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Vagal
Sensory Innervation of the Heart
The vagus nerve appears to mediate certain aspects of anginal
pain in some patients. As stated previously, pain resulting from
ischemic heart disease commonly is referred to the chest, left
arm, and sometimes the right arm. Less frequently, pain is referred
to the neck and jaw (Sampson
and Cheitlin, 1971).
Surgical sympathectomies
in humans usually abolish or relieve angina pectoris (Lindgren
and Olivecrona, 1947; Meller
and Gebhart, 1992; White and
Bland, 1948); however, sympathectomy occasionally unmasks
pain referred to the neck and jaw, or uncovers stress-induced
pain in these regions. The more rostral location of this pain
led investigators to suggest that vagal afferent fibers may be
involved in the transmission of nociceptive information from the
thoracic region and also from other regions.
We suspected
that nociceptive vagal information might converge onto spinothalamic
tract cells of the upper cervical segments that received somatic
information from the regions of the neck and jaw. Results of a
study in monkeys showed that stimulation of cardiac receptors
with algogenic chemicals injected into the pericardial sac predominately
increased activity of spinothalamic tