Download Mechanism of relation among heart meridian, referred cardiac pain

Vol. 45 No. 5
SCIENCE IN CHINA (Series C)
October 2002
Mechanism of relation among heart meridian, referred
cardiac pain and heart
RONG Peijing () & ZHU Bing (
)
Institute of Acupuncture-Moxibustion, CATCM, Beijing 100700, China
Correspondence should be addressed to Zhu Bing (email: [email protected])
Received March 27, 2001
Abstract It has been demonstrated that an important clinical phenomenon often associated with
visceral diseases is the referred pain to somatic structures, especially to the body area
of homo-segmental innervation. It is interesting that the somatic foci of cardiac referred pain were
often and mainly distributed along the heart meridian (HM), whereas the acupoints of HM have
been applied to treat cardiac disease since ancient times. The purpose of this study was to investigate the neural relationship between the cardiac referred pain and the heart meridian.
Fluorescent triple-labeling was injected into the pericardium, some acupoints of HM and lung meridian (LM, for control). The responses of the left cardiac sympathetic nerve and of the EMG in left
HM and LM were electrophysiologically studied, when the electrical stimuli were applied to the
acupoints of left HM and to the left cardiac sympathetic nerve. More double-labeled neurons in
HM-heart, not in LM-heart, were observed in the ipsilateral dorsal root ganglia of the spinal segments C8-T3. Electric stimulation of the acupoints of left HM was able to elicit more responses of
left cardiac sympathetic nerve than that of the LM-acupoints. Electric stimulation of the left cardiac
sympathetic nerve resulted in stronger activities of EMG-response in the acupoints of left HM than
in LM-acupoints. We conclude that double-labeling study has provided direct evidence for the existence of dichotomizing afferent fibers that supply both the pericardium and HM. Electrophysiological results show that HM is more closely related functionally to heart. These findings provide a
possible morphological and physiological explanation for the referred cardiac pain and HM-heart
interrelation.
Keywords: dichotomization of DRG, heart, heart meridian, somato-visceral connection.
Chinese ancient physicians and philosophers put forward the meridian doctrine of the traditional Chinese medicine, based on extensive clinical phenomena and therapeutic regularity. It is
composed of 14 lines distributed in limbs and trunk. The relationship between meridians (soma)
and zang-fu organs (viscera) is a key content of the meridian doctrine, which comprises the morpho-functional unit of somatic-visceral connections. The heart meridian (HM), one of the 14 meridians, is distributed along the medial part of the hand and brachium from first finger to the axilla.
All the nine acupoints in HM can be used for treating coronary heart disease. Surprisingly, the
somatic foci of referred cardiac pain are identical with the course of HM. We suggest that they
have common biological bases.
From the physiological viewpoint, one basis for convergence of visceral and somatic sensory
information onto central neurons may be by way of the shared primary afferent fibers, and another
basis of viscero-somatic convergence would be at the level of neurons in the central nervous sys-
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tem that are activated independently by visceral and somatic primary afferent fibers[1]. The latter
mechanism would be most consistent with the “convergence-projection theory” of referred pain.
Visceral pain is usually referred to a somatic area innervated by the homo-segments of spinal cord
that receive the input from the originating viscus[2]. The connection between meridians and zangfu
organs is also the basis of convergence of visceral and somatic afferent onto central neurons in the
same spinal cord segments. Meridian-zangfu and somato-visceral connections might be the common realization in different times and in different scientific background.
In the present study, we investigated the morphological and neurophysiological mechanisms
of referred cardiac pain, heart and meridian-heart interrelation. A preliminary report of this work
has appeared in an abstract form[3].
1 Methods
Experiments were performed on Sprague-Dawley rats weighing 200300 g anesthetized
with urethane. Fluorescent dyes (Sigma), Bisbenzimide (Bb), Propidium Iodide (PI) and Fast Blue
(FB) were composed in a saturated solution.
