© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

4.8.6.4 Direct Synaptic Monitoring

The synaptic cleft between the axonal presynaptic terminal and the dendritic postsynaptic membrane is 10-20 nm in most synapses, although in the vertebrate myoneural junction it may be as large as 100 nm. Contact area per bouton is ~1 micron², giving a total gap volume of ~10⁷-10⁸ nm³. The density of acetylcholine receptors is highest in muscles along the crests and upper thirds of the junctional folds (~10,000/micron²), and is lowest in the extrasynaptic regions (~5/micron²).⁸⁰² (Other neurotransmitters exist; Table 7.2 and Section 7.4.5.6.) Each action potential discharge triggers the release of ~10⁴-10⁵ molecules of acetylcholine into the gap volume of an active neuromuscular junction (diffusion time ~1 microsec), raising c_ligand from near zero to ~3 x 10^-4 molecules/nm³ (~0.0005 M)⁵³¹ in ~1 millisec, followed by near-complete hydrolyzation by acetylcholinesterase during the 1-2 millisec refractory period. Into the gap volume may easily be inserted a ~10⁵ nm³ neurotransmitter concentration sensor (Section 4.2.3) able to measure ~100 acetylcholine molecules in ~1 millisec (Eqn. 4.5), thus detecting pulses at the fastest discharge rate. A similar device could be used to precisely regulate neuro-transmitter concentration at the junction, and hence the neural signal itself, under nanodevice control (Section 7.4.5.6). Simple electrochemical and mechanochemical artificial synapses have been demonstrated.⁴⁹⁹

Last updated on 17 February 2003