Question

Explain how action potentials are conducted by an unmyelinated axon.

Short answer

Action potentials in unmyelinated axons are conducted through a sequence of events, starting with the resting state where the membrane maintains a potential of around -70 mV. Depolarization occurs when a stimulus causes a change in membrane potential, reaching a threshold voltage and allowing Na+ ions to enter the cell, temporarily making the membrane potential positive. The action potential then propagates continuously along the axon as each adjacent membrane area becomes depolarized, opening voltage-gated Na+ channels. Repolarization follows, with voltage-gated K+ channels opening, allowing K+ ions to flow out and returning the membrane potential to its negative resting state. The refractory period during repolarization ensures that action potentials propagate unidirectionally and allows for recovery before another action potential occurs.

Step-by-step solution

Understand the resting state of the neuron

Before an action potential occurs, the neuron is at its resting state. At this stage, the electrical potential difference across the neuron's membrane, known as the resting membrane potential, is approximately -70 mV. The neuron achieves this negative charge through the active transport of potassium ions (K+) and sodium ions (Na+) using ion channels and the Na+/K+ pump, which maintains a higher concentration of K+ ions inside the cell and a higher concentration of Na+ ions outside the cell.

Depolarization: Triggering an action potential

An action potential occurs when a stimulus causes a localized change in the membrane potential, resulting in a brief depolarization of the membrane. When the membrane potential reaches a threshold voltage (around -55 mV), voltage-gated Na+ channels open, allowing Na+ ions to rush into the cell. This causes the membrane potential to rapidly become positive, up to about +40 mV.

Propagation: Spreading the action potential along an unmyelinated axon

For an action potential to be conducted along an unmyelinated axon, the depolarization must spread from one point on the membrane to the next. The influx of Na+ ions during depolarization creates a localized positive charge inside the cell. This charge difference causes localized currents to flow within and between the intracellular and extracellular spaces. As a result, the adjacent membrane area becomes depolarized, triggering more voltage-gated Na+ channels to open and initiating another action potential. In this way, the action potential propagates in a continuous manner along the entire length of the unmyelinated axon.

Repolarization: Returning to the resting state

After the depolarization phase, the neuron starts the process of repolarization, returning its membrane potential back to the resting state. Voltage-gated K+ channels open, allowing K+ ions to flow out of the neuron, which brings the membrane potential back towards its initial negative value. Meanwhile, the voltage-gated Na+ channels close, stopping the influx of Na+ ions.

Refractory period and the importance in consecutive action potentials

During repolarization, there is a brief period called the refractory period, during which the neuron is less sensitive or insensitive to new stimuli. This period ensures that action potentials propagate unidirectionally along the axon, preventing them from spreading in both directions, and allowing for a short period of recovery before the neuron can generate another action potential. In summary, action potentials in unmyelinated axons are conducted through a sequence of events: the resting state, depolarization, propagation, repolarization, and the refractory period. The continuous propagation of action potentials along an unmyelinated axon differs from that in myelinated axons, where action potentials leap between the nodes of Ranvier, a phenomenon known as saltatory conduction.

Key concepts

Neuron Resting State

In the realm of neuroscience, understanding the 'neuron resting state' is crucial for grasping how our nervous system functions. At rest, a neuron's membrane potential is typically around -70 millivolts (mV), creating a state of polarization due to an imbalance of ions inside and outside the cell. This balance is primarily maintained by the sodium-potassium pump which actively transports potassium ions (K+) into the cell while removing sodium ions (Na+). This exchange maintains a high concentration of K+ inside and a high concentration of Na+ outside, setting the stage for the neuron to become 'excitable' when appropriately stimulated.

Imagine a sprinter in the starting blocks, this is the neuron at rest - poised and ready for the race, or in this case, for the transmission of an action potential. Understanding this resting state is foundational as it is the contrasting silence before the electrical storm of nerve signal transmission.

Membrane Depolarization

The next act in this nerve performance is 'membrane depolarization.' When a neuron receives a signal, if strong enough to reach a critical threshold (around -55 mV), it triggers a fascinating molecular reaction. Voltage-gated sodium channels spring into action, opening quickly and permitting Na+ ions, which were in higher concentration outside the neuron, to flood into the cell. This sudden influx of positively charged ions shifts the membrane potential from negative to positive, often reaching up to +40 mV.

Think of this like a flash of lightning—a rapid, dramatic change in the environment. This stage is the initial spark that ignites the action potential, sending a wave of electrical energy along the neuron's axon. The process of depolarization is a critical step for the action potential to occur and serves as the starting pistol for the signal to be propagated.

Action Potential Propagation

Just as a wave travels across the ocean, 'action potential propagation' is the journey of the nerve impulse along the unmyelinated axon. As the Na+ ions enter during depolarization, they change the local electrical charge of the cell, causing nearby voltage-gated sodium channels to open in a domino effect. The action potential thus travels as a wave, one segment of the membrane after the other flipping from resting state to a depolarized state.

This sequential firing of action potentials ensures messages are transmitted over long distances across the neuron. Unlike in myelinated axons, where the action potential jumps from node to node, unmyelinated axons support a continuous, smooth wave-like propagation. Understanding this process brings clarity to how signals in the nervous system can travel without the 'insulation' provided by myelin.

Neuron Repolarization

After the charge reversal of depolarization, a neuron must somehow return to its original restful state—a process called 'neuron repolarization.' This is achieved when voltage-gated potassium channels open, allowing K+ ions to escape the cell. This outward flow of positively charged ions brings the membrane potential back down, aiming for that initial -70 mV. During this stage, sodium channels also begin to close, ceasing the entry of Na+ ions.

The restoration of the negative membrane potential is like a reset for the neuron, preparing it for the possibility of receiving and transmitting another action potential. This ebb and flow between depolarization and repolarization are fundamental to neuron communication, comparable to the inhaling and exhaling in the respiratory cycle.

Refractory Period Neuron

Lastly, the 'refractory period neuron' represents the neuron's down-time—like a recharging period after it has fired. During the refractory period, the neuron resists generating a new action potential. This phase ensures that each action potential is a distinct signal, preventing the backward flow of the impulse and enforcing a one-way transmission along the axon.

There are two types of refractory periods: The absolute refractory period, wherein a second action potential is impossible, and the relative refractory period, where a greater-than-normal stimulus is required. This mechanism is akin to a recovery phase after intense exertion, allowing the neuron to restore its ion concentrations and be ready for the next signal with the aid of the sodium-potassium pump. This refractory period is instrumental in the precise and orderly flow of neural messages.