The N-lines of skeletal muscle.

The N-lines of skeletal muscle, although known for a century, have remained elusive and neglected structures. Their fortuitous appearance in diverse experiments on beef sternomandibularis muscle, involving stretch, heat denaturation, and myosin extraction, have been collected and related to the literature. Treatments with pyroantimonate and formamide solutions have proved a more reliable way of visualizing N-lines. It is concluded that there are at least seven N-lines: an N1-line always near the Z-line, four N2-lines in the mid-I-band, and two N3-lines at the extremity of the I-filaments or in the "gap." There is evidence for suspension of both N2- and N3-lines on G-filaments.