Question

How does myelination affect the propagation of an action potential? a. It speeds propagation by increasing the density of voltage-gated channels all along the axon. b. It slows propagation by increasing electrochemical gradients favoring Na \(^{+}\) entry. c. It speeds propagation by preventing cations from leaking out across the membrane as they spread down the axon. d. It slows propagation by restricting voltage-gated \(\mathrm{Na}^{+}\) channels to gaps in the myelin (nodes of Ranvier)

Short answer

Myelination speeds up the propagation of an action potential by preventing cations from leaking out across the membrane as they spread down the axon. The correct answer is (c).

Step-by-step solution

Understand Myelination

Myelination is a process in which a protective layer called the myelin sheath wraps around the axon of a neuron. This increases the speed of impulse transmission along the axon. Step 2: Understanding Action Potential

Understand Action Potential

Action potential is the temporary change in electrical potential across the cell membrane of a neuron, in response to a stimulus. It is a way in which electrical signals, or nerve impulses, are transmitted along the axon of the neuron. Step 3: Review the Statements and Match the Correct One a. Incorrect: Myelination does speed up propagation but not by increasing the density of voltage-gated channels all along the axon. b. Incorrect: Myelination speeds up propagation, not slowing it down. It also doesn't increase electrochemical gradients favoring Na+ entry. c. Correct: Myelination speeds up propagation by preventing the cations from leaking out across the membrane, enabling faster transmission of action potentials down the axon. d. Incorrect: While the statement about restricting voltage-gated Na+ channels to the gaps in myelin (nodes of Ranvier) is correct, this does not slow down propagation but rather speeds it up. Step 4: Conclusion

Conclude and Choose the Correct Option

After understanding the process of myelination and its impact on action potential propagation, we can determine that the correct answer is (c): Myelination speeds up the propagation of an action potential by preventing cations from leaking out across the membrane as they spread down the axon.

Key concepts

Neural Impulse Transmission

Neural impulse transmission is the process by which neurons communicate with each other. At the core of this transmission is the action potential, an electrical pulse that travels along the neuron's axon. The action potential is initiated when the neuron receives a strong enough signal, which causes a sudden influx of sodium (Na+) ions into the neuron. This influx changes the electric charge across the neuron's membrane, creating the action potential. The signal then travels down the axon until it reaches the axon terminal, where it triggers the release of neurotransmitters that can influence other neurons.

The speed and efficiency of neural impulse transmission are critical for the proper functioning of the nervous system. Delays or disruptions in this process can affect everything from muscle coordination to cognitive abilities. For example, in neurological conditions like multiple sclerosis, impaired impulse transmission due to the loss of myelination can lead to symptoms ranging from physical weakness to difficulties in speech.

Axon Myelination

Axon myelination is the process by which myelin, a fatty substance, coats the axons of certain neural cells. This myelination has a profound impact on the speed and efficiency of neural impulse transmission.

The myelin sheath acts as an insulator, allowing the action potential to 'jump' across the axon from one node of Ranvier to the next in a process called 'saltatory conduction'. This is much faster than the 'walking' pace that would occur if the entire axon was non-myelinated. Myelination also conserves energy for the neuron since the voltage-gated channels only need to open at the nodes of Ranvier, rather than all along the axon.

• Saltatory conduction increases the speed of impulse transmission
• Myelin conserves energy for the neuron and increases transmission efficiency

Understanding the importance of myelination helps explain why certain diseases that affect the myelin sheath can have such debilitating effects on nervous system function.

Nodes of Ranvier

The nodes of Ranvier are tiny gaps in the myelin sheath along the axon of a neuron. These gaps are crucial for the process of rapid signal transmission.

Despite accounting for a small fraction of the axon's length, they are strategically placed to boost efficiency. At each node, the axon membrane is exposed to the extracellular fluid, allowing for ions to flow in and out, thus regenerating the action potential. Without these nodes, the action potential would weaken as it travels down the axon. The nodes serve as 'recharging stations' that ensure the signal remains strong enough to reach its destination.

• Nodes of Ranvier are essential for maintaining the strength of the action potential
• They facilitate the rapid transmission of electrical signals along myelinated axons

This ingenious setup ensures that the nervous system can transmit information quickly and with a high degree of fidelity, allowing for complex behaviors and rapid responses to stimuli.

Electrochemical Gradients

Electrochemical gradients are the basis for the neural impulse transmission. They describe the difference in concentration and charge of ions across a cell membrane.

In the context of neurons, these gradients drive the action potentials. Sodium (Na+) and potassium (K+) ions have concentrated gradients across the neuronal membrane, maintained by the sodium-potassium pump -- a complex protein that actively transports these ions to keep the gradient. When a neuron is stimulated, Na+ channels open, allowing Na+ ions to rush into the neuron, following their electrochemical gradient. This influx of positive charge depolarizes the neuron's membrane, leading to the propagation of an action potential. Later, K+ channels open, allowing K+ to exit the neuron and repolarize the membrane, thus restoring the resting state.

• Electrochemical gradients enable the movement of ions that generate action potentials
• The sodium-potassium pump maintains these gradients
• Ions flow in and out of the neuron in response to changes in membrane potential

The manipulation of these gradients is a fundamental aspect of neuronal function and is critical for proper neural communication.