Dual projections of the ventromedial lamina VI and the medial lamina VII neurones in the second sacral spinal cord segment to the thalamus and the cerebellum in the cat

Projections from the lowest lumbar and sacral-caudal segments to the cerebellar cortex in the rat: An anterograde tracing study

The crossed spinocerebellar tracts originate from neurons in the basolateral part of lamina V, the sacral nuclei of Stilling and the ventrolateral part of the ventral horn of the L6 to caudal segments. The present study examined their projection areas in the cerebellar cortex by using anterograde labeling of mossy fiber terminals with biotinylated dextran in the rat. Labeled terminals were distributed bilaterally in lobules I-V of the anterior lobe. They were most abundant in the apical parts of the lateral vermis and the intermediate region of lobules Ib and IIa, and the rostral side of lobule IIb. The number of labeled terminals in lobules Ib-IIb accounted for 56% and 81%, respectively, of the total 9783 and 7045 labeled terminals. The number of labeled terminals decreased in lobules III to V. In the posterior lobe labeled terminals were distributed exclusively to lobules VIIIa and VIIIb and copula pyramidis. The present study demonstrates that spinocerebellar neurons of the sacral-caudal segments project primarily to the lateral part of lobules I and II, and less densely to lobules III-V and VIII, and copula pyramidis. The projection pattern was essentially similar to that observed in the cat.

Properties of bilateral spinocerebellar activation of cerebellar cortical neurons

We aimed to explore the cerebellar cortical inputs from two spinocerebellar pathways, the spinal border cell-component of the ventral spinocerebellar tract (SBC-VSCT) and the dorsal spinocerebellar tract (DSCT), respectively, in the sublobule C1 of the cerebellar posterior lobe. The two pathways were activated by electrical stimulation of the contralateral lateral funiculus (coLF) and the ipsilateral LF (iLF) at lower thoracic levels. Most granule cells in sublobule C1 did not respond at all but part of the granule cell population displayed high-intensity responses to either coLF or iLF stimulation. As a rule, Golgi cells and Purkinje cell simple spikes responded to input from both LFs, although Golgi cells could be more selective. In addition, a small population of granule cells responded to input from both the coLF and the iLF. However, in these cases, similarities in the temporal topography and magnitude of the responses suggested that the same axons were stimulated from the two LFs, i.e., that the axons of individual spinocerebellar neurons could be present in both funiculi. This was also confirmed for a population of spinal neurons located within known locations of SBC-VSCT neurons and dorsal horn (dh) DSCT neurons. We conclude that bilateral spinocerebellar responses can occur in cerebellar granule cells, but the VSCT and DSCT systems that provide the input can also be organized bilaterally. The implications for the traditional functional separation of VSCT and DSCT systems and the issue whether granule cells primarily integrate functionally similar information or not are discussed.

Cortico-cerebellar coherence during a precision grip task in the monkey

We studied the synchronization of single units in macaque deep cerebellar nuclei (DCN) with local field potentials (LFPs) in primary motor cortex (M1) bilaterally during performance of a precision grip task. Analysis was restricted to periods of steady holding, during which M1 oscillations are known to be strongest. Significant coherence between DCN units and M1 LFP oscillations bilaterally was seen at approximately 10-40 Hz (contralateral M1: 25/87 units; ipsilateral: 9/87 units). Averaged coherence between DCN units and contralateral M1 LFP showed a prominent approximately 17-Hz coherence peak and an average phase of approximately -pi/2 radians, implying that the DCN units fired around the time of maximal depolarization of M1 cells. The lack of a time delay between DCN and M1 activity suggests that the cerebellum and cortex may form a pair of phase coupled oscillators. Although coherence values were low (mean peak coherence, 0.018), we used a computational model to show that this probably resulted from the nonlinearity of spike generating mechanisms within the DCN. DCN unit discharge and DCN LFPs also showed significant coherence at approximately 10-40 Hz, with similarly low magnitude (mean peak coherence, 0.012). The average coherence phase was -2.5 radians for the 6- to 14-Hz range and -1.1 radians for the 17- to 41-Hz range, suggesting different frequency-specific underlying mechanisms. Finally, 4/40 pairs of simultaneously recorded DCN units showed a significant cross-correlation peak, and 16/40 pairs showed significant unit-unit coherence. The extensive oscillatory synchronization observed between cerebellum and motor cortex may have functional importance in sensorimotor processing.

