Smooth muscle, located in the walls of hollow organs such as blood vessels, the bladder, uterus, and gastrointestinal tract, is a non-striated muscle used to move substances within the body by contractions. These contractions are usually involuntary and occur without any stimulation of nerves, however the speed of these contractions is much smaller than that of skeletal (or striated) muscles. Smooth muscles are made up of fibres that are much smaller than skeletal muscle fibres. They are classified into either single- or multi-unit smooth muscles. In single-unit muscles, the muscle fibres are arranged into dense sheets or bands. Although the fibres are roughly parallel, they are packed irregularly and often occur so that the narrow portion of one fibre lies against a wide part of the adjacent fibre. Between the two fibres is a connection, or junction, that serves as a low-resistance pathway for electrical signals that spread quickly throughout the tissue. Multi-unit smooth muscles however, do not have these connections. They are instead joined with connective tissue fibres.