The brain contains millions of neurons, the nerve cells that transmit signals within the brain and to the body. Neurons connect through a series of cellular junctions, called synapses, that allow neurons to transmit chemical signals to each other. Once a cell receives a chemical signal, the cell undergoes a number of changes that lead to the transmission of a bioelectrical signal through the cell, called an action potential. Action potentials rely on the activity of a number of elements to propagate a bioelectrical signal.
Sodium
One key element in a nerve cell bioelectrical signal is sodium. Sodium ions can be found dissolved in water in and around nerve cells, and can pass into and out of the cell through proteins at the cell surface, called sodium ion channels. Before the initiation of an action potential, the environment around the nerve cell contains a higher concentration of sodium than the cell itself. When nerve cell receive a chemical signal from its neighbor, sodium channels in the receiving nerve begin to open, and sodium rushes into the nerve cell. This rush of sodium signals the beginning of an action potential, and starts the bioelectric signal within the cell. This step of the action potential is called depolarization, according to Georgia State University.
Potassium
The second half of the action potential is carried out by potassium. At the beginning of the action potential, potassium occurs at relatively high concentrations within the nerve cell. After sodium has rushed into the cell during depolarization, potassium ion channels open, allowing potassium to rush out of the cell. This outflux of potassium allows for a repolarization of the nerve cell and allows the cell to return to its rest state, according to Eastern Kentucky University. This depolarization and repolarization involving sodium and potassium occurs several times along the length of the receiving neuron, until the signal reaches the body of the cell and triggers a nerve response.
Calcium
Bioelectrical signals also play a role in the release of chemicals into nerve synapses, allowing for the transmission of chemical signals and the initiation of an action potential. Calcium, another type of ion, helps control the release of chemical signals into the synapse, where they can initiate an action potential. To transmit a chemical signal, calcium ion channels on the surface of a neuron open, allowing calcium ions to flow into the cell. This influx of calcium causes chemicals called neurotransmitters to be released into the synapse, where they can bind to the receiving neuron and and initiate a bioelectric action potential.
Calcium plays a role in signaling to a long chain of nerve cells. When one cell begins an action potential, the release of calcium allows that cell to release a chemical signal to its neighboring cells, initiating action potentials in those neighboring cells. This begins a domino affect, transmitting bioelectric signals from one neighbor cell to another, eventually linking millions of cells within the brain to allow for the brain's bioelectrical current.
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