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Notes: Summary The morphology of single C3–C4 propriospinal neurones (PNs) including the cell body, dendritic tree, axonal trajectory and the pattern of projection and termination of axonal collaterals in the C3–C4 segments was investigated by intra-somatic or intra-axonal injection of horseradish peroxidase. All the C3–C4 PNs could be antidromically activated from the lateral funicle in C6 and the lateral reticular nucleus but not from Th13. Another criterion was that they received monosynaptic excitation from corticospinal fibres in the contralateral pyramid. Twenty-four C3–C4 PNs were successfully stained. They were located in the lateral part of laminae VI–VIII except for two neurones which were located in lamina V and two in lamina IX. Five to eleven dendrites originated from the cell bodies and extended throughout laminae IV–VIII and even into the white matter in the transverse plane and up to 3 mm rostro-caudally. The axonal trajectory from the cell body was usually curved before reaching the lateral funicle. The bifurcation of the stem axon into a descending and an ascending branch was mostly observed in the white matter close to or at the border between the white and grey matter at the level of the cell body. The ascending and descending axonal branches maintained their location in the same part of the lateral funicle. Sixteen out of 24 stem axons gave off collaterals in the grey matter and/or in the white matter. One to five collaterals were given off from the axons in the grey matter. Boutons were found in a restricted region in the intermediate zone from lamina VI to the border between laminae VII and VIII, in the lateral part of laminae V–IX, in the middle and medial parts of laminae VI–VIII. The termination in the vicinity of large neurones in lamina VIII suggests that long PNs receive collateral projections from the C3–C4 PNs. The finding that some collaterals terminated laterally in lamina IX is in agreement with electrophysiological observations that spinal accessory motoneurones receive disynaptic pyramidal excitation which is mediated via C3–C4 PNs. The collateral projection from the C3–C4 PNs to lateral and medial regions in laminae VI-VII is discussed in relation to feed-forward and feed-back inhibitory control of the C3–C4 PNs.

Notes: Abstract Trajectory formation of unrestrained forelimb target-reaching was investigated in relation to the effect of a change in target location. Sagittal displacement of the target (6 cm in each direction) gave a selective change of velocity in the x direction (protraction) with an increase or decrease at larger and shorter distances, respectively. In the case of a double-peaked x velocity profile, the change was mainly with respect to the first major component. The shape of the y (sideways) and of the z (lifting) velocity profiles were both almost unchanged, but the onset of the movement in the z direction changed with the x distance. Vertical displacement (4 cm up or 5 cm down) gave increased velocity in the z direction (lifting) when the target was above the normal mid-position and decreased velocity when the target was lower. The velocity was changed with constant rate of rise, so that the rise time increased when the target was elevated and shortened when the target was lowered (pulse width control policy). The change in the z velocity was not selective. In cats with a double-peaked x velocity profile, the second component decreased when the target was elevated and increased when it was lowered. With excessive lowering of the target (14 cm down), the first x velocity component was very much reduced in amplitude so that protraction depended mainly on the second x velocity component. In the cat with a unimodal x velocity profile, a second component appeared in the x and net velocity profiles when the target was excessively lowered. The velocity profile in the y direction changed when the target was lowered so that the horizontal movement path became straighter. Sideways displacement (10–13 cm) produced adduction/abduction, with only moderate changes in x and z velocity profiles. The results are discussed with reference to the angular movements in the elbow and shoulder.

