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• W2027612601 abstract "Non-technical summary The arterial baroreflex is a closed-loop, negative feedback control system that senses baroreceptor pressure and controls systemic arterial pressure (AP) to attenuate perturbations in AP. The total arc of the baroreflex consists of two subsystems: the neural (baroreceptor pressure input to sympathetic nerve activity (SNA)) and peripheral (SNA input to AP) arcs. We show that although the spontaneous baroreflex transfer function obtained by closed-loop analysis has been believed to represent the neural arc function, it is inappropriate for system identification of the neural arc but is essentially appropriate for the peripheral arc under resting condition, when compared with open-loop transfer functions that have good predictabilities of time-series output dynamics from input signals. Our results indicate that in the spontaneous baroreflex system under closed-loop conditions, the peripheral arc (feedforward) function predominates over the neural arc (feedback) function, probably because of the SNA component that is independent of the baroreceptor pressure input. Abstract Although the dynamic characteristics of the baroreflex system have been described by baroreflex transfer functions obtained from open-loop analysis, the predictability of time-series output dynamics from input signals, which should confirm the accuracy of system identification, remains to be elucidated. Moreover, despite theoretical concerns over closed-loop system identification, the accuracy and the predictability of the closed-loop spontaneous baroreflex transfer function have not been evaluated compared with the open-loop transfer function. Using urethane and α-chloralose anaesthetized, vagotomized and aortic-denervated rabbits (n= 10), we identified open-loop baroreflex transfer functions by recording renal sympathetic nerve activity (SNA) while varying the vascularly isolated intracarotid sinus pressure (CSP) according to a binary random (white-noise) sequence (operating pressure ± 20 mmHg), and using a simplified equation to calculate closed-loop-spontaneous baroreflex transfer function while matching CSP with systemic arterial pressure (AP). Our results showed that the open-loop baroreflex transfer functions for the neural and peripheral arcs predicted the time-series SNA and AP outputs from measured CSP and SNA inputs, with r2 of 0.8 ± 0.1 and 0.8 ± 0.1, respectively. In contrast, the closed-loop-spontaneous baroreflex transfer function for the neural arc was markedly different from the open-loop transfer function (enhanced gain increase and a phase lead), and did not predict the time-series SNA dynamics (r2; 0.1 ± 0.1). However, the closed-loop-spontaneous baroreflex transfer function of the peripheral arc partially matched the open-loop transfer function in gain and phase functions, and had limited but reasonable predictability of the time-series AP dynamics (r2, 0.7 ± 0.1). A numerical simulation suggested that a noise predominantly in the neural arc under resting conditions might be a possible mechanism responsible for our findings. Furthermore, the predictabilities of the neural arc transfer functions obtained in open-loop and closed-loop conditions were validated by closed-loop pharmacological (phenylephrine and nitroprusside infusions) pressure interventions. Time-series SNA responses to drug-induced AP changes predicted by the open-loop transfer function matched closely the measured responses (r2, 0.9 ± 0.1), whereas SNA responses predicted by closed-loop-spontaneous transfer function deviated greatly and were the inverse of measured responses (r, −0.8 ± 0.2). These results indicate that although the spontaneous baroreflex transfer function obtained by closed-loop analysis has been believed to represent the neural arc function, it is inappropriate for system identification of the neural arc but is essentially appropriate for the peripheral arc under resting conditions, when compared with open-loop analysis. The arterial baroreflex plays a crucial role in circulatory control by its dynamic system characteristics (Eckberg & Sleight, 1992; Rowell, 1993). The baroreflex is a closed-loop, negative feedback control system that constantly senses arterial pressure (AP) by