Neurons are the fundamental units of the nervous system, specialized for communication throughout the body. They transmit information rapidly and precisely. This communication relies on the generation and propagation of electrical signals.
The Neuron’s Resting State
A neuron at rest maintains a negative electrical charge inside compared to its outside, a state known as the resting membrane potential, typically around -70 millivolts (mV). This electrical difference is established by the unequal distribution of ions across the neuron’s cell membrane. Potassium ions (K+) are more concentrated inside the cell, while sodium ions (Na+) and chloride ions (Cl-) are more concentrated outside.
The membrane’s selective permeability to these ions, largely due to “leak” channels, plays a significant role in maintaining this potential. Neurons possess more potassium leak channels than sodium leak channels, allowing potassium to exit the cell more readily than sodium enters. The sodium-potassium pump actively works to maintain these concentration gradients by expelling three sodium ions from the cell for every two potassium ions it brings in, using energy in the form of ATP. This continuous action helps keep the inside of the neuron negatively charged relative to the outside.
Reaching the Activation Point
A neuron becomes activated when its membrane potential shifts sufficiently from its resting state. This change typically begins with a stimulus, which causes local alterations in the membrane potential, known as graded potentials. These graded potentials are proportional to the strength of the stimulus; a stronger stimulus results in a larger graded potential. For instance, the binding of neurotransmitters to ligand-gated ion channels on the neuron’s dendrites can lead to the movement of ions across the membrane, creating these localized changes.
If these depolarizing graded potentials, which make the inside of the cell less negative, summate and reach a specific voltage, they trigger an action potential. This critical voltage is called the “threshold potential,” typically ranging from -55 mV to -50 mV. When the membrane potential reaches this threshold, it causes voltage-gated sodium channels to open rapidly, initiating the action potential. This opening allows a sudden influx of positively charged sodium ions into the cell, which further depolarizes the membrane and pushes the neuron past its threshold.
The Action Potential: A Rapid Signal
Once the threshold potential is reached, a rapid and self-propagating electrical event, known as an action potential, begins. The first phase, called depolarization, involves a swift influx of positively charged sodium ions into the neuron through voltage-gated sodium channels that open at threshold. This rapid entry of sodium ions causes the inside of the neuron to become positively charged, often reaching around +30 mV.
Following depolarization, the repolarization phase occurs, during which the membrane potential quickly returns to a negative state. This is primarily due to the inactivation of voltage-gated sodium channels and the opening of voltage-gated potassium channels. As potassium ions rush out of the cell, the positive charge inside decreases, restoring the negative membrane potential. This sequence of events allows the electrical signal to propagate along the axon, transmitting information to other neurons or target cells.