A neuron, or nerve cell, is the fundamental unit of the nervous system that transmits information using electrical signals. This capability relies on polarization, a state where the neuron maintains an electrical charge difference across its outer membrane. This electrical potential is a maintained state of readiness, much like a charged battery, allowing the cell to send signals with speed and precision.
The Polarized Resting State
When a neuron is not sending a signal, it is in a polarized resting state. This state is defined by the resting membrane potential, a stable charge of approximately -70 millivolts (mV) where the inside of the cell is negative compared to the outside. The potential is created and maintained by the specific distribution of ions across the membrane.
The primary ions involved are positively charged sodium (Na+) and potassium (K+), along with large, negatively charged proteins inside the neuron. The cell maintains a high concentration of Na+ outside and a high concentration of K+ inside. This imbalance is actively managed by the sodium-potassium pump, which uses energy to shuttle three Na+ ions out for every two K+ ions it brings in.
The neuron’s membrane has selective permeability due to “leaky” potassium channels that are always open. These channels allow K+ ions to diffuse out of the cell, moving down their concentration gradient. As these positive ions exit, they leave behind negatively charged proteins, making the cell’s interior more negative and establishing the resting potential.
Generating an Electrical Signal
An action potential is generated when a stimulus causes the neuron’s membrane potential to become less negative and reach a threshold of around -55 mV. Once this threshold is crossed, the neuron fires an action potential in an all-or-none fashion. This means the signal is fired at its full intensity or not at all.
The first phase of an action potential is depolarization. Upon reaching the threshold, voltage-gated sodium channels open, and Na+ ions flood into the cell. This influx of positive ions causes a rapid reversal of the membrane’s polarity, with the inside of the neuron reaching a positive charge of about +30 mV to +40 mV.
This state is short-lived, as the sodium channels quickly close and inactivate. Next, voltage-gated potassium channels open, initiating the repolarization phase. K+ ions rush out of the cell, driven by their concentration gradient and the positive internal charge, which restores the negative charge inside the neuron.
The membrane potential may briefly become more negative than its resting state, a phase called hyperpolarization, because the potassium channels are slow to close. Afterward, the sodium-potassium pump re-establishes the original ion concentrations. This returns the neuron to its polarized resting state, ready for the next signal.
How Neuron Structure Enables Signaling
A neuron’s structure is specialized for signaling. The primary receivers are the dendrites, tree-like extensions from the cell body covered in receptors. These receptors detect incoming signals from other neurons and convert them into small electrical changes that travel toward the cell body.
If the cumulative signals trigger an action potential, the signal is transmitted down the axon. The axon is a long projection that acts as the neuron’s transmission cable, carrying the impulse away from the cell body. The action potential propagates as a self-sustaining wave, with the depolarization of one segment triggering the next, ensuring the signal travels without weakening.
To increase transmission speed, many axons are wrapped in an insulating layer called the myelin sheath. This sheath has small gaps called the Nodes of Ranvier. This structure allows the action potential to “jump” from one node to the next in a process called saltatory conduction, which significantly speeds up signal transmission compared to unmyelinated axons.
Disruptions to Neuron Polarization
Disruptions to the precise control of neuron polarization can have profound consequences. Various substances and conditions can interfere with the balance of ions and the function of channels, preventing neurons from firing correctly.
An example is tetrodotoxin, a neurotoxin found in pufferfish that physically blocks voltage-gated sodium channels. By preventing the influx of Na+ ions, it makes it impossible for neurons to depolarize and fire action potentials. This leads to a paralysis of muscles, including those required for breathing, which can be fatal.
Local anesthetics like lidocaine also block sodium channels, but in a temporary and localized manner. When applied to a specific area, lidocaine prevents pain-sensing neurons from transmitting signals to the brain. This produces the numbing effect used for medical procedures.
Certain diseases also disrupt neuron polarization. In multiple sclerosis (MS), the immune system attacks and destroys the myelin sheath surrounding axons in the central nervous system. This demyelination impairs saltatory conduction, slowing or blocking the transmission of nerve signals and causing widespread neurological symptoms.