cara15...
"Assume that it has already been explained how reception of a
neurotransmitter catalyzes either an EPSP or IPSP, but go into
detail regarding the mechanism by which the action potential
is carried along the axon."
There is an excellent discussion of the overall process in a
multimedia project on the website of Williams College in
Massachesetts, copyright 2000 Betty Zimmerberg:
"The cell body, or soma, of a neuron is like that of any other
cell, containing mitochondria, ribosomes, a nucleus, and other
essential organelles. Extending from the cell membrane, however,
is a system of dendritic branches which serve as receptor sites
for information sent from other neurons. If the dendrites
receive a strong enough signal from a neighboring nerve cell,
or from several neighboring nerve cells, the resting electrical
potential of the receptor cell's membrane becomes depolarized.
Regenerating itself, this electrical signal travels down the
cell's axon, a specialized extension from the cell body which
ranges from a few hundred micrometers in some nerve cells, to
over a meter in length in others. This wave of depolarization
along the axon is called an action potential. Most axons are
covered by myelin, a fatty substance that serves as an insulator
and thus greatly enhances the speed of an action potential.
In between each sheath of myelin is an exposed portion of the
axon called a node of Ranvier. It is in these uninsulated areas
that the actual flow of ions along the axon takes place."
http://www.williams.edu:803/imput/introduction_main.html
In other words, once an EPSP or IPSP has been generated, the
receptor cell regenerates its resting electrical potential,
which creates enough of a difference in charge to propel
the depolarization in a wave of ion flow which occurs in the
nodes of Ranvier, insulated by a sheath of myelin, just as
a difference in electrical potential causes electrons to flow
through an insulated copper wire in your computer. The actual
process is more about the conversion of chemical energy into
electrical energy, as we shall see.
This process is repeated as the action potential travels from
cell to cell through either electrical synapses, which are
specialized channels called gap junctions, or through chemical
synapses, whereby chemical neurotransmitters, either excitatory
or inhibitory, travel through the synaptic cleft which separates
two cells.
The discussion goes on to detail how this process is carried out
repeatedly in 4 steps, with a thorough discussion of each step:
1. Synthesis and Storage of Neurotransmitters
http://www.williams.edu:803/imput/I.html
2. Neurotransmitter Release
http://www.williams.edu:803/imput/II.html
3. Recognition by Neurotransmitter Postsynaptic Receptors
http://www.williams.edu:803/imput/III.html
4. Inactivation of Neurotransmitters
http://www.williams.edu:803/imput/IV.html
The homepage for the project is here:
http://www.williams.edu:803/imput/
On the page which describes the third step, the differences
between ionotropic receptors, also referred to as
ligand-gated ion channels, and Metabotropic receptors,
also called G-protein linked receptors, is discussed.
The mechanisms whereby depolarization and the resultant
ion transfer occurs, differ between the two, and each is
considered at length.
http://www.williams.edu:803/imput/III.html
Now, as to the actual details of the ion transmission through
the nodes of Ranvier, there is an excellent discussion on the
website of Raymond Walters College at the University of
Cincinnati, ©2003, Kenneth R. Koehler, in which he describes
how "...the nerve impulse 'jumps' down the axon (this is known
as 'saltatory' conduction)", through a series of electrochemical
processes involving sodium and potassium ions. I won't presume
to reproduce it here, so please read the complete disquisition
on this webpage:
http://www.rwc.uc.edu/koehler/biophys/4d.html
If you follow the links leading from the bottom of that page,
he then proceeds to explain basic electrical circuitry, in
preparation for the following page, in which he simulates the
activity of electrical transmission through the axon using
a circuit diagram of resistors and capacitors. Brilliant!:
http://www.rwc.uc.edu/koehler/biophys/4h.html
You may also locate additional useful links in the search
results which I've linked to below.
Please do not rate this answer until you are satisfied that
the answer cannot be improved upon by means of a dialog
established through the "Request for Clarification" process.
A user's guide on this topic is on skermit-ga's site, here:
http://www.christopherwu.net/google_answers/answer_guide.html#how_clarify
sublime1-ga
Searches done, via Google:
"synaptic transmission
://www.google.com/search?q=%22synaptic+transmission
"node of Ranvier" "ion transfer"
://www.google.com/search?q=%22node+of+Ranvier%22+%22ion+transfer%22 |