Neurologic Velcro

Neurons must in some way respond to mechanical stimuli or you wouldn’t have a sense of touch.  A remarkable ion channel called piezo2 in the axon membrane opens in response to stretch of the axon causing a nerve impulse to fire.  They are huge proteins with anywhere from 2,100 to 4,700 amino acids.  Three come together to form an ion channel.  Each monomer contains 35 or so transmembrane segments, arranged in an arc.  (If I could ever figure out how to get an image from a paper or captured on the web into a WordPress document, I’ll update this post, and I’m going to work on it this coming weekend. ) In the meantime content yourself with figure 1 from this https://www.nature.com/articles/s41586-019-1505-8.  The 3 arcs are called blades and are made mostly of alpha helices.   The complex is huge, fitting into a circle of diameter 270 Angstroms.   The structure distorts the membrane, indenting it so the ion is channel formed by the junction of the 3 monomers is closed.  Stretch the membrane and the indentation disappears opening the channel, ions flow inside the axon and a nerve impulse is fired.

Enter Neuron vol. 111 pp. 3211 – 3229 ’23 which has fantastic pictures of 3 types of mechanically sensitive sense organs — the Pacinian corpuscle, the Meissner corpuscle and the lanceolate complex found around hair follicles.  Each picks up a different type of mechanical stimulus, yet they all use the same ion channel — piezo2.

The answer was totally unexpected — each axon has hundreds to thousands of small (1 micron) projections (containing piezo2) looking exactly like velcro (see figure 8 p. 3225) and these are tacked to the nonneuronal cells of the sense organ by adherens junctions, so that when any part of the sense organ is moved piezo2 is stretched and the axon fires.  Again apologies — I hope to get these figures in here over the weekend.

So the specificity of the sense organ for the type of mechanical stimulus doesn’t lie in the neuron’s axon which is the same in all 3, but in the non-neuronal cells it is attached to.