The provided GENESIS script models the synaptic connectivity between two types of neurons in the mammalian neocortex: the Layer 2/3 regular spiking pyramidal neurons (P23RSa) and the Layer 2/3 low-threshold spiking interneurons (I23LTS). This modeling aims to simulate the synaptic transmission dynamics and structural connectivity patterns observed in biological neural circuits.

Biological Basis of the Model

Neuron Types

• P23RSa (Layer 2/3 Regular Spiking Pyramidal Neurons):

  • These are excitatory neurons characterized by pyramidal shapes and typical regular-spiking behavior.
  • They play a crucial role in the propagation of information across cortical layers and in associational cortical networks.

• I23LTS (Layer 2/3 Low-Threshold Spiking Interneurons):

  • These are inhibitory interneurons that typically exhibit low-threshold firing patterns and are involved in shaping the dynamics of excitatory-inhibitory balance in the cortex.
  • They are important for modulating the excitability of local circuits and providing feedback inhibition.

Synaptic Connections

The model encompasses two primary types of synaptic transmission:

• AMPA Receptor-Mediated Synapses:

  • The fast excitatory postsynaptic potentials (EPSPs) are mediated by AMPA receptors, which are glutamate-gated ion channels.
  • The code assigns weights and delays to mimic the speed and strength of synaptic transmission typically observed in AMPA-mediated EPSPs.

• NMDA Receptor-Mediated Synapses:

  • These synapses involve slower, voltage-dependent EPSPs mediated by NMDA receptors, which allow calcium influx and are critical for synaptic plasticity such as long-term potentiation (LTP).
  • This is crucial for the timing and integration of synaptic inputs, often relating to processes like learning and memory.

Connectivity and Synaptic Plasticity

• The script models the probability and spatial constraints of synaptic connections, simulating the biological scenario where synaptic contacts are not uniform but follow certain probabilities and spatial distributions.
• Connectivity patterns are determined using masks (boxes) that represent the spatial structure of synaptic inputs and outputs, reflecting the anatomical and functional architecture of cortical circuits.

Synaptic Transmission Dynamics

• Synaptic delays modeled in the code reflect the time it takes for action potentials to propagate from the axons of P23RSa neurons to the synapses on I23LTS neurons, simulating biological delays due to factors like axonal conduction velocity and synaptic vesicle release.
• The use of stochastic distributions (e.g., Gaussian for delays) represents the biological variability inherent in synaptic transmissions.

Weight Assignment

• The code also models synaptic weights and their decay parameters, which can be associated with synaptic strength and plasticity mechanisms in the neuron.
• Weighting functions likely simulate phenomena like synaptic scaling and pattern completion by emphasizing stronger or weaker synaptic connections within the network.

Overall, this GENESIS script simulates a biologically-relevant cortical microcircuit with detailed connectivity and synaptic response properties mirroring real-world nerve cell interactions. Through its structuring and parameterization, the model reflects critical aspects of neural physiology, particularly how neurons communicate within cortical layers.