Neurones in the brain stem of the cat excited by vagal afferent fibres from the heart and lungs.

Abstract

Extracellular recordings were made from 164 neurones in the nucleus tractus solitarius and dorsal motor vagal nucleus of the chloralose-anaesthetized cat. 139 neurones were excited synaptically and 25 non-synaptically by electrical stimulation of cardiac and pulmonary vagal branches. Synaptically excited neurones fall into two populations, one activated solely by myelinated afferent fibres and a second activated solely by non-myelinated afferent fibres. 94 neurones were synaptically excited by afferent fibres in a single vagal branch while 45 were excited by stimulation of two or three branches. Neurones responding to volleys in myelinated afferent fibres were located in both medial and lateral regions of the nucleus tractus solitarius whilst those excited by non-myelinated afferent fibres were restricted to the medial region. Consistent differences in the locations of neurones excited by stimulation of either cardiac or pulmonary or by single or several branches could not be distinguished.

Convergence properties of solitary tract neurones driven synaptically by cardiac vagal afferents in the mouse

“…2) together with the latency difference between evoked synaptic potentials from the vagus nerve and left ventricle is indicative of unmyelinated or C fibre inputs with an estimated conduction velocity of around 0·6 m s¢ (see Results). Previous studies in the cat demonstrated that electrical stimulation of a cardiac branch of the vagus nerve evoked excitatory responses in NTS neurones which were mediated by either myelinated or unmyelinated vagal afferents (Donoghue et al 1981;Bennett et al 1985). The finding that the innervation of the left ventricle is predominantly by unmyelinated vagal fibres in the cat (see Hainsworth, 1991, for review) is consistent with the present observations in mice.…”

“…The latency and fibre-type mediating converging inputs from baro-and chemoreceptors to veratridine-driven NTS neurones were not identified in the present study. However, based on the absence of a mixed convergence from both myelinated and unmyelinated vagal inputs to NTS neurones driven orthodromically by electrical stimulation of the cardiac branch of the vagus (Donoghue et al 1981;Bennett et al 1985), it is suggested that the converging baro-and chemoreceptor inputs may be predominantly unmyelinated.…”

“…In addition, the finding of both DIS and DS NTS neurones is consistent with the results of Hines, Toney & Mifflin (1994), who described two populations of NTS neurones responding to either chemical stimulation of cardiopulmonary afferents or mechanical distension of the right atrium. Previous NTS studies were unable to characterize DIS and DS NTS neurones as they only employed electrical stimulation of a cardiac branch of the vagus nerve (Donoghue et al 1981;Bennett et al 1985Bennett et al , 1988Jones et al 1995). Indeed, this study is one of the first (see Hines et al 1994) to selectively stimulate both chemically and mechanically sensitive cardiac receptors to characterize NTS neurones, and, in this regard, the WHBP appears to be a viable preparation for studying the central integration of cardiac vagal afferent inputs.…”

“…The finding that the innervation of the left ventricle is predominantly by unmyelinated vagal fibres in the cat (see Hainsworth, 1991, for review) is consistent with the present observations in mice. Since the cardiac vagal branches in the cat contain afferents from both the ventricles and atria as well as the oesophagus and lungs (Bennett et al 1985), the inputs to NTS described by Bennett et al (1985) and Donoghue et al (1981) might originate from receptors of non-cardiac origin. There are no previous reports describing the response characteristics of NTS neurones during chemical or mechanical stimulation of the left side of the heart.…”

Cardiovascular responses to stimulation of pulmonary C fibres in the cat: their modulation by changes in respiration.

“…There are known 59 M. DE BURGH DALY AND E. KIRKMAN differences in the reflex cardiac effects resulting from stimulation of receptors discharging into myelinated and non-myelinated vagal afferent fibres (Spyer, 1981;. This has led to the hypothesis that there may be two separate systems within the central organization of the brain stem, which is based on differences in the anatomical distribution of primary afferent neurones within the nucleus of the tractus solitarius influenced by myelinated and non-myelinated fibres (Bennett et al 1985). Such an hypothesis, however, cannot fully explain the present findings because most of the respiratory modulation of excitatory inputs to the cardiac vagal motoneurones takes place at the level of the neurones themselves (Spyer, 1981(Spyer, , 1982, although the inhibitory effects of lung inflation, mediated by pulmonary stretch receptors, may 'gate' excitatory inputs to the cardiac vagal motoneurones earlier in the vagal pathway (Potter, 1981).…”