MT Mechanolonic Transducers Moran and Varela

Biological sensory systems ranging from human inner ears to honey bee hairplate receptors use sensory cilia to transduce mechanical deformation into cognitive information like position in space ("proprioception"). Harvard biologists Moran and Varela (1971) studied the proprioceptive transduction of mechanical deformation in the legs of cockroaches. There, tactile spines contain mechanoreceptor structures called "campaniform sensilla" which consist of a single bipolar neuron from whose dendritic tip extends a modified cilium containing 350 to 1000 microtubules. Moran and Varela determined that this parallel bundle of MT was directly involved in the transduction of mechanical deformation to the neuron.

All neurons are mechanical receptors, being stretch sensitive to cause ionic currents. In the cockroach campaniform sensilla, the cilium is the only bridge and transduction does not occur when MT are incapacitated with tubulin binding drugs like colchicine and vinblastine (Moran and Varela, 1971). MT are thus directly involved in proprioceptive determination of the system's spatial relation to its environment. Moran and Varela saw two possible roles for MT in their mechanoreceptor function: as passive translation rods, or as generators of ionic/electrical currents. They suggested that MT behaved as "mechanochemical engines driven backward." Compression or bending of MT would cause release of bound ions from MT subunits, resulting in an ion flux. Moran and Varela saw these ionic currents capable of generating membrane depolarization, being perhaps the first to suggest that MT can regulate membranes. Each tubulin dimer reversibly binds 16 calcium ions so ionic fluxes of significant current could result from an active assembly of parallel MT. This process may be analogous to the coordinated release of calcium ion waves by sarcoplasmic reticulum in muscle cells which triggers actin-myosin contractile interactions. Calcium waves are also thought to regulate the bending and waving of cilia and flagella, and can regulate the cytoskeletal "ground substance" by coupling to sol-gel states. Moran and Varela's contribution was to observe that MT could release ions such as calcium in a controlled and modifiable manner useful for intracellular communication.

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