In the morphological experiments, following anesthetization a longitudinal incision, about 3
cm in length, was made in the rostral portion of the ventral abdominal wall exposing the central
tendon of the diaphragm. The position of the pericardial sac was located by identifying the base of
the heart, which could be visualized through this thin central tendon. The pericardial sac was injected transdiaphragmatically with 15 µL of Bb and the site was then sealed with paraffin wax,
which set as the needle was withdrawn to prevent leakage of the dye from the injection site. The
incision was sutured and local antibiotics were used routinely. PI and FB were respectively
microinjected into three HM acupoints (HT 1-3) on the medial side of the left brachium and three
lung meridian (LM, for control) acupoints (LU 1, 3, 4) on the lateral side of the left brachium
(each acupoint for 5 µL, about 24 mm in depth). It was noted that the points injected both in
HM and LM should be located in the identical height anatomically. The injection sites were sealed
with paraffin wax as the needle was withdrawn and the animal again allowed recovering. After an
appropriate survival of 24 h, the rat was anesthetized and perfused transcardially with 4% sodium
citrated saline until the perfusate was clear, which was followed by 4% paraformaldehyde in
phosphate buffer (0.1 mol/L, pH 7.4). The left dorsal root ganglia (DRG) of the spinal segments
cervical 6 (C6) to thoracic 4 (T4) were removed and immersed in a 4 solution of 15% sucrose in
phosphate buffer. Sections (30 µm) were made by a cryostat at −20. The sections were then
viewed using a Nikon-E600 microscope with a fluorescence vertical illuminator producing ultraviolet light of 365 nm excitation wavelength. Photomicrographs were made utilizing the same
microscope.
In the electrophysiological experiments, following surgery (tracheotomy) under anesthetics
of 10% urethane (1 g/kg, ip), the animals were artificially ventilated through a tracheal cannula;
the rate and volume of ventilation were adjusted to maintain a normal acid:base equilibrium as
assessed with a capnometer. Heart rate was monitored continuously and core temperature was
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maintained at 37 ± 1.0 by means of a homeothermic blanket system. Following anesthetization
a longitudinal incision, about 2 cm in length, was made in the anterior cervical wall for exposing
and isolating the left cardiac branch of sympathetic post-ganglionic fibers for stimulating or recording with a bipolar electrode. For electric stimulation and recording electromyography (EMG)
responses of acupoints, a pair of noninsulated needle electrodes were inserted s.c. in HT3 and HT
7 acupoints, about 35 mm in depthon the medial side of the left brachium, and another pair of
electrodes were inserted in LU5 and LU9 acupoints on the lateral side of the left brachium. The
electric stimulus consisted of a volley of 25 rectangular pulses of 0.1 ms duration and was delivered every 30 s from a constant current stimulator. The stimulus intensity was initially adjusted
to 1 mA and gradually increased to 10 mA. Such stimulation procedure might evoke stable cardiac
sympathetic nerve and EMG responses. The cardiac sympathetic nerve responses and the EMG
responses were fed to a storage oscilloscope to allow monitoring the experiments, and to a computerized system (Power-Lab system) for on-line digitization of data. The individual responses
were plotted either against time to allow the study of their temporal evolution or against stimulus
strength to build recruitment curves.
For each sequence, numerical data concerning the effects of electro-acupuncture stimuli and
cardiac sympathetic nerve stimuli were expressed as spike numbers. Global mean results were
performed analysis of variance and were always presented as means ± SE. Paired t test and linear
regression analysis were used for studying the significance of variations; a P value below was
considered being significant.
