Fast sodium channel gating supports localized and efficient axonal action potential initiation

Area of Science:

• Neuroscience
• Cellular Neuroscience
• Axonal Physiology

Background:

• Action potentials (APs) are crucial for neuronal communication.
• While AP initiation in myelinated axons is linked to high sodium channel density in the initial segment, mechanisms in unmyelinated axons remain unclear.
• Hippocampal mossy fibers are unmyelinated axons critical for memory formation.

Purpose of the Study:

• To investigate the density and gating properties of sodium channels in the proximal unmyelinated axons of mouse hippocampal mossy fibers.
• To elucidate the role of these properties in action potential initiation and propagation.

Main Methods:

• Recording of sodium currents from axonal and somatic membrane patches of adult mouse hippocampal mossy fibers.
• Computational modeling to simulate action potential initiation and propagation dynamics.

Main Results:

• Sodium channel density in the proximal axon is approximately 5 times higher than in the soma.
• Sodium channel activation and inactivation kinetics are approximately 2 times faster in the proximal axon compared to the soma.
• Modeling confirmed that fast activation localizes AP initiation to the proximal axon, and fast inactivation enhances energy efficiency during APs.

Conclusions:

• Unmyelinated mossy fiber axons possess specialized sodium channel density and gating properties.
• These adaptations facilitate robust action potential initiation and propagation with minimized current flow.
• Findings provide insights into the bioenergetics of neuronal signaling in the hippocampus.