STRIATED SPHINCTERS
The few striated muscles which serve as sphincters have fibre diameters about 1/4 the diameter of the skeletal fibres.
Soleus muscle XS. Note capillaries
surrounding large fibres. There is
very little connective tissue between
fibres.
External anal sphincter. Note very small fibres:
diameters 3-4X less than for soleus muscle and
abundant connective tissue (elastin, collagen)
between blocks of muscle fibres.
The modal XS area of soleus and plantaris fibres
(c. 6000 sq. microns) is some 10 - 15X greater
than for the fibres of the external anal sphincter (400 mm2).
The fibres in the external anal and urethral
sphincters in man tend to be slow, unlike those in
rats which tend to be fast.