In: Anatomy and Physiology
A resting (non-signaling) neuron has a voltage across its membrane called the resting membrane potential, or simply the resting potential. The resting potential is determined by concentration gradients of ions across the membrane and by membrane permeability to each type of ion.
In a resting neuron, there are concentration gradients across the membrane for Na+ and K+. Ions move down their gradients via channels, leading to a separation of charge that creates the resting potential.
Membrane potential is a potential gradient that forces ions to passively move in one direction: positive ions are attracted by the ‘negative’ side of the membrane and negative ions by the ‘positive’ one.
There are two things required for establishing a membrane potential: (1) ion concentration gradient across the membrane, and (2) selective ion channels in the membrane.
The concentration gradient causes the ion to move from the compartment with the higher ion concentration to the compartment with the lower ion concentration.
Na+/K+-pump is an electrogenic transmembrane ATPase located in the outer plasma membrane of cells. The Na+/K+-ATPase pumps 3 sodium ions out of cells while pumping 2 potassium ions into cells. Both cations move against their concentration gradients.
There is an ionic imbalance between the inside and outside of a cell, with K+ concentration high inside the cell and low outside, while the Na+ concentration is low inside the cell and high outside. The cell membrane is differentially permeable to K+ and Na+, being more permeable to K+ than to Na+ in the resting state. Thus, there is a continuous tendency to loose K+. To maintain this ionic gradient, the cell membrane has a Na+/K+ pump which requires a source of energy, ATP.
If there is no concentration gradient for Na+ or K+, across the plasma membrane, the ratio of the extracellular to intracellular concentration terms becomes 1. Hence, there will be no potential difference across this membrane, even if the channels for the ion were fully open and ions will diffuse according to membrane potential only.