# Biological Basis of the Code: Hippocampal/Cortical HH Channels The provided code models the electrical activity underlying the generation of action potentials in neurons, specifically within the context of the fast sodium (Na⁺) and potassium (K⁺) currents that are key to action potential dynamics. This model is based on modifications to the classical Hodgkin-Huxley (HH) framework, adapted by Traub for simulating neuronal behavior in hippocampal and cortical pyramidal neurons. Here is a breakdown of the biological concepts embedded within the code: ## Neuronal Action Potentials Action potentials are rapid, transient changes in the membrane potential of neurons, allowing for the propagation of neural signals. These are primarily driven by the coordinated opening and closing of voltage-gated ion channels on the neuron's membrane. ## Ion Channels and Their Conductances The two main types of ion channels modeled here are: - **Na⁺ Channels:** Responsible for the depolarizing phase of the action potential. They allow Na⁺ ions to diffuse into the cell, which drives the membrane potential toward the Na⁺ equilibrium potential (`ena` in the code, set to 50 mV). - **K⁺ Channels:** Responsible for the repolarization phase of the action potential. They allow K⁺ ions to exit the cell, moving the membrane potential back toward the resting state closer to the K⁺ equilibrium potential (`ek`, set to -90 mV). The code assigns conductance parameters - `gnabar` for Na⁺ and `gkbar` for K⁺ - that determine the maximum possible conductances for these channels. ## Gating Variables The opening and closing of these ion channels are governed by gating variables: - **`m, h, n`:** These variables represent the probability of channel states: - `m`: Activation of Na⁺ channels. - `h`: Inactivation of Na⁺ channels. - `n`: Activation of K⁺ channels. These gating variables transition between states based on voltage-dependent kinetics, capturing how the likelihood of the channels being open changes with the membrane potential (`v`). ## Temperature Effects The activity of ion channels can be temperature-dependent. The parameter `tadj` accounts for this by scaling the rate constants involved in gating transitions based on an assumed Q10 coefficient (set to 3). This reflects how biochemical reactions, including those controlling ion channel kinetics, accelerate with increased temperature. ## Traub Modification The model incorporates a shifted voltage convention (`vtraub`) to align with convention used in Traub's work on hippocampal pyramidal cells. This adjustment permits the code to more accurately simulate neuron-specific behavior tuned to Traub’s empirical findings in hippocampal networks. ## Functional Components The code calculates the infinite values (`m_inf`, `h_inf`, `n_inf`) and time constants (`tau_m`, `tau_h`, `tau_n`) for each gating variable based on the membrane potential. The model approximates these using exponential functions, reflecting their biologically nonlinear dynamics. ## Summary The code is an adaptation of the Hodgkin-Huxley model, focusing on accurately depicting the biophysics of hippocampal and cortical neurons by simulating how Na⁺ and K⁺ channels regulate the action potential cycle. This detailed approach allows researchers to understand and predict the electrical properties of neurons relevant to the functioning of the hippocampus, a brain region crucial for learning and memory.