a, Forces operating on an actin bundle underneath the cell surface. A curved actin bundle stretched between focal adhesions has a tension T (red arrows), imposing an outer force towards the plasma membrane (orange arrows). From the tension on a bundle at the cell edge and its curvature radius (R), it is possible to determine the plasma membrane tension. b, Two-dimensional illustration of the actin bundle, plasma membrane, cortex and cytoplasm mechanical model under the action of an AFM tip (blue). The green layer represents the plasma membrane and the cortex. The springs simulate the response of the cytoplasm. The red arrows are the direction of the tension of the actin bundle. The force applied by the tip induces a deformation on the plasma membrane and cell cortex and a reaction force from the bundle. By measuring local stiffness with AFM it is possible to determine the tension on the bundle using a. Adapted from ref. 4, Springer Nature Ltd.