baroreceptors and quickly regulates systemic AP physiologically to attenuate perturbations in AP (Eckberg & Sleight, 1992; Rowell, 1993). The total arc baroreflex system consists of two subsystems: the neural and peripheral arcs (Kamiya et al. 2005b, 2008a, 2010; Kawada et al. 2010). The neural arc subsystem represents central processing from baroreceptor pressure to efferent sympathetic nerve activity (SNA), whereas the peripheral arc subsystem represents processing from SNA to systemic AP via peripheral circulatory organs including heart, kidney and blood vessels (Fig. 1) (Ikeda et al. 1996; Kamiya et al. 2005b). Functional structure of arterial baroreflex system A, theoretical considerations of the coupling of baroreflex neural and peripheral arcs. Although baroreflex is a negative feedback control system that senses AP by baroreceptors and regulates AP, we opened the loop by changing baroreceptor pressure independent of AP. By measuring SNA, we divided the baroreflex system into the neural arc (from baroreceptor pressure input to efferent SNA via central nervous system) and the peripheral arc (from SNA input to AP via cardiovascular organs system). B, block diagram of open-loop baroreflex system. Because of vascular isolation of carotid-sinus regions, CSP is independent of systemic AP. Noise is introduced to the neural and/or peripheral arcs. C, block diagram of closed-loop-spontaneous baroreflex system, where CSP equals AP. Noise is introduced to the neural and/or peripheral arcs. Because of the closed-loop nature, changes in AP (and thus, in CSP) control SNA via neural arc transfer function (Hn), which in turn modulate AP via peripheral arc transfer function (Hp). CSP, carotid sinus pressure; SNA, sympathetic nerve activity; AP, arterial pressure; NN, unknown noise in the neural arc; PN, unknown noise in the peripheral arc. Transfer function analysis is a powerful tool to determine the dynamic characteristics of biosystems. This analysis has revealed the dynamic causality mainly in ‘open-loop’ biosystems, including cerebral autoregulation (Zhang et al. 2002), renal vascular function (DiBona & Sawin, 2003, 2004), heart rate control (Ikeda et al. 1995) and cutaneous circulation (Kamiya et al. 2008b). We have applied the transfer function analysis to characterize the ‘closed-loop’ arterial baroreflex system, in which we used the open-loop and white-noise pressure perturbation techniques to overcome the difficulties of closed-loop system identification (see Appendix A) (Ikeda et al. 1996; Kawada et al. 2002; Kamiya et al. 2005b, 2008a). We have reported that the neural arc transfer function (Hn) has derivative and high-cut filter characteristics with a pure delay, indicating that more rapid change of arterial pressure results in greater response of SNA to pressure change (Kawada et al. 2002; Kamiya et al. 2005b), whereas the peripheral arc transfer function (Hp) has second-order low-pass filter characteristics with a pure delay (see Appendix B) (Kawada et al. 2002; Kamiya et al. 2005b). However, at least two important issues remain to be elucidated. First, a hallmark of the transfer function, the predictability of time-series output dynamics from input signals (Ikeda et al. 1995; Kamiya et al. 2008b), has not yet been investigated in the baroreflex system. Accurate system identification of the transfer function yields good predictability, whereas inappropriate system identification results in poor predictability. In the present study, we tested the first hypothesis that the open-loop baroreflex transfer functions of the neural and peripheral arcs are capable of predicting time-series SNA and AP output dynamics from baroreceptor pressure and SNA inputs, respectively. Second, identifying transfer functions is theoretically difficult under closed-loop and spontaneous resting baroreflex conditions. The reason is that unknown noises in the neural and peripheral arcs would interfere with the accuracy of system identification in closed-loop-spontaneous conditions, in contrast to open-loop transfer function identification where the interfering effects of noises would be eliminated by the open-loop and white-noise pressure perturbation techniques (Ikeda et al. 1996; Kawada et al. 2002; Kamiya et al. 2005b, 2008a) (see Appendix A). Although earlier interesting studies have applied a simplified (open-loop-like) calculation of transfer function to closed-loop-spontaneous resting baroreflex condition in humans (Cooke et al. 1999, 2009; Ogoh et al. 2009) and animals (Orea et al. 2007) without opening the loop, whether the reported transfer functions are actually capable of predicting time-series output dynamics has not been verified. In addition, the accuracy and limitation of closed-loop-spontaneous baroreflex transfer functions remain unclear from the viewpoint of comparing with open-loop transfer functions. In the present study, we tested the second hypothesis that the closed-loop-spontaneous baroreflex transfer function is limited to predict baroreflex dynamics compared with the open-loop transfer function. In the present study, by artificially controlling intra-carotid sinus pressure (CSP) and recording renal SNA and systemic AP, we identified the open-loop baroreflex transfer functions by introducing CSP perturbation according to a binary random (white-noise) sequence. We also determined the closed-loop-spontaneous baroreflex transfer functions by matching CSP with systemic AP. We then compared the characteristics and predictability of these transfer functions. Our results confirmed good predictability of the open-loop baroreflex transfer functions, and unexpectedly indicated that the closed-loop-spontaneous transfer function approximately matched the open-loop transfer function for the peripheral arc but deviated markedly from the open-loop transfer function for the neural arc. Thus, the closed-loop-spontaneous baroreflex transfer function is inappropriate for system identification of the neural arc but is partially appropriate for the peripheral arc under resting condition, compared with the open-loop analysis. These findings may have great impact, because the closed-loop-spontaneous baroreflex transfer function has been believed to represent the neural arc function (Orea et al. 2007; Cooke et al. 2009; Ogoh et al. 2009). Animals were cared for in strict accordance with the Guiding Principles for the Care and Use of Animals in the Field of Physiological Science approved by the Physiological Society of Japan and the National Cerebral and Cardiovascular Center Research Institute, and the ethical regulations and policies of The Journal of Physiology (Drummond, 2009). Ten Japanese white rabbits weighing 2.4–3.3 kg were initially anaesthetized by intravenous injection (2 ml kg−1) of a mixture of urethane (250 mg ml−1) and α-chloralose (40 mg ml−1). Anaesthesia was maintained by continuously infusing the anaesthetics at a rate of 0.33 ml kg−1 h−1 using a syringe pump (CFV-3200, Nihon Kohden, Tokyo). The rabbits were mechanically ventilated with oxygen-enriched room air. Bilateral carotid sinuses were isolated vascularly from the systemic circulation by ligating the internal and external carotid arteries and other small branches originating from the carotid sinus regions. The isolated carotid sinuses were filled with warmed physiological saline pre-equilibrated with atmospheric air, through catheters inserted via the common carotid arteries. CSP was controlled by a servo-controlled piston pump (model ET-126A, Labworks; Costa Mesa, CA, USA). Bilateral vagal and aortic depressor nerves were sectioned in the middle of the neck region to eliminate reflexes from the cardiopulmonary region and the aortic arch. Systemic AP was measured using a high-fidelity pressure transducer (Millar Instruments; Houston, TX, USA) inserted retrograde from the right common carotid artery below the isolated carotid sinus region. A catheter was inserted into the right femoral vein to infuse phenylephrine and nitroprusside. Body temperature was maintained at around 38°C with a heating pad. The left renal sympathetic nerve was exposed retroperitoneally. A pair of stainless steel wire electrodes (Bioflex wire AS633, Cooner Wire) was attached to the nerve to record renal SNA. The nerve fibres peripheral to the electrodes were ligated tightly and crushed to eliminate afferent signals. The nerve and electrodes were covered with a mixture of silicone gel (Silicon Low Viscosity, KWIK-SIL, World Precision Instrument, Inc., FL, USA) to insulate and immobilize the electrodes. The pre-amplified SNA signal was band-pass filtered at 150–1000 