The Mechanical Properties of Nerves, the Size of the Action Potential, and Consequences for the Brain

The action potential is widely regarded as a purely electrical phenomenon. However, one also finds mechanical and thermal changes that can be observed experimentally. In particular, nerve membranes become thicker and axons contract. The spatial length of the action potential can be quite large, ranging from millimeters to many centimeters. This suggests the use of macroscopic thermodynamics methods to understand its properties. The pulse length is several orders of magnitude larger than the synaptic gap, larger than the distance of the nodes of Ranvier and even larger than the size of many neurons such as pyramidal cells or brain stem motor neurons. Here, we review the mechanical changes in nerves, we discuss theoretical possibilities to explain them and implications of a mechanical nerve pulse for neurons and for the brain. In particular, the contraction of nerves leads to the possibility of fast mechanical synapses.