Related Experiment Videos
Structural features of crayfish phasic and tonic neuromuscular terminals
M J King1, H L Atwood, C K Govind
1Life Sciences Division, Scarborough College, University of Toronto, Ontario, Canada.
The Journal of Comparative Neurology
|September 2, 1996
Summary
Phasic crayfish axon terminals release more transmitter due to larger active zones and synaptic complexity, not synapse number. These structural differences explain the high transmitter output of phasic terminals.
Area of Science:
- Neuroscience
- Cell Biology
- Muscle Physiology
Background:
- Phasic and tonic excitatory axons innervate crayfish limb extensor muscles.
- Phasic terminals exhibit significantly higher transmitter release per unit length than tonic terminals.
Purpose of the Study:
- To investigate the fine structural differences between phasic and tonic excitatory axon terminals in crayfish.
- To correlate structural characteristics with the known differences in transmitter release.
Main Methods:
- Comparative analysis of axon terminal fine structure using horseradish peroxidase (HRP) labeling.
- Quantitative assessment of mitochondrial volume, synaptic vesicle density, synapse number, active zone dimensions, and synaptic complexity.
Main Results:
- Phasic terminals are thinner with single unbranched mitochondria, while tonic terminals are thicker with multibranched mitochondria.
- Tonic terminals have a greater overall synaptic volume and more synapses/active zones per terminal length.
- Phasic synapses show higher proportions of multiple active zones and longer individual active zones, suggesting a role in greater transmitter release.
Conclusions:
- Synapse and active zone number do not account for the higher transmitter output of phasic terminals.
- Active zone size and synaptic complexity (multiple active zones) are likely causal factors in the enhanced transmitter release of phasic terminals.
- Additional factors, potentially electrical and biochemical, may also contribute to the large differences in transmitter output.