Central Role of Subthreshold Currents in Myotonia

Abstract

It is generally thought that muscle excitability is almost exclusively controlled by currents responsible for generation of action potentials. We propose that smaller ion channel currents that contribute to setting the resting potential and to subthreshold fluctuations in membrane potential can also modulate excitability in important ways. These channels open at voltages more negative than the action potential threshold and are thus termed subthreshold currents. As subthreshold currents are orders of magnitude smaller than the currents responsible for the action potential, they are hard to identify and easily overlooked. Discovery of their importance in regulation of excitability opens new avenues for improved therapy for muscle channelopathies and diseases of the neuromuscular junction. ANN NEUROL 2020;87:175-183.

Figure 1:

Neuromuscular transmission in healthy and myasthenia gravis muscle. In normal muscle, the endplate potentials that trigger muscle action potentials go beyond the muscle fiber action potential threshold. This extra depolarization is known as the safety factor. During repetitive firing of the nerve, the endplate potential amplitude undergoes a normal decline in amplitude such that the safety factor decreases. However, as long as the endplate potential triggers sufficient depolarization to reach action potential threshold, there is no failure of neuromuscular transmission. On the right, myasthenia gravis is shown, in which the number of acetylcholine receptors is reduced. The safety factor is reduced such that, by the sixth endplate potential, it is insufficient to reach action potential threshold to trigger a muscle action potential. This failure of neuromuscular transmission is responsible for weakness. Note, all traces were generated using records from mouse muscle fibers. Since suprathreshold endplate potentials and action potentials cannot be recorded at the same time, the recordings in the top and bottom panels were obtained from different fibers. Action potentials in the traces were triggered by sustained injection of current rather than repeated brief depolarization as occurs during voluntary activation of muscle. The recordings in the right column were obtained from healthy wild type muscle and edited to mimic the changes seen in myasthenia gravis. For example, the endplate potential traces (which were generated from endplate current recordings) in the right column are the same as those in the left column but were scaled down 30% and the action potential stimulation was cut short to mimic failure of neuromuscular transmission.

Figure 2:

Two contributors to the depolarization that triggers myotonia. On top is an intracellular recording of action potentials from a normal mouse skeletal muscle fiber. In normal muscle, as soon as voluntary firing of muscle action potentials stops, muscle hyperpolarizes and relaxes. On the bottom are action potentials from a myotonic mouse muscle. Unlike normal muscle, there is continued firing of action potentials following cessation of voluntary firing. The cause of involuntary firing is a combination of a steadily increasing depolarization (green) such that the membrane potential does not return to the resting membrane potential (RMP, indicated by a thin gray line) between action potentials, and a transient depolarization (red) which occurs prior to each myotonic action potential.

Figure 3:

Distinct currents trigger repetitive firing during voluntary contraction versus myotonia. The top trace shows the muscle fiber’s action potentials during voluntary firing vs. myotonia. A vertical, dotted red line indicates the time at which voluntary firing ceases and myotonia begins. The bottom trace shows the depolarization type responsible for the repetitive firing of muscle action potentials. During voluntary contraction, repeated firing of the motor neuron triggers repeated endplate potentials, each of which triggers a muscle action potential. When voluntary contraction ends, motor neuron firing stops, as do endplate potentials. At this time, there is a switch in the type of current responsible for firing the muscle action potential, from endplate potentials to a combination of steady (green) and transient (red) depolarizations. The subthreshold oscillations in membrane potential in the bottom right of the figure were generated from a real recording of myotonia by erasing the part of the trace more depolarized than action potential threshold and drawing the missing part of the oscillation by hand.

Figure 4:

Block of subthreshold depolarization could treat myotonia without causing the weakness that can accompany block of fast-inactivating Na channels. The top trace shows the effects of treating myotonia by raising action potential threshold via block of fast-inactivating Na channels with mexiletine. Note that the action potentials become steadily smaller, due to reduced muscle excitability, until they fail. This is in contrast to the muscle action potentials in myasthenia gravis depicted in Fig. 1, which are normal; then suddenly disappear when the endplate potentials fail to reach threshold. While block of fast-inactivating Na⁺ channels can effectively eliminate myotonia by raising threshold above the voltage reached by subthreshold depolarizations, it runs the risk of causing failure of neuromuscular transmission, such that there is weakness. In the lower trace, myotonia is eliminated by blocking transient subthreshold depolarization. Alternately, if the steady subthreshold depolarization could be blocked it would also be possible to eliminate myotonia. As subthreshold currents play little to no role in setting action potential threshold, activation of muscle during voluntary contraction is unaffected such that there is no weakness. AP = action potential.

Figure 5:

A potential role of subthreshold currents in neuromuscular transmission. Shown on the left is failure of neuromuscular transmission occurring in a situation, such as myasthenia gravis, in which the endplate potential is not large enough to reach action potential threshold. In the absence of a subthreshold current, depression of the endplate potential causes failure of neuromuscular transmission. On the right is shown how gradual activation of a subthreshold current (shown in red) during repetitive firing of muscle might counteract depression of the endplate potential to maintain neuromuscular transmission.