Neuron function question

Kalman Rubinson kr4 at nyu.edu
Fri Oct 24 10:48:32 EST 2003


On Fri, 24 Oct 2003 15:22:36 GMT, "David Todtman"
<dtodtmanREMOVETHHIS at shaw.ca> wrote:

>I am trying to understand an element of neuron action.
>
>At the outset, I want to thank anyone who takes the time to help me out
>here.
>
>The text I am using notes two types of neuronal action:  ionic and
>metabolic. 

You might try a more explicit text.

> An ionic action occurs when enough neurotransmitter molecules
>(or other ligands) bind with postsynaptic cell receptors.  I gather that the
>binding is an electro-chemical bond and when enough electro-chemical bonds
>occur, the electrical potential of the postsynaptic cell changes rapidly and
>the cell is said to "fire."  Is this more or less the general idea?  I do
>not have to understand this process in a deeply technical way.

Very generally, it's correct.  A better term for them is "ionotropic"
receptors since these receptors affect neuronal function by directly
opening their ionic channels.

>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."  

These might better be called "metabotropic" since they influence
neuronal function by second messengers which can affect neuronal
metabolism, modulate ionotropic receptor channels or, even, nuclear
operations.

>Are there a specific class of receptors that are 'metabolic receptors' as opposed to
>'ionic receptors'?

Yes and no.  Some ionotropic receptor channels can also have
metabotropic effects along with their iotropic ones.  In addition,
some metabotropic receptors can phosphorylate ionotropic channels and
change their properties.

>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?

Yes.  They are generally molecules involved in the cell's internal
processes such as cAMP, cGMP, calcium, etc.. 

>Do the these ligands pass into the cell thus altering the internal environment?
 
They are not necessarily ligands and they are already inside.  The
metabotropic receptor is typically a transmembrane protein which binds
the transmitter on its external portion and change/releases something
from its internal portion.

>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?

Influencing nuclear functions to up-regulate receptor and/or channel
synthesis.

>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 do not know of such off-hand but imagine it's possible.

Kal



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