Question

Suppose you stimulate an axon so that you generate an action potential at both ends at the same instant. Describe the propagation of these action potentials. What happens when they meet?

Short answer

The action potentials propagate along the axon towards each other. Upon meeting, they cease to propagate because the area of the axon's membrane where they meet is in a refractory period, making it impossible or difficult to initiate another action potential.

Step-by-step solution

Understanding Action Potentials

An action potential is a self-propagating wave of electrochemical activity that travels along the membrane of a neuron. It is initiated when the membrane potential (the electrochemical gradient across the neuron's membrane) reaches a certain threshold. This results in a rapid influx of positively charged sodium ions, causing the interior of the neuron to become positively charged relative to the outside.

Propagation of Action Potentials

In this scenario, action potentials are initiated simultaneously at both the ends of the axon. Each action potential will travel along the axon towards the opposite end. This is due to the local currents generated by the influx of sodium ions, which depolarize the next, adjacent region of the membrane and prompt it to produce its own action potential.

Interaction of the Action Potentials

As the action potentials travel towards each other, the question arises about what happens when they meet. Neurons display a refractory period following each action potential, during which it is either impossible (absolute refractory period) or difficult (relative refractory period) for the neuron to initiate another action potential. Thus, when two action potentials meet, they do not pass through each other. Instead, they stop progressing because the section of the axon membrane where they meet is in the refractory period and therefore cannot depolarize again.

Key concepts

Action Potential

Action potentials are crucial for the functioning of neurons. They are electrical impulses that allow neurons to communicate with each other. These impulses are generated when a neuron receives a stimulus strong enough to reach a threshold level, resulting in a change in electrical charge. This change triggers the opening of sodium channels on the neuron's membrane.
The influx of sodium ions into the cell causes the inside to become positively charged compared to the outside. This shift is known as depolarization. After reaching a peak, the sodium channels close, and potassium channels open, allowing potassium ions to exit the cell and restoring the negative charge inside. This repolarization phase is essential for resetting the neuron's membrane potential. The action potential travels along the axon by depolarizing adjacent sections of the membrane, creating a wave of electrical activity that moves down the neuron.

Axon

The axon is a vital part of a neuron, responsible for transmitting the action potential from the cell body to the axon terminals. Structurally, it is a long, thin extension that ensures fast communication over long distances within the nervous system.
Axons are often covered by a myelin sheath, a fatty layer that serves as insulation, helping increase the speed of impulse conduction. This myelin allows the action potential to jump between gaps in the sheath known as nodes of Ranvier, a process called saltatory conduction.

• The axon's ability to relay signals efficiently depends on its diameter; thicker axons conduct signals faster than thinner ones.
• The axon ends in multiple branches known as axon terminals, which form connections with other neurons, muscles, or glands at synapses.

The axon's role is crucial because it allows neurons to communicate and coordinate responses throughout the body.

Membrane Potential

Membrane potential is the voltage difference across a neuron's membrane, resulting from the distribution of charged ions. It represents the potential energy needed to send a signal along the neuron.
In its resting state, a neuron's membrane potential is generally around -70 millivolts, realized by the difference in ion concentration inside and outside the neuron. This resting potential is maintained by sodium-potassium pumps that actively transport ions against their concentration gradients. When a stimulus disrupts the resting potential, causing depolarization, an action potential can be initiated if the voltage surpasses a particular threshold.

• A change in membrane potential is necessary for the propagation of action potentials.
• The membrane potential must be restored after an action potential passes to prepare for future signal transmission.

Understanding membrane potential is fundamental because it underlies the neuron's capability to generate electrical signals used in neural communication.

Refractory Period

The refractory period is a crucial concept to ensure orderly signal transmission within neurons. After an action potential occurs, the neuron enters this phase, during which it temporarily cannot generate another action potential.
This period is divided into two parts:

• **Absolute Refractory Period**: Immediately following the action potential, the neuron is unable to initiate another action potential, no matter how strong the stimulus. This is due to the inactivation of sodium channels.
• **Relative Refractory Period**: After the absolute phase, the neuron can begin another action potential, but a stronger-than-normal stimulus is required. This phase happens as potassium channels are closing and the neuron's membrane is approaching its resting state.

The refractory periods ensure action potentials only travel in one direction along the axon and help control the frequency of nerve impulses. This regulation is essential for precise and accurate neural communication.