Acoustic responses and electrotonic properties of morphologically-defined neurons in dorsal cochlear nucleus of unanesthetized, decerebrate gerbils: An experimental and modeling study

The dorsal cochlear nucleus (DCN) is a laminated neural structure that consists of several morphologically different neuron types. The complexity of its intrinsic neural circuitry suggests that the DCN is capable of performing sophisticated signal processing. Yet little is known about the physiological characteristics such as acoustic response and electrotonic membrane properties of any particular DCN neuron type. The purpose of this study was to gain such knowledge and establish a relationship between cell physiology and morphology, which is important in understanding the DCN neural circuitry and function. To do so, intracellular single-unit recording and marking experiments were conducted on decerebrate gerbils using HRP- or neurobiotin-filled micropipettes. Intracellular responses to acoustic (tone and broadband noise bursts) and electric stimuli (depolarizing and hyperpolarizing current pulses) were recorded and associated with cell morphology. Acoustic responses were classified according to the response map scheme (type I to type V). To study the electrotonic membrane properties of DCN neurons, a compartmental neuron model was developed, from which electrotonic parameters such as membrane time constant and electrotonic dendritic length were estimated from responses to hyperpolarizing current injections.;Recorded DCN cells include fusiform, giant and cartwheel cells. The results have demonstrated a correlation between the morphology of these neurons and their acoustic responses. While most fusiform and giant cells were associated with type III and type IV unit response properties, respectively, cartwheel cells were found to have unique and much weaker acoustic responses. Some features of membrane properties were also correlated with cell morphology, but to a lesser degree. Nearly all fusiform and giant cells fired only simple action potentials and had monotonic driven rate vs. current level curve, whereas all cartwheel cells discharged complex action potentials and had nonmonotonic rate-current relationship. Results from the electrotonic study suggest that DCN neurons may not be reliably distinguished from each other by their electrotonic parameters. A statistical analysis has also shown the limits of applying electrotonic models to DCN neurons. Overall, these results have revealed novel information about the characteristics of DCN neurons, and are a step further toward our understanding of the neural circuitry and functional mechanism of the DCN.