Weighting of synaptic responses

[Rose Reich]

Hi. I am a first year medical student who has a keen interest in 
neuroscience. Prior to my medical course my interest was mainly fueled by
computational neural networks, but now i am trying to build (in my own mind)
a physiological parallel to traditional Feedforward Networks. 
My biggest stumbling block has been my lack of correct physiological 
terminology, making looking up data difficult, so i seek your help.. :)

Q: In a modeled neural network the input into a neuron is regulated via 
   a weighting mechanism. In "real" brains is this performed by regulating
   Na+ concentration within the dendrite thus changing ion gradient, having
   a nett effect on the input level at the hillock ? or is it done some other
   way ?

Q: How is this weighting controlled? In a modeled neural net one simply 
   adjusts the weighting vector, but how is this achieved in real brains ?


Thanks in advance. 

Joshua Reich

(ps: these are not questions that are for any assignment, they are for my 
own interest. :)   )