694
Baroreceptor Dynamics and Their Relationship to
Afferent Fiber Type and Hypertension
ARTHUR M. BROWN, WILLIAM R. SAUM, AND SYOZO YASUI
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SUMMARY Static characteristics of baroreceptors differ depending on whether receptors are connected to
myelinated or unmyelinated axons (MBs and UBs) and whether they come from normotensive or spontaneously
hypertensive rats (NTRs and SHRs). Dynamic characteristics are incompletely known and were examined using an in
vitro rat aortic arch-aortic nerve preparation in which static characteristics are similar to those present in vivo. Small
amplitude sinusoidal pressures were applied at frequencies varying from 0.1 to 20 Hz over the range of linear
responses to pressure. MBs from SHRs and NTRs show peaking in curves relating gain to frequency, gain being the
ratio of sinusoidal discharge-rate amplitude and sine wave pressure amplitude. Similar curves from UBs of NTRs and
SHRs are overdamped and show no such peaking. In MBs, the response phase leads initially, starts to lag around
resonant frequencies, and falls behind progressively thereafter. In UBs, the phase also leads initially, then lags
monotonically to about —60°. The discharge rectifies at high frequencies probably as the result of nonlinear threshold
properties of the spike-initiating zone. The dynamic response curves of SHR and NTR aortas also were determined
and found to be flat to much higher frequencies than those of the baroreceptors. Thus aortic wall dynamics do not
limit the frequency range of the baroreceptors. There are no differences between NTRs and SHRs in the dynamics of
either their aortas or their baroreceptors. The differences in dynamics between MBs and UBs may be due to
differences in mechanical coupling related to structural differences between their endings.
CARDIOVASCULAR baroreceptors are connected to
the central nervous system by either myelinated or unmyelinated axons.1 The reflexes elicited by electrical stimulation of the two groups, which will be referred to as MBs
and UBs, differ quantitatively and qualitatively. Electrical
excitation of unmyelinated afferents in the aortic nerve
produces more profound depressor reflexes than excitation of myelinated afferents (Fig. 1 in paper by Douglas
and Ritchie2). Moreover, electrical excitation of myelinated cardiac vagal afferents may produce tachycardia,
whereas excitation of both myelinated and unmyelinated
cardiac vagal afferents elicits marked depressor effects.3
These results emphasize the differences in the central
connections of MBs and UBs but do not address the
question of the functional differences between the two
groups of receptors. Recently, Thoren and Thoren et al.
have compared the static characteristics of aortic MBs and
UBs and found that the threshold is higher and the
sensitivity to suprathreshold pressures is lower in UBs.
The dynamic characteristics have not been compared,
however. A comparison would clearly be important since
cardiovascular pressures are normally phasic and the
discharge of MBs and UBs has frequency-dependent
components.- In addition, the dynamic characteristics of
baroreceptors are largely unknown except for a few receptors which have been studied over limited frequency and
From the Department of Physiology and Biophysics, University of
Texas Medical Branch, Galveston, Texas.
Dr. Saum's present address is: Department of Pharmacology, Georgetown University, School of Medicine-School of Dentistry, Washington,
DC.
Supported by National Institutes of Health Grants HL-16657 and HL18798.
Address for reprints: Dr. Arthur M. Brown, Department of Physiology
and Biophysics, University of Texas Medical Branch, Galveston, Texas
77550.
Received August 11, 1977; accepted for publication January 6, 1978.
pressure ranges.6"8 For these reasons, a thorough examination of the dynamic characteristics of MBs and UBs
seems warranted. A comparison of the dynamic characteristics of baroreceptors from normotensive and spontaneously hypertensive rats (NTRs and SHRs, respectively)
also seems desirable, since they have different static
characteristics which are related to resetting and reduced
sensitivity of SHR baroreceptors.9i l0 This comparison
might provide more insight into the mechanisms underlying hypertensive resetting. Therefore, the dynamic characteristics of MBs and UBs from NTRs and SHRs 4-6
months of age were examined using small sinusoidal
pressures and small pressure steps. The suitability of the
linear analysis presently used was verified. The essential
observations are that the gain of MBs shows peaking and
the phase a clear inflection at the resonant frequencies,
whereas the gain of UBs is overdamped and the phase
declines gradually. These responses are not limited by the
dynamic properties of the aortic walls. There are no
differences in dynamics between baroreceptors of SHRs
and NTRs. The highly distinctive differences in dynamics
between MBs and UBs are interpreted according to the
structural differences reported for the two groups.
