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Related Experiment Video

Updated: May 31, 2026

Membrane Potentials, Synaptic Responses, Neuronal Circuitry, Neuromodulation and Muscle Histology Using the Crayfish: Student Laboratory Exercises

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Crustacean phasic and tonic motor neurons.

Andrew G Millar¹, Harold L Atwood

¹Department of Physiology, Faculty of Medicine, University of Toronto, Toronto, Ontario, M5S 1A8, Canada.

Integrative and Comparative Biology

|June 18, 2011

Summary

Phasic and tonic motor neurons in crustaceans exhibit distinct synaptic properties for locomotion. Differences in neurotransmitter release are primarily due to the exocytotic machinery's calcium sensitivity, not vesicle pool size or synapse structure.

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Published on: November 25, 2014

Area of Science:

Neuroscience
Motor Neuron Physiology
Crustacean Locomotion

Background:

Crustacean motor neurons are specialized for distinct locomotive functions.
Tonic neurons support continuous activity, while phasic neurons mediate rapid bursts.
Significant differences exist in synaptic responses between phasic and tonic neurons.

Purpose of the Study:

To investigate the factors underlying the large differences in initial neurotransmitter release between phasic and tonic motor neurons.
To identify the key mechanisms differentiating synaptic output in these specialized neurons.

Main Methods:

Electrophysiological recordings at neuromuscular junctions of crustacean motor neurons.
Analysis of synaptic parameters including quantal release, synaptic depression, and facilitation.
Comparative analysis of potential contributing factors like synapse size, active zone characteristics, and vesicle pools.

Main Results:

Phasic neurons show significantly higher quantal release per synapse and muscle fiber compared to tonic neurons.
Phasic neurons exhibit faster synaptic depression and less short-term facilitation.
Factors such as synapse size, active zone dimensions, and readily releasable vesicle pools do not fully explain the observed differences in initial transmitter output.

Conclusions:

The primary determinant of the large difference in initial neurotransmitter release between phasic and tonic motor neurons is likely the sensitivity of the exocytotic machinery to intracellular calcium.
Ongoing research is focused on elucidating the molecular underpinnings of these presynaptic terminal differences.