In the small smooth muscle fibers the SR is very sparse indeed. There is no T-tubular system, but in-pocketings of the sarcolemma form caveolae (not caveoli) which are associated with the SR elements. Smooth muscle action potentials show a wide range of properties, some being less Ca2+ dependent (normal APs) and others being more so - very much like the cardiac AP with a plateau corresponding to a period of Ca2+ entry. The AP in many smooth muscles is not altered by removal of extracellular Na+ but is abolished by removal of Ca2+. Because of the small fibre size and large surface/volume ratio, quite significant quantities of Ca2+ can enter from the extracellular fluid (ECF) to supplement that released from the SR. Like cardiac muscle (but unlike skeletal), smooth muscle contraction is weakened by hypocalcaemia.
As in cardiac muscles, smooth muscle fibres may be joined by gap junctions to provide electrical continuity between fibres and tight junctions for adherence between cells. These are not, however, consolidated into intercalated disks.
There are no cross-striations, because the filaments and Z-line material are organised in a less orderly arrangement. Alpha actinin forms dense bodies attached to the cell membrane or to an intracellular scaffolding of intermediate (desmin) filaments. As in striated muscle, the thin (actin) filaments are attached to the alpha actinin (dense bodies) and interdigitate between the thick (myosin) filaments. This arrangement allows for considerable shortening of the smooth muscle fibres. These muscles contract for long periods to maintain tone (sphincters) or to cause movement (e.g. peristalsis) with minimal expenditure of energy. (A few sphincters, as noted before, are composed of striated muscle.)
There are two types of smooth muscle. In the first (right, bottom), fibres are inter-connected with each other via gap junctions, allowing the spread of electrical excitation through the sheet of cells. The cells may show spontaneous rhythmic contractions and relaxations in step with periodic membrane depolarisation and repolarisation (a basic electrical rhythm - BER). Sympathetic and/or parasympathetic axons course through the tissue, at intervals forming swellings or varicosities in which synaptic vesicles accumulate. Synaptic gaps are large. This is known as unitary or visceral smooth muscle (gut, uterus) since all the fibres in a layer act as a single unit. In the second type (left, bottom), gap junctions are rare, synaptic gaps are narrow and fibres may act more or less independently of each other. No BER is shown. This type of muscle is known as multiunit smooth muscle (e.g. ciliary muscle, iris, arrector pili; walls of large blood vessles).