Effects of volleys in cortico-spinal tract fibres on ventral spino-cerebellar tract cells in the cat

Do spinocerebellar neurones forward information on spinal actions of neurones in the feline red nucleus?

We recently demonstrated that feline ventral spinocerebellar tract (VSCT) neurones monitor descending commands for voluntary movements initiated by pyramidal tract (PT) neurones as well as locomotor movements relayed by reticulospinal (RS) neurones. The aim of the present study was to examine whether VSCT neurones likewise monitor descending commands from the red nucleus (RN). Extracellular records from the spinal border (SB) subpopulation of VSCT neurons revealed that a third (31%) of SB neurones may be discharged by trains of stimuli applied in the RN. Moreover, when RN stimuli failed to discharge SB neurones they facilitated activation of some of these neurones by RS and/or PT neurones, while activation of other SB neurones was depressed. We propose that the facilitation and depression of actions of RS neurones by RN neurones might serve to reflect a higher or lower excitability of motoneurones and therefore a likely higher or lower efficacy of the RS descending commands, prompting the cerebellum to adjust the activation of reticulospinal neurones. Activation of SB neurones by RN stimuli alone would also allow monitoring and adjusting the RN descending commands. Intracellular records from SB neurones revealed both monosynaptic and disynaptic EPSPs and disynaptic IPSPs evoked by RN stimuli. The disynaptic actions remained following transection of axons of reticulospinal neurones within the medullary longitudinal fascicle (MLF) and were therefore taken to be relayed primarily by spinal neurones, in contrast to EPSPs and IPSPs evoked by PT stimuli found to be relayed by reticulospinal rather than spinal neurones.

Processing information related to centrally initiated locomotor and voluntary movements by feline spinocerebellar neurones

Feed-back information on centrally initiated movements is processed at both supraspinal and spinal levels and is forwarded by a variety of neurones. The aim of the present study was to examine how descending commands relayed by reticulospinal neurones are monitored by a population of spinocerebellar tract neurones. Our main question was whether a spinal border (SB) subpopulation of ventral spinocerebellar tract (VSCT) neurones monitor actions of reticulospinal neurones with input from the mesencephalic locomotor region (MLR) as well as from pyramidal tract (PT) neurones. In the majority of intracellularly recorded SB neurons, stimuli applied in the MLR and in the medullary pyramids evoked EPSPs in parallel with EPSPs evoked by stimulation of axons of reticulospinal neurones in the medial longitudinal fascicle (MLF). In extracellularly recorded neurones short trains of stimuli applied in the ipsilateral and contralateral pyramids potently facilitated discharges evoked from the MLF, as well as EPSPs recorded intracellularly. In both cases the facilitation involved the disynaptic but not the monosynaptic actions. These results indicate that reticulospinal neurones activating SB neurones (or more generally VSCT neurones) are co-excited by axon-collaterals of other reticulospinal neurones and by fibres stimulated within the MLR and PTs. The study leads to the conclusion that these spinocerebellar neurones monitor descending commands for centrally initiated voluntary as well as locomotor movements relayed by reticulospinal neurones. Thereby they may provide the cerebellum with feed-back information on the likely outcome of these commands and any corrections needed to avoid errors in the issuing movements.

A trans-spinal loop between neurones in the reticular formation and in the cerebellum

Voluntary limb movements are initiated in the brain but the neurones responsible for activating the muscles (motoneurones and interneurones) are located in the spinal cord. The spinal cord also contains neurones that provide the brain, and especially the cerebellum, with continuous information on effects of the descending commands. We show that one population of such neurones provide the cerebellum with information on how likely the brain's commands (mediated by descending reticulospinal neurones) are to be executed as planned, depending on the degree of inhibition of motoneurones. They may therefore play an important role in preventing errors in activation of motoneurones and thereby help the brain to correct its signals to the spinal cord before such errors have been committed.

Collateral actions of premotor interneurons on ventral spinocerebellar tract neurons in the cat

Strong evidence that premotor interneurons provide ventral spinocerebellar tract (VSCT) neurons with feedback information on their actions on motoneurons was previously found for Ia inhibitory interneurons and Renshaw cells, while indications for similar actions of other premotor interneurons were weaker and indirect. Therefore the aim of the present study was to reexamine this possibility with respect to interneurons relaying actions of group Ib afferents from tendon organs and group II afferents from muscle spindles. In all, 133 VSCT neurons in the L3-L5 segments (including 41 spinal border neurons) were recorded from intracellularly in deeply anesthetized cats to verify that stimuli applied in motor nuclei evoked monosynaptic inhibitory postsynaptic potentials (IPSPs) attributable to stimulation of axon collaterals of premotor interneurons. IPSPs were found in over two thirds of the investigated neurons. When intraspinal stimuli were preceded by stimuli applied to a muscle nerve at critical intervals, IPSPs evoked from motor nuclei were considerably reduced, indicating a collision of nerve volleys in axons of interneurons activated by group I and group II afferents. In individual VSCT neurons monosynaptic IPSPs were evoked from both biceps-semitendinosus and gastrocnemius-soleus motor nuclei, in parallel with disynaptic IPSPs from group Ib and group II as well as group Ia afferents. These observations indicate that individual VSCT neurons may monitor the degree of inhibition of both flexor and extensor motoneurons by premotor interneurons in inhibitory pathways from group Ib and group II afferents to motoneurons. They may thus be providing the cerebellum with feedback information on actions of these premotor interneurons on motoneurons.