Notes: Summary Collateralization and termination of single C3-C4 propriospinal neurones (PNs) have been studied in the C6-Th1 segments of the cat using two methods: threshold mapping for antidromic activation of C3-C4 PNs and intra-axonal injection of horseradish peroxidase. Low threshold points for antidromic activation of C3-C4 PNs were found in the region of different motor nuclei in lamina IX both at one level and at different segmental levels, in all parts of lamina VII, in the lateral part of lamina VI and in the dorsal and ventral parts of lamina VIII. Collaterals were found from C6 to Th1. A marked decrease of conduction velocity of the stem axon occurred in the caudal region of termination, while it was almost constant in the rostral region of termination. HRP was injected iontophoretically in C6-Th1 into stem axons of neurones, which were activated antidromically from the ventral part of the lateral funiculus in C5/C6, from the lateral reticular nucleus (LRN) and monosynaptically from the corticospinal fibres (stimulated in the contralateral pyramid) which were transected in C5/C6. Reconstruction of successfully stained stem axons, revealed collaterals with terminals on presumed motoneurones in different parts of lamina IX and on interneurones in laminae IV–VIII. These findings confirm previous results which showed monosynaptic projections from C3-C4 PNs to forelimb motoneurones and Ia inhibitory interneurones. With respect to termination in the region of the motoneurones in lamina IX and in the region of Ia inhibitory interneurones in lamina VII, three patterns were found: 1) termination mainly in lamina IX (n=1) 2) termination in laminae IX and VII (n=15) and 3) termination mainly in lamina VII (n=2). However, in some cases the same stem axon gave off collaterals which terminated either on motoneurones in lamina IX or on presumed Ia inhibitory interneurones in lamina VII. Furthermore, when the stem axons had collaterals which terminated in different motor nuclei only some of these collaterals had additional terminations on presumed Ia inhibitory interneurones. This result suggest that C3-C4 PNs do not follow a strict Ia pattern of reciprocal innervation. It is tentatively proposed that the difference of innervation may be related to the type of multi-joint movement, such as target-reaching with the forelimb, which has been shown to be controlled by the C3-C4 PNs. Termination in laminae VI, VIII and different parts of lamina VII indicates that C3-C4 PNs also project to other types of neurones than motoneurones and Ia inhibitory interneurones. Injection of wheat germ agglutinated horseradish peroxidase (WGA-HRP) laterally in laminae VI-VII in C3 and C4 caused anterograde labelling of axonal bundles from neurones in these segments. Labelled axons were found mainly in the lateral funiculus with the highest density in the ventral part. These axons could be traced throughout the forelimb segments and also to the LRN.

Notes: Abstract Trajectory formation of unrestrained forelimb target-reaching was investigated in six cats. A Selspotlike recording system was used for three-dimensional recording of the position of the wrist every 3 ms with the aid of two cameras detecting infrared light emitted from diodes taped to the wrist. These measurements allowed reconstruction of movement paths in the horizontal and sagittal planes and velocity profiles in the direction of the cartesian x, y and z co-ordinates. Horizontal movement paths were smoothly curved, segmented or almost linear. Sagittal movement paths were sigmoid. The net velocity profile was usually bell-shaped with longer deceleration than acceleration, but for some slow movements the velocity profile had a plateau. When the net velocity profile was bell-shaped, the averaged sagittal movement paths and normalized x (protraction) and z (lifting) velocity profiles were virtually superimposable for fast and slow movements: thus, movement speed was changed by parallel scaling of protraction and lifting. Comparison of movement paths and velocity profiles amongst the different cats revealed considerable differences. The ż profile was unimodal in one cat and double peaked in five cats: the second component was pronounced in two cats and small in the other three. The ż profile was unimodal and, except for one cat, it had later onset and summit than the first component of the x profile. In contrast to the interindividual differences, there was a high degree of intraindividual constancy over 6–12 months. It is postulated that the interindividual variability depends on chance differences established early during learning of the task and that the imprinted pattern remains, resulting in intra-individual constancy.

