What is the Resting Potential of a Neuron?
Polarization of Resting Neurons Prepares Them to Transmit Impulses
Apr 12, 2009
Before it is stimulated to transmit an electrical impulse, the membrane of a neuron has a slight polarization. The fluid inside the axon has a negative charge relative to the extra-cellular fluid. This polarized state is known as the resting potential. It prepares a neuron for the propagation of an action potential or nerve impulse.
The Resting Potential of a Neuron
The cell membrane of a neuron has charged particles (ions) on either side. When the neuron is at rest (not actively transmitting an impulse), the outside of the membrane has a net positive charge because it has more positive ions than negative ones. The inside has a net negative charge, because it has more negative ions than positive ones.
A membrane potential is simply a voltage that exists across a membrane due to this unequal distribution of charges. When a neuron is at rest, the resting potential is about -70 millivolts (mV), indicating that the inside is negative relative to the outside. The specific ions that are responsible for the charge difference are positive sodium and potassium ions and negatively-charged protein molecules and chloride ions.
Normally, the process of diffusion ensures a uniform distribution of particles in a medium by moving ions from areas of high concentration to areas of low concentration. The reason that the resting potential can exist is because cell membranes are impermeable and do not allow the flow of ions in and out of the cell except through ion-specific channels.
These ion channels are trans-membrane protein structures that form water-filled conduits. When they are open, particular ions can diffuse through them in response to the concentration gradient that exists between the inside and the outside of the cell membrane. When they are closed, there is no diffusion of ions across the membrane.
The Sodium-Potassium Pump
At rest, there is a higher concentration of sodium ions outside the membrane and a higher concentration of potassium ions inside the membrane. This imbalance is created and maintained by the sodium-potassium pump which actively transports ions against their concentration gradients (opposite to the diffusional force).
The pump forces two potassium ions into the cell for every three sodium ions it pumps out of the cell. This results in a net decrease in the number of positive charges inside the membrane, making it more positive. The pump uses energy in the form of ATP to do this work.
A few potassium channels remain open all of the time, allowing potassium ions to leak across the membrane. Potassium ions do not diffuse out of the cell to the extent that there is equilibrium on either side of the membrane because the diffusional force is opposed by an electrostatic force. Negatively-charged molecules inside the cell exert an attractive force on the positive potassium ions, causing most of them to remain inside the cell.
Transmission of Nerve Impulses
A nerve impulse or action potential occurs when a neuron sends information along the axon towards the axon terminal, ultimately for transmission to other neurons or effector cells. The action potential is created by a depolarizing current. The movement of sodium and potassium ions across the cell membrane passes the electrical current along the cell.
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