Search

The organization of facilitatory convergence from cutaneous afferents (Skin) and the corticospinal tract (pyramidal tract, Pyr) in pathways to forelimb motoneurones of mainly distal muscles was studied in anaesthetized cats by analysing postsynaptic potentials (PSPs), which were spatially facilitated by combinations of stimuli to the two sources at different time intervals. Conditioning Pyr volleys facilitated Skin-evoked PSPs of fixed (1.2-3.6 ms) central latencies (Skin PSPs), suggesting that disynaptic and polysynaptic skin reflex pathways are facilitated from the pyramidal tract. The shortest latencies (1.2-1.7 ms) of pyramidal facilitation suggested direct connection of pyramidal fibres with last order neurones of skin reflex pathways. Conditioning Skin volleys facilitated Pyr-evoked PSPs of fixed, mostly disynaptic latencies (1.0-2.5 ms; Pyr PSPs), suggesting that pyramido-motoneuronal pathways are facilitated from Skin at a premotoneuronal level. The shortest pathway from skin afferents to the premotor neurones appeared to be monosynaptic. Although Pyr and Skin volleys were mutually facilitating, the facilitation curve of Pyr PSPs and that of Skin PSPs were discontinuous to each other, with the peak facilitation at different Skin-Pyr volley intervals. Transection of the dorsal column (DC) at the C5/C6 border had little effect on the latencies or amplitudes evoked by maximal stimulation and the pyramidal facilitation of Skin PSPs. In contrast, the facilitation of Pyr PSPs by Skin stimulation was greatly decreased after the DC transection, and the facilitation curve of Pyr PSPs was continuous to that of Skin PSPs, with no separate peak. Latencies of Pyr PSPs ranged similarly to those in DC intact preparations. More rostral DC transection (C4/C5 border) reduced Skin-facilitated Pyr excitatory PSPs (EPSPs) less than C5/C6 lesions, suggesting that the C5 segment also contains neurones mediating Skin-facilitated Pyr EPSPs. The results show that convergence from skin afferents and the corticospinal tract occurs at premotor pathways of different cervical segments. We suggest that corticospinal facilitation of skin reflex occurs mostly in the brachial segments and Skin facilitation of cortico-motoneuronal effects takes place largely in the rostral cervical segments and partly in the brachial segments.

This study explored the locations and input-output properties of a large population of putative premotor neurones of skin reflex pathways in the cat. These neurones, interneurones excited by forelimb skin afferents and antidromically from the T1 motor nucleus (MN) and/or the lateral funiculus (LF, C8/T1 border), termed antidromic cells, were extracellularly recorded at C6-8. Selection of this site was based on data showing that cells retrogradely HRP labelled from the T1 MN were most numerous in C6-8 and the observation that transection of LF at the C8/T1 border abolished most skin-evoked postsynaptic potentials of T1 motoneurones. Antidromic cells were located in laminae IV-V, VI and VII. The latencies of antidromic excitation ranged from 0.4 to 1.8 ms, with a tendency for laminae IV-V cells to show longer latencies than laminae VI and VII cells. Latency of skin-evoked excitation ranged from 0.6 ms (IV-V cells), 0.8 ms (VI) and 1.4 ms (VII) to greater than 5 ms. The sum of the ortho- and antidromic latencies (estimated central latency) of individual cells explained the central latencies of skin-evoked postsynaptic potentials in T1 motoneurones. Skin-evoked firing responses (average of eight to ten cells) were earliest and largest in laminae IV-V antidromic cells, and latest and smallest in lamina VII cells. The antidromic cells who received inputs from muscle afferents and descending tracts. The following three results support the suggestion that the sampled antidromic cells are mostly premotor neurones. (1) Projection to the T1 MN via LF was verified in six laminae IV-VII antidromic cells, as tested with threshold mapping for antidromic excitation. (2) Three skin-excited axons of the middle LF projected to T1 MN, as revealed by intra-axonal staining (HRP). (3) PHA-L injection in laminae I-V of C8 anterogradely labelled terminals in lamina IX and LF axons at T1. It is suggested that last-order neurones of skin reflex pathways to T1 motoneurones are widely distributed in laminae IV-VII of C6-8 and consist of a variety of neurones with different locations and input patterns.

1. The spatial patterns of reflexes elicited by localized pressure stimulation of the foot skin were analysed by recording electromyographic activities of various hindlimb muscles or muscle nerve discharges in cats anaesthetized with sodium pentobarbitone. 2. Reflex discharges evoked by stimulation of the central pad occurred mainly in physiological toe extensors located in the foot. Stimulus-response relationships of single motor units revealed characteristically wide ranges of graded response and recruitment. 3. Within the central pad, the strongest excitation was evoked from the central lobe and was distributed to extensors of all four toes. Excitation from the medial and the lateral lobes was usually asymmetrical and weaker in toe muscles of the stimulated side. It is suggested that the weakness was in part due to concomitant inhibition. 4. Stimulation of a toe pad caused marked suppression of central pad-evoked activity of toe extensors with a highly specific spatial pattern. The inhibition was strongest in extensors of its own toe, and gradually weaker in the more distant toes. Weak excitation was occasionally evoked in extensors of the most medial or lateral toes, when the most lateral or the most medial toe-pad, respectively, was stimulated. 5. A similar pattern of relfex to that from the toe pad was evoked from the claw base and the hairy toe dorsum of each digit. 6. Reflex effects, both inhibitory and excitatory, from the central and toe pads, claw bases and toe dorsum were maintained during prolonged stimuli, indicating that slowly adapting receptors contributed to these reflexes. 7. It is concluded that stimulation of localized skin areas of the foot, particularly the pads, evokes highly specialized reflexes, which may be important in controlling movements of individual digits.

Premotor neurones mediating skin reflex actions onto cat forelimb motoneurones at T1 were identified by observing their monosynaptic effects on motoneurones by means of spike-triggered averaging. Both excitatory and inhibitory premotor neurones, with mono- or polysynaptic inputs from skin afferents, were identified at C7 to rostral C8, and were found mostly in laminae V-VI. They received excitatory inputs from corticospinal and rubrospinal tract fibres.

Implantation of an HRP-pellet into motor nuclei at the T1 segment resulted in retrograde labelling of laminae V-VII neurones densely at C6-C8 in cats. Electrophysiological experiments showed the presence of neurones in this region that received inputs from skin afferents and cortico- and rubrospinal tracts, sent axons descending in the lateral funiculus, and distributed in the motor nucleus at T1. Termination in T1 motor nuclei was also verified by intra-axonal staining with HRP. The input and output properties of these neurones indicated that they can be premotor neurones of skin reflex pathways to T1 motoneurones.