THE RESTING MEMBRANE POTENTIAL, Exams of Physiology

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THE RESTING MEMBRANE

POTENTIAL

• Our goal is to understand
• (1) how the membrane potential is established, and
• (2) how the membrane potential is maintained.

• The resting membrane potential of a cell is the electrochemical state of a cell (neuron) at rest, i.e the membrane permeability at rest.
• It is the transmembrane potential of the resting cell-at which there is no net movement of a particular ion across the cell membrane.
• It is the potential that would be maintained if there were no action potentials, synaptic potentials,or other active changes in the membrane potential.
• The resting membrane potential refers to a situation in which the cell is at rest and no perturbations have been done to change the potential.
• We will examine all of these ideas in this discussion.

What is the value of the normal resting membrane potential?

• The value of the membrane potential varies from cell to cell; depending on the cell type and ranges from about -20 mV to - mV.
• For example, in a typical neuron, its value is -70 mV, in a typical skeletal muscle cell,.its value is -90 mV, and in most other mammalian cells, the membrane potential is around -50 mV (-0. V).
• In most cells the resting potential has a negative value, which by convention means that there is excess negative charge inside compared to outside.
• In other words, the resting potential has the inside of the neuron 70mV more negative than the extracellular space (V stands for voltage, which is a measure of potential or the difference in electric charge between two places).

• All cell membranes produce electrical signals by ion movements, but transmembrane potential is particularly important to neurons, because rapid changes in the membrane potential of neurons bring about the nervous impulse , which is the basis of neuronal signaling.
• In muscle cells, changes in the membrane potential bring about contraction.
• In endocrine cells, changes in the membrane potential bring about release of hormones.
• Details of this will be given in the Physiology section of the NEUROSCIENCE SECTION IN BLOCK 3.

• Importance of resting potential
• The cell’s ability to fire an action potential is due to the cell’s ability to maintain the cellular resting potential at approximately - 70 mV (-.07 volt).
• The basic signaling properties of neurons are determined by changes in the resting potential.
• This is the basis of cell to cell communication ( Chemical synapses and the rest.)

• It is important to keep in mind that the potential difference occurs at the level of the cell membrane.
• This is because biological membranes can act to allow separation of electrical charge, by separating solutions in two compartments by the very short-distance, non-conducting, hydrophobic core of the membrane (=3 nm).
• Charge separation across the membrane leads to an electric field across the membrane.
• This electric field gives rise to the measured membrane potential.

• RECALL that…..
• The inside of the cell is called the intracellular space. The outside of the cell is called the extracellular space. The cell membrane is made of two layers of fat (called lipid bilayer ),and particles can not pass through this lipid bilayer. (Figure 3.3). However, ion transport proteins in the membrane form channels for particles (Figure 3.6 & 3.7). These channels can open and close which allow particles to enter and leave the cell.

• For determination of membrane potentials, the two most important types of membrane ion transport proteins are ion channels and ion pumps.
• Ion channel proteins create paths across cell membranes through which ions can pass.
• They have selectivity for certain ions, thus, there are potassium, chloride-, and sodium-selective ion channels.
• Different cells and even different parts of one cell (dendrites, cell bodies, nodes of Ranvier) have different amounts of various ion transport proteins.
• Typically, the amount of certain potassium channels is most important for control of the resting potential (as we will see).
• Some ion pumps such as the Na+/K+ATPase are electrogenic, that is, they produce charge imbalance across the cell membrane and can also contribute to the membrane potential.

Ion Channels are…

• a class of integral proteins that span the cell membrane
• permit transient current flow and thereby facilitate depolarization,or hyperpolarization of the cell membrane.
• Ion channels in the plasma membrane of neurons and muscles contributes to their excitability.
• When open, ions move down their concentration gradients

• Given that the membrane is permeable only to certain ions (e.g., K+), these forces create what is called a resting potential.
• [Potential means a separation of charge. In this case the separation is across the membrane.Resting means that no current is flowing across the membrane.]
• These simple principles create the resting potential:
• 1. Concentration gradient moves ions from high to low concentration 2) Electrical force moves ions with same charge away from each other and ions with opposite charge towards each other. 3) Na+/K+ pump moves Na+ to the extracellular space. It can't get back in because the membrane is impermeable to Na+. 4) The membrane is permeable to K+ (i.e., it flows either way) 5) Large negatively charged proteins (A-) are stuck inside the neuron

• Na+ and K+ are driven intracellularly to A- by the electrical force.
• The concentration gradient pushes Na+ intracellularly and K+ extracellularly.
• The Na+/K+ pump moves Na+ extracellularly
• Because of this, the extracellular space has lots of Na+ and ends up relatively positive compared to the intracellular space where there are lots of A-.

There is an excess of positive charges outside and negative charges inside the membrane.This gives rise to an electrical potential difference, which ranges from about 60 to 70 mV. This potential difference is maintained because the lipid bilayer acts as a barrier to the diffusion of ions

Resting Potential

Figure 12–8 (Navigator)