State-related inhibition by GABA and glycine of transmission in Clarke's column

During the state of active sleep (AS), Clarke's column dorsal spinocerebellar tract (DSCT) neurons undergo a marked reduction in their spontaneous and excitatory amino acid (EAA)-evoked responses. The present study was performed to examine the magnitude, consistency of AS-specific suppression, and potential role of classical inhibitory amino acids GABA and glycine (GLY) in mediating this phenomenon. AS-specific suppression of DSCT neurons, expressed as the reduction in mean spontaneous firing rate during AS versus the preceding episode of wakefulness, was compared across three consecutive sleep cycles (SC), each consisting of wakefulness (W), AS, and awakening from AS (RW). Spontaneous spike rate did not differ during W or RW between SC1, SC2, and SC3. AS-specific suppression of spontaneous firing rate was found to be consistent and measured 40.3, 31.5, and 41.6% in SC1, SC2, and SC3, respectively, indicating that such inhibition is marked and stable for pharmacological analyses. Microiontophoretic experiments were performed in which the magnitude of AS-specific suppression of spontaneous spike activity was measured over three consecutive SCs: SC1-control (no drug), SC2-test (drug), and SC3-recovery (no drug). The magnitude of AS-specific suppression during SC2-test measured only 11.7 or 14.6% in the presence of GABA(A) antagonist bicuculline (BIC) or GLY antagonist strychnine (STY), respectively. Coadministration of BIC and STY abolished AS-specific suppression. AS-specific suppression of EAA-evoked DSCT spike activity was also abolished in SC2-test after BIC or STY, respectively. We conclude that GABA and GLY mediate behavioral state-specific inhibition of ascending sensory transmission via Clarke's column DSCT neurons.

Distribution of the sacral neurones of origin of the ascending spinal tracts with axons passing through the lateral funiculi of the lowermost thoracic segments: an experimental HRP study in the cat

The locations of 249 cell bodies of the ascending tract neurones in the grey matter of S1-S3 segments of the spinal cord were reconstructed by histochemical staining, after their axons (or axonal collaterals) at the level of the Thl3 segment were injected with horseradish peroxidase (HRP). In three cats in which the injections of HRP were restricted to the lateral part of the lateral funiculi (llf), about 84% of 159 retrogradely labelled cells were found on the contralateral side, while about 16% were located ipsilaterally. They were the most numerous in S2, S3 and S1 segments, respectively, and the neurones were distributed mainly in the lateral laminae I-VII, medial laminae V, VI and lamina VIII. In three other animals in which the injections of the marker were limited to the dorsal part of the lateral funiculi (dlf), 84 of the 90 ascending tract neurones were found to be distributed in the S2 and S3 segments both ipsi- (lateral laminae III-V) and contralaterally, (lateral laminae IV and V as well as the medial laminae VII and VIII) in similar numbers. The remaining six of the 90 cells with only contralateral projections at the dorsolateral funiculus at Thl3 were scattered within the S1 segment. These data are consistent with the results of studies on sacral spinocerebellar, spinothalamic and spinoreticular projections, as well as the localization of sacral spinocervical and priopriospinal neurones. They may also imply the importance of the bilateral fiber course of the neurones of origin of ascending tracts in the S2 and S3 segments within the dorsolateral funiculus.