2
Results
Neurons labeled with fluorescent dyes were observed in the left DRG (C6-T4) of 11 rats and
were mainly moderate (1530 µm) and small (<15 µm) cells. The fluorescence characteristics of
the different tracers briefly described that, when the preparations were viewed under light of 365
nm wavelength, Bb containing neurons possessed a blue-green fluorescing nucleus while the cytoplasm was unstained or fluoresced only slightly; PI and Fb stained the cytoplasm orange-red and
blue, respectively. Single-labeled neurons in HM and LM injection were 31.94% and 26.40%, and
restricted mainly to C6-T1 and C6-8 segments, respectively; the neurons labeled following heart
injection were 26.83% and the majority of single-labeled neurons was mainly restricted to segments T1-2 in the range of C6-T4 distributions. Fig. 1 shows the mean numbers of double-labeled
neurons within the C6- T4 ganglia of 11 rats in 3 groups. HM-heart and LM-heart injection double-labeled neurons were 6.81% and 3.97% and mainly in C8-T3 and C7-8 segments, respectively;
HM-heart double-labeled neurons were much more in comparison with the control LM-heart injection (P < 0.05). Only a few double-labeled neurons were observed in HM-LM injection (2.77%)
and triple-labeled cells in HM-LM-heart injection were 1.28%. The distinction of Fb and Bb double-labeled neurons (HM-heart injection) is shown in plate I-3, in which the blue-green fluorescing nucleus and light-blue fluorescing cytoplasm can be clearly viewed; in plate I-4, the Fb and PI
double-labeled neuron (LM-heart injection) of fluorescing the blue-green nucleus and orange-red
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cytoplasm can be distinguished. Plate I-1 is PI labeled neurons injected into the LM acupoint and
plate I-2 is Bb labeled neurons injected in heart.
2.1 Left sympathetic responses induced by ipsilateral HM and LM acupoints stimuli
2.1.1 General characteristics of the sympathetic activities. In the majority of condition, the
cardiac sympathetic nerve did not present spontaneous activity at the beginning of the testing sequences and a proportion developed residual
activity; this activity consisted of irregular
discharges of low magnitude. The cardiac
sympathetic nerve responded to electric
stimulation applied percutaneously to ipsilateral HM acupoints and two reflex components could be evoked in supramaximal electrical stimulation. Early and late reflex potentials had a latency about 20ü40 and 100ü
Fig. 1. Distributions of double-labeled neurons in every dorsal
root ganglia. , HM-heart; , LM-heart; , HM-LM.
150 ms, respectively. In the viewpoint of Sato,
these responses were induced by the afferent myelinated (A) fibers and unmyelinated (C) fibers
activations[4].
2.1.2 Different thresholds of cardiac sympathetic nerve responses to stimulation applied to HM
and LM acupoints. The cardiac sympathetic nerve gave the responses to electroacupuncture applied to ipsilateral HM acupoints with a mean threshold of (2.92 ± 0.38) mA; while same responses were elicited with a mean threshold of (4.92 ± 0.54) mA in LM acupoints stimulation, and
the difference was highly significant (P < 0.05, n = 18).
Fig. 2. Stimulations in HM-(left) and LM-(right) acupoints elicited
ipsilateral cardiac sympathetic nerve responses. Stimulation of HM
acupoints elicited larger sympathetic nerve responses in 2- to 8-mA
range than that of LM acupoints.
2.1.3 Different magnitudes of cardiac
sympathetic nerve responses to stimulation
applied to HM and LM acupoints. All of
the cardiac sympathetic nerve responses
were elicited by HM and LM acupoints
stimuli. This is illustrated by an individual
example in fig. 2. It can be seen that the
magnitudes of the responses of the cardiac
sympathetic nerve were directly related to
the stimulation applied to HM and LM
acupoints at 28 mA. These findings are
corroborated by the cumulative results presented in table 1, showing that both HM and LM acupoint stimulations monotonically increased
the cardiac sympathetic nerve discharges in response to the application of electrical stimuli at 2
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8 mA. 4-mA HM acupoint stimulation induced 11.62 ± 2.30 spikes in the sympathetic nerve,
while the same stimulation of LM acupoints only produced 7.15±2.07 spikes. It is interesting to
note that stimulation of HM acupoints elicited larger sympathetic nerve responses in 2- to 8-mA
range than that of LM acupoints, and the difference was highly significant (P 0.050.02, n =
13).
