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PHGY 209 Nerves and synapses.doc

Course Code
PHGY 209
Ann Wechsler

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[“not a single atom that is in your body today was there when that event took place” – Steve Grand (computer
-spinal cord=highway of processing information (relaying up to brain
and down to periphery of body) and mediates automated activities (ex.
-afferent fibers or sensory neurons bring information from periphery to
spinal cord (ex. skin sensations)
-efferent fibers or motor neurons allow movement
-nervous system comprises about 100 billion neurons.
-neurons use electricity to propagate information from a place to
-there is probably from a hundred trillion to a quadrillion of synapses in
human nerv.syst.
∙soma: cell body, containing nucleus, where
metabolic and synthetic activities take place,
maintaining neuron in living state
∙dendrites: “antennas” of neuron, for information
to flow in
-axon: unique, for information to flow out (initial
segment inserts in cell soma, presynaptic
terminal=swelling at axon’s end)
Resting membrane potential: the inside of a neuron has a slightly negative voltage(=electrical pressure) compared to
the outside (-60mV to -70mV, tiny fraction of unpaired negatives charges, less than 1%). Neurons use it as a starting
point for propagating electrical signals.
[almost all cells have a resting membrane potential]
-at rest, neurons still have
great permeability to
potassium ions (because of
leak potassium channels), but
Ca&others can’t flow in nor out.

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-negative voltage of neurons is then due to
high concentration of potassium ion
-When the chemical and electrical gradients are
equal, the system is at equilibrium(quickly
attained). The membrane potential at equilibrium
is described by the Nernst equation.
-Nernst equation tells us exactly what the voltage
is going to be
W/ the Nernst equation, we find E(Na)=+50mV and E(Cl)=-70mV
The resting membrane potential is a bit more positive than EK, because there is a small inward leak of Na+, which
pushes the membrane slightly toward ENa.
-The membrane potential is determined by concentration gradients and relative permeabilities of membrane to
different physiological ions.
-The dominant permeability makes greatest contribution to the membrane potential. (At rest, the dominant
permeability is to potassium, so the membrane potential is close to EK.)

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-Action potential is a jump in membrane potential, from negative to positive compared to the outside (all or none
-Three types of channels:
•leak potassium channels
•voltage-gated sodium channels
•voltage-gated potassium channels (accelerate falling phase, open w/ depolarization of axon, open slowly)
-These flows don’t change very much ions concentration (too small numbers), sodium-potassium pump are
continuously in action (maintaining gradients in background).
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