The influence of ulnar nerve blockade on skin microvascular blood flow

Microvascular research is seriously hampered by the great temporal and spatial variability of the measured skin blood flow and variation in sympathetic vasomotor reflexes within and between persons. Therefore skin vasomotor reflexes were studied before and after ulnar nerve blockade within the same person, resulting in a temporal complete denervation of the fifth finger and partial denervation of the fourth finger. Skin temperature and laser Doppler flux (LDF) were registrated to measure predominantly arteriovenous shuntflow. Measurements were performed on the palmar tip of the second and fifth finger in nine healthy volunteers, at baseline, and during a sympathetic reflex test (i.e. inspiratory gasp) and postural response test. Beat-to-beat digital blood pressure was recorded from the third and fourth finger by a Finapres device. Baseline capillary blood cell velocity (CBV) was measured at the nailfold of the second and the fifth finger. After ulnar blockade baseline skin temperature, LDF and CBV increased significantly, with respectively (mean +/- SE) 3.2 +/- 0.9 degrees C, 20.9 +/- 5.9 relative perfusion units and 0.79 +/- 0.40 mm-1 s. The percentage LDF decrease of the fifth finger during inspiratory gasp was 48.2 +/- 5.3% before and 3.1 +/- 0.9% after blockade. The postural response test showed a decrease in LDF of the fifth finger with no significant difference before and after blockade, respectively 12.3 +/- 14.7% and 8.0 +/- 2.7%, while no difference was found in the increase in digital blood pressure in the denervated fourth finger compared to both the same finger before blockade and to the third non-blocked finger.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological studies of thoracic spinocerebellar tract neurons in relation to respiratory movement

The differential roles of thoracic spinocerebellar tract (SCT) neurons with axons ascending in ipsi- (uncrossed) and contralaterally (crossed) spinal cord and their activities during respiratory movement were examined by extracellular recordings in the T9-T12 spinal segments of the anaesthetized cat. A total of 36 uncrossed and 7 crossed SCT neurons showed rhythmic discharges in relation to either spontaneous or artificial respiration. Uncrossed neurons were located in and around Clarke's column and thus are cells of origin of the dorsal spinocerebellar tract (DSCT). Crossed neurons were located in laminae VII and VIII. Almost all DSCT neurons were modulated during artificial respiration. Nineteen DSCT neurons showed high-frequency discharges during chest expansion and 15 DSCT neurons showed high-frequency discharges during the chest retraction phase of artificial respiration. Their respiration-related activity maintained the same phase relation and firing patterns after vagotomy. The phase relationship of neural rhythmicity to chest movement during artificial respiration and spontaneous breathing was the same in 14 neurons examined. Artificially induced pneumothorax caused a marked decrease of respiration-related modulation, and severing of a single nerve to the appropriate muscle caused a marked decrease of modulation, suggesting that respiration-related rhythmic activity of DSCT neurons is induced by the extension of respiratory muscles in the chest wall during both spontaneous and artificial respiration. Crossed SCT neurons showed rhythmic activity in phase with the central respiratory rhythm as indexed by phrenic nerve activity. Two neurons received an excitatory influence and five an inhibitory influence in the inspiratory phase from the centre. Four neurons in the latter group also received excitatory inputs from the periphery during chest expansion. Since inspiration brings chest expansion during spontaneous breathing, the central and peripheral inputs seem antagonistic in function. These neurons seemed to signal a discrepancy between the chest wall movement and the central respiratory rhythm. Functional differences between uncrossed and crossed SCTs in the lower thoracic segments in regard to the central and peripheral inputs as observed here are similar to those observed in the lumbar SCTs during locomotion.

Bilateral influences from the motor cortex on lumbar ventral horn interneurons

Microelectrode recordings were made of discharges of ventral horn interneurons (VHIN) in segments L6-7 of the spinal cord during bilateral stimulation of the motor cortex in cats. The overwhelming majority of VHIN was shown to be activated by influences from the contralateral cortex, and about half of them also by ipsilateral influences. Clear correlation was established between convergence of afferent and cortical influences: Neurons with inputs from ipsilateral afferents only were activated, irrespective of their other characteristics, by descending influences from the contralateral cortex only, whereas bilateral cortical influences converged on cells with bilateral afferent connections. It is suggested that VHIN with bilateral segmental and supraspinal connections are important integrative elements in the mechanism of bilateral coordination of motor responses of different degrees of complexity.