Table 1 Comparison of cardiac sympathetic nerve responses induced by stimulations in ipsilateral HM- and LM-acupoints
HM-acupoints
LM-acupoints
2.2
2 mA
4.231.49
1.540.87
4 mA
11.622.30
7.152.07
6 mA
18.623.23
12.082.51
8 mA
20.852.99
14.072.61
Left cardiac sympathetic nerve stimuli induced acupoints EMG responses in HM and LM
2.2.1 General characteristics of the EMG activities. When the needle electrode was inserted
into the muscle, it evoked violent and irregular discharges of action potentials, only with a brief
lasting of about 500 m and small amplitude. Electric stimulation of the cardiac sympathetic nerve
elicited an EMG reflex multiphasic response in the ipsi-lateral HM and LM acupoints with 20
80 ms latencies for duration of 100200 ms; while these responses occasionally maintained 1 s
duration in the condition of supra-maximal stimulation.
2.2.2 Different thresholds of EMG reflex responses of HM and LM acupoints induced by
stimulation of cardiac sympathetic nerve. The left cardiac sympathetic nerve stimulation elicited
EMG reflex response in the ipsilateral HM acupoints with a mean threshold of (1.75 ± 0.22) mA,
while same responses elicited in LM acupoints required the stimulation with a mean threshold of
(3.25 ± 0.41) mA. This difference was highly significant (P < 0.02, n = 12).
2.2.3 Different magnitudes of EMG reflex responses both at HM and LM acupoints elicited by
ipsilateral cardiac sympathetic nerve stimulation. Electric stimulation of the cardiac sympathetic
nerve elicited EMG reflex responses both at HM and LM acupoints in simultaneous application of
a Double-Amplifier-Power-Lab system. The
magnitudes of the responses of EMG were
directly related to the stimulation applied to
cardiac sympathetic nerve in 26 mA range,
Fig. 3. Left cardiac sympathetic nerve stimulations in different strength induced EMG responses at both HM- and
LM-acupoints. Cardiac sympathetic nerve stimulations elicited
larger EMG responses at HM acupoints in 26 mA range.
and the responses increased as stimulus
strength increased. These findings are corroborated by the cumulative results presented
in fig. 3, showing that the cardiac sympathetic
nerve stimulations monotonically increased
EMG discharges in response to the application
of electrical stimuli in 26 mA range. 4-mA
sympathetic nerve stimulation induced 11.17 ±
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1.96 action potentials at HM acupoints, while same stimulation produced only 4.67 ± 1.26 action
potentials at LM acupoints. It is interesting to note that the cardiac sympathetic nerve stimulations
elicited larger EMG responses at HM acupoints in 26 mA range than those at LM acupoints.
This difference was highly significant (P 0.010.05, n = 12).
3
Discussion
Single-, double- and triple-labeled neurons were observed in C6-T4 DRG following injection
of fluorescent tracers into the pericardium and left medial (heart meridian) or lateral (lung meridian) brachium. Our results clearly indicate that more dichotomizing fibers that supply both the
pericardium and the medial brachium exist and a closer relation between heart meridian and heart
is observed. Dichotomizing afferents fibers have been proposed as a possible explanation for referred pain [1]. Cardiac pain is frequently referred to the medial but not the lateral aspect of the left
brachium, and the referred region is identical with the route of the heart meridian, which suggests
the existence of a similar mechanism between heart meridian and cardiac activity.
A number of different mechanisms have been suggested in order to explain the somatic localization of the sensory experience evoked from viscera. All these models take into account the
clinical observation that visceral pain is usually referred to a somatic area innervated by the same
spinal segments that receive the input from the originating viscus[2]. Therefore, all the neurophysiological interpretation of visceral pain is based on viscero-somatic integration, that is, the convergence of inputs from viscera and from somatic structures onto sensory neurons whose activation
leads to the experience of somatic pain [2]. The simplest form of viscero-somatic convergence occurs in primary afferent fibers with multiple peripheral branches that innervate sensory receptors
in skin, muscle and viscera. Electrophysiological and double-labeling studies have provided direct
evidence for the existence of dichotomizing peripheral afferents that supply both visceral and somatic structures. Bahr et al.[5] have reported that such dichotomizing primary afferent neurons
constitute 18% of the total number of cells in the DRG of the cat. But in the rat, Dawson et al.[6]
demonstrated that, by means of intraneural injection of tracers, the frequency of pre-spinal
somato-visceral convergence averaged 2%. Alles and Dom[7] have observed dichotomizing afferent fibers in ipsi-lateral DRG neurons of spinal cord segments C8, T1 and T2, when two tracers
were respectively injected into the pericardium and the brachium. This finding provides a possible
morphological explanation for referred cardiac pain. We would therefore like to propose the dichotomizing pericardial and heart meridian in the medial brachium afferent as an alternative anatomical base for explanation of both the referred cardiac pain and heart meridian-heart.
