In the axon model of Figure 23.50, if the thickness of the myelin sheath were increased, the propagation speed of nerve impulses would A. Increase. B. Decrease. C. Remain the same.

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FIGURE 23.50 A circuit model of nerve-impulse propagation along myelinated axons. (a) An electrical model of a myelinated axon The ion channels are located at the nodes. When they are triggered, the potential at the node changes rapidly to +40 mV. Thus this section of the axon acts like a battery with a switch.. Myelin sheath rig Conducting fluid The myelin sheath acts like the dielectric of a capacitor C between the conducting fluids inside and outside the axon. (b) Signal propagation in the myelinated axon 1. An action potential is triggered at this node; we close the switch. ww 2. Once the switch is closed, the action potential emf drives a current down the axon and charges the capacitance of the membrane. www Ion channels ww 20 H The conducting fluid within the axon acts like a resistor R. 3. When the voltage on the capacitor exceeds a threshold, it triggers an action potential at this node the next switch is closed.