Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
22.214.171.124 Neurothermal Sensing
While neurons come in many shapes and sizes (Chapter 25), our "exemplar" ~14,000-micron3 neuron discharging ~90 mV into an input impedance of ~500 Kohms produces ~0.2 microampere current per pulse and generates a continuous (average) 100-300 pW of waste heat as measured experimentally (Table 6.8). The discharge rate of 5-100 Hz can produce brief surges up to ~2000 pW during a high-frequency train, but the duty cycle of such trains is far less than 100%, reducing time-averaged dissipation to the observed 100-300 pW range.
Single impulses are measured experimentally to produce 2-7 microkelvin temperature spikes in cold or room-temperature mammalian non-myelinated nerve fibers801 and ~23 microkelvins in non-myelinated garfish olfactory nerve fibers3482 at an energy density ranging from 270-1670 joules/m3-impulse from 0-20°C.3483-3485 In non-myelinated fibers the initial heat occurs in two temperature-dependent phases: a burst of positive heat, followed by rapid heat reabsorption (called the negative heat).3484 The positive heat derives from the dissipation of free energy stored in the membrane capacity, and from the decrease in entropy of the membrane dielectric with depolarization.3483,3484 An L ~ 20-micron neuron in good thermal contact with an aqueous heat sink at 310 K has thermal conductance L Kt ~ 10-5 watts/K, so trains of 5-100 Hz impulses lasting 1 second should raise cellular temperature by 10-30 microkelvins; up to ~200 microkelvin thermal spikes from such trains have been observed experimentally.801 These events are easily detectable by nanoscale thermal sensors capable of ~1 microkelvin sensitivity up to ~1 KHz (Section 4.6.3). The ~microkelvin heat signature of individual impulses or very short pulse trains can probably be temporally resolved because the minimum pulse repetition time is ~10 millisec (at 100 Hz) which is much longer than the thermal time constant for an L ~ 20-micron neuron (thermal conductivity Kt = 0.528 watts/m-K and heat capacity CV = 3.86 x 106 joules/m3-K for brain tissue; Table 8.12) which is tEQ ~ L2 CV / Kt ~ 3 millisec.
Last updated on 17 February 2003