Hi David,
I think I cannot give you detailed information on all your aspects and
questions, but for a few I may be able to help you out a bit.
>> The text I am using notes two types of neuronal action: ionic and
> metabolic. An ionic action occurs when enough neurotransmitter
> molecules (or other ligands) bind with postsynaptic cell receptors.
A receptor is often a biomembrane channel molecule. So when the specific
transmitter binds to the outer end of the channel it causes a
conformational change of the receptor which most often leads to channel
opening. Because of the different ionic concentrations inside and outside
of the cell this can cause an Na+ ions inflow, depolarizing the cell.
When enough channels are opened the depolarisation may grow large enough
to cause firing of action potentials.
> Now for a second question. The text says that metabolic actions occur
> gradually--hours, days, weeks, or months. Then, the text says, "When
> ligands bind to these receptors, they operate by activating what are
> referred to as _second messenger systems_ within the cytoplasm of the
> cell." What is not explained in the text is "these receptors." Are
> there a specific class of receptors that are 'metabolic receptors' as
> opposed to 'ionic receptors'?
This is a different kind of receptor which does not necessarily serve as
a channel. But it also has an outer and an inner part. The outer part
binds the messenger (a chemical messenger molecule, for example GABA or
Acetylcholine, but others are possible and at the momemnt I don't know
which messengers exactly belong to these type of receptors).
When a messenger is bound on the outside surface the receptor starts a
cascade of metabolic activities inside the cell. Most often a rise in
inner cell cGMP concentration is caused. A risen cGMP level brings
several changes in neuron metabolism and this can cause changes which
last longer.
In both cases the ligand on the outer parts of the receptors becomes
inactive after short while. It is destroyed by enzyms in the synaptic
gap.
> Now, here is another query. The text says that the second messenger
> systems within the alter the cell's internal chemical environment.
> Okay but what specifically are the "messengers"? Particular
> molecules? Do the these ligands pass into the cell thus altering the
> internal environment?
Most often the chemical messenger molecules don't get into the cell
themselves. But nonetheless this case also exists in the form of gaseous
CO molecules acting as messengers.
> And finally, one result of metabolic action can be alteration in the
> number of receptor cites. In simple terms, what are some ways that
> this may occur? I.e., I believe that receptors are protein structures
> and does it happen that a type of metabolic action results in
> reductions in the cell's ability to manufacture the receptors?
I don't know exactly in this case. But there also are ways to temporarily
disable specific receptors. So there are receptors which need specific
concentrations of ions or cGMP to be active, they may even be inactivated
by a different than normal pH level.
I hope, my answers have been useful to you,
I can't give any guarantees for their correctness, of course...
Kind regards,
Dominik
