Poster abstracts
Poster number 91 submitted by Sarah Hulbert
Interactions between corticospinal and reticulospinal outputs determine muscle response in the upper limbs and trunk
Sarah M. Hulbert (Biophysics Graduate Program), John A. Buford (Division of Physical Therapy, College of Medicine, The Ohio State University)
Abstract:
It is known that both corticospinal and reticulospinal tracts individually contribute to the stability and movement of the upper limbs and trunk. Using stimulus-triggered averaging (StimTA) of motor cortex and pontomedullary reticular formation (PMFR) outputs, we tested the hypothesis that these regions also interact to produce effects (suppression or facilitation) in upper limbs and trunk. In our paired pulse paradigm, three areas of the cortex-primary motor cortex (M1), supplementary motor area (SMA), and dorsal premotor cortex (PMd)-were electrically stimulated at varying time shifts with respect to the electrical stimulation of the PMRF during a bilateral reaching task. Specifically, three paradigms were used: 1) Cortical area stimulated before PMRF, 2) Both areas stimulated simultaneously, 3) The PMRF stimulated before the cortical area. These experiments were conducted in three non-human primates, M. fascicularis .
We studied 12 bilateral pairs of muscles in the upper limbs and trunk. By comparing EMG results from the individual stimulation of the cortex or the PMRF to EMG results from the paired pulse paradigms, we were able to identify muscle responses dominated by a single region (either cortex or PMRF) as well as responses indicative of complex interactions between the regions.
As expected, we were able to detect several instances where stimulation of a single region, usually the PMRF, resulted in a significant muscle response and that same region was shown to be responsible for the muscle responses in each of the paired pulse paradigms. One of the most prominently observed complex interactions was the gating of the cortex by the PMRF. That is, individual stimulation of the cortex revealed a significant muscle response, as did individual stimulation of the PMRF. However, in at least one of the paired pulse paradigms, the muscle response was significantly attributed to the PMRF only, indicating that signals from the PMRF negated signals coming from the cortical areas.
This shows that corticospinal and reticulospinal systems can interact to produce muscle recruitment. Therefore, a systems approach is needed to fully understand the outputs of these motor systems.
Keywords: Pontomedullary reticular formation, motor cortex, reaching, stimulus-triggered averaging