Biological Effects - RF/Microwave Interaction with Biological Tissues - page 109

intermediaries. They have several ion-bonding sites, and by changing their con-
figuration as a function of the occupied site, they excite a specific target
enzyme. The Ca 2+ ions use specific channels to cross the membrane along the
concentration gradient. These voltage-gated channels are normally closed in
excitable cells, but they open in response to the action potential, that is, the
transmembrane voltage pulse induced by the arrival of a messenger on the
cellular surface. The membrane resting potential is -90 mV, but it may reach
+40 mV. The channel opens from -30 mV. The phenomenon lasts for a mil-
lisecond and allows the passage of about 3000 Ca 2+ ions, after which the
outward migration of K + ions returns the potential to its equilibrium value.
The calcium ion flux excites the endoplasmic reticulum, which itself liberates
Ca 2+ ions. The evacuation of the ions must take place against the concentra-
tion gradient. It is supported by the enzyme ATPase, or calcium pump, which
acquires the necessary energy by dissociating ATP molecules [13].
A new model of the neuronal membrane electrical activity was presented
in 1994 [71]. The main differences with previous models consist both in taking
into account the temperature dependence of the various parameters and in
inserting the synaptic inputs described as ionic channels. The model consists
of two interconnected schemes: (1) a circuital model, representing the ionic
currents crossing the membranes, and (2) a block model, representing the
intracellular calcium concentration dynamics. It provides a deterministic and
stochastic analysis . In the deterministic model the time course of the mem-
brane voltage and the firing frequency are univocally determined once given
the initial conditions. The insertion of the synaptic inputs in the model causes
stochastic time fluctuations of the instantaneous membrane voltage and of the
interspike interval, without inducing variations in the spike shape. The model
yields a good simulation of some known responses of the membrane in terms
of its firing frequency and resistance, validated by experimental results. It can
be used in a variety of applications and may be particularly promising in the
study of the interaction between EM fields and neuronal membrane activity
for the comprehension of interaction mechanisms. The field-induced modifi-
cations of the membrane stochastic behavior can be simulated by suitable
alterations of specific model parameters according to the type of stimulus (fre-
quency, wave shape, incident energy, etc.).
The model has been used for analyzing microscopic effects of signals of
GSM on voltage-dependent membrane channels of one single cell by com-
puter simulation [72]. The theory is based on Markov finite-state models. The
effect of mobilophony signals has been investigated on calcium, potassium,
and sodium channels. Pulsed signals are shown to act more on the reduction
of the opening probability than on CWs. The low-frequency components of
the signals induce a variation of 30% of the opening probability in Na + and
K + channels and 60% in Ca 2+ channels. It is interesting to observe that the two
low-frequency sine waves of GSM, at 8.3 and at 217 Hz, respectively, act much
less on the opening probability than the composite pulsed GSM signal, in
which the frequencies 8.3 and 217 Hz are present.