This study aimed to define the characteristics of the long loop reflex (LLR) in patients with spinocerebellar degeneration (SCD) and motor neuron disease (MND), and observe changes in LLR caused by thyrotropin releasing hormone (TRH), a facilitator of cerebellar and motor neurons. The markers used for LLR were: V1-2 peak Latency (the latency between the V1 and V2 peaks); V2 peak-P24 Latency (the latency between the V2 peak and P24 of a somatosensory evoked potential); V2 Amplitude, and V2 Square (the area of the V2 wave). V1-2 peak Latency was significantly longer, and V2 Amplitude was significantly lower than the control in SCD. We attributed these alterations of the LLR to cerebellar ataxia, since all SCD cases had cerebellar ataxia, and extrapyramidal symptoms were only present in one SCD case; the MND cases with motor neuron disturbance showed no significant difference from the control. TRH injection resulted in an increase in V2 Square and a decrease in V2 peak-P24 Latency in SCD and other neurological disease patients. We regarded these changes as activation of the LLR by TRH. With TRH therapy, activation of the LLR coincided with improvement of cerebellar ataxia in SCD. Symptomatic improvement, however, was not observed and the LLR changes were not stable in MND. These results suggest that TRH-induced activation of LLR is caused by the activation of cerebellar function, and indirectly concerns with upper motor neurons because V2 Square increased in MND without pyramidal tract signs.