Discovery on the profile of postsynaptic potentials in human motoneurons

Postsynaptic potential (PSP) investigations on humans are based on indirect recordings since it is impossible to record directly from human motoneurons. This indirect estimation of a PSP can be acquired by recording the muscle activity with electromyography (EMG) in response to stimulation of specific sensory fibers. On the other hand, it is possible to use direct methods on in vitro animal models by injecting predetermined PSPs into regularly discharging motoneurons and evaluate their output by the methods used in human studies. In this thesis, these direct recordings from animal studies have been integrated into indirect human studies to reveal the human PSP profile more clearly. The animal data were collected from a previous research with permission. In that study, current waveforms were injected into regularly discharging hypoglossal motoneurons to mimic PSPs in rat brain slices. The motoneuron background discharge rate and PSP duration output were procured from these experiments to investigate their correlation. The duration of the PSPs were detected with a discharge-based method Peristimulus Frequencygram (PSF) since it was shown to estimate the underlying PSPs more correctly than probability-based methods, which are used widely in the literature. A significant correlation was found between the background discharge rate and the duration of EPSP and IPSPs with linear regression. Since the injected PSP duration was predetermined in these studies, the motoneuron background discharge rate that delivers the exact PSP duration was obtained from linear regression analysis. This discharge rate was thought to reflect the PSP duration more correctly in human experiments. A similar discharge rate versus PSP duration correlation plot was constructed in three EPSP and three IPSP experiments. H-reflex, transcranial magnetic stimulation (TMS) induced motor evoked potential (MEP), and masseter stretch reflex experiments were performed to study excitatory postsynaptic potential (EPSP) profiles; and Renshaw inhibition, masseter inhibition, and cutaneous silent period (CSP) experiments were used to study inhibitory postsynaptic potential (IPSP) profiles. The duration of the PSPs was also found by PSF together with cumulative sum (CUSUM) analysis to indicate the timing of stimulus-induced significant events accurately. The background discharge rate effect on the duration of PSP was found significant only in IPSP experiments. The motoneuron background discharge rate acquired from rat brain slice studies was used in human regression analyses to estimate the genuine IPSP duration in human. The estimated IPSP duration was found to be 192.10 ms in CSP, 43.94 in Renshaw, and 191.20 ms in masseter inhibition experiments. The background effect on EPSP duration was not significant, probably because obtaining the falling phase of the EPSP was a great challenge. Therefore, more experiments are necessary with rearranged stimulus intensities. The combination of the direct and indirect methods was a novel approach and suggests more understanding of synaptic connections between neurons; thus, it may procure more insight into the functioning of neuronal circuits in normal and pathological conditions.