Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!spool.mu.edu!munnari.oz.au!uhccux!uhunix1.uhcc.hawaii.edu From: bjones@uhunix1.uhcc.hawaii.edu (Brad Jones) Newsgroups: bionet.neuroscience Subject: Re: Current flow in neurons (was Re: Auditory Impulse Travel and Distance) Message-ID: <13628@uhccux.uhcc.Hawaii.Edu> Date: 25 Jun 91 13:32:23 GMT References: <1991Jun18.033141.11511@agate.berkeley.edu> <13584@uhccux.uhcc.Hawaii.Edu> <1991Jun24.175406.24957@agate.berkeley.edu> Sender: news@uhccux.uhcc.Hawaii.Edu Reply-To: bjones@uhunix1.uhcc.hawaii.edu (Brad Jones) Distribution: bionet Organization: University of Hawaii Lines: 35 In article <1991Jun24.175406.24957@agate.berkeley.edu> brp@dino.berkeley.edu (Bruce Raoul Parnas) writes: >In the strictly mathematical sense, this conduction is nearly instantaneous- >any set of equations used to model this system will immediately show a >non-zero response at all points in the cell. However, is this practically >useful? ....[section deleted]... >In this sense, the regenerative phase of spiking neurons makes them conduct >SIGNALS faster than their non-spiking counterparts which (as was mentioned in >Brad's message) are at the mercy of membrane capacitance. So, it seems to >me that it all really depends on how we decide to define conduction in nerves. Yes, this is a good point. _Information_ transfer in a non-spiking neuron is not necessarily faster than it would be in a spiking neuron. Many variables affect how fast the input signal reaches a level sufficient to release transmitter at a distant site. In order to maximize the length constant, a neuron may have a very high membrane resistance which will increase the time constant and lead to a slower rise time of the passive signal at a distant site. Still, for a short neuron having a membrane potential near or above the activation threshold for Ca channels in the presynaptic membrane (e.g. retinal bipolar cells), information transfer must be faster without spikes than it would be with them. This is because there is no inter-spike refractory period associated with the passive conduction. Also, the speed of passive conduction is very important in myelinated nerve where discrete spiking nodes are separated by non-spiking regions a millimeter or so in length. Of course the specialization that makes this useful is the decreased membrane capacitance and increased membrane resistance afforded by the Schwann cell wrapping. -- -------- Brad Jones -- bjones@uhunix.uhcc.hawaii.edu - bjones@uhunix.bitnet Bekesy Laboratory of Neurobiology, Pacific Biomedical Research Center University of Hawaii, Honolulu, HI 96822