Activation of the primary motor cortex by somatosensory stimulation in adult rats is mediated mainly by associational connections from the somatosensory cortex
Section snippets
Experimental procedures
A total of 47 Sprague–Dawley rats of either sex (body weight 200–300 g) were anaesthetized with i.p. injections of a mixture of ketamine (100 mg/kg, Cp-Pharma) and Rompun (xilasine; 8 mg/kg, Bayer AG). Supplementary doses were given at hourly intervals. Over the left hemisphere, craniotomy exposed the vibrissal muscle field in MI (see Hall and Lindholm[17]); both MI and the barrel field of SI were sometimes exposed, leaving a strip of bone between the two holes. The exposed cortex was covered with
Responses in primary motor cortex to somatosensory input
At the maximum point of neural activation in the vibrissal field of MI cortex, a total of 275 responsive and non-responsive units were isolated by electrical stimulation of the whisker-pad. The laminar distribution of all these units (Fig. 1A) corresponded to the average numerical distribution pattern of neurons in the motor cortex of rat.[38]A subpopulation of 105 neurons (38%) showing evoked activity of the types shown in Fig. 1B exhibited a bilaminar distribution in cortical depth, with
Discussion
The present data demonstrate that in anaesthetized adult rats electrical stimulation of the whisker-pad activates the trigeminal afferents so as to induce complex response patterns in a subpopulation of MI neurons. The mean latency of the first activation period of MI neurons was 11.21±3.09 ms, which is longer by 2.0–2.5 ms than the SI responses observed.[4]There are two basic possibilities concerning how trigeminal activity could be transferred to MI neurons. First, at least two thalamic nuclei
Conclusion
Taken together, the results characterize the responses in MI to somatosensory stimulation as follows:
- 1.
VL cannot relay the MI input responsible for the short-latency somatosensory activation in MI. A Po contribution in the relaying of short-latency somatosensory responses is not likely either, with regard to the results of Diamond et al.[13]
- 2.
The present study has provided data which suggest that indirect input through associations from SI to MI mediates the short-latency (11.21±3.09 ms) activation
Acknowledgements
The authors are grateful to Mrs H. Böttcher and to Mr F. Gyulai for expert histological and technical assistance, respectively. This work was supported by grants from DFG (Wo. 279/8-5 and SFB 406:TPA9) and OTKA (T/016752). T. Farkas received a DAAD stipend during his stay in Göttingen.
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