarcomeres as in striated muscle types. 2. Cardiac muscle, also called myocardium, makes up the walls of the heart and is responsible for the pumping of blood. Cardiac muscle contraction is involuntary, unlike skeletal muscle, although it is characterised by striations similar to those in skeletal muscle which are due to the organised distribution of myofbrils within the cell sarcomere. 3. Skeletal muscle, also known as striated or voluntary muscle, is connected to the skeleton and is responsible for generating movement. Contraction is under voluntary control, and as a result the skeletal muscle is highly innervated by motoneurons: every muscle fbre is contacted by a motoneuron at the neuromuscular junction, which transmits the signal to contract.