Methods
Experiments were performed upon male rats of the
Wistar-Kyoto strain (NTRs) or Okamoto-Aoki strain
(SHRs) of 4-6 months of age. The details of the preparation and recording methods have been described elsewhere,5' 9t 10 but several modifications have since been
incorporated and for convenience the essentials are shown
in Figure 1. The aortic segment was adjusted to approximate its in vivo position. Each receptor reported on in the
present study was tested with pressure steps of 10-30 mm
Hg and pressure sine waves. The sinusoidal analysis was
695
BARORECEPTOR DYNAMICS/Brown et al.
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performed in the steady state over a range of 0.1-20 Hz
at amplitudes of 10-60 mm Hg and at several mean
arterial pressures. Each sinusoidal analysis was bracketed
by static studies. Efforts were directed mainly toward
testing the response over its linear range which was
assessed from the static pressure-response curves.
Aortic segments from four NTRs and four SHRs were
excised in exactly the same way as in the receptor experiments in order to examine the dynamic properties of the
aortic walls. A chord of each aortic wall was measured
using a piezoelectric sonomicrometer" which has a measured flat (±5%) frequency response to 40 Hz. One
crystal was fixed to the aortic segment at a site corresponding to the terminal nerve branches depicted schematically
in Figure 1, and the other crystal was fixed directly across
from it on the opposite aortic wall. The mean pressures
and pulse amplitudes used as forcing functions covered
the range of pressures used in the receptor experiments
and included mean values between 80 and 150 mm Hg in
NTRs and between 110 and 180 mm Hg in SHRs.
response periods were averaged for a given stimulus x(t),
and the response y(t) was fitted by the method of least
mean squares error. Thus, the pertinent analytical expressions are:
x(t) = p sin (277ft) + P,, (mm Hg)
(1A)
y(t) = a sin (2irft + 0) + C (impulses/sec) (IB)
where Po = mean pressure
p = modulation pressure amplitude
C = mean discharge rate
a = modulation discharge amplitude
f = test frequency (Hz); f = 1/T with T being
one cycle period (sec) and 0 is the response phase shift
(phase lead when 0 > 0, phase lag when 0 < 0) and K =
a/p represents the gain of the system. 0 and K are plotted
against the test frequencies in graphs describing the frequency response transfer function of the linearized system.
The measurements of amplitude and phase derived from
the curve-fitting program were checked with direct measurements and there was close agreement.
Data Analysis
The spike trains were converted to instantaneous frequencies as described before." When sinusoidal pressures
were used, the instantaneous frequency curves were analyzed on a PDP 11/70 using an average frequency method
which divides a sine wave into 72 bins each covering 5° of
the cycle.12 The interspike interval in each bin is then
averaged over a number of cycles. Between 10 and 20
Sources of Error
Measurements of phase are less accurate than measurements of amplitude because the inaccuracies associated
with assigning interspike intervals to bins seem to have
larger effects on phase measurements. The delays due to
conduction in unmyelinated fibers have insignificant effects upon measurements of phase even at high frequen-
PRINTERPLOTTER
VALVE
FIGURE 1 Experimental arrangement for studying rat aortic baroreceptors in vitro. The computer analysis is done off-line.
CIRCULATION RESEARCH
696
cies. The filtering dynamics of the recording equipment
were not limiting, since they were negligible compared to
those of the baroreceptors. An absence of spikes in a part
of the cycle occurred at higher frequencies and was
associated with nonlinear discharge behavior termed rectification. 12'13 This reduced the goodness of fit made by
eye and the computer-averaged fitted functions were far
more satisfactory.
