Collection 1: solitary dietary fiber baroreceptor tape
Recordings were obtained from 173 separate single baroreceptor fibres in 26 animals. Initial examination of response curves from these fibres suggested that they could be described using a minimum of four categories. This allowed responses to be grouped by two distinct characteristics (Fig. 2). The second characteristic was the absence or presence of silence (a pressure/pressure range over which a fibre did not fire). Every fibre was placed in a category (types A–D) based on these two response characteristics. For every fibre we quantified five variables. These were the threshold pressure (P th) at which activity began to increase with increasing pressure, and the firing rate at this threshold (F th); the pressure (P maximum) at which activity reached a maximum (F max); and gain (G) as the ratio (F max ? F th)/(P max ? P th). Values of these variables for the four different fibre types were compared using ANOVA followed by Fisher’s LSD.
a,b Classification scheme for single fibre baroreceptor response curves in the left ADN. Below a threshold pressure (Pth), types A and B demonstrate either no activity (a) or a constant activity (b), and above Pth, activity increases with increasing pressure. Below a nadir, decreasing pressure is associated with an increase in activity. In type C, there is no activity at the nadir, while type D fibres, like type B, are not silent at any pressure (Fth ? 0). All four types of fibre reach a maximum firing frequency (Fmax) at Pmax
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Table 1 shows the categorical distribution of fibres and the values of five measured variables. Almost 50% of fibres (type A) had a ‘classic’ sigmoidal response curve with silence below P th. Over half the total population did not respond in this manner; curves from 30% of the fibres demonstrated a nadir (Fig. 3a; types C and D), and 40% were never silent over the ramp pressure range (Fig. 3a; types B and D). The features possessed by types A–D most likely explain the overall shape of the multifibre response curves (Fig. 1), which have a nadir and show activity at all pressures tested. Fibres of type B (sigmoidal response curve with no silence at P th) had a significantly higher maximum firing rate than the other three fibre types (P < 0.01). Type D fibres (nadir in the response curve with no silence at P th) had lower gain than either type A or C (P < 0.01). It should be noted that since the pulse characteristics were not controlled, values for P th and P max are not necessarily comparable with those from other studies. On the other hand, our data do reflect natural responses, and we have assumed that variability in pulse characteristics occurred randomly.
a Mean response curves for the four fibre types defined in Fig. 2. Since the Pth for individual fibres differed, the sharp features seen in the response of an individual fibre are smeared in the pooled graphs. However, it is clear that types B and D (40% of total) show substantial firing at all pressures, and that types C and D (30% of total) have a nadir at low pressure, below which firing frequency increases while pressure decreases. b An estimate of the relative contribution of each type of baroreceptor to a response curve of a multifibre record from an intact ADN. The curves in this graph were obtained by finding the product of the type mean response curve (a) and number of fibres contributing to the type mean. While activity from type A fibres (48%) predominates this multifibre response curve, the contribution of all types of fibres (those never silent and those possessing a low pressure nadir) can be seen clearly in the response labelled ALL, which reflects this weighted contribution of the four fibre types