Hz. These nerve signals were full-wave rectified and low-pass filtered with a cut-off frequency of 30 Hz to quantify the nerve activity. After the surgical preparation, all animals (n= 10) were maintained supine. The overall scheme of the experimental design is shown in Fig. 2. Protocols 1–4 were conducted in randomized order at intervals of at least 5 min, while protocol 5 was done finally. In all protocols, bilateral CSP was controlled by a servo-controlled piston pump (Kawada et al. 2002). The SNA, CSP and AP were recorded at a sampling rate of 200 Hz using a 12-bit analog-to-digital converter. Data were stored on the hard disk of a dedicated laboratory computer system. Experimental design In system identification studies, open-loop (protocol 1, CSP was perturbed according to a binary random sequence) and closed-loop-spontaneous (protocol 2, CSP was matched with systemic AP) baroreflex transfer functions were identified from experimental data. In predictability studies, the predictive power of the above transfer functions was tested using independent data (protocols 3, 4 and 5). Protocol 3 and 4 were open-loop and closed-loop-spontaneous baroreflex conditions, respectively. Protocol 5 was pharmacological pressure intervention by phenylephrine and nitroprusside infusions in closed-loop condition. TF, transfer function; CSP, carotid sinus pressure; SNA, sympathetic nerve activity; AP, arterial pressure. Before these protocols, operating AP and SNA in baroreflex closed-loop condition were determined. First, CSP was matched with systemic AP to close the baroreflex loop. After at least 5 min of stabilization, the variables were recorded for 10 min, and the average AP over 10 min was defined as the operating AP under closed-loop condition. System identification studies Protocol 1 was performed to identify the open-loop baroreflex transfer functions. After at least 5 min of stabilization, CSP was randomly assigned at 20 mmHg above or below the operating AP every 500 ms according to a binary random (white-noise) sequence, in which the input power spectrum of CSP was reasonably flat up to 1 Hz (Kawada et al. 2002). The variables were recorded for 10 min and stored for analysis. Protocol 2 was performed to determine the closed-loop-spontaneous baroreflex transfer functions by a convenient method of applying the same calculation as that used in the open-loop condition of protocol 1 (see Appendix A). CSP was matched with systemic AP to close the baroreflex loop. After at least 5 min of stabilization, the variables were recorded for 10 min and stored for analysis. Predictability studies Protocols 3, 4 and 5 were performed to investigate the predictability of baroreflex transfer functions. In protocol 3 (open-loop), CSP was randomly assigned at 20 mmHg above or below the operating AP. The variables were recorded for 10 min and stored for analysis. In protocol 4 (closed-loop), CSP was matched with systemic AP to close the baroreflex loop. After at least 5 min of stabilization, the variables were recorded for at least 10 min and stored for analysis. Protocol 5 was also performed to investigate the predictability of baroreflex transfer functions during sequential pharmacological pressure interventions in the closed-loop condition. CSP was matched with systemic AP. After at least 2 min of stabilization, phenylephrine hydrochloride (3 μg kg−1) was bolus infused through a venous catheter inserted into the right femoral vein, followed 1–2 min later by sodium nitroprusside (4 μg kg−1) and then 1–2 min later by the second phenylephrine hydrochloride infusion (4 μg kg−1). The variables were recorded continuously for at least 10–11 min and stored for analysis. SNA signal was normalized by the following steps. First, 0 arbitrary unit (a.u.) was assigned to the post-mortem noise level. Second, 100 a.u. was assigned to the SNA signals averaged over 10 min before protocols. Last, the other SNA signals in protocols 1–5 were then normalized to these values. Although individual noise may be present in the neural and peripheral arc subsystems, the effects of noise on the calculations of transfer functions are eliminated by open-loop operation and white-noise-like perturbation of CSP (see Appendix A, Fig. 1A). It should be noted that since protocol 2 was a closed-loop and spontaneous