What must be true of the concentration of potassium and sodium inside the neuron?

The sodium/potassium (Na+/K+) pump  introduces two potassium ions that were outside and expels three sodium ions that were inside the neuron. After that, the sodium ions cannot re-enter the neuron, because the membrane is not permeable to it. This means 3 positive charges are lost every time the pump works and this makes that in the outside there are more positive loads that in the interior, creating a difference of potential. When the nervous impulse reaches a neuron in a state of rest, the membrane is depolarized, and this opens the channels for sodium, thereby the polarity is inverted. At the end, the neuron must return to the initial rest potential and the membrane is repolarized, closing the channels for sodium.

Explanation:

A neuron is a type of cell found in the central nervous system that is able to receive, process, decode and send information in the form of electrical or chemical signals through the process of synapses. This allows the transmition of information to other cells in the form of nerve impulses, which translates into "instructions" for the rest of the organism. So during the synapse, this process can be of two types:

The information between neurons is transmitted by changes in the polarity of the cell membranes, due to the presence of neurotransmitters that alter this ionic concentration inside the cell. So they are responsible for depolarizing the membrane of the neuron that receives the nerve impulse, opening the channels for sodium that remained closed before. The difference in ion concentration produces a difference in the potential between the exterior of the membrane and the interior of the cell, and it is achieved by the action of the sodium/potassium (Na+/K+) pump  which introduces two potassium ions that were outside and expels three sodium ions that were inside the neuron. After that, the sodium ions cannot re-enter the neuron, because the membrane is not permeable to it. This means 3 positive charges are lost every time the pump works and this makes that in the outside there are more positive loads that in the interior, creating a difference of potential. It is said that the neuron is in rest potential, ready to receive a nervous impulse.

When the nervous impulse reaches a neuron which is in a state of rest, its membrane is depolarized, and this opens the channels for sodium, thereby the polarity is inverted. After this, the interior of the neuron reaches an electropositive value, with respect to the exterior.  Initally, the value is about -70 millivolts (negative the interior with respect to the outside).  When there is a change of potential of 120 millivolts more than the interior, the action potential has been reached and there is a transmission of the nervous impulse to the next neuron.

At the end, the neuron must return to the initial rest potential and the membrane is repolarized, closing the channels for sodium. Enzymes destroy neurotransmitters and the rest potential is reached when the sodium is expelled by the Na+/K+ pump.

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explanation: