In my "earlier" years in Madison, my research interests focused on understanding the circuitry of the auditory brainstem i.e. what information do cells extract from their input about the auditory environment, what mechanisms do they use for doing this, and where they then send this information. My experimental approaches for analyzing these features have utilized both physiological and anatomical methods using an in vivo anesthetized cat preparation and an in vitro rodent brain slice preparation. The primary method for recording from nerve cells utilized sharp glass microelectrode that penetrate and record intracellular voltages from single nerve cells or their axons. Such an approach in vivo allows one to present computer-controlled auditory stimuli to the animal's ears while monitoring the intracellular response of the cell to these auditory stimuli. As you might imagine, recording the intracellular response from single neurons in the brain of an animal is a rather daunting task and the intracellular recordings are often short- lived. In the slice preparation however, longer, more stable recordings, using either sharp or patch electrodes, are more easily achieved but there is no ear attached to the slice so auditory stimulation is not possible. Such an approach does, however, allow one to monitor intracellular response of cells to synaptic inputs that are driven electrically, to pharmacologically manipulate these synaptic inputs, and also to characterize the cell's membrane features, ie. what channels the cell possesses for passing charged ions across its membrane and how these influence the output of the cell. In both methods a small molecule (Biocytin) is in solution inside the microelectrode and can be injected into the cell. The Biocytin diffuses everywhere in the cell and can be made electron dense so that the physiologically characterized cell can subsequently be analyzed anatomically and categorized at both the light and electron microscopic level. More recently, together with 2 graduate students and 2 post-docs in my lab (Ed Bartlett, Brenda Hefti, Luis Populin, and Sherry Feig) I have started working on higher auditory CNS centers, namely the thalamus (medial geniculate body - MGB) and cortex, and their interactions. Again, we are using both in vivo and in vitro recording methods and light and electron microscopic techniques. In these experiments we have already uncovered some interesting and unique features of these higher order centers. As examples 1) unlike other sensory systems, the ascending sensory input to the auditory thalamus is composed of both excitatory glutamatergic and inhibitory GABAergic connections indicating a fundamental processing difference. 2) Unlike the visual and somatosensory cortices, the thalamocortical recipient layer of the auditory cortex is not populated by spiny stellate cells again indicating a fundamental difference in the cortical processing of ascending auditory information.

In summary, my research has focused on the clarification of the structure and function of the individual components making up the circuitry of the auditory CNS. In future years I intend to pursue the new work on the thalamus and cortex and its interactions with the ascending and descending auditory pathways while continuing my interest in the circuitry of the auditory brainstem and its function in auditory processing.

Recent Publications

Joris, P.X., Smith, P.H. and Yin, T.C.T. (1998) Coincidence detection in the Auditory System: 50 years after Jeffress. Neuron, 21:1235-1238.

Bartlett, E. L. and Smith, P.H. (1999) Anatomical, intrinsic, and synaptic properties of dorsal and ventral division neurons in the rat medial geniculate body. J. Neurophysiol. 81:1999-2006.

Hefti, B.J. and Smith, P.H. (2000) Anatomy, physiology and synaptic responses of layer V cells in rat auditory cortex: characterization and study of inhibition through intracellular GABAA blockade. J. Neurophysiol. 83:2626-2638..

Smith, P.H. and Populin, L. (2001) Fundamental differences in the thalamocortical recipient layer of the cat auditory and visual cortices. J. Comp. Neurol. 436:508-519.

Hefti, B.J. and Smith, P.H. (2002) Distribution and kinetic properties of GABAergic inputs to layer V pyramidal cells in rat auditory cortex. J. Assoc. Res. Otolaryngol. 4:1-16.