baroreflex condition, unknown noise, if present in the neural and peripheral arc subsystems, would affect the accuracy of system identification (see Appendix A, Fig. 1B). Based on earlier studies (Cooke et al. 1999, 2009; Ogoh et al. 2009), we applied a simplified (open-loop-like) calculation of transfer function to the closed-loop-spontaneous resting baroreflex condition, and estimated the closed-loop-spontaneous baroreflex transfer functions from AP input to SNA in the neural arc (Hn-closed-spon) and from SNA to AP in the peripheral arc (Hp-closed-spon), together with coherence functions and step responses (see Appendix A). In protocols 3 and 4, we calculated the predicted time-series output dynamics (SNA and AP) from measured input signals (CSP/AP and SNA in the neural and peripheral arc, respectively), using eqn (4) and impulse response obtained from the transfer functions in protocols 1 and 2. The predicted output was scatter-plotted, and compared with the actually measured output by calculating the linear correlation coefficient (r) and root mean square (RMS). The analysis was performed using the data at arbitrarily selected 1 and 3 min in protocols 3 and 4, respectively. In protocol 5, similar to protocol 3 and 4, we calculated the predicted time-series output dynamics of SNA from measured pressure input signals (CSP/AP in the neural arc) during pharmacological interventions. The predicted SNA was scatter-plotted, and compared with the actual SNA measurements by calculating r and RMS. The analysis was performed using the data for 10–11 min. Since AP was determined by interventions (phenylephrine and nitroprusside infusions) and not by SNA, we did not calculate the predicted AP dynamics from the measured SNA signals. All data are presented as means ± SD. Paired t test and repeated measures analysis of variance with post hoc multiple comparisons were used to compare variables as appropriate. Differences were considered significant when P < 0.05. Figure 3 shows a typical example of the open-loop system identification of baroreflex transfer functions in protocol 2. CSP was perturbed according to a binary random (white-noise) sequence at 500 ms intervals (Fig. 3A, green line). When CSP was increased, SNA decreased, and vice versa. In the frequency domain, the input power spectrum of CSP was reasonably flat up to 1 Hz (Fig. 3B, green line). Open-loop transfer function A, typical representative data of one rabbit in protocol 2, showing time series of carotid sinus pressure (CSP), sympathetic nerve activity (SNA) and systemic arterial pressure (AP) during CSP perturbation in open-loop baroreflex condition. CSP is changed according to a binary random (white-noise) signal with a switching interval of 500 ms. B, input power spectrum of CSP (green line) is reasonably flat up to 1 Hz. Autospectra of SNA (top line) and systemic AP (bottom line) are also shown. The arrowhead indicates a peak of SNA autospectrum at 0.4 Hz. C, open-loop transfer functions of the neural arc (Hn-open) from CSP input to SNA (left panels) and of the peripheral arc (Hp-open) from SNA input to AP (right panels) identified in the same animal as in A. The gain (top), phase (second), coherence (third) and normalized random error (Error, bottom) functions are shown. Units of gain are [a.u. mmHg−1] for the neural arc and [mmHg a.u.−1] for the peripheral arc, respectively. D, step responses (Step res.) derived from the transfer functions shown in C. The units are [a.u.] for the neural arc and [mmHg] for the peripheral arc, respectively. E, open-loop transfer functions of the total arc (Htotal-open) from CSP input to AP identified in the same animal as in A. The gain (top), phase (second), coherence (third) and normalized random error (Error, bottom) functions are shown. Unit of gain is [mmHg mmHg−1]. F, step response (Step res.) derived from the transfer function shown in E. The unit is [mmHg]. a.u., arbitrary unit. The open-loop transfer function of the neural arc from CSP input to SNA (Hn-open; Fig. 3C, left panels) showed that the gain increased as the frequency of CSP perturbation increased between 0.01 Hz and 0.4 Hz, indicating dynamic high-pass characteristics. The phase approached –π at the lowest frequency, indicating a negative SNA response to CSP changes, and lagged as the