In article <SLEHAR.91Jun18084520 at park.bu.edu> slehar at park.bu.edu (Steve
Lehar) writes:
>The reason why the brain uses neural spiking, and encodes signal
>magnitude as spiking frequency is exactly to avoid the degredation
>with distance that is experienced by the alternative method of neural
>signaling, i.e. the density of ions of a particular charge.
>>The ions, injected at the site of neural input must diffuse passively
>along the neuron, which works ok as long as they don't have to diffuse
>too far. When you get one of those neurons with an extremely long
>axon however, there may be little or no charge left by the time the
>signal gets to the end, so the signal decays with distance.
While correct in a general sense, this description is not technically
accurate. Passive electrical conduction in neurons does not occur at
the diffusion rate of the ions in the cell. Electrical conduction in
neurons occurs just as in wires: at the speed of light. The charged
partical doesn't have to move the entire distance of current flow, it
bumps a neighboring ion which bumps another and so on. The reason
there is spatial decrement of electrical signals in neurons is because
current leaks out across the membrane capacitor as the current flows
along the cell.
>again to recover. That gulp of ions diffuses outward, and what
>happens next depends critically on the density of electrically gated
>channels in the local viscinity. If the next one is too far away,
>then the charge will not be strong enough to trigger it, and the
>charge diffuses slowly in space and time.
^^^^^^ ^^^^^^^^ ^^^^^^ ^^ ^^^^^ ^^^ ^^^^
As I said, the charge travels at the speed of light. The amplitude
decrement is due to the current loss at the membrane capacitance. The
temporal slowing is also due to the distributed capacitance. But,
_and_this_is_important_, when current is injected at a point in a
neuron the initial deflection will be measured at all points of the
cell nearly instantaneously. The _rise_time_ is what is slowed by the
membrane capactitance, so more distant sites show a more slowly rising
voltage change than sites near the current injection. The reason this
is important is that non-spiking neurons are able to conduct electrical
signals much faster than spiking ones (but not as far). Conduction
velocity in non-spiking neurons is near-instantaneous. Conduction
velocity in spiking neurons depends on the rise time of the voltage
change at the sodium channels, and the sodium channel density and
distribution.
--
--------
Brad Jones -- bjones at uhunix.uhcc.hawaii.edu - bjones at uhunix.bitnet
Bekesy Laboratory of Neurobiology, Pacific Biomedical Research Center
University of Hawaii, Honolulu, HI 96822
