Lack of Monosynaptic Corticomotoneuronal EPSPs in Rats: Disynaptic EPSPs Mediated Via Reticulospinal Neurons and Polysynaptic EPSPs Via Segmental Interneurons

Abstract

In the rat, some findings have been taken to suggest the existence of monosynaptic corticomotoneuronal (CM) connections. Because this connection is believed to be largely responsible for the ability to make independent digit movements in primates and man, it has been inferred that the monosynaptic CM connection in the rat is likewise important for skilled prehension. Comparison of intra- and extracellular recordings from forelimb motoneurons in anesthetized rats, revealed no monosynaptic CM excitatory postsynaptic potentials (EPSPs). The fastest descending excitation in forelimb motoneurons was disynaptically mediated via a corticoreticulospinal pathway and slowly conducted excitation via corticospinal fibers and segmental interneurons. The findings stress the importance of di- and trisynaptic excitatory corticofugal pathways to forelimb motoneurons in the control of skillful digit movements.

In Vivo Recordings of Bulbospinal Excitation in Adult Mouse Forelimb Motoneurons

“…For comparison, the onset of the volley has been adjusted to the horizontal line. In case of the corticospinal volley, it can be seen that the amplitude in C 2 -C 4 was virtually identical and decreased gradually in C 5 -C 7 , which suggests that there was a major termination in the caudal cervical segments similar to the rat (Alstermark et al 2004). In contrast to the rat (Alstermark et al 2004), the volley remained rather synchronized in the caudal segments C 5 -C 7 , which indicates that the corticospinal fibers are fairly homogenous with respect to size.…”

“…In case of the corticospinal volley, it can be seen that the amplitude in C 2 -C 4 was virtually identical and decreased gradually in C 5 -C 7 , which suggests that there was a major termination in the caudal cervical segments similar to the rat (Alstermark et al 2004). In contrast to the rat (Alstermark et al 2004), the volley remained rather synchronized in the caudal segments C 5 -C 7 , which indicates that the corticospinal fibers are fairly homogenous with respect to size. The conduction velocity was 8.7 Ϯ 2.4 (SD) m/s (n ϭ 10); similar velocity was recently found by Tanaka et al (2004), which is similar to the slow component in the rat (Alstermark et al 2004).…”

“…This pattern of termination is supported by labeling of corticospinal fibers in the adult mouse (T. Isa, personal communication). With respect to laminae VI-VII, the termination in the mouse differs compared with the rat, where it has been found also laterally and is larger in amplitude (Alstermark et al 2004). Figure 1C shows a recording from the cord dorsum of the descending corticospinal (top), and for comparison, the medial longitudinal fasciculus (MLF; bottom) volleys (onset indicated by filled arrows) evoked by electrical stimulation (100 A).…”

“…We have recently analyzed pyramidal excitation in the adult rat using intracellular recording from forelimb motoneurons (Alstermark et al 2004), and the work by Kuno and colleagues inspired us to attempt a similar analysis in the adult mouse. It will be shown that pyramidal stimulation evokes surprisingly weak excitation in forelimb motoneurons via the corticospinal pathway (different to rat) and that strong excitation is mediated via a fast reticulospinal pathway (similar to rat).…”

“…A major difficulty for neurophysiological investigations has been to perform intracellular recordings from motoneurons in vivo in the adult mouse. Since the work by Kuno and colleagues (Huizar et al 1975;Kuno 1976) on the electrophysiological properties of spinal motoneurons in normal and dystrophic mice, no publications using intracellular recordings in vivo can be found in PubMed.We have recently analyzed pyramidal excitation in the adult rat using intracellular recording from forelimb motoneurons (Alstermark et al 2004), and the work by Kuno and colleagues inspired us to attempt a similar analysis in the adult mouse. It will be shown that pyramidal stimulation evokes surprisingly weak excitation in forelimb motoneurons via the corticospinal pathway (different to rat) and that strong excitation is mediated via a fast reticulospinal pathway (similar to rat).…”