Editing Muscle cell (section)

===Smooth muscle cells===
{{Main|Smooth muscle}}{{Further|Basal electrical rhythm}}
[[Smooth muscle cells]] are so-called because they have neither myofibrils nor sarcomeres and therefore no [[striated muscle tissue|striations]]. They are found in the walls of hollow [[organs]], including the [[stomach]], [[intestines]], [[urinary bladder|bladder]] and [[uterus]], in the walls of [[blood vessel]]s, and in the tracts of the [[Respiratory system|respiratory]], [[Urinary system|urinary]], and [[reproductive system]]s. In the [[eye]]s, the [[ciliary muscle]]s dilate and contract the [[iris (anatomy)|iris]] and alter the shape of the [[lens (anatomy)|lens]]. In the [[skin]], smooth muscle cells such as those of the [[arrector pili]] cause [[hair]] to stand erect in response to [[cold|cold temperature]] or [[fear]].<ref name="open">{{cite web |title=Smooth muscle |date=6 March 2013 |url=https://opentextbc.ca/anatomyandphysiologyopenstax/chapter/smooth-muscle/ |access-date=10 June 2021|last1=Betts |first1=J. Gordon |last2=Young |first2=Kelly A. |last3=Wise |first3=James A. |last4=Johnson |first4=Eddie |last5=Poe |first5=Brandon |last6=Kruse |first6=Dean H. |last7=Korol |first7=Oksana |last8=Johnson |first8=Jody E. |last9=Womble |first9=Mark |last10=Desaix |first10=Peter }}</ref>

Smooth muscle cells are spindle-shaped with wide middles, and tapering ends. They have a single nucleus and range from 30 to 200 [[Micrometre|micrometer]]s in length. This is thousands of times shorter than skeletal muscle fibers. The diameter of their cells is also much smaller which removes the need for [[T-tubule]]s found in striated muscle cells. Although smooth muscle cells lack sarcomeres and myofibrils they do contain large amounts of the contractile proteins actin and myosin. Actin filaments are anchored by [[Smooth muscle#Dense bodies|dense bodies]] (similar to the [[Sarcomere#Bands|Z discs]] in sarcomeres) to the sarcolemma.<ref name="open"/>