|คาสิโนไทย||3. Signal Amplification by Microtubules||MT|
Sensitivity of Neurons to Weak Electric Fields
In the resting state, the membrane potential field (E) is on the order of 107 V/m, as obtained from the following formula,
E = Vm / d
where Vm is the resting membrane voltage (~ 70 mV) and d is the membrane thickness (~ 7 nanometer). Surprisingly, some neurons are sensitive to the external electric field as small as 1 V/m (คาสิโนไทยFrancis et al., 2003; Reato et al., 2010). Voltage-gated ion channels depend on the membrane potential field, which changes only a tiny fraction by superposition with the small external field. Therefore, the significant effects on neural activity must not arise from the direct interaction between the external field and ion channels. The neuron should possess certain mechanism to "amplify" the external signal, which is necessary for wireless communication because the transmitting EM waves also produce very weak electric fields.
The EM wave consists of oscillating electric and magnetic fields. Given the radiation power, it is possible to estimate the electric field strength at certain distance from the source. A formula is available on this website. Although the formula is accurate only at large distance, our purpose is to get a rough estimate. For power = 0.01 W and distance = 5 cm, the formula gives E = 10 V/m. This field strength is negligible compared with the membrane potential field, but is sufficient to influence neural activity as demonstrated experimentally (Chapter 8). The underlying mechanism for signal amplification is discussed below.
Signal Amplification by Microtubules
Neuronal excitability is fundamentally governed by the opening and closing of ion channels, which in turn depend on the membrane voltage. By definition, the voltage between two points is given by the integration of electric fields from one point to another. In a nerve membrane, the electric fields may come from various sources, including ions in the intracellular and extracellular solutions, charges on surface molecules and the microtubules. The effects of surface charges on channel gating and excitability have been reported (คาสิโนไทยCukierman et al., 1988; Genet and Cohen, 1996), but the contribution from microtubules was largely ignored. As shown below, a microtubule can significantly modulate the membrane potential field (and thus excitability) when it localizes near the membrane (Figure 1).
Let rm be the distance between the center of a tubulin dimer and the middle of the membrane. The electric field produced at the membrane by the tubulin dimer is given by
E = kQ/rm2
where k is the Coulomb's constant and Q represents the effective charge on a tubulin dimer. Assuming Q = 12 e–, at rm = 40 nm, we have E ~ 107 N/C = 107 V/m, which is on the same order of magnitude as the resting membrane potential field. As the tubulin (or microtubule) moves away from the membrane, it should have significant impact on the membrane potential field (Figure 1), and thus the channel gating. Generally speaking, the microtubule moving closer to the membrane is equivalent to membrane hyperpolarization while moving away from the membrane has the same effects as depolarization.
In non-neurons and most neuronal compartments, microtubules are tightly packed in the cytoskeleton. They do not have much space to move around. Nor can a small field cause significant translocation. The only exception is the axon initial segment (AIS) where microtubules are flexible to move (คาสิโนไทย). Furthermore, microtubules are highly negatively charged and AIS contains high density of voltage gated ion channels for the initiation of action potentials (Buffington and Rasband, 2011). These features make AIS the ideal place for microtubules to amplify the effects of external electric fields on neuronal excitability.
Author: Frank Lee