Properties of axons of sympathetic preganglionic neurons of the lower thoracic spinal cord

Morphology of sympathetic preganglionic neurons in the neonatal rat spinal cord: an intracellular horseradish peroxidase study

Understanding the central neural control of autonomic functions requires a knowledge of the morphology of the preganglionic neurons, for the location of the dendritic arborizations of these neurons will indicate which central pathways may have access to them. In the present study, individual sympathetic preganglionic neurons in the neonatal rat spinal cord have been examined by the intracellular injection of horseradish peroxidase (HRP) in an in vitro preparation. Seventeen HRP-labeled preganglionic neurons in thoracic segments T1-T3 were examined in detail; of these, 12 somata were located in the intermediolateral cell column (IML), one in the lateral funiculus (LF), two in the intercalated nucleus (IC), and two at the border between IML and IC. All of the neurons had extensive dendritic arborizations arising from an average of six primary dendrites; the average total dendritic length for these cells was 2,343 microns. The morphology of preganglionic neurons differed depending on the location of their cell bodies. Preganglionic neurons located in the IML were essentially two-dimensional: the cells had some dendrites that coursed rostrocaudally for 300-500 microns within the IML and others that coursed mediolaterally, extending to the lateral surface of the cord and close to the central canal. Axons of these cells coursed ventrally from the cell body and exited from the spinal cord at the first ventral root caudal to the cell body. No intraspinal axon branches were observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Light microscopic observations on the morphology of sympathetic preganglionic neurons in the pigeon, Columba livia

Experiments were performed in anesthetized, immobilized, artificially respirated pigeons (Columba livia). Extracellular recordings from 56 antidromically activated and collided sympathetic preganglionic neurons were obtained. Eleven cells were intracellularly labeled with horseradish peroxidase and reconstructed at the light microscopic level. Electrophysiologically there were no statistical differences between labeled and unlabeled neurons. Four different somatic shapes were observed: fusiform, pyriform, multipolar and stellate. Nine of 11 cells were located within the principal preganglionic cell column (column of Terni), the other two were within nucleus intercalatus spinalis. Principal column neurons exhibited planar, horizontally aligned dendritic arbors with major extensions directed rostrocaudally. Unexpectedly, the majority of these cells also had dendritic branch projections which spanned the entire width of the ipsilateral zona intermedia. Contralateral dendritic terminal arborizations were evident in seven neurons. Intercalatus neurons were multipolar-shaped and exhibited a notably different dendritic arrangement from principal column preganglionic cells. The dendrites of intercalated cells coursed obliquely within the transverse spinal cord axis, giving rise to major dendritic extensions into the base of the dorsal horn, the dorsolateral funiculus, and the dorsal aspects of the ventral horn. Irrespective of somatic subnuclear location, the morphology of preganglionic dendrites was similar: (1) Largely primary, secondary, and tertiary processes were smooth. (2) Fine caliber proximal and distal elements appeared beaded or "varicose." (3) Distal processes gave rise to thin-stalked, spine-like appendages. The axons of preganglionic neurons arose from cell bodies as well as primary and secondary dendrites. The axons of two cells branched intraspinally. The present findings provide detailed descriptions of the somatic structures and accompanying dendritic trees of preganglionic neurons within nucleus intercalatus. The observations also include anatomical evidence showing the intraspinal collateralization of sympathetic preganglionic axons. In general, avian sympathetic preganglionic neurons located within the principal cell column appear to be structurally homologous to their mammalian counterparts within the intermediolateral cell column of thoracic spinal cord.

Electrophysiological properties of sympathetic preganglionic neurons in the cat spinal cord in vitro

Intracellular recordings were obtained from sympathetic preganglionic neurons of the intermedio-lateral nucleus of the adult cat in slices of upper thoracic spinal cord maintained in vitro. The neurons were identified by their antidromic responses to stimulation of various ipsilateral sites. Sites from which antidromic responses could be evoked were the white ramus, the ventral root, the ventral root exit zone, the white matter between the latter and the outer edge of the tip of the ventral horn, the lateral edge of the ventral horn. Resting membrane potential was --61.3 +/- 1.6 mV (mean +/- SEM), input resistance 67.5 +/- 3.7 M omega, time constant 11.5 +/- 1.2 ms. The amplitude of the action potential generated by antidromic or direct stimulation was 77.4 +/- 2.3 mV. Threshold for direct spikes was 18.2 +/- 1.8 mV. The action potential had an average duration of 3.03 +/- 0.16 ms. It showed a prominent "hump" on the falling phase. The action potential had a tetrodotoxin (TTX)-sensitive and a TTX-resistant component. The latter was abolished by cobalt. Tetraethylammonium, cesium and barium prolonged the action potential duration which acquired a plateau-shape. A prolonged after-hyperpolarization (AHP) followed the sympathetic preganglionic neuron spike. Following a single spike, AHP duration and peak amplitude were 2.8 +/- 0.3 s and 16.6 +/- 0.7 mV, respectively. The AHP was abolished by cesium or barium, but enhanced by tetraethylammonium. An AHP followed the TTX-resistant spike. EPSPs and IPSPs could be generated by focal stimulation. The EPSP triggered spikes when threshold (15.0 +/- 2.0 mV) was reached.(ABSTRACT TRUNCATED AT 250 WORDS)

Do sympathetic preganglionic neurones have a recurrent inhibitory mechanism?

In anaesthetized and immobilized cats 32.5% of the intermediolateral sympathetic preganglionic neurones (B2-SPN) in T3, T8-9 and L2 spinal segments can be re-excited antidromically by repeated stimuli only at very long interstimulus intervals (16 ms and more) considerably exceeding the refractoriness of the somata and axons of these neurones. This was observed in the absence of any association with subnormality or other peculiarities B2-SPN axons. In this group of B2-SPNs with prolonged time interval between repeated antidromic discharges, orthodromic spikes effectively inhibited the generation of antidromic ones for a period which was substantially longer than the possible collision time. The preceding antidromic activation of a fraction of the B2-SPN segmental pool partly inhibited the orthodromic reactions of the others in this segmented elicited by stimulation of segmental afferent fibres or spinal descending pathways. These data definitely indicate that some of the B2-SPN group have a recurrent inhibitory mechanism.