VOL.
mmHj
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Results
60
The barorecptors were divided into four groups, baroreceptors from normotensive and spontaneously hypertensive rats and baroreceptors having myelinated or unmyelinated axons. Of receptors with myelinated axons, 17
were examined in 11 NTRs and 8 in 6 SHRs. Of receptors
with unmyelinated axons, 4 were examined in 4 NTRs and
2 in 2 SHRs.
The mean systolic blood pressures measured by the
occlusive tail cuff method" were 110-125 mm Hg in
unanesthetized NTRs and 185-200 in unanesthetized
SHRs. The conduction velocities of the receptors' axons
ranged from 5 to 25 m/sec in MBs and 0.3 to 1.8 m/sec in
UBs. The static characteristics of these four groups of
baroreceptors have been described and are similar to
those found in vivo.5-9 As we have already reported,9
fibers can be studied for as long as 6 hours without any
change in their responses. Steady discharge in response to
a pressure step is attained within a few seconds and
persists for at least 3 minutes, which is the longest
duration we have used. The major differences between
SHR and NTR baroreceptors are that SHR baroreceptors
have higher pressure thresholds, but threshold and maximum asymptotic discharge frequencies are similar. The
major differences between MBs and UBs from either
NTRs or SHRs are the UBs have higher pressure thresholds and much lower threshold and maximum asymptotic
discharge frequencies.
40
y
A,
\ A i\ A A
V
80 _
a
X
/V
20
0
0.8
1.6
2.4
3.2
TIME
4.0
4.8
300
360
(SEC)
5
o
60
120
180
240
DEGREES
6 HZ
] i 0 mm Hg
160
Baroreceptor Response to Sine Waves of Pressure
FIGURE 2 Response of an NTR baroreceptor with a myelinated
axon )mb) to pressure sine waves 5 mm Hg in amplitude at
frequencies of I (A) and 6 Hz (B). The top panels show neural
discharge on the top trace and pressure sine waves below. The
middle panels show the instantaneous frequency which is the
inverse period of the discharge (above) and pressure sine wave.
Note the differences in the pressure calibrations between the two
middle panels (done to avoid running the pressure wave through
the instantaneous frequency points in B), and also note that
frequency is the only difference between the two pressure sine wave
forcing functions. The bottom panels show the average frequency
values obtained by adding 20 successive cycles, represented by the
filled circles. The solid lines are the curves fitted to the data by the
least mean squares error method using Equation IB in the text.
Note the rectification that appears at higher frequencies and the
computed sine wave fit at these frequencies.
MAY
I HZ
*
Pressure sine waves applied over the linear response
range elicited corresponding periodic discharge activity in
MBs and UBs (Figs. 2 and 3), the UBs having lower
discharge frequencies and higher static thresholds. At low
stimulus frequencies, the instantaneous discharge frequen-
42, No. 5,
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120
.0
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0
0.2
0.4
0.6
TIME
0.8
(SEC)
160
140
120
>
u
2
20
DEGREES
1.0
1.2
1978
BARORECEPTOR
O.I HZ
]40mmHg
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*0
SO 40 70 80
TIME (SEC)
»0
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I
o
DEGREES
DYNAMICS/SWHTI
et al.
697
cies were sinusoidal (Fig. 2A). There was greater scatter
in the UB frequencies, as was anticipated from earlier
studies on their static discharge characteristics.5 The average values of amplitude and phase for 10-20 periods were
readily fitted either by eye or by the computer. When
higher frequencies were used while maintaining amplitude
and mean pressures constant, the discharge no longer
occurred during the falling part and nadir of the sine
waves (Figs. 2B and 3B). The interruption of discharge
persisted despite increases in mean pressure of 5-15 mm
Hg. The effect was not related to a change in the static
characteristics since the steady state pressure-response
curves were identical before and after each sinusoidal
analysis performed in the present experiments. This phenomenon has been reported for other mechanoreceptors12'13
and is called rectification. It reflects the nonlinear nature
of neural discharge in sensory receptors that occurs when
the generator potential which responds sinusoidally falls
below values predicted from consideration of the static
threshold. When rectification occurred, the amplitude and
phase of the instantaneous frequency curves determined
by eye were not as accurate as those determined by the
computer.
