Diane Daly Ralston

The distribution of dorsal root axons in laminae I, II and III of the macaque spinal cord: a quantitative electron microscope study.

This study examines the projection of dorsal root fibers to the upper dorsal horn of the monkey lumbar spinal cord utilizing degeneration and autoradiographic methods. The animals survived dorsal rhizotomy for periods varying from 18 hours to 28 days. Electron microscopy reveals the earliest degeneration to be neurofilamentous alteration of large synaptic profiles in lamina III and the inner zone of the substantia gelatinosa (IIi). This degeneration begins 18 hours after rhizotomy, reaches a peak at three days postoperatively and disappears by the end of the first week. Degenerating myelinated axons in the spinal gray matter, dorsal column white matter and Lissauers tract first appear three days postoperatively.

The primate dorsal spinothalamic tract: evidence for a specific termination in the posterior nuclei (Po/SG) of the thalamus.

&NA; The spinothalamic tract in primates and other mammals arises primarily from cells in lamina 1 of the dorsal horn, from lamina V cells and to a lesser extent from other laminae. Most of the neurons of lamina I respond only to noxious mechanical or thermal stimuli. Spinothalamic tract (STT) cells of lamina V tend to respond to both innocuous and noxious stimuli. Recent studies have suggested that the classical STT in the anterolateral quadrant (ALQ) contains primarily the axons of lamina V cells and that the axons of lamina I cells travel more dorsally in the dorsolateral quadrant (DLQ) to constitute the dorsal spinothalamic tract (DSTT). Using the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA‐HRP) injected into the spinal cord in conjunction with a contralateral anterolateral cordotomy, we have found there is a substantial projection of the DSTT to the posterior nuclei of the caudal‐ventral thalamus, designated Po/SG. This projection is almost entirely abolished when the lesion includes the area of spinal cord white matter at the level of the denticulate ligament. Larger lesions that destroy the ALQ and much of the lateral column white matter, but that spare the dorsolateral column white matter in the region of the corticospinal tract, abolish all transport of WGA‐HRP to the thalamus. We conclude that the spinothalamic pathway in the non‐human primate encompasses a continuous fiber bundle that extends dorsally to include the region of lateral column white matter opposite the denticulate ligament and that the more dorsal aspect of this pathway projects primarily to Po/SG of thalamus.

Anatomical evidence for red nucleus projections to motoneuronal cell groups in the spinal cord of the monkey

In 4 rhesus monkeys wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections were made in the mesencephalic tegmentum. In 3 cases with injections involving the red nucleus (RN), rubrospinal fibers descended mainly contralaterally to terminate in laminae V, VI and dorsal VII of the spinal cord and in the lateral motoneuronal cell groups at the level of the cervical and lumbosacral enlargements. In all 4 cases the area of the interstitial nucleus of Cajal (INC) was injected, which resulted in labeled interstitiospinal fibers in the medial part of the ipsilateral ventral funiculus of the spinal cord. The results indicate that there is no major qualitative difference between the mesencephalic (RN and INC) and motor cortical projections to the spinal cord.

Transneuronal changes of the inhibitory circuitry in the macaque somatosensory thalamus following lesions of the dorsal column nuclei.

The inhibitory circuitry of the ventroposterolateral nucleus (VPL) of the macaque somatosensory thalamus was analyzed in normal animals and in those surviving for a few days or several weeks following a unilateral lesion of the cuneate nucleus, the source of medial lemniscal (ML) axons carrying information from the contralateral upper extremity. Inhibitory synaptic terminals in the VPL were defined as those that contain flattened or pleomorphic synaptic vesicles and that can be shown to be immunoreactive for γ‐aminobutyric acid (GABA). There are two types of these profiles: F axon terminals that arise from neurons of the thalamic reticular nucleus, and perhaps from VPL local circuit neurons (LCNs); and the dendritic appendages of LCNs that form presynaptic dendrites (PSDs). ML terminals normally have extensive synaptic interactions with PSDs but not with F axon terminals. Electron microscopic analyses revealed that cuneatus lesions resulted in a rapid loss of ML terminals and a statistically significant reduction in both F and PSD synaptic profiles. Confocal scanning microscopy also demonstrated a profound loss of GABA immunoreactivity in the deafferented VPL. These changes persisted for more than 20 weeks, without any evidence of reactive synaptogenesis of surviving sensory afferents or of inhibitory synapses. The changes in GABA circuitry are transneuronal, and the possible mechanisms that may underlie them are discussed. It is suggested that the altered GABAergic circuitry of the VPL in the monkey may serve as a model for understanding changes in somatic sensation in the human following peripheral or central deafferentation.

Ultrastructural evidence for direct monosynaptic rubrospinal connections to motoneurons in Macaca mulatta

The magnocellularis division of the red nucleus of the Macaca mulatta, a midbrain structure involved in processing motor information, is known by light microscopic analysis to project, via the rubrospinal tract, to the contralateral intermediate horn of the spinal cord. Physiological studies, however, provide additional evidence for direct monosynaptic connections to motoneurons subserving distal musculature. This electron microscopic study demonstrates, by analyzing the anterograde transport of 5% wheatgerm agglutinin-horseradish peroxidase injected into the red nucleus, the presence of labeled terminals synapsing upon somata and proximal dendrites of motoneurons in the lateral portion of the ventral horn of the cervical enlargement of the spinal cord. We conclude that this anatomical evidence confirms the presence of direct monosynaptic connections to spinal motoneurons in the primate.

Identification of dorsal root synaptic terminals on monkey ventral horn cells by electron microscopic autoradiography

SummaryThe projection of dorsal root fibres to the motor nucleus of the macaque monkey spinal cord has been examined utilizing light and electron microscopic autoradiography. Light microscopy demonstrates a very sparse labelling of primary afferent fibres in the ventral horn. Silver grains overlying radioactive sources are frequently clustered into small groups, often adjacent to dendritic profiles. Under the electron microscope, myelinated axons and a few large synaptic profiles containing rounded synaptic vesicles were overlain by numerous silver grains. These labelled profiles made synaptic contact with dendrites 1–3 μm in diameter. The labelled profiles did not contact cell bodies or large proximal dendrites of ventral horn neurons. Frequently, small synaptic profiles containing flattened vesicles were presynaptic to the large labelled terminals and it is suggested that these axoaxonal synapses may mediate presynaptic inhibition of the primary afferent fibres. The relationship of the present findings to previously published physiological and anatomical studies is discussed.

Non-myelinated axons are rare in the medullary pyramids of the macaque monkey

Previous electron microscopic studies of the medullary pyramids have concluded that non-myelinated axons constitute about 30-60% of all axons in the pyramid of the rat, and about 8-15% in the cat and monkey. Physiological studies of pyramidal tract axons have not found fibers conducting in the range predicted for non-myelinated axons, less than 1 m/s. This present study of the primate pyramid demonstrates that most of the profiles which could be interpreted as being non-myelinated axons when viewed in cross-section, are actually astroglial processes when examined in longitudinal section. We conclude that non-myelinated axons constitute less than 1% of the pyramidal tract axons in the old world adult primate.