Functional and anatomic differentiation between parvicellular and magnocellular regions of red nucleus in the monkey

Single unit recording in awake monkeys was used to search for functional differences between the two divisions of the red nucleus, and anatomical tracing of WGA-HRP was used to investigate inputs to the two divisions. We studied a total of 323 units in 4 red nuclei of two monkeys. Recording sites were identified in histological sections by the locations of lesions and the reconstruction of electrode tracks. Of the units in the RNm 98.5% discharged in high frequency bursts during movement. Only 52% showed reliable responses to somatosensory stimulation, and the responses observed were weaker than the movement-related discharge. None of the units recorded in the RNp showed strong movement-related discharge, and 51% were completely unresponsive during both motor and sensory tests. A dorsolateral group of medium-sized cells that overlaps the rostral half of the main RNm and the caudal pole of RNp appears to represent an extension of the magnocellular region. Retrograde transport of WGA-HRP indicated that some of these cells are rubrospinal neurons. Furthermore, the discharge properties of dorsolateral neurons are like the main RNm neurons, except for lower discharge rates and smaller spike amplitudes. Mouth movements are strongly represented in the dorsolateral region. Anterograde transport of WGA-HRP from the motor cortex demonstrated dense terminal label in RNp as contrasted with light label in RNm. Retrograde transport of WGA-HRP from RNm labeled many more cells in the cerebellar interpositus nucleus than in motor cortex. We concluded that input to RNm from the cerebellum is the likely source of the strong movement-related activity recorded from cells in the RNm. The absence of appreciable movement-related activity in parvicellular red nucleus provides a clear functional distinction between this division and the magnocellular division of the red nucleus.