Separate sodium and calcium spikes in the same axon

Area of Science:

• Marine Biology
• Neurophysiology
• Animal Locomotion

Background:

• Aglantha digitale, a hydromedusa, exhibits two distinct swimming behaviors: slow, endogenous swimming for feeding and fast, predator-evoked escape swimming.
• Both swimming types involve bell contraction and water expulsion, differing in intensity and distance covered.
• Giant motor axons directly innervate the bell musculature, mediating these contractions.

Purpose of the Study:

• To investigate the neurophysiological mechanisms underlying the two distinct swimming behaviors in Aglantha digitale.
• To determine how a single set of giant motor axons can mediate two different motor outputs.

Main Methods:

• Electrophysiological recordings from giant motor axons.
• Analysis of impulse conduction properties during different swimming behaviors.

Main Results:

• Giant motor axons in Aglantha digitale can conduct two distinct types of impulses.
• Fast swimming is mediated by a rapid, Na+-dependent action potential.
• Slow swimming relies on a low-amplitude, Ca2+-dependent spike.
• This represents the first documented instance of an axon capable of dual impulse propagation modes.

Conclusions:

• The dual impulse conduction capability of Aglantha digitale's giant motor axons provides a mechanism for generating distinct swimming behaviors.
• This finding offers a physiological role for low-potential Ca2+ activation in neuronal signaling.
• The study reveals novel insights into the neural control of locomotion in marine invertebrates.