frequency increased (Fig. 3C, left panels). The coherence was over 0.8 between 0.03 to 0.4 Hz except at around 0.35 Hz (Fig. 3C, left panels). The step response (Fig. 3D, left panel) of SNA in response to CSP consisted of an initial decrease followed by partial recovery and then steady state. The open-loop transfer function of the peripheral arc from SNA input to AP (Hp-open, Fig. 3C, right panels) showed that the gain decreased as the frequency increased, indicating low-pass characteristics. The phase approached zero at the lowest frequency, indicating a positive AP response to SNA changes, and lagged as the frequency increased. The coherence was over 0.8 between 0.01 to 0.3 Hz except at around 0.2 Hz (Fig. 3C, right panels). The step response (Fig. 3D, right panel) of SNA to CSP was a gradual increase to steady state. The transfer function of baroreflex total arc from CSP input to systemic AP identified in the open-loop condition (Fig. 3E) showed that the gain decreased as the frequency increased, indicating low-pass characteristics that were milder than Hp-open. The phase approached –π at the lowest frequency, indicating negative feedback system characteristics of baroreflex (negative AP response to CSP changes). The phase lagged as frequency increased. The transfer function of total arc was almost consistent with multiplication of tandemly arranged open-loop transfer functions of neural (Hn-open) and peripheral (Hp-open) arcs (Fig. 1A and B), at the frequency where their coherence functions were high. Figure 4 shows a typical example of the closed-loop-spontaneous transfer functions simplified, calculated in protocol 3 by applying open-loop-like calculations to closed-loop-spontaneous data. The data were obtained from the same animal as in Fig. 3. Closed-loop-spontaneous transfer function A, typical representative data of protocol 3, showing time series of CSP, SNA and systemic AP in closed-loop-spontaneous baroreflex condition, where CSP is matched with systemic AP. The data were obtained from the same animal as in Figure 3. B–D show exactness of good match between CSP and systemic AP. B, autospectrum of CSP (green line) overlaps with that of AP (black line). Autospectrum of SNA (top line) is also shown. The arrowhead indicates a peak in the SNA autospectrum at 0.4 Hz. C, beat-to-beat waveform of CSP (green line) overlaps with that of AP (black line). D, the transfer functions from CSP to systemic AP. Gain (top), phase (middle) and coherence (bottom) functions are shown. Unit of gain is [mmHg mmHg−1]. E, the closed-loop-spontaneous transfer functions of the neural arc (Hn-closed-spon) from CSP (=AP) input to SNA (left panels) and of the peripheral arc (Hp-closed-spon) from SNA input to AP (right panels) identified in the same animal as in A. The gain (top), phase (second), coherence (third) and normalized random error (bottom) functions are shown. Units of gain are [a.u. mmHg−1] for the neural arc and [mmHg a.u.−1] for the peripheral arc, respectively. F, step responses (Step res.) derived from the transfer functions. The units are [a.u.] for the neural arc and [mmHg] for the peripheral arc, respectively. a.u., arbitrary unit; CSP, carotid sinus pressure; SNA, sympathetic nerve activity; AP, arterial pressure; Step res., step response. In E and F, the open-loop transfer functions and derived step responses are included for reference (red lines). We closed the baroreflex loop by matching CSP with systemic AP. The exact match of the two parameters was demonstrated by autospectrum (Fig. 4B) and beat-to-beat waveform (Fig. 4C), both showing overlapping of CSP (green line) and systemic AP (black line). The exact match was further confirmed by the transfer functions from CSP to systemic AP (Fig. 4D), which showed that the gain, phase and coherence functions were maintained constant at 1, zero and 1, respectively. The closed-loop-spontaneous transfer function of the neural arc (Hn-closed-spon) from CSP (that equalled AP) to SNA (Fig. 4E, left panels, black line) was markedly different from the open-loop transfer function (Hn-open, red line) with respect to gain, phase, coherence and step response. The increase in gain versus frequency was steeper; the gain was thus higher and the coherence was lower in Hn-closed-spon compared with Hn-open. The phase led