As frequency increased, the number of impulses per
cycle decreased, but the number of impulses per second
remained constant for MBs and UBs in either NTRs or
SHRs (Fig. 4). Again, the level of discharge is lower in
UBs. Similar observations have been reported by other
investigators.7-8 At very high frequencies, the number of
impulses per second rose in UBs.
Linear Range of Baroreceptor Responses
1.0 HZ
]40mmHg
z
160
10
It
12
TIME
13
14
(SEC)
E
E
The linear range of the response was estimated for each
receptor from steady state pressure step-response
curves9'10 (Fig. 5). Increasing the amplitudes of the steps
produced linear increases in discharge over the range of
pressures used for the linear analyses. The receptor discharge rose sharply in response to a step input of pressure,
but with a definite time duration, and then relaxed slowly
to a steady level. Although the fast phase gave rise to a
pronounced overshoot, there were no detectable oscillations during the slow phase; many UBs, however, showed
an irregular steady state discharge.5
Linearity was also examined by measuring the receptor
response to increases in amplitude of the pressure sine
waves. Increases in amplitude were less than 40 mm Hg
and did not pass through either threshold pressure or the
pressure for maximum asymptotic discharge (saturation).
Under these conditions, the amplitude of the receptor
response increased linearly with increases in sine wave
amplitude and there was no change in phase (Fig. 6).
Similar results occurred in three baroreceptors with mye-
U
z
w
O
0
60
120
ISO
240
DEGREES
300
360
FIGURE 3 Response of an NTR baroreceptor with an unmyelinated axon (UB) to pressure sine waves 20 mm Hg in amplitude at
frequencies of 0.1 (A) and I Hz (B). Arrangement as for Figure 2.
Compared to MBs, there is more scatter in the instantaneous
frequency data, the amplitude of the discharge sine wave response
is reduced, the mean discharge frequency is reduced, and the
pressure threshold is increased.
CIRCULATION RESEARCH
698
A
VOL.
42, No. 5,
MAY
1978
B
400
100
200
80
100
oi
60
40
40
u
U
20
u
10
1/5
UJ
to
=5
20
Q_
a.
5
10
0.1
0.2
0.4
!
2
FREQUENCY
4
10
0.1
20
0.2
0.4
1
2
FREQUENCY
(HZ)
4
10
20
(HZ)
FIGURE 4 The effects of pressure sine wave frequencies upon baroreceptor discharge per cycle (A) and per second (B). • . MB; • , UB.
The values for p and Po (see Equation I A) are 18 and 99 for the MB and 16 and 180 for the UB. There is an inverse relationship between
spikes/cycle and frequency but the discharge/unit time is unchanged over most of the frequencies. The log-log scales are used for data
compression.
linated axons (two NTRs and one SHR) and in two
baroreceptors with unmyelinated axons (one NTR and
one SHR).
Frequency Response Characteristics of MBs and UBs
from Normotensive and Spontaneously Hypertensive
Rats
Gain and phase plots for individual MBs and UBs from
NTRs and SHRs were normalized for comparison. The
gain of MBs increased at about 1 Hz, peaked between 5
and 9 Hz, and decreased rapidly at higher frequencies.
The average values ±1 SEM for 37 runs in 17 NTR MBs
and 27 runs in 8 SHR MBs are shown in Figure 7. There
were no significant differences in the gain curves of SHR
and NTR baroreceptors with myelinated axons. The corresponding average phase curves are shown in Figure 8.
There was a small phase lead at low frequencies which
converted to phase lag at the resonant frequencies and fell
rapidly to values of around -120°. Again, there were no
significant differences between MBs from SHRs and
NTRs.
The gain curves of UBs differed quite markedly from
B
u
LU
80
LSES/
UJ
60
80
60
3
a.
