The provided code is a computational model simulating the release of neurotransmitters from inner hair cells (IHCs) and the generation of action potentials in the auditory nerve fibers. These processes are critical components of the auditory pathway in the biological system. Here’s a breakdown of the biological basis of the code provided:
Inner Hair Cells (IHCs): IHCs are sensory cells located in the cochlea of the inner ear. They convert mechanical sound vibrations into electrical signals. This process involves the opening of ion channels and the influx of ions like calcium, which then leads to neurotransmitter release.
Neurotransmitter Release: The script simulates how the IHCs release neurotransmitters based on 'release probabilities'. The probabilities model the stochastic nature of neurotransmitter release at synapses. The release times are determined using these probabilities, which are pre-calculated and loaded into the simulation.
Auditory Nerve Fibers: Once neurotransmitters are released by the IHCs, they bind to receptors on the auditory nerve fibers, triggering action potentials (spikes) that propagate the auditory signal to the brain.
Ion Channels: The auditory nerve model within the script specifically mentions the insertion of K+ (potassium) and Na+ (sodium) channels into the fibers. These channels are crucial for generating and propagating action potentials. The potassium and sodium ions play key roles in creating the electrical changes necessary for nerve impulses.
Action Potentials: The code models action potential generation by setting thresholds (10 mV). Each action potential corresponds to a spike in the auditory nerve, simulated using the APCount mechanism.
Fiber Characteristics: The code assigns random lengths to the Lu and Lh parameters of the fibers, reflecting biological variability. These parameters could represent structural characteristics like the length of dendritic fields or segments of the nerve fibers that influence conduction velocity and the spatial extent of ion channels.
Synaptic Current: The code models excitatory postsynaptic currents using a double exponential function with specified time constants (0.1 and 0.3 ms), which reflect the rise and decay kinetics of synaptic currents in biological synapses. The synaptic weight and reversal potential (0 mV) are set, showing the strength and direction of ionic flow across the synapse.
Modulation of Conductance: Ascribed conductance values (gmax_k and gmax_na) in relation to fiber length Lh represent changes in ionic channel availability, assuming a constant number of channels, which influences excitability and conduction properties of the fibers.
Sound Stimulation: The script models a sound stimulus of a specific length (5 ms) and frequency (10,000 Hz), reflecting a controlled auditory environment to study how IHC behavior translates into auditory nerve signals in response to specified sound pressure levels (dB SPL).
Simulation Timeline: The script runs simulations over a precise timeline from the start to stop times, reflecting the time course of neural events following sound stimulus and neurotransmitter release.
In summary, this code encapsulates key aspects of auditory processing, from sensory transduction in inner hair cells to the generation and propagation of neural signals in auditory nerve fibers, underscoring the detailed interaction of ions, channels, and synapses in the auditory system.