<Li> The MLC is also known as the regulatory light chain and actively participates in muscle contraction . Two MLC isoforms are found in smooth muscle, and they are encoded by different genes, but only one isoform participates in contraction . </Li> <Li> The MLC is also known as the essential light chain . Its exact function is unclear, but it's believed that it contributes to the structural stability of the myosin head along with MLC . Two variants of MLC (MLC) exist as a result of alternate splicing at the MLC gene . </Li> <P> Different combinations of heavy and light chains allow for up to hundreds of different types of myosin structures, but it is unlikely that more than a few such combinations are actually used or permitted within a specific smooth muscle bed . In the uterus, a shift in myosin expression has been hypothesized to avail for changes in the directions of uterine contractions that are seen during the menstrual cycle . </P> <P> The thin filaments that form part of the contractile machinery are predominantly composed of α - and γ - actin . Smooth muscle α - actin (alpha actin) is the predominant isoform within smooth muscle . There are also lots of actin (mainly β - actin) that does not take part in contraction, but that polymerizes just below the plasma membrane in the presence of a contractile stimulant and may thereby assist in mechanical tension . Alpha actin is also expressed as distinct genetic isoforms such as smooth muscle, cardiac muscle and skeletal muscle specific isoforms of alpha actin . </P>

Where do smooth cells get most of their calcium to stimulate contraction