The roles of inhibition in hierarchical processing in the auditory system and the response features of inferior colliculus neurons revealed by in vivo whole cell recordings

The inferior colliculus (IC) receives inputs, which include both excitatory and inhibitory projections, from almost all lower auditory nuclei and thus is the major integration center in the auditory pathway. This dissertation evaluated three questions in order to understand the roles of inhibition in shaping response features of IC neurons, and how the influences of inhibition in the IC compare to the roles that inhibition plays in shaping the response properties of neurons in two lower nuclei, the dorsal nucleus of the lateral lemniscus (DNLL) and intermediate nucleus of the lateral lemniscus (INLL). The first question asks what response features are expressed by cells in the DNLL and INLL, and to what degree those features are shaped by inhibition. The second question asks whether the response properties of IC cells are inherited from lower nuclei and to what degree are they created in the IC by the interactions of excitatory and inhibitory projections. To investigate these questions, I recorded single-unit activity evoked by the same set of stimuli in the three nuclei both before and while inhibition was blocked by iontophoresis of receptor blockers. The results showed that the response features of DNLL and INLL neurons are homogeneous and weakly influenced by inhibition. The response features of IC neurons, however, are heterogeneous and markedly shaped by inhibitory inputs. Thus, DNLL and INLL properties were either largely inherited or shaped by the convergence of excitatory projections, while IC response features were largely created in the IC by inhibition. The third question asks what intrinsic and synaptic features underlie sound-evoked response properties in IC neurons. These features were evaluated with in vivo whole-cell recordings. Consistent with extracellular studies, a variety of tuning features were revealed among the IC population by whole-cell recordings, but the spectral extent of subthreshold excitatory and inhibitory inputs was surprisingly broad. Particular attention was given to a subset of IC neurons with special features in synaptic responses, discharge features and their intrinsic properties. These features provide unique insights into how sound-evoked synaptic response features and intrinsic properties might interact to produce the discharge features of these novel cells.