X-Message-Number: 3890 Date: Wed, 22 Feb 1995 21:20:27 -0800 From: John K Clark <> Subject: SCI.CRYONICS Uploading In 3876 (Joseph J. Strout) Wrote: >The voltage exists between the inside and outside of the cell, >across the membrane. It spreads by the movement of >electrons, as in any conductor. In a conductor like a metal, current is caused by the movement of electrons but neurons are not made of metal. Free electrons can not exist in an aqueous solution, especially one that has a lot of chlorine in it as neurons do. A chlorine atom would grab a free electron and become an Cl- ion so fast it makes my head spin. A chlorine ion is 65 thousand times as heavy as an electron, a potassium ion is even heavier and that makes them slow. >You accurately described the propagation of the action >potential, but the spread and integration of PSPs is different >because it does not depend on voltage-gated ion channels. That's true, but I'm sure you know there are 2 types of channels, voltage gated ion channels and ligand gated ion channels. It's the ligand gated ion channels that change the membrane potential by opening up and allowing ions to cross the membrane when a neurotransmitter from a presynaptic neuron binds to it. Ligand gates work well with PSP because they are transducers, the more of the external neurotransmitter the stronger the ion current, but they don't work at light speed and they deal in ions not electrons. The only other thing I can figure your talking about is the cabal properties of dendrites, that is the passive spread of electrical signals with no amplification from voltage gated channels. Trouble is dendrites make terrible cables, their resistance is high but much worse their electrical capacitance is very high so the signal is slow even by biological standards. Because the membrane has such a high capacitance it takes a long time for membrane potential to build up from ion current flow. The high capacitance also distorts the signal, turning a sharp spike into a gentle rise and fall after traveling only a millimeter or two. >These signals are short-range but (lightspeed) fast. As you point out, in the cell body the grand postsynaptic potential is the spatial integration of all the excitations, but the signal ( at least the one that is passed on by the axon hillock through the axon for long distances) is also the result of a TEMPORAL integration of the excitations. The size of the net PSP depends on the rate of firing of the incoming excitations so can't be light speed fast. The bottom line is that signals can not be processed and transmitted from the synapses to the axon hillock at anything close to light speed and could benefit from a speed up as much as the action potential would. More generally, I don't understand why you think timing problems represents an insurmountable obstacle in speeding up the brain. Computer designers come across this problem constantly and have always been able to find clever solutions like catching. If information arrives before another part is ready for it just store it until needed, that way all the parts are kept busy and time is not wasted. John K Clark Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=3890