(see also "Several Insights into muscle contraction mechanism" part 1 and 2)

         The N line is a nebulous striation  within the I band in the skeletal muscle fiber (later identified as N2 line). As the non-contracting muscle is lengthened, the N2 lines moves away from the Z line, maintaining the same proportional ratio between the N-N distance and the sarcomere length (Page, S.G.1968 J.Physiol. 197, 709-715).

 

        

 

           According to Kuan Wang (one of the greatest specialists in the domain)

1980  http://www.pnas.org/content/77/6/3254.full.pdf or

1988  http://jcb.rupress.org/content/107/6/2199.full.pdf).

the N2 line is part of a structure extending from the Z line to the M line (in the middle of sarcomere).

          Dan Eremia (1985) postulated (for the first time in literature) the necessary existence of threads connecting the so called "free ends" of thin filaments to the Z line at the opposite edge of the sarcomere. He named this structure "ultra-thin N-filament" and considered that the N2 line is part of this N-filament.

          Indeed, the N line divides the distance between Z line and the  "free ends" of thin filaments (T) in two segments (ZN and NT) whose ratio is constant.

          

 

     

 

 

 

ZZ (µm)

 

 

 

ZN / NT  (Eremia)

 

 

 

ZN / NM  (Wang)

2,2

0,31 / 1,025 = 0,30

0,31 / 1,1 = 0,28

2,4

0,37 / 1,225 = 0,30

0,37 / 1,2 = 0,31

2,6

0,43 / 1,425 = 0,30

0,43 / 1,3 = 0,33

2.8

0,49 / 1,625 = 0,30

0,49 / 1,4 = 0,35

3,0

0,55 / 1,825 = 0,30

0,55 / 1,5 = 0,36

3,2

0,61 / 2,025 = 0,30

0,61 / 1,6 = 0,38

3,4

0,67 / 2,225 = 0,30

0,67 / 1,7 = 0,39

3,6

0,73 / 2,425 = 0,30

0,73 / 1,8 = 0,40

 

 

 

 

 

 

 

  

 

         

          Dan Eremia proposed that N-filaments are made up by nebulin (in skeletal muscle fibers) or by nebulette (in skeletal muscle fibers).