Cooperation of muscle and cutaneous afferents in the feedback of contraction to peroneal motoneurons.

Peroneal motoneurons were recorded intracellularly in anesthetized cats during sustained submaximal contractions of peroneus brevis muscle (PB) elicited by repetitive electrical stimulation of motor axons in the distal portion of cut ventral root filaments. Mechanical stimulation of the territory innervated by the superficial peroneal nerve (SP) was applied during contraction to assess the influence of afferents from this territory on the contraction-induced excitation of motoneurons. In 21 peroneal motoneurons in which PB contraction evoked excitatory potentials, a stimulation engaging mechanoreceptors located in the skin around toes was found to either enhance (in 12 motoneurons) or reduce (in 9 motoneurons) the contraction-induced excitatory potentials. Among positive effects, six showed simple summation of the responses to each individual stimulus, suggesting a convergence of afferent pathways on motoneurons. In six other motoneurons, complex interactions were observed, as may result from convergence at a premotoneuronal level. Among negative effects, a single instance was observed of inhibitory facilitation, as may result from convergence of cutaneous and muscular, possibly Ib, afferents on inhibitory interneurons. Several pathways, mediating either facilitory or inhibitory influences, are available for cooperation of muscle and cutaneous input, allowing flexibility of motoneuron activation in different tasks.

Effects on peroneal motoneurons of cutaneous afferents activated by mechanical or electrical stimulations.

The postsynaptic potentials elicited in peroneal motoneurons by either mechanical stimulation of cutaneous areas innervated by the superficial peroneal nerve (SP) or repetitive electrical stimulation of SP were compared in anesthetized cats. After denervation of the foot sparing only the territory of SP terminal branches, reproducible mechanical stimulations were applied by pressure on the plantar surface of the toes via a plastic disk attached to a servo-length device, causing a mild compression of toes. This stimulus evoked small but consistent postsynaptic potentials in every peroneal motoneuron. Weak stimuli elicited only excitatory postsynaptic potentials (EPSPs), whereas increase in stimulation strength allowed distinction of three patterns of response. In about one half of the sample, mechanical stimulation or trains of 20/s electric pulses at strengths up to six times the threshold of the most excitable fibers in the nerve evoked only EPSPs. Responses to electrical stimulation appeared with 3-7 ms central latencies, suggesting oligosynaptic pathways. In another, smaller fraction of the sample, inhibitory postsynaptic potentials (IPSPs) appeared with an increase of stimulation strength, and the last fraction showed a mixed pattern of excitation and inhibition. In 24 of 32 motoneurons where electrical and mechanical effects could be compared, the responses were similar, and in 6 others, they changed from pure excitation on mechanical stimulation to mixed on electrical stimulation. With both kinds of stimulation, stronger stimulations were required to evoke inhibitory postsynaptic potentials (IPSPs), which appeared at longer central latencies than EPSPs, indicating longer interneuronal pathways. The similarity of responses to mechanical and electrical stimulation in a majority of peroneal motoneurons suggests that the effects of commonly used electrical stimulation are good predictors of the responses of peroneal motoneurons to natural skin stimulation. The different types of responses to cutaneous afferents from SP territory reflect a complex connectivity allowing modulations of cutaneous reflex responses in various postures and gaits.

Indications for GABA-immunoreactive axo-axonic contacts on the intraspinal arborization of a Ib fiber in cat: a confocal microscope study.

Confocal microscopy was used to detect GABA-immunoreactive axo-axonic appositions, indicating possible synaptic contacts, on Ib fiber terminals in the lumbosacral spinal cord. A Ib fiber from posterior biceps-semitendinosus muscles was labeled by intra-axonal ejection of tetramethylrhodamine dextran (red), and serial sections of S1-L7 spinal cord segments were processed for GABA immunocytochemistry revealed by fluorescein isothiocynate (green). Appositions between GABA-immunoreactive structures and the labeled fiber appeared as yellow spots because of the presence of both fluorochromes in small volumes (0.3 * 0.3 * 0.5 micrometer(3)) of tissue. These spots were identified as probable axo-axonic contacts when: (1) they were observed in two to four serial confocal planes, indicating that they did not occur by chance; and (2) their sizes, shapes, and locations were similar to those of axo-axonic contacts found on Ia terminals, known to bear presynaptic boutons, and resembled the axo-axonic synapses described in electron microscope studies of Ib boutons in Clarke's column. A total of 59 presumed axo-axonic contacts was observed on two Ib collaterals, representing an estimated 20% of the total complement. In a three-dimensional reconstruction of one collateral, they were mostly located in terminal positions, and some branches bore more contacts than others. Such differential distribution could not result from chance appositions between GABAergic structures and Ib arborization and further supported the identification of axo-axonic contacts. Segmental Ib collaterals bear axo-axonic synapses that might ensure differential funneling of information toward different targets.

Differential distribution of immunoreactivity in the adult rat spinal cord revealed by the monoclonal antibody, Py: a light and electron microscopic study.

The monoclonal antibody Py has previously been shown to be a useful marker for subpopulations of neurons in the rat brain. However, the distribution of Py immunoreactivity in other regions of the CNS and PNS is not known. Here, we present a light and electron microscopic investigation into the distribution of Py immunoreactivity in the adult rat spinal cord, dorsal root ganglia, and peripheral nerves. Py immunoreactivity was associated with cytoskeletal elements in the cell body and dendrites of large-diameter neurons (particularly motoneurons, Clarke's nucleus neurons, and some dorsal root ganglion cells). Small-diameter neurons of lamina II (substantia gelatinosa) were Py negative. Py immunoreactivity was also detected in some populations of nerve fibers, notably axons located in the corticospinal tract, axons in the region of the white matter bordering the gray matter (presumably propriospinal axons), and also motor axons of the ventral root, but not in peripheral nerve. Dorsal roots were largely unstained. The present observations suggest a possible involvement of the Py antigen in the function or maintenance of the cytoskeleton of some populations of neurons and that the antibody may be a potentially useful tool for studying lesion-induced cytoskeletal alterations, particularly in alpha-motoneurons and Clarke's nucleus neurons.