The code provided attempts to model the biological phenomenon known as "paired-pulse facilitation" (PPF), a form of short-term synaptic plasticity observed at synapses. This process is characterized by an increased response of a postsynaptic neuron to a second stimulus following a first stimulus when the interval between the two stimuli is short, typically tens of milliseconds. Here's a breakdown of the biological basis:

Biological Phenomenon

• Paired-Pulse Facilitation (PPF): This is a form of synaptic plasticity where two consecutive synaptic stimuli result in a larger postsynaptic potential for the second pulse compared to the first. It occurs due to residual calcium remaining in the presynaptic terminal following the first pulse, which enhances neurotransmitter release during the second pulse.

Key Biological Concepts in the Code

1. Stimulation Timing (isi): The code uses an inter-stimulus interval (isi) of 50 milliseconds (0.05 in the code), which is a typical interval where PPF effects can be observed.

2. Postsynaptic Response: The references to voltage measurements (somaVavg, primVavg, secVavg, and tertVavg) suggest that the model records the membrane potential across different compartments of the neuron, such as the soma and various dendritic branches, to study the change in postsynaptic potential due to paired stimulations.

3. Presynaptic Activation (presyn_ext): The use of setfield for presyn_ext likely models the activation of an external input to the synapse located at secdend11, simulating the injection of presynaptic signals (analogous to synaptic input) at specific times.

4. Usage of Calcium Dynamics: While calcium (Ca²⁺) itself is not explicitly annotated in the code provided, the paired-pulse setup often implicates the role of calcium dynamics. Typically, PPF increases neurotransmitter release at subsequent stimulations due to the accumulation of calcium ions from the first stimulation.

Modeling Objectives

The objective appears to be to replicate the physiological conditions that lead to paired-pulse facilitation, and to capture the dynamics of synaptic efficacy changes in response to closely timed stimuli. The model likely aims to produce data reflecting the impact of inter-stimulus intervals on synaptic responses, which can be interpreted to understand the underlying mechanisms of synaptic plasticity, such as calcium dependency and neurotransmitter release probability.

Conclusion

In summary, this code models paired-pulse facilitation by applying closely timed stimuli to a synapse and measuring the resulting postsynaptic potentials. The simulation seeks to mimic the increased synaptic strength seen biologically and contributes to understanding how synaptic behaviors like facilitation and depression can affect neuronal communication and network function.