PHY 4327 Lecture Notes - The Membranes, Exosphere, In Reality

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25 Nov 2012
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NROB60 Chapter 4
Introduction
The frequency and pattern of action potentials constitute the code used by neurons to transfer information
from one location to another.
Properties of the Action Potential
The Ups and Downs of an Action Potential
o See Fig. 4.1.
o The first part, called the rising phase, is characterized by a rapid depolarization of the membrane.
This change in membrane potential continues until Vm reaches a peak value of about 40 mV
o The apart of the action potential where the inside of the neuron is positively charged with respect to
the outside is called the overshoot.
o The falling phase of the action potential is a rapid repolarization until the membrane is actually more
negative than the resting potential
o The last part of the falling phased is called the undershoot (after-hyperpolarization).
o Finally, there is a gradual restoration of the resting potential
o The action potential lasts about 2 milliseconds (msecs).
The Generation of an Action Potential (all-or-none)
o Consider you stepping on a thumbtack
o The initial chain of events is therefore:
1) The thumbtack enters skin
2) The membrane of the nerve fibers in the skin is stretched
3) Na+ - permeable channels open.
a. Because of the large concentration gradient and the negative charge of the cytosol,
Na+ ions enter the fiber through these channels
b. The entry of Na+ depolarizes the membrane (i.e. the cytosolic surface of the
membrane becomes negative)
c. If this depolarization (generator potential) achieves a critical level, the membrane
will generate an action potential. The critical level of depolarization that must be
crossed in order to trigger an action potential is called threshold.
o The depolarization that causes action potentials arises in different ways in different neurons
The Generation of Multiple Action Potentials
o The rate of action potential generation depends on the magnitude of the continuous depolarizing
current
o The firing frequency of action potentials reflects the magnitude of the depolarizing current
This is one way that stimulation intensity is encoded in the nervous system
o There is a limit to the rate at which a neuron can generate action potentials
The maximum firing frequency is about 1000 Hz
Once an action potential is initiated, it is impossible to initiate antoehr for about 1 msec.
This period is called the absolute refractory period.
It can be relatively difficult to initiate another action potential for several milliseconds after the
end of the absolute refractory period
During the relative refractory period, the amount of current required to depolarize
the neuron to action potential threshold is elevated above normal
The Action Potential, In Theory
Depolarization of the cell during the action potential is caused by the influx of sodium ions across the
membrane, and repolarization is caused by the efflux of potassium ions.
Membrane Currents and Conductances
o See Fig. 4.4.
o The membrane of this cell has three types of protein molecules: sodium-potassium pumps, potassium
channels and sodium channels.
o Begin by assuming that both the potassium channels and the sodium channels are closed and that
the membrane potential, Vm, is equal to 0 mV.
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