Neurons generate electricity using ion currents
The human body, including the brain, is an electric system, which generates electric currents. All our functions are mediated by electric currents or signals including our cardiac function and the movement of our limbs for instance. An electric current is defined as a flow of charged particles. These can be electrons in a wire or ions in biological systems. Ions are atoms that have lost or gained electrons, thereby becoming electrically charged, such as the sodium ion (Na+), the calcium ion (Ca2+), the potassium ion (K+), and the chloride ion (Cl-).
Neurons are cells that can generate electric currents. They possess ion channels on their surface which play a fundamental role by their opening and closing. Upon the arrival of an excitation signal, sodium channels open, leading to a sodium ion flow (current) towards the inside the cell. Due to this positive charge inflow, the electric potential of the neuron rises to a high value of +30 mV. An electric impulse has been generated. Shortly after, potassium channels open, resulting in a potassium ion flow (current) towards the outside of the cell. Due to this positive charge outflow, the electric potential of the cell decreases to its initial (rest) value of -70 mV. The electric impulse has been stopped. It is said that the neuron has generated an action (electric) potential and that the neuron has “fired”.
Figure 1: Neurons generate electricity through ion currents which constitute electric currents.
Figure 2 : Propagation of the electric current along the axon of a neuron (from Wikipedia).
The pulse (action potential) that has been generated on the neuron at one specific site, travels along the axis of the neuron and reaches its terminal, which is found in close proximity to another neuron (Figure 1). Neighboring neurons have close-contact sites called “synapses” (Figure 2). When the travelling pulse reaches the terminal of a neuron, it can activate local calcium channels, thereby inducing calcium increase on the inside of the neuron locally. Calcium will trigger the release of substances called neurotransmitters in the synapse. The neurotransmitters will be released from vesicles as shown in Figure 2 and will bind receptors at the neighboring neuron in a process that triggers an action potential there. The initiating mechanism will be the same as that described previously, that is the opening of sodium channels. Some of the most common neurotransmitters are dopamine, epinephrine, GABA, glutamate, epinephrine and serotonin.
As more and more neighboring neurons are triggered to generate electric impulses, an additive strong electric current is generated and amplified. If the bioamplification is strong enough, a biological outcome will occur.
It can be understood that calcium plays a fundamental role in neurotransmission and by extension in the amplification of the initial biological signal in order to mediate a biological response. In the absence of sufficient amplification, there will be no biological outcome. Therefore, means of controlling neurotransmission through this specific ion will have the most decisive outcomes.
Parallelly, the generation of the electric pulse (action potential) is orchestrated by complex patterns of activation and deactivation of different channels, including those for sodium, calcium potassium, and chloride. Electrochemical gradients of these ions (Figure 2 - left) play a central role in the process. All the currents are integrated to generate the electric activity of the body, including that of the brain, where we refer to brain rhythms or brain waves.
By manipulating the ions and their electrochemical gradients by electromagnetic means e.g. by energizing the ions and regulating their availability in proximity to the channels, or in general by imitating the conditions required for channel activation and deactivation, we may be able to code a specific neural sequence that transmits a certain neural signal.
The fundamental role of ion channels in physiology is reflected by the fact that they currently represent the second drug action target (after receptors [*]), meaning the second category of targets that the medicinal and pharmaceutical research wishes to address for the purpose of curing disease.