Spinoreticular neurons in the second sacral segment of the feline spinal cord

In continuation of previous electrophysiological studies on the location of ascending tract neurones within the second sacral segment of the feline spinal cord, the spinoreticular projections of these neurones have been investigated. Following electrical stimulation of the axonal terminals of 37 spinoreticular neurons via a tungsten electrode placed stereotactically in the contralateral nucleus reticularis gigantocellularis, antidromic potentials from their cell bodies were recorded with glass microelectrodes both extra- and intracellularly. The axons of these neurones were additionally excited from the dorsolateral funiculi of the contralateral (n = 37) and ipsilateral (n = 30) side at the lowermost thoracic spinal level. The latencies of antidromic excitation from the brainstem to the second sacral segment ranged from 3.2 to 11.8 ms (mean, 5.9 ms), whereas the corresponding axonal conduction velocities were between 27.1 and 100 m/s. The neurones examined in this study were found to be situated in the medial lamina VII of Rexed and the area adjacent to the central canal (n = 13), the medial lamina VIII (n = 12), medial laminae V and VI (n = 10) and in laminae II and III (n = 2). Three medium-sized (40-60 microm) of triangular- or oval-shaped neurones were visualized in medial laminae VII and VIII following the intracellular labelling with horseradish peroxidase.

Origin, course, and laterality of spinocerebellar axons in the North American opossum, Didelphis virginiana

Spinocerebellar axons have been studied extensively in placental mammals, but there have been no full reports on their origin, laterality, or spinal course in any marsupial. We have used the North American opossum (Didelphis virginiana) to obtain such information and to ask whether any spinocerebellar neurons innervate both the anterior and posterior lobes of the cerebellum through axonal collaterals. To identify spinal neurons that project to the cerebellum, we employed the retrograde transport of Fluoro-Gold (FG) from the anterior lobe, the main target of spinocerebellar axons. In some cases, cerebellar injections of FG were combined with hemisections of the rostral cervical or midthoracic spinal cord, so that laterality of spinocerebellar connections could be established. To determine whether single neurons project to both the anterior lobe and the posterior lobe, injections of Fast Blue (FB) into the anterior lobe were combined with injections of Diamidino yellow (DY) or rhodamine B dextran (RBD) into the posterior lobe, or vice versa. Following injections of FG into the anterior lobe, neurons were labeled throughout the length of the spinal cord, which differed in laminar distribution and laterality of their projections. Among other areas, neurons were labeled in the central cervical nucleus, the nucleus centrobasalis, Clarke's nucleus, the dorsal horn dorsal spinocerebellar tract area, the spinal border region, and Stilling's nucleus. When anterior lobe injections of FB were combined with injections of RBD or DY into the posterior lobe, or vice versa, some double-labeled neurons were present in all major spinocerebellar groups. Cerebellar injections of FG also retrogradely labeled spinocerebellar axons, allowing us to document their locations in the gray matter as well as within the periphery of the lateral and ventral funiculi at all spinal levels. A few spinocerebellar axons also were found in the dorsal funiculus (a dorsal column-spinocerebellar tract), which appeared to originate from neurons in the dorsal part of Clarke's nucleus from the ninth thoracic segment to the first lumbar segment. Our results indicate that spinocerebellar axons in the marsupial opossum are generally comparable in origin, course, and laterality to the same axons in the placental mammals studied to date.

Lamina VII and VIII neurons of the S2 segment bilaterally projecting to the C6 segment of the spinal cord in the cat

Intracellular and extracellular recordings of antidromic action potentials were applied to investigate neurons of the S2 segment projecting to the C6 segment of the cat spinal cord. The cell bodies were located in laminae VII and VIII of the gray matter while axons ascended in lateral funiculi. Thirty-two out of the total 45 neurons were found to project to the C6 segment bilaterally, seven ipsilaterally and six contralaterally. The axonal conduction velocities were in the 42-96 m/s range and in some neurons were significantly lower in distal parts of axons, supposing that some neurons may give off collateral branches to various segments of the spinal cord. It is discussed if the investigated neurons form a part of the propriospinal system or if their cervical projections are only collaterals of long tracts ascending to supraspinal levels. The organisation of the presented connections between spinal enlargements indicates their contribution in complex mechanisms of co-ordination of movements of the limbs.