In this week's journal, Lenz et al. undertake a unique set of experiments in awake humans undergoing thalamic surgery for movement disorders or chronic pain. Interestingly, they uncovered two distinct pain response pathways. The first was binary, in which painful sensations were either present or absent and had a large projected field on the body. This pathway is likely to act as a general alarm system for harmful stimuli. The second was an analog (graded) system, in which non-painful sensations--mechanical or thermal, for example--eventually gave way to pain as the stimulus increased.
2. Revisiting the GPe-STN-GPi loop in Parkinsonian monkeys: Role of external pallidal segment in primate Parkinsonism: comparison of the effects of MPTP-induced Parkinsonism and lesions of the external pallidal segment
Soares et al. now compare MPTP-induced parkinsonism with specific Gpe lesions in monkeys to reassess the GPe-STN-GPi loop. After MPTP, activity in GPe decreased and increased in STN and GPi. However, GPe lesions did not induce parkinsonism. While decreased activity of the GPe may be necessary for the induction of parkinsonism, it does not appear to be sufficient. The authors suggest that changes in the pattern of firing, not just the rate, may be important.
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