Nano-Mechanical Probing of Synaptic Activity at Dendritic Spines

Abstract

Dendritic spines are small structures found covering the dendrites of neurons, forming excitatory synapses throughout the brain. These structures are a fundamental element in neural circuits, allowing many synaptic inputs to be processed by a single neuron. Due to their sub-micron size, however, it is very difficult to study the individual synaptic properties of spines using conventional methods such as electrophysiology and optical measurements. In particular, direct measurement of the membrane depolarization that occurs in the spines is needed to fully understand their function. While probing spines with Atomic Force Microscopy (AFM) we have observed spontaneous, mechanical events at the spine head which last for ∼10ms. During these events, the apparent height of spines increases abruptly with a temporal pattern that exhibits features similar to action potentials. Using different AFM recording modes we have measured changing forces between the AFM tip and spine head that accompany these height changes. We have used these measurements to test a mechanical model of the spine that can explain the observed behavior. Our observations suggest a strong electromechanical coupling phenomenon occurring specifically at spines that may facilitate non-invasive probing of individual synaptic activity by AFM.