1
u
40
QUENCY
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0.4
20
UJ
O
;
10
o
TIME
(SEC)
12
ul
20
8
12
16
20
PRESSURE
24
28
32
(mm Hg]
FIGURE 5 Baroreceptor discharge in response to small pressure steps, the values of which (in mm Hg) are alongside each step response
(A). The baseline pressure was 115 mm Hg. The peak, • , and steady state, • , responses are linearly related to the amplitude of the step (B).
The sine wave analyses were performed over this range of pressures.
BARORECEPTOR DYNAMICS/flrovw et al.
U 80
uj
A A AA AA
s,-' V
V V V v
120
m
3
to
v
ioo £
o
UJ
3
4
TIME
5
(SEC)
I
E
to
=3
Q_
699
those of MBs. There was no peaking and the gain started
to deteriorate at frequencies above 0.1 Hz and continued
to fall monotonically. There was a small phase lead at low
frequencies and the phase fell continuously, reaching
values of about -60° at the highest frequencies tested
here. We have compared the gain and phase behaviors of
four UBs from NTRs (nine runs) and two UBs from SHRs
(six runs), and they were similar. The average gain and
phase values for these six units are shown in Figure 9.
We also tested the effects of mean pressure Po on the
dynamic response curves. Pressures that saturated receptor discharge or dropped it below threshold were avoided.
In the linear range, changes of 20-40 mm Hg in Po had no
effect on the dynamic response curves of MBs or UBs
from SHRs and NTRs. Thus, as long as the operating
point is within this range, it does not affect the dynamic
characteristics of the baroreceptors for all cases.
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Frequency Response Characteristics of Excised Aortas
2 20
iZ.
0
I
2
3
4
TIME
5
6
(SEC)
FIGURE 6 Baroreceptor discharge (points) in response to increases in amplitude of pressure sine waves (solid lines). Between
A and B the amplitude was increased 2.5 times from 4 to 10 mm
Hg. The response amplitude (peak to peak) correspondingly
increased approximately 2.5-fold from 18 impulse/sec to 49 impulse/sec.
The dynamic responses of excised aortic segments were
compared in four NTRs and four SHRs. The piezoelectric
crystals measured similar aortic chords in the two groups.
The response of the chords to pressure sine waves ranging
from 10 to 40 mm Hg in amplitude was sinusoidal and
followed frequencies from 0.1 to 15 Hz with only small
reductions in gain which became somewhat larger at 20
Hz (Fig. 10). The dynamic ranges have also been examined when mean pressures ranging from 80 to 150 mm Hg
in NTRs and 110 to 180 mm Hg in SHRs were used, and
the results were similar to those shown in Figure 10.
B
NTRs
CD
CO
-a
-a
<
-8
- 8
- 10
0.4
-10
2
4
10
FREQUENCY
20
40
(HZ)
100
200
0.4
2
4
10
FREQUENCY
20
40
100
200
(HZ)
FIGURE 7 Gain curves of MBs from normotensive and spontaneously hypertensive rats. A: Average gain curves normalized to the value at
the lowest frequency tested for 17 MBs from 11 NTRs. Gain or K is defined in text. Error bars are ± I SEM. B: Average gain curves for 8
MBs from 6 SHRs.
CIRCULATION RESEARCH
700
VOL. 42, No. 5, MAY 1978
B
40
NTRs
40
SHRs
20
20
|
0
o
)EGREE
UJ
LU
- 20
•
.
- 20
- 40
PHASE
-40
<
X
a.
•
- 60
- 80
- 100
- 100
- 120
- 120
- 140
0.2
0.4
1
2
4
FREQUENCY
10
I
i • • •
0.1
20
i
0.2
. , , , , . . .
0.4
.
1
2
. . . . . . . .
4
FREQUENCY
(HZ)
•
10
20
(HZ)
FIGURE 8 Average phase curves for MBs from normotensive (A) and spontaneously hypertensive (B) rats. Data were obtained from the
same experiments shown in Figure 7.
These results will be included in a forthcoming report of a
much more extensive study on the wall characteristics of
aortas from NTRs and SHRs (Andresen, Krauhs, and
Brown, unpublished observations). Thus, the frequency
response range of the excised aortic segments is considerably greater than that of the aortic baroreceptors con-
tained therein. There were no differences in the normalized gain curves between SHR and NTR aortas (Fig. 10).
