MYOFIBRIL SIZE VARIATION ALONG THE LENGTH OF EXTRAOCULAR-MUSCLE IN RABBIT AND RAT .1. ORBITAL LAYER

It is generally assumed that a muscle fiber is structurally uniform al ong its length. That assumption is not consistent with the observed va riation of myofibrillar profile size along the length of both singly i nnervated fibers (SIFs) and multiply innervated fibers (MIFs) in the o rbital (outer) layer of extraocular muscle (EOM), Muscle fibers were r econstructed in serial sections along the orbital layer of rabbit and rat EOM, For both the SIFs and MIFs, myofibril profile size was smalle st (narrowest) near the endplate, In the SIFs of rat, for example, the myofibril profiles were 28% wider at a distance of 1.5 mm from the en dplate than at the endplate itself, Measures of profile size included the mean intercept length and the mean shortest path from test points within the profile to the profile boundary, The possible effect of sar comere length variation was controlled by normalizing the myofibrillar profile size data to a constant spacing of the myosin filament lattic e, This morphometric approach was also used to quantify the further in crease of profile size that occurs in the end portions of the orbital MIFs where the myobrillar organization is typically ill-defined.