Computer simulations of neuron-glia interactions mediated by ion flux (Somjen et al. 2008)

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Accession:113446
"... To examine the effect of glial K+ uptake, we used a model neuron equipped with Na+, K+, Ca2+ and Cl− conductances, ion pumps and ion exchangers, surrounded by interstitial space and glia. The glial membrane was either “passive”, incorporating only leak channels and an ion exchange pump, or it had rectifying K+ channels. We computed ion fluxes, concentration changes and osmotic volume changes. ... We conclude that voltage gated K+ currents can boost the effectiveness of the glial “potassium buffer” and that this buffer function is important even at moderate or low levels of excitation, but especially so in pathological states."
Reference:
1 . Somjen GG, Kager H, Wadman WJ (2008) Computer simulations of neuron-glia interactions mediated by ion flux. J Comput Neurosci 25:349-65 [PubMed]
Model Information (Click on a link to find other models with that property)
Model Type: Neuron or other electrically excitable cell; Electrogenic pump; Glia;
Brain Region(s)/Organism:
Cell Type(s): Astrocyte;
Channel(s): I Na,p; I Na,t; I T low threshold; I A; I K; I K,Ca; Na/Ca exchanger; Na/K pump;
Gap Junctions:
Receptor(s): NMDA;
Gene(s):
Transmitter(s): Ions;
Simulation Environment: NEURON;
Model Concept(s): Epilepsy; Calcium dynamics; Sodium pump;
Implementer(s):
Search NeuronDB for information about:  NMDA; I Na,p; I Na,t; I T low threshold; I A; I K; I K,Ca; Na/Ca exchanger; Na/K pump; Ions;
TITLE calcium pump 
COMMENT
uit aren borgdorff's proefschrift pagina 30:
V(ca2+i)=Vmax/(1+Km/Ca2+)^h
met Vmax = 352uM/s, Km = 6.9 uM and h=1.1

zie parameters ook in PARAMETER-box
pomp uit granule cellen met gemiddelde diam= 11.1 +/- 0.15 um
Om de granule cel-specifieke waarde Voor Vmax te gebruiken in 
een CA1 cel moet het omgezet worden in algemenere eenheid, nl.
van uM/s naar i/um2:

diam was 11.1 um -> r=5.55 um ->
opp=387 um en inhoud = 716 um3, -> inhoud/opp = r/3 = 1.85 um



ENDCOMMENT

NEURON {
	SUFFIX capump
	USEION ca READ cai WRITE ica	
	RANGE ica
	GLOBAL Vmax, Km, hill, Vconv, carest
}

UNITS {
	(mA) = (milliamp)
	(mV) = (millivolt)
	FARADAY = (faraday) (coulombs)

}

PARAMETER {
	Vmax = 352 (uM/s) 		: units of Borgdorff
	vol_surf_ratio = 1.85 (um) 	: assume r=5.55 um and sphere
	Km = .0069 (mM)			: 6.9 uM
	hill = 1			: hill is 1.1, no significant diff from 1
	scale = 1e-4
}

ASSIGNED {
	ica (mA/cm2)
        cai (mM) 
	Vconv
	carest
}

INITIAL {
	VERBATIM
	cai = _ion_cai;
	carest = _ion_cai;
	ENDVERBATIM
	ica =  pumprate(cai)*scale	
}

BREAKPOINT {
	ica =  pumprate(cai)*scale
}

FUNCTION pumprate (ci) {
	Vconv = Vmax*vol_surf_ratio*FARADAY*2*(1e-4)
	if (fabs(ci-carest) < 1e-7) {
	  pumprate = (ci-carest)*Vconv/Km
	}else{
	  pumprate = Vconv/(1+Km/(ci-carest))
	}
}

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