Discussion
The present experiments on the dynamic characteristics
of baroreceptors were interpreted using linear analysis;
B
60
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20
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o
HASE
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- 80
0.1
1
\
\
- 60
-20
-40
-60
-80
- 12
0.4
1
2
FREQUENCY
20
(HZ)
-100
0.1
0.2
0.4
1
2
FREQUENCY
4
(HZ)
10
20
FIGURE 9 Gain and phase curves of unmyelinated baroreceptors (UBs). A: Average gain curves of UBs from four NTRs and two SHRs
normalized as in Figure 7. There were no differences in individual gain curves between SHRs and NTRs. B: Average phase curves of UBs
from the same experiments used in A.
BARORECEPTOR DYNAMICS/Broww et al.
B
,.5
1.5
NTRs
o
O
10
701
SHRs
10
<
I 0.5
o
2 0.5
o
15
10
FREQUENCY
20
(HZ)
10
FREQUENCY
15
20
(HZ)
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FIGURE 10 Dynamic characteristics of excised aortic segments from NTRs and SHRs. The gain is the ratio of the amplitude of the
sinusoidal changes in chord dimensions and the amplitude of the sinusoidal pressure wave and is normalized to the lowest frequency values.
Mean pressures were 130 mm Hg for the NTRs and 150 mm Hg for the SHRs, and the pressure sine wave amplitude was 20 mm Hg. Data
are mean ± I SEM from four NTRs (A) and four SHRs (B). The dynamic range of the aortic wall is much greater than that of the aortic
baroreceptors (compare with Figs. 7 and 9A). The dynamic ranges were at least as great at other mean pressures ranging from 80 to 150 mm
Hg in NTRs and from UOto 180mm
HginSHRs.'4
this is justified since a linear range of responses has been
clearly identified. The approach we have taken has been
only infrequently reported for baroreceptors and usually
over very limited ranges of frequencies and mean and
modulating pressures.6"8 Our experiments produced several new results. Baroreceptors with myelinated axons
(MBs) show peaking in their gain curves, and baroreceptors with unmyelinated axons (UBs) have overdamped
responses. The static characteristics of MBs and UBs also
differ.5 In contrast, baroreceptors from normotensive and
spontaneously hypertensive rats 4-6 months in age have
similar dynamic characteristics, but their static characteristics are quite dissimilar.9 While the gain of SHRs should
be less at frequencies approaching zero, our range for
comparison did not extend below 0.4 Hz where differences in gain may have occurred.
The dynamic characteristics are probably due to mechanical filtering arising either from the coupling of the
receptors to the vessel wall or from the receptors themselves. The dynamic ranges of the aortic wall and the
receptors' axons for all four categories of baroreceptor
examined presently extend beyond the limited response
range of the receptors5'» (Figs. 7, 9A, and 10). The
dynamic response is also unlikely to be limited by the
receptor membrane, since mechanoreceptor generator
potentials following frequencies of 300 Hz have been
reported.14' ' 5
At higher frequencies, rectification appears. As noted
already, this is related to the nonlinear nature of the
discharge from sensory receptors —when the generator
potential falls below threshold for spike initiation, discharge ceases. Rectification may cause or contribute to
the phasic discharge recorded from the entire carotid sinus
nerve trunk at pressures well above threshold. An earlier
interpretation of this observation, namely, that baroreceptors operate normally only at or near threshold, therefore
needs to be reevaluated.8
The transient response to step stimuli has an initial rise
which is rapid but not instantaneous; this was either
overlooked or ignored previously.6 The transient develops
an overshoot followed by a slow relaxation to a steady
state discharge. These qualitative features are similar in
MBs and UBs. 5 ' 9 The slow phase is a monotonically
decreasing function showing no sign of oscillation. This
suggests that the slow process in MBs and UBs may not
differ greatly. Quantitative assessment of the fast phase is
difficult to make accurately in the time domain. The
equivalent examination of frequency response data at
intermediate to higher frequencies is much easier because
the region of interest is expanded. Indeed, this has revealed the significant difference in the fast phase between
MBs and UBs, as we have already noted. The nature of
this difference poses a hypothesis that the fast phase is
due to a second order filter, which again may well be
common for MBs and UBs, except for its damping factor
