Chapter 12 - Section 12.4 - Study Guide - Assess Your Learning Outcomes - Page 471: 8

One action potential can trigger another in a chain reaction, constituting a nerve signal in an unmyelinated nerve fiber through a process known as saltatory conduction. This process is essential for the efficient and rapid propagation of nerve signals along the axon while preventing the signal from traveling backward to the neurosoma (cell body). However, it's important to note that saltatory conduction is more characteristic of myelinated nerve fibers, whereas unmyelinated fibers rely on continuous conduction. Here's how one action potential triggers another and how a chain reaction of action potentials constitutes a nerve signal in an unmyelinated nerve fiber: **1. Initiation of Action Potential:** - An action potential is initiated when a stimulus causes the membrane potential at the trigger zone (axon hillock) to reach the threshold level (typically around -55 to -50 mV). When the threshold is reached, voltage-gated sodium channels open, leading to the rapid depolarization of the membrane. **2. Depolarization and Propagation:** - During the rising phase of the action potential, the membrane depolarizes as sodium ions rush into the neuron, making the inside of the neuron more positive. **3. Continuous Conduction:** - In unmyelinated nerve fibers, the action potential propagates through a process called continuous conduction. - As sodium ions enter the neuron at the site of the action potential initiation, they diffuse sideways (laterally) to adjacent regions of the membrane, depolarizing those regions. - This depolarization triggers the opening of voltage-gated sodium channels in the adjacent membrane region, initiating another action potential there. - This process of depolarization and initiation of action potentials continues sequentially along the length of the unmyelinated axon. **4. Chain Reaction:** - As each action potential is generated, it triggers the next one in line. This sequential activation of adjacent membrane regions creates a chain reaction of action potentials, allowing the nerve signal to propagate along the entire length of the unmyelinated nerve fiber. **5. Refractory Period Prevents Backward Propagation:** - The refractory period, which consists of both the absolute and relative refractory periods, prevents the action potential from traveling backward toward the neurosoma. - During the absolute refractory period, the sodium channels are inactivated and cannot reopen, ensuring that the action potential can only move forward. - During the relative refractory period, the membrane potential is more negative than usual, requiring a stronger stimulus to initiate another action potential in the backward direction. - Together, these refractory periods enforce the directional flow of the nerve signal. In summary, in unmyelinated nerve fibers, one action potential triggers another through continuous conduction, where depolarization at one site initiates the next action potential in the adjacent membrane region. The refractory period ensures that the signal can only propagate in one direction, preventing it from traveling backward to the cell body. This sequential activation of action potentials constitutes a nerve signal that can be propagated along the entire length of the unmyelinated nerve fiber.

One action potential can trigger another in a chain reaction, constituting a nerve signal in an unmyelinated nerve fiber through a process known as saltatory conduction. This process is essential for the efficient and rapid propagation of nerve signals along the axon while preventing the signal from traveling backward to the neurosoma (cell body). However, it's important to note that saltatory conduction is more characteristic of myelinated nerve fibers, whereas unmyelinated fibers rely on continuous conduction. Here's how one action potential triggers another and how a chain reaction of action potentials constitutes a nerve signal in an unmyelinated nerve fiber: **1. Initiation of Action Potential:** - An action potential is initiated when a stimulus causes the membrane potential at the trigger zone (axon hillock) to reach the threshold level (typically around -55 to -50 mV). When the threshold is reached, voltage-gated sodium channels open, leading to the rapid depolarization of the membrane. **2. Depolarization and Propagation:** - During the rising phase of the action potential, the membrane depolarizes as sodium ions rush into the neuron, making the inside of the neuron more positive. **3. Continuous Conduction:** - In unmyelinated nerve fibers, the action potential propagates through a process called continuous conduction. - As sodium ions enter the neuron at the site of the action potential initiation, they diffuse sideways (laterally) to adjacent regions of the membrane, depolarizing those regions. - This depolarization triggers the opening of voltage-gated sodium channels in the adjacent membrane region, initiating another action potential there. - This process of depolarization and initiation of action potentials continues sequentially along the length of the unmyelinated axon. **4. Chain Reaction:** - As each action potential is generated, it triggers the next one in line. This sequential activation of adjacent membrane regions creates a chain reaction of action potentials, allowing the nerve signal to propagate along the entire length of the unmyelinated nerve fiber. **5. Refractory Period Prevents Backward Propagation:** - The refractory period, which consists of both the absolute and relative refractory periods, prevents the action potential from traveling backward toward the neurosoma. - During the absolute refractory period, the sodium channels are inactivated and cannot reopen, ensuring that the action potential can only move forward. - During the relative refractory period, the membrane potential is more negative than usual, requiring a stronger stimulus to initiate another action potential in the backward direction. - Together, these refractory periods enforce the directional flow of the nerve signal. In summary, in unmyelinated nerve fibers, one action potential triggers another through continuous conduction, where depolarization at one site initiates the next action potential in the adjacent membrane region. The refractory period ensures that the signal can only propagate in one direction, preventing it from traveling backward to the cell body. This sequential activation of action potentials constitutes a nerve signal that can be propagated along the entire length of the unmyelinated nerve fiber.