## Biological Basis of the Code The code segment provided is part of a computational model focused on specific neural circuitry in the somatosensory cortex (SSC). It attempts to replicate biological aspects of synaptic connectivity within the neocortex, particularly focusing on excitatory and inhibitory inputs to the dendrites of cortical neurons. Here’s a breakdown of the relevant biological concepts: ### 1. **Synaptic Composition and Density** - **Excitatory and Inhibitory Synapses**: The model distinguishes between excitatory and inhibitory synapses, key player types in neural communication. Excitatory synapses increase the likelihood of neuron firing (typically utilizing glutamate as a neurotransmitter), while inhibitory synapses decrease this likelihood (often using GABA). - **Spatial Distribution**: The model specifies different densities of synapses along the proximal and distal dendrites, which is crucial because dendritic location affects the influence of synaptic inputs on neuronal output. Proximal dendrites (closer to the soma) have 62 excitatory and 33 inhibitory synapses per 100 microns, while distal dendrites (further from the soma) feature 117 excitatory and only 5 inhibitory synapses. ### 2. **Source of Synaptic Inputs** The code specifies ratios for the possible sources of excitatory inputs, which in turn reflects the diverse afferent connections typical in the SSC: - **LGN (Lateral Geniculate Nucleus)**: Represents thalamic inputs (15% of proximal, 7% of distal), crucial for sensory information relay. - **Other SSC Inputs from L4 and L6**: - **L4**: Accounts for a significant portion of the excitatory input (35% of proximal, 29% of distal), highlighting its role in thalamocortical and intracortical processing. - **L6**: Provides extensive inputs (50% of proximal, 64% of distal), reflecting its integrative functions and feedback to the thalamus. ### 3. **Incomplete Accounting of Synapses** The model acknowledges that not all synapses are accounted for, consistent with experimental findings (e.g., by Ahmed et al. 1994), pointing to additional sources of synaptic inputs that are not yet identified or characterized. ### 4. **Functional Relevance** This model aims to capture the complexity of synaptic integration in SSC neurons, crucial for sensory processing and plasticity. The variation in synapse density and input source across dendritic locations reflects the intricate balance of excitation and inhibition that shapes neuronal firing and, hence, cortical information processing. In summary, the code encapsulates a portion of the intricate synaptic architecture and functional connectivity present in the neocortical microcircuitry, an essential aspect of the SSC's role in sensory perception and processing.