There are two types of smooth muscle fibres:
Single-Unit (Visceral)
Single-unit smooth muscle occurs in sheets that surround small arteries, veins and hollow organs such as the stomach, intestines, uterus, and urinary bladder. The contractions produced by single-unit muscle are auto-rhythmic and involuntary . The fibres connect to one another by gap junctions, forming a network through which muscle action potentials can spread. In response to a stimulus (hormone, neurotransmitter, autorhythmic signal), the muscle action potential spreads to neighbouring fibres which then contract in unison, AS A SINGLE UNIT.
Multi-Unit
Multi-unit smooth muscle fibres have individual neuromuscular junctions and few gap junctions. As stated, if a visceral muscle fibre receives a stimulus, this causes the contraction of many adjacent fibres. However, stimulation of one multi-unit fibre causes contraction of that fibre only. Multiunit smooth muscle fibres are found in the walls of large arteries, in airways to the lungs, in the arrector pili muscles that attach to hair follicles, in the muscles of the iris that adjust pupil diameter, and in the ciliary body that adjusts focus of the lens in the eye.
Smooth Muscle Contraction
Smooth muscle contraction is also based on the sliding filament mechanism. In addition to actin and myosin filaments, which are arranged in a lattice-shaped framework, intermediate filaments are attached at either end to dense bodies in the sarcolemma. Dense bodies are similar in function to Z-discs. During contraction, the intermediate filaments pull on the dense bodies to produce further shortening of the muscle fibre. As smooth muscle contracts, it also rotates like a corkscrew turns. The fibre twists in a helix as it contracts and rotates in the opposite direction as it relaxes. The principles of smooth muscle contraction are similar to skeletal muscle however, contraction of the fibres starts much slowly and lasts much longer. Smooth muscle can also shorten and stretch to a greater extent than the other muscle types. Increased concentration of Ca++ in the cytosol of smooth muscle initiates contraction just like striated muscle. Ca++ flows into smooth muscle cytosol from both interstitial and small amounts of a sarcoplasmic reticulum. Because there are no T-tubules, it takes longer for the Ca++ to reach the filaments in the centre of the fibre. This accounts in part, for the slow onset of contraction as mentioned above.
114
Made with FlippingBook Publishing Software