An electrical impulse, scientifically known as an action potential, is the fundamental method of communication within the nervous system. This rapid, temporary change in the electrical voltage across a neuron’s membrane allows information to be transmitted over long distances with remarkable speed and fidelity. The entire nervous system relies on these swift signals to transfer sensory input, motor commands, and cognitive data. Understanding this impulse involves examining the neuron’s preparatory state, the mechanism of the signal itself, and how it moves and connects to the next cell.
The Resting State of a Neuron
Before a signal can fire, the neuron must maintain a state of readiness called the resting membrane potential, which is typically around -70 millivolts (mV). The negative sign indicates that the inside of the neuron is slightly more negative than the surrounding fluid. This charge difference is established by creating an imbalance of electrically charged particles, or ions, across the cell membrane.
A high concentration of sodium ions (\(\text{Na}^+\)) is kept outside the cell, while potassium ions (\(\text{K}^+\)) are maintained inside. The cell membrane is more permeable to potassium, allowing some \(\text{K}^+\) to leak out, which contributes to the negative charge inside. The sodium-potassium pump actively works to preserve this ion imbalance by using energy to continuously exchange ions across the membrane, moving three \(\text{Na}^+\) ions out for every two \(\text{K}^+\) ions it brings in.
Generating the Signal: The Action Potential
The electrical impulse is generated when the neuron’s membrane potential rapidly shifts from its negative resting state to a positive charge and then quickly returns to negative. This process begins when the neuron receives enough stimulation to reach a specific voltage threshold, often around -55 mV. This threshold acts as a trigger, ensuring the action potential is an “all-or-nothing” event.
Once the threshold is reached, voltage-gated sodium channels snap open, initiating the depolarization phase. Since sodium is highly concentrated outside the cell, positive \(\text{Na}^+\) ions rush into the neuron, causing the internal voltage to spike dramatically and become positive, reaching a peak of about +30 mV. This rapid influx of positive charge is the electrical impulse itself.
Almost as quickly as the sodium channels open, they become inactivated, and the repolarization phase begins. During repolarization, voltage-gated potassium channels open, allowing positive \(\text{K}^+\) ions to flow rapidly out of the cell. This outflow of positive charge causes the membrane potential to drop back down toward its negative resting value. The slow closing of potassium channels often leads to a brief hyperpolarization phase, which constitutes the refractory period, ensuring the signal travels in only one direction down the neuron.
How the Signal Travels
The action potential is a wave of electrical change that must travel the entire length of the neuron’s long extension, the axon. The movement of the impulse along the axon is called propagation, where the electrical change at one point triggers the same event in the adjacent segment of the membrane. The speed of this transmission is greatly increased in axons that are wrapped in a fatty insulating layer called the myelin sheath.
The myelin sheath prevents the current from leaking out and allows the signal to travel much faster. This insulation is not continuous; it is broken up by tiny, uninsulated gaps known as the Nodes of Ranvier. Instead of the impulse having to regenerate continuously along the entire axon, the signal appears to jump from one node to the next, a process termed saltatory conduction.
The voltage-gated sodium channels are highly concentrated only at these uninsulated nodes. The electrical signal travels quickly and passively through the insulated segments, and then the full action potential is regenerated only at each node. This hopping mechanism significantly boosts the speed of signal transmission, allowing impulses to move at speeds up to 150 meters per second.
Communication Between Cells
Once the electrical impulse reaches the end of the axon, it must transmit its message to the next cell at a specialized junction called the synapse. The electrical signal cannot simply jump the small gap, or synaptic cleft, between the two cells.
The arrival of the action potential at the axon terminal triggers a conversion from an electrical signal to a chemical one. This electrical change causes the release of chemical messengers known as neurotransmitters, which are stored in tiny sacs within the terminal. The neurotransmitters diffuse across the synaptic cleft and bind to specific receptor proteins on the membrane of the receiving cell.
This binding causes a change in the receiving cell’s membrane, such as opening ion channels. The resulting change in the receiving cell’s voltage determines whether the signal is passed along, causing it to fire its own action potential, or whether the signal is inhibited. After delivery, the neurotransmitters are rapidly cleared from the synapse, ensuring that the next impulse can be sent cleanly.