as frequency increased, while the step response oscillated (Fig. 4F, left panel) in Hn-closed-spon, which were markedly different from Hn-open. In contrast to the neural arc, the closed-loop-spontaneous transfer function for the peripheral arc (Hp-closed-spon) from SNA to AP (Fig. 4E, right panels, black line) approximated that of the open-loop transfer function (Hp-open, red line). The gain (except at 0.02–0.05 Hz) and phase were similar up to 0.3 Hz, although the coherence was lower in Hp-closed-spon than in Hp-open (common feature for both neural and peripheral arcs). The step response was similar to that of Hp-open except for a slower time constant (Fig. 4F, right panel). Because of the closed-loop condition, the gain and phase functions of Hp-closed-spon were the inverse of those of Hn-closed-spon. Since CSP exactly matched systemic AP in this closed-loop-spontaneous baroreflex condition, the transfer function of total arc baroreflex from CSP input to systemic AP was calculated as all-pass filter without modulating phase (Fig. 4D). This is greatly different from the transfer function of the total arc identified from open-loop experiments (Fig. 3E). The closed-loop-spontaneous transfer functions (Fig. 5A, blue lines) (Hn-closed-spon and Hp-closed-spon) obtained from all animals (n= 10) in protocol 2 were compared with the open-loop transfer functions (Fig. 5A, red lines) in protocol 1. The step response was also compared between closed-loop-spontaneous (Fig. 5B, blue line) and open-loop experiments (Fig. 5B, red line). Comparison between open-loop and closed-loop-spontaneous transfer functions Solid and dashed lines represent the mean and mean + SD, respectively, obtained from all animals (n = 10). A, red lines are open-loop transfer functions of the neural (Hn-open, left panels) and peripheral arcs (Hp-open, right panels) identified in protocol 1. Blue lines are closed-loop-spontaneous transfer functions (blue lines) of the neural (Hn-closed-spon, left panels) and peripheral arcs (Hp-closed-spon, right panels) identified in protocol 2. The gain (top), phase (second), coherence (third) and normalized random error (bottom) functions are shown. Units of gain are [a.u. mmHg−1] for the neural arc and [mmHg a.u.−1] for the peripheral arc, respectively. The closed-loop-spontaneous baroreflex transfer function for the neural arc is markedly different from the open-loop transfer function, whereas that for the peripheral arc partially matches the open-loop transfer function. B, step response (Step res.) calculated from the open-loop (red lines) and closed-loop-spontaneous (blue lines) transfer functions. The units are [a.u.] for the neural arc and [mmHg] for the peripheral arc, respectively. a.u., arbitrary unit; CSP, carotid sinus pressure; SNA, sympathetic nerve activity; AP, arterial pressure; Step res., step response. In the neural arc (Fig. 5A and B, left panels; Table 1), closed-loop-spontaneous transfer functions (Hn-closed-spon, blue lines) were markedly different from open-loop transfer functions (Hn-open, red lines), similar to the example shown in Fig. 4E. The difference was characterized by an enhanced increase of gain versus frequency (slope), a phase lead and an oscillation of step response. In contrast, in the peripheral arc (Fig. 5A and B, right panels; Table 2), closed-loop-spontaneous transfer functions (Hp-closed-spon) were similar to open-loop transfer functions (Hn-open) in gain, phase and step response. The transfer function of the baroreflex total arc from CSP input to systemic AP in the open-loop condition was identified as having low-pass filter characteristics with negative feedback in all animals. In contrast, the total arc transfer function in the closed-loop-spontaneous condition had all-pass filter characteristics without modulating phase in all animals. The ability of the neural arc transfer functions (determined by protocols 1 and 2) to predict output dynamics (SNA) from given input signals (CSP) in the" @default.
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• W2027612601 title "Closed-loop spontaneous baroreflex transfer function is inappropriate for system identification of neural arc but partly accurate for peripheral arc: predictability analysis" @default.
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