The Journal of comparative neurology
-
The mechanism by which A-fibres sprout into lamina II of the dorsal horn of the adult rat after peripheral nerve injury, a region which normally receives input from noci- and thermoreceptive C-fibres alone, is not known. Recent findings indicating that selective C-fibre injury and subsequent degenerative changes in this region are sufficient to induce sprouting of uninjured A-fibres have raised the possibility that the structural reorganisation of A-fibre terminals is an example of collateral sprouting, in that deafferentation of C-fibre terminals alone in lamina II may be sufficient to cause A-fibre sprouting. Primary afferents of the sciatic nerve have their cell bodies located predominantly in the L4 and L5 dorsal root ganglia (DRGs), and the A-fibres of each DRG have central termination fields that show an extensive rostrocaudal overlap in lamina III in the L4 and L5 spinal segments. ⋯ Neither intact nor peripherally injured A-fibres were seen to sprout into denervated lamina II after L5 rhizotomy. Sprouting was only ever seen into regions of lamina II containing the terminals of peripherally injured C-fibres. Therefore, it seems that the creation of synaptic space within lamina II is not the explanation for A-fibre sprouting after peripheral nerve section or crush, emphasising that injury-induced changes in C-fibres and subsequent chemotrophic effects in the superficial dorsal horn are the likely explanation.
-
Comparative Study
Premotor neurons for vertical eye movements in the rostral mesencephalon of monkey and human: histologic identification by parvalbumin immunostaining.
In the monkey, premotor neurons for vertical gaze are located in the mesencephalic reticular formation: the rostral interstitial nucleus of the medial longitudinal fascicle (riMLF) contains medium-lead burst neurons, and the interstitial nucleus of Cajal (iC) acts as integrator for the eye-velocity signals to eye-position signals. Both nuclei lie adjacent to each other and are similar in appearance at the transition zone in Nissl-stained sections, which makes a delineation of the functionally different nuclei difficult in human. For a neuropathologic analysis of degenerative changes in saccadic disorders of patients, the histologic identification of the riMLF and the iC is important. ⋯ The immunocytochemical staining of human brainstem sections revealed the riMLF as a cluster of medium-sized, elongated parvalbumin-positive cells, with a similar appearance and at a similar location as that in monkey: a wing-shaped nucleus dorsomedial to the red nucleus, rostral to the traversing tractus retroflexus, dorsally bordered by the thalamo-subthalamic paramedian artery. The adjacent iC could be distinguished easily by its more densely packed, round parvalbumin-immunoreactive neurons. The exact identification of premotor neurons of the vertical system in the normal human brain provides a reference basis for the neuropathologic analysis of vertical gaze disorders at a cellular level.
-
Two different populations of projection neurons are intermingled in the cerebellar nuclei. One group consists of small, gamma-aminobutyric acid-containing (GABAergic) neurons that project to the inferior olive, and the other group consists of larger, non-GABAergic neurons that provide an input to one or more, usually premotor, centers in the brainstem, such as the red nucleus, the thalamus, and the superior colliculus. All cerebellar nuclear neurons are innervated by GABAergic Purkinje cells. ⋯ Nine out of 367 investigated BDA-labeled terminals were observed to be presynaptic to a WGA-HRP-labeled profile as well as to a gold lectin-labeled profile. This indicates that nuclear cells that project to the inferior olive as well as those that project to premotor centers are under the influence of the same Purkinje cells. Such an arrangement would suggest an in-phase cortical modulation of the activation patterns of the inhibitory cells that project to the inferior olive and excitatory cells that project to premotor nuclei, which could explain why olivary neurons, especially those of the rostral part of the dorsal accessory olive, appear to be unresponsive to stimuli generated during active movement.
-
To examine the mechanisms responsible for the more rapid nerve regeneration observed after a previous (conditioning) nerve injury, adult rats were subjected to a midthigh sciatic nerve transection by using one of three protocols designed to facilitate or restrict nerve regeneration: 1) ligation, in which transected axons were prevented from regenerating; 2) cut, in which transected axons were permitted to extend into peripheral target tissue but were separated from the denervated peripheral nerve stump; and 3) crush, in which axons could regenerate normally through the denervated distal nerve tract. The affected dorsal root ganglia (DRG) were subsequently removed, dissociated, and cultured for up to 3 days, and the timing of neurite initiation, rate of outgrowth, and arborization pattern of previously injured neurons were compared with control DRG. ⋯ The primary effect of all conditioning lesions on cultured DRG neurons appeared to be to advance the timing of morphogenesis, resulting in conditioning-lesioned neurons that exhibited characteristics consistent with control neurons that had been cultured for an additional day or more. A secondary effect of conditioning lesions on neurite outgrowth rates was dependent on the local environment of the axons prior to culturing.
-
There is strong evidence that neural circuits underlying certain rhythmic motor behaviors are located in the spinal cord. Such local central pattern generators are thought to coordinate the activity of motoneurons through specific sets of last-order premotor interneurons that establish monosynaptic contacts with motoneurons. After injections of biotinylated dextran amine into the lateral and medial motor columns as well as the ventrolateral white matter at the level of the upper and lower segments of the lumbar spinal cord, we intended to identify and localize retrogradely labelled spinal interneurons that can likely be regarded as last-order premotor interneurons in rats. ⋯ Interneurons projecting to the lateral motor column at the level of the L1-L2 segments were also located in laminae V-VII, but most of them were concentrated in the middle one-third or in the lateral half of this area. Following injections into the medial motor column at the level of the L1-L2 segments, the majority of labelled neurons were confined to the medial aspect of laminae V-VII and lamina VIII, and the proportion of neurons that were found contralateral to the injection site was strikingly higher than in the other experimental groups. The results suggest that the organization of last-order premotor interneurons projecting to motoneurons, which are located at different areas of the lateral and medial motor columns and innervate different muscle groups, may present distinct features in the rat spinal cord.