Question

1) Is myelin continuously being made?

2) can you come up with a good analogy for the time course of the action potential?

Answer 1

1) the myelin sheath is a multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction. Yes it is covering layer for some axon and being contineously made.

2) Action potential (AP) An electrical signal that propagates along the membrane of a neuron or muscle
fiber (cell); a rapid change in membrane potential that involves a depolarization followed by a repolarization. Also called a nerve action potential or nerve impulse as it relates to a neuron, and a muscle action potential as it relates to a muscle fiber.

An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state. An action potential
has two main phases: a depolarizing phase and a repolarizing phase (Figure 1). During the depolarizing phase, the negative membrane potential becomes less negative, reaches
zero, and then becomes positive. During the repolarizing phase, the membrane potential is restored to the resting state of -70 mV. Following the repolarizing phase there may be an after-hyperpolarizing phase, during which the membrane potential temporarily becomes more negative than the resting level.
Two types of voltage-gated channels open and then close during an action potential. These channels are present mainly in the axon plasma membrane and axon terminals. The first channels that open, the voltage-gated Na+ channels, allow Na+ to rush into the cell, which causes the depolarizing phase. Then voltage-gated K+ channels open, allowing K+ to flow out, which produces the
repolarizing phase. The after-hyperpolarizing phase occurs when the voltage-gated K+ channels remain open after the repolarizing phase ends.
An action potential occurs in the membrane of the axon of a neuron when depolarization reaches a certain level termed the threshold (about -55 mV in many neurons). Different neurons may have different thresholds for generation of an action potential, but the threshold in a particular neuron usually is constant. The generation of an action potential depends on whether a particular stimulus is able to bring the membrane potential to threshold. An action potential will not occur in response to a subthreshold stimulus, a stimulus that is a weak depolarization that cannot bring the membrane potential to threshold (Figure 2).

However, an action potential will occur in response to athreshold stimulus, a stimulus that is just strong enough to depolarize the membrane to threshold (Figure 2). Several action potentials will form in response to a suprathreshold stimulus, a stimulus that is strong enough to depolarize the membrane above threshold (Figure 2). Each of the action potentials caused by a suprathreshold stimulus has the same amplitude (size) as an action potential caused by a threshold stimulus. Therefore, once an action potential is generated, the amplitude of an action potential is always the same and does not depend on stimulus intensity. Instead, the greater the stimulus strength above threshold, the greater the frequency of the action potentials until a maximum frequency is reached as determined by the absolute refractory period .