Could an action potential produce few or more neurotransmitters based on the stimulus received?

Question

I reckon that if you would be able to widen the AP width, it would produce more neurotransmitters in that larger time interval. Is that correct? Or does the neuron have a standard amount of neurotransmitters it releases in order to pass the signal post-synaptically?

(bonus: could it NOT result in neurotransmitter release, or does the definition of an AP include the release of neurotransmitters?)

Answer 1

Short answer

• Widening of the action potential increases neurotransmitter release;
• Generally, an action potential results in the release of about one vesicle of neurotranmitters;
• An action potential does not have to lead to neurotransmitter release - the chance being anywhere between 9 - 100%, depending on the synapse under investigation.
  
  

Background

Neurotransmitters are released in packets, called neurotransmitter vesicles:

^{Source: Science blogs}

Vesicles are released via a mechanism involving Ca²⁺. When an action potential reaches the synaptic terminal, voltage-gated Ca²⁺ channels open, which leads to Ca²⁺ influx. The increased Ca²⁺ in the terminal leads to release of vesicles.

A study by Sabatini & Regehr (1997) showed that widening of the presynaptic action potential in rat cerebellar cells resulted in enhanced Ca²⁺ influx, which in turn greatly enhanced neurotranmitter release.

According to a study in rat hippocampal cells it was shown that, on average, a single action potential releases approximately 0.5% of the vesicle pool from the presynaptic terminal. The presynaptic vesicle pool was estimated to be 100 - 200 vesicles, which hence added up to 0.5 - 1 vesicle released per action potential (Ryan & Smith, 1995).

An earlier study of Rosenmund et al. (1993) confirms these findings in hippocampal neurons, and investigated the variability between different synapses in greater detail. They found that the chance that a single action potential released neurotranmitters was 9 - 54%. They reasoned that larger synaptic terminals, or terminals triggered by long-term potentiation have a higher chance of releasing neurotransmitter. Moreover, such terminals with enhanced release probability may also have a higher chance of releasing multiple vesicles per action potential. They predicted that 54% of release events from such enhanced terminals may release multiple vesicles per action potentials, whereas just 9% of normal terminals may do this.

References
- Rosenmund et al. Science 1993; 262: 754-6
- Ryan & Smith, Neuron 1995; 14: 983-9
- Sabatini & Regehr, J Neurosci 1997; 17: 3425-35

Answer 2

Neurons encode the "largness" of the stimulus in firing frequency. Neurotransmitters are stored in vesicules near the end of the axon. It has been shown that neurotransmitter release follows Poisson-distribution and that usually a single "packet" (quantum) is released - this is known as quantal release. Although the actual number of molecules in a single vesicle vary a bit, and ocasionally multiple vesicles may be emptied, it is accepted that for a single AP a neuron releases a single vesicle. These two pages can give you more info and details: http://neuroscience.uth.tmc.edu/s1/chapter04.html http://neuroscience.uth.tmc.edu/s1/chapter05.html

Also this link provides introductionary info on the topic

A great image from :http://onlinelibrary.wiley.com/doi/10.1113/jphysiol.2006.123224/full