£; i.e., £ < 1 or underdamped for MBs and £ > 1 or
overdamped for UBs. Further quantitative study is needed
to substantiate the above theory that attributes the observed dynamic difference ultimately to a single parameter
{•
Interpretation and Significance of the Functional
Differences between Baroreceptors with Myelinated or
Unmyelinated Axons
A tentative explanation of the differences in the dynamic and static characteristics of UBs and MBs may
come from the structural differences between their endings. An electrophysiological explanation is unlikely, since
the amount of current required to discharge a UB should
be smaller, thereby reducing threshold, which is inconsistent with our observations. Complex, unencapsulated baroreceptor endings (CUE) are usually connected to axons
which are myelinated in the nerve trunk (MBs) although
their terminations may be unmyelinated.16- " On the other
hand, axons that are unmyelinated in the nerve trunk are
usually connected to simple endings. CUEs have extensive
702
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branching and small protrusions, free of covering Schwann
cells, which are connected to the nearby elastin and
collagen fibers.16-17 Thus they may be tightly coupled to
the vessel wall. Whereas the details of the connections
between simple endings and the vessel wall are not known,
it is unlikely that they are coupled so tightly. These
structural differences could be responsible for the higher
static threshold, reduced sensitivity, and overdamped dynamic response found in UBs and the opposite results
found in MBs. It seems intuitively reasonable to associate
"tight endings" and "loose endings," respectively, with a
lower threshold and a higher one. In this regard, for
instance, one can imagine a possible effect of tissue
wrinkling. The structural differences also appear to support our theory which attributes the filtering difference
between MBs and UBs to the damping factor £ of the
hypothetical common second order filter. This is because,
compared to tight endings of MBs, loose endings of UBs
should dissipate a given mechanical energy more quickly
and thus result in a greater damping effect.
The physiological significance of the differences in the
dynamics between MBs and UBs is not known. Some
speculation seems justified, since baroreceptor discharge
patterns have frequently been used to account for sinoaortic Blutdruck-characteristics.7-8 It is of interest that MBs
of the rat show resonant enhancement for pressure signals
at or near the heart rate usually present in these animals,
namely, 4-5 Hz, and it would be interesting if a similar
correlation were present in other species with different
resting heart rates. The gain of MBs is 4 times that of UBs
at these frequencies, and UBs clearly provide poor feedback to the central nervous system (CNS) with respect to
heart rate information. The well-known frequency dependence of carotid sinus reflexes18"20 may also be related
to the resonant characteristics of MBs. On the other hand,
the observation that the input to the CNS per unit time is
relatively unaffected over a wide range of frequencies in
both UBs and MBs, despite the large differences between
their dynamic responses, needs to be considered as well.
Implications of the Dynamic and Static Characteristics of
NTR and SHR Baroreceptors
The similarities of the dynamic responses of NTR and
SHR baroreceptors indicate some similarities in mechanical filtering, since the latter is probably mainly responsible
for the dynamic responses we have observed. The differences in static characteristics, namely, resetting and reduced sensitivity, may also have a mechanical explanation.
Thus the coupling to the aortic wall of both MBs and UBs
from SHRs may be reduced equivalently. An alternative
possibility, however, is that baroreceptor resetting is due
VOL.
42, No. 5,
MAY
1978
to intrinsic changes in the receptors themselves.21 This
problem will be examined further in a paper (Andresen,
Krauhs, and Brown, unpublished observations) which
compares the static and dynamic characteristics of NTR
and SHR aortas as they relate to hypertensive resetting.
Acknowledgments
We thank Tim Farrell for expert technical assistance.
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Baroreceptor dynamics and their relationship to afferent fiber type and hypertension.
A M Brown, W R Saum and S Yasui
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Circ Res. 1978;42:694-702
doi: 10.1161/01.RES.42.5.694
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