Tracing of frog sensory‐motor synapses by intracellular injection of horseradish peroxidase.

Abstract

SUMMARY1. Monosynaptically connected primary afferent fibres and motoneurones of the isolated spinal cord of the frog were injected with horseradish peroxidase (HRP). Six labelled afferent fibre-motoneurone pairs were reconstructed-and. subjected to detailed analysis.2. Frog motoneurones possess eight to twelve dendritic arrays displaying some dorso-ventral asymmetry. Dorsal dendrites exhibit a rostro-caudal extent of 1-7-2-6 mm (average 2-2 mm).3. Primary afferent fibres bifurcate in the dorsal funiculus. First-order collaterals emanate from the main ascending and descending branches, at an average distance of 407 ,sm. The average number of boutons per collateral is 670.4. To reach a contacting bouton the presynaptic spike must pass on average five bifurcations and then zero to twelve boutons en passant, attached to a single terminal collateral branch. The structural equivalent of the axon cylinder of the collateral tree roughly preserves cross-sectional area. The branch power ranged between 1-15 and 3*35 (average 2 06). Primary afferent fibres usually form clusters of contacting boutons (contact regions).Connexions between an afferent fibre and a motoneurone comprise from five to twenty-three contact regions (average 12-5). Each contact region contains one to twelve contacting boutons (average 3 3).6. In two of three experiments contacting boutons were found to be significantly larger than non-contacting boutons. The average diameter of the former was 2-6 ,um (range 1-2-4-0).7. In five out of six cases more than one collateral belonging to the same fibre participated in the connexion with a given motoneurone.8. The average number of contacting boutons per motoneurone and collateral is 19-1. It was estimated that each collateral could supply not more than thirty-five motoneurones. This would be less than 8-5 % ofthe motoneurones with their dendrites which cross the termination space of a single collateral. The average number of contacting boutons forming one primary motoneurone connexion was 41-5 (range 21-72).

Relation between structural and release parameters at the frog sensory‐motor synapse.

“…It reveals that same tendency which was found previously for the much larger population of single-fibre e.p.s.p.s recorded at frog sensory-motor synapses (Shapovalov & Shiriaev, 1980). The ratio of the mean amplitude of the electrical and chemical components was used as an index for describing the connexion, and all six connexions were numbered according to this coefficient value (see also Grantyn et al 1984).…”

“…When a motoneurone and a sensory fibre which directly contact one another are both injected with horseradish peroxidase (HRP), the subsequent reconstruction of labelled elements allowed the number and distribution of synaptic contacts on the identified post-synaptic cell to be determined (Grantyn, Shapovalov & Shiriaev, 1982, 1984. In the present paper we compare the structural and quantal parameters determined at the same sensory-motor connexion.…”

Anurans

“…Even more attention has been paid to anuran motoneurons, particularly to their massive dendritic trees (Sala y Pons 1892;Ebbesson 1976;Szekely 1976;Bregman and Cruce 1980;Rosenthal and Cruce 1985a;Antal et al 1986bAntal et al , 1992. The innervation by primary afferents of lumbar lateral motoneurons is well known in anurans, particularly due to the elegant work on the isolated spinal cord of Rana ridibunda by the St. Petersburg group (Grantyn et al 1982(Grantyn et al , 1984aMotorina et al 1982a,b). The innervation by primary afferents of lumbar lateral motoneurons is well known in anurans, particularly due to the elegant work on the isolated spinal cord of Rana ridibunda by the St. Petersburg group (Grantyn et al 1982(Grantyn et al , 1984aMotorina et al 1982a,b).…”

Bulbospinal inhibition of PAD elicited by stimulation of afferent and motor axons in the isolated frog spinal cord and brainstem

“…10). However, it must be pointed out that axon collaterals in frog motoneurons have been seen only occasionally (Sala y Pons 1892; Silver 1942;Grantyn et al 1984). In fact, quite recently Shupliakov et al (1990) have injected HRP in single motoneurons that produced VR-DRPs following intracellular stimulation and only in 1 out of 22 motoneurons were they able to stain an axon collateral.…”

“…Morphological evidence supporting this proposal is, however, scarce. Although collaterals of motor axons have been described in the frog spinal cord (Sala y Pons 1892; Silver 1942;Grantyn et al 1984), their existence has been questioned (see Szekely 1976;Shupliakov et al 1990). Electrophysiological studies have not been very successful either, because it has not been possible to record the activity of spinal interneurons that respond to antidromic stimulation of ventral roots or motor nerves (but see Glusman and Rudomin 1974).…”