Fig. 3.

Rest tension in the axon and its self regulation. (A) Scanning electron micrograph of a micro mechanical force sensor (spring constant, k = 3.5 nN/μm) used to measure the force response of embryonic Drosophila axons. An x-y-z piezo stage held the sensor and brought the probe into contact with the axon to form nonspecific adhesion and apply stretch. The force, F = kx, on the probe was measured from the deflection, x, of the force sensing beams. The tension, T, in the axon was obtained from the force balance at the point of contact. (B) We dissected an embryo (after ≈16 h of embryogenesis) and removed the fat cells from around the fifth axon (from the posterior end of the embryo). We then used the probe to stretch the axon and measure its tension. The stretch was measured from geometry. The interval between the 2 data points was 50 s. Extrapolation of the force-stretch curve to 0 stretch point gave an estimate of the rest tension of ≈1 nN. (C) A probe pushed an axon at mid length in <1 s, and then held the stretch with time. The corresponding tension in the axon was measured as a function of time. (D) The probe was quickly released from a similarly stretched axon after its tension had relaxed to the rest value. The axon was overstretched as soon as the probe was removed. The time lapse images of the axon show that it shortened its length with time linearly with a velocity of 5 nm/s. The axon recovered its initial length in ≈10 min. R² values of the linear fits in B and D are shown.