How the Brain Works
The brain consists of over 86 billion cells called neurons that send signals to each other to communicate. The neuron can be divided into three parts: the dendrites, the soma, and the axon. Dendrites receive signals, the soma is the main cell body, and the axon sends signals.
Action Potentials
Action potentials are the basis of neuron communication. Neurons at rest have a voltage of around -70 mV compared to the outside of the cell. Their voltage depends on the amount of potassium and sodium ions in and out of the cell. At rest, there are more sodium ions out of the cell than potassium ions in the cell, making the cell negatively charged. An action potential happens at -55 mV. At this time, many voltage-gated sodium ion channels open and polarize the neuron even more, to around 30 mV. At that point, the neuron begins to repolarize by opening many voltage-gated potassium channels, letting potassium ions exit. When an action potential happens, neurotransmitters are released.
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Graded Potentials
When neurotransmitters are released, they bind to ligand-gated channels on other neurons. This causes a graded potential, which is a small increase in voltage that does not immediately cause an action potential unless there is more stimulation. Graded potentials that are caused by neurotransmitters from other neurons are called postsynaptic potentials, with the fired neuron being the presynaptic neuron. Postsynaptic potentials last longer than action potentials because they do not open as many ion channels. Postsynaptic potentials can be excitatory (EPSPs) or inhibitory (IPSPs). Because graded potentials do not cause a rush of ion channel opening and closing, they are able to last longer than action potentials.
Pyramidal Neurons and the Cortex
The outer layer of the brain is known as the cerebral cortex, and is the easiest for BCIs to read and influence. It is the part of the brain that deals with motor functions, sensory processing, and most forms of thinking. The cortex also has special neurons known as pyramidal neurons. A pyramidal neuron's long dendrites (the part of the cell that receives signals) allow for longer electric current when receiving a signal without having to activate an action potential.
Electromagnetic Properties
When an action potential happens, many voltage-gated ion channels open. These voltage gated channels activate linearly down the axon, with each channel depolarizing one part of the neuron and causing the next one to open. This creates an electric current.
As the positive charge travels in the cell, some things occur. The outside of the cell at that part becomes relatively negative, this is known as the ‘sink’ part of the neuron. Since the ‘sink’ part is now negative, the other part of the neuron is now relatively more positive. This is the ‘source’ part of the neuron. These opposite but equal charges are known as dipoles. The dipole creates an electric field that moves from the 'source' part into the direction of the ‘sink’ part. The dipole moves in the opposite direction of the current in the cell. Multiple dipoles create stronger fields that can be read through the scalp and skull.
As per the laws of electromagnetism, this current also creates a magnetic field. The right-hand rule describes this behavior. Looking at a picture of a thumbs-up or a thumbs-down, the current runs in the direction of the thumb and the magnetic field runs circularly in the direction of the curled fingers. Unlike electric fields, magnetic fields are not affected by the skull and tissue.