Notes: Summary 1. The effect of stimulating the contralateral pyramid has been investigated with intracellular recording from 128 long propriospinal neurones (long PNs) in the C3-Th1 segments of the cat. Long PNs were identified by the antidromic activation from the Th13 segment. They were located in laminae VII–VIII of Rexed. Single pyramidal stimulation evoked monosynaptic EPSPs in 15/40 of the long PNs in cats with intact pyramid. In 15 other long PNs, a train of three to four pyramidal stimuli evoked EPSPs with latencies indicating a minimal disynaptic linkage. The remaining 25% of the long PNs lacked mono- or disynaptic pyramidal EPSPs. In a few cases longer latency excitation was observed. 2. The location of the intercalated neurones which mediate the disynaptic pyramidal EPSPs was investigated by making four different lesions of the corticofugal fibres: 1) at the border of the C5 and C6 segments, 2) at the border of the C2 and C3 segments, 3) at the caudal part of the pyramid; three mm rostral to the decussation and 4) at the level of the trapezoid body. Stimulation of the corticofugal fibres was made either rostral to lesion 3 (rPyr) in order to activate neurones in a cortico-bulbospinal pathway or caudal to lesion 3 (cPyr) to activate neurones in a corticospinal pathway. In the former case, in one experiment, stimulation was made in the pyramid between lesions 3 and 4 (double pyramidal lesion). In case of cPyr stimulation, lesions 1 and 2 were added sequentially in order to investigate if the corticospinal excitation was mediated via C3–C4 PNs. All lesions were made mechanically, except lesion 2 which in some of the experiments was performed by reversible cooling. 3. Stimulation in the pyramid rostral to lesion 3 and in between lesions 3 and 4 evoked disynaptic EPSPs in the long PNs, which shows that they were mediated via reticulospinal neurones. Stimulation in cPyr after lesion 3 elicited disynaptic EPSPs, which remained after lesion 1 but were abolished after adding lesion 2. It is concluded that the disynaptic cPyr EPSPs were mediated via intercalated neurones in the C3–C4 segments. 4. When the disynaptic cPyr EPSP was conditioned with a single volley in nucleus ruber and/or in tectum, it was markedly facilitated, especially when the conditioned volley was applied simultaneously with the effective cPyr volley. The results show that the intercalated neurones in the C3–C4 segments receive monosynaptic convergence from cortico-, rubro- and tectospinal] fibres. Stimulation in the lateral reticular nucleus (LRN) evoked monosynaptic EPSPs. These EPSPs had similar latencies and shapes as those previously recorded in forelimb motoneurones and which have been shown to be due to activation of ascending branches of the C3–C4 PNs. This finding in addition to the striking similarity of the descending input pattern of long PNs as compared to the forelimb motoneurones strongly suggest that short C3–C4 PNs project both to long PNs as well as to forelimb motoneurones. 5. Spatial facilitation of disynaptic EPSPs in long PNs was also observed between rPyr volleys and tectal volleys. The results suggest that common reticulospinal neurones which project to the long PNs receive monosynaptic convergence from corticofugal and tectofugal fibres but in some of the reticulospinal neurones the main input is cortical and in others tectal. Monosynaptic EPSPs were evoked from the medial part of the reticular formation, from 2 mm caudal to 6 mm rostral of the obex level. These EPSPs were presumably due to direct activation of reticulospinal neurones. 6. Convergence of disynaptic excitation mediated by cortico-propriospinal and cortico-reticulospinal routes was observed in about 12% of the long PNs. Convergence of monosynaptic corticospinal and disynaptic corticoreticulospinal and/or cortico-propriospinal input was observed in about 15% of the long PNs. 7. The role of the monosynaptic pyramidal input and disynaptic corticoreticulospinal and cortico-propriospinal (mediated by short C3–C4 PNs) inputs to long PNs is discussed in relation to postural control during movements of head and forelimb.

Notes: Summary Excitation of dorsal neck motoneurones evoked by electrical stimulation of primary trigeminal afferents in the Gasserian ganglion has been investigated with intracellular recording from α-motoneurones in the cat. Single stimulation in the Gasserian ganglion ipsi-and contralateral to the recording side evoked excitatory postsynaptic potentials (EPSPs) in motoneurones innervating the lateral head flexor muscle splenius (SPL) and the head elevator muscles biventer cervicis and complexus (BCC). The gasserian EPSPs were composed of early and late components which gave the EPSPs a hump-like shape. A short train of stimuli, consisting of two to three volleys, evoked temporal facilitation of both the early and late EPSP components. The latencies of the gasserian EPSPs ranged from 1.6 to 3.6 ms in SPL motoneurones and from 1.6 to 5.8 ms among BCC motoneurones. A rather similar latency distribution between 1.6 and 2.4 ms was found for ipsi- and contralateral EPSPs in SPL and BCC motoneurones, which is compatible with a minimal disynaptic linkage between primary trigeminal afferents and neck motoneurones. Systematic transections of the ipsi- and contralateral trigeminal tracts were performed in the brain stem between 3 and 12 mm rostral to the level of obex. The results demonstrate that both the ipsi- and contralateral disynaptic and late gasserian EPSPs can be mediated via trigeminospinal neurones which take their origin in the nucleus trigeminalis spinalis oralis. Transection of the midline showed that the contralateral trigeminospinal neurones cross in the brain stem. Systematic tracking in and around the ipsilateral trigeminal nuclei demonstrated that the axons of ipsilateral trigeminospinal neurones descend just medial to and/or in the medial part of the nucleus. Spinal cord lesions revealed a location of the axons of the ipsilateral trigeminospinal neurones in the lateral and ventral funiculi. Interaction between the ipsi- and contralateral gasserian EPSPs showed complete summation of the disynaptic EPSP component, while the late components were occluded by about 45%. These results show that the disynaptic EPSPs are mediated by separate trigeminospinal neurones from the ipsi- and contralateral side, while about half of the late EPSPs are mediated by common neurones which receive strong bilateral excitation from commissural neurones in the trigeminal nuclei. Spatial facilitation was found in the late gasserian EPSP but not in the disynaptic gasserian EPSP by conditioning stimulation of cortico- and tectofugal fibres. Disynaptic pyramidal and tectal EPSPs, which are mediated by reticulospinal neurones, were facilitated by a single stimulation in the gasserian ganglion at an optimal interval of 2 ms. It is suggested that primary trigeminal afferents can excite the reticulospinal neurones via a disynaptic trigeminoreticular pathway.