The physiological and functional organization of medial geniculate body (MGB) has been one of the key issues in the understanding of central auditory system. Recent studies have led to the theory that physiologically and functionally auditory thalamus can be subdivided into lemniscal (primary) and nonlemniscal (secondary) pathways. However, it has been lacked of direct recording of acoustic evoked responses in unit neuron and the corresponding relationship between MGB neuronal memberane response patterns and anatomical locations to approve the hypothesis.
To demonstrate the membrane response patterns and frequency tuning properties in
MGB neurons and further investigate the functional contradistinction of lemniscal and nonlemniscal systems, we examined the’ neuronal auditory response features to noise and tonal stimuli and labelled the target MGB neurons by in vivo intracellular recordings in anaesthetized guinea' pigs. We correlated the response patterns, the tuning properties and the anatomical locations of MGB neurons. The main finds are showed below:
1)Neurons with either a single EPSP or spike response pattern to repeated noise stimuli located in lemniscal MGB, while neurons with sustained or burst responses were mostly located in nonlemniscal MGB. Neurons with inhibitory response patterns to acoustic stimuli were located in nonlemniscal MGB. ON, OFF/ON-OFF firing neurons were located in the central lemniscal, outside or around the border of lemnical MGB.
2)Frequency tuning curves in lemniscal neurons is sharp and specific while those in nonlemniscal neurons broad and irregular.
Based on both the results of the present study and extensive studies, our result confirmed that the lemniscal MGB reserved the representation of frequency consistent with the tonotopic organization of the basilar membrane and precisely kept the response time and tuning property to acoustic signals. Multiple discharge patterns in nonlemniscal neurons indicated that diverse encoding mechanisms exist in the complex sensory information processing nuclei. Tuning characteristics to tonal stimuli suggested that nonlemniscal neurons may selectively switch off this pathway, avoid the influence of less important information and prepare for more biological significant information.
The present study has both important theoretic and practical significance. For the theoretic significance; studies in the physiological property and function of MGB directly affect auditory and other high-level sensory processes; as to the practical significance,research in the response mechanisms of auditory nucleus helps us to understand and analyze biological behavior and has important application value in rehabilitation medication and clinical practice.