Acupuncture treatment of coronary heart disease is a common therapy, and one of its main
actions is to relieve angina pectoris. Acupuncture or somatic physical and chemical stimulations
are able to produce satisfactory analgesia, especially in the condition of these stimuli application
at homo-segmental level with relative visceral organ. Selze and Spenser[8] reported that the
somatic stimulations could strongly inhibit afferent activities of viscera. It produced an inhibition
of transmission by depolarization of the afferent terminals and is involved in pre-synaptic
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SCIENCE IN CHINA (Series C)
inhibition. Several groups[9
ü11]
Vol. 45
have clearly observed that the visceral afferent activities in some
neurons of dorsal horn in the spinal cord can be strongly depressed by homo-segmental somatic
stimuli. Our pervious results have already confirmed that the nociceptive reflex and painful sensation were inhibited by the electroacupuncture applied at the homotopic acupoint in the strength
less than pain threshold[12].
In general viewpoint, the bases of meridians-viscera connection are involved in somato-sympathetic reflex, and acupuncture at HM-acupoints is able to treat some cardiac diseases. We observed that, in the present study, electric stimulation of left HM-acupoints evoked greater reflex
responses of the ipsi-lateral cardiac sympathetic nerve than the same stimulation applied to the
LM-acupoints. Left cardiac sympathetic activity was especially able to strengthen the myocardial
function and to produce the coronary vasodilatation in muscular vasoconstriction[13]. These reflex
responses elicited by the stimulation applied to HM-acupoints are helpful to the treatment of coronary heart disease.
Sato[14], on the basis of reviewing the majority data, has indicated that somatic afferent nerve
stimulation can regulate various visceral functions by reflex responses and the effects of somatic
afferent stimulation are dependent upon the particular organs and on the spinal afferent segments.
These responses include both generalization (e.g. in cerebral cortical blood flow, heart rate, and
adrenal medullary hormonal secretion and splenic immune function) and strong segmental organization. The responses of sympathetic and parasympathetic efferent nerves to the somatovisceral reflexes depend on the individual organ, the segmental site being stimulated and the nature or intensity of the stimulation. Somato-sympathetic reflexes are strongly segmental in the
spinalized animal preparation.
The result, that electric stimulation of the left cardiac sympathetic nerve resulted in strong activities of EMG-response in the acupoints of left HM than in the acupoints of left LM, indicates
that somato-sympathetic reflexes have clearly the characteristic of segmental distribution[15]. The
various studies[16
ü19]
tend to suggest that the myotonic band appears along some meridians or
along longitudinal muscles in the condition of visceral pathologies, which is in the homo-segmental innervation with the somatic structures.
Acknowledgements This work was supported by the Climbing Program of Chinese Committee of Sciences and State
Administration of Traditional Chinese Medicine.
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RONG Peijing et al.: Heart meridian & referred cardiac pain
Vol. 45
Plate I
Photomicrograph of DRG double-labeled neurons. 1, PI single labeled neurons injected into the LM acupoint; 2, Bb single
labeled cells injected in heart; 3, HM-heart Fb and Bb double-labeled neurons viewed the blue-green fluorescing nucleus and
light-blue fluorescing cytoplasm; 4, LM-heart Fb and PI double-labeled neurons viewed the blue-green fluorescing nucleus
and orange-red fluorescing cytoplasm.