The biological conduction model describes the highly regulated process by which the nervous system transmits electrical signals, known as impulses, across the length of nerve cells or neurons. This rapid communication system is foundational to virtually all bodily functions, governing complex thought, sensory perception, and muscle movement. The entire process relies on precise changes in the electrical charge across the neuron’s membrane, moving the signal forward with speed and fidelity.
Setting the Stage: The Resting Potential
Before a neuron fires an electrical signal, it maintains a steady, polarized state known as the resting potential. This baseline condition establishes the necessary electrochemical gradients for rapid signal transmission. The inside of the neuron is held at an electrical charge of approximately -70 millivolts (mV) relative to the surrounding fluid.
This negative charge is established and maintained by the sodium-potassium (\(text{Na}^+/text{K}^+\)) pump, a specialized protein embedded in the cell membrane. The pump actively transports three positively charged sodium ions (\(text{Na}^+\)) out of the cell for every two potassium ions (\(text{K}^+\)) moved in, creating a net loss of positive charge inside. This action establishes high concentrations of sodium outside the neuron and high concentrations of potassium inside.
The difference in ion concentration is amplified by numerous potassium “leak” channels, which are always open. Since the membrane is far more permeable to potassium than to sodium in this resting state, potassium ions slowly diffuse out of the cell down their concentration gradient. This contributes to the negative electrical charge inside, defining the stable, polarized resting state.
Generating the Impulse: The Action Potential
The nerve impulse, or action potential, is a swift, all-or-nothing electrical event that temporarily reverses the resting membrane potential. This signal is initiated when a stimulus causes the membrane potential to rise from -70 mV toward the threshold potential, typically around -55 mV. Reaching this threshold triggers a rapid chain of events driven by voltage-gated ion channels.
The initial phase is depolarization, marked by a massive influx of positively charged sodium ions (\(text{Na}^+\)) into the cell. As the membrane reaches the threshold, voltage-gated sodium channels open, causing sodium ions to rush inward driven by both the electrical and concentration gradients. This influx of positive charge causes the membrane potential to rapidly swing up to a peak positive value, often reaching +30 mV, completing the rising phase.
Almost as quickly as they open, the voltage-gated sodium channels become inactivated, halting the flow of \(text{Na}^+\) into the cell. This inactivation marks the beginning of the repolarization phase, during which the membrane potential returns to its negative resting value. This return is facilitated by the opening of voltage-gated potassium channels, which open more slowly than the sodium channels.
Potassium ions (\(text{K}^+\)) rush out of the cell down their concentration gradient, carrying positive charge out and restoring the negative potential inside the neuron. Because the voltage-gated potassium channels often remain open slightly longer than necessary, the membrane briefly overshoots the -70 mV baseline. This brief period of hyperpolarization, where the potential may dip to -80 mV or lower, places the neuron into a refractory state. During this time, it is significantly more difficult to initiate a second action potential, ensuring the signal travels in only one direction down the axon.
Speed and Efficiency of Signal Travel
Once generated, the action potential must propagate down the length of the axon to reach the next cell. The mechanism involves the positive charge from the current action potential depolarizing the adjacent patch of membrane, triggering the next action potential in sequence. The speed at which this process occurs determines the efficiency of nervous system communication.
One method of signal transmission is continuous conduction, which occurs in axons that lack a myelin sheath. In this slower process, the action potential is regenerated sequentially at every point along the axon membrane. While reliable, this constant opening and closing of ion channels across the entire surface requires more time. Propagation speeds are modest, typically ranging from 0.5 to 10 meters per second.
A faster and more efficient mechanism is saltatory conduction, which occurs exclusively in myelinated axons. Myelin is a fatty layer of insulation wrapped around the axon by glial cells, similar to the plastic coating on an electrical wire. This sheath prevents the flow of ions across the membrane, meaning the action potential cannot be regenerated in the myelinated segments.
The signal instead “jumps” rapidly from one gap in the myelin to the next, a process named for the Latin word saltare, meaning “to leap.” These gaps, known as the Nodes of Ranvier, are the only places where the voltage-gated sodium and potassium channels are densely concentrated. The signal is regenerated only at these nodes, allowing the electrical current to travel passively and much faster through the insulated sections. This jumping propagation increases conduction speeds, often reaching 100 to 150 meters per second.
Axon diameter also influences signal speed, regardless of myelination. A larger diameter axon provides less internal resistance to the flow of the electrical current. This reduced resistance allows the charge to spread more quickly down the length of the axon. Therefore, a large, myelinated axon represents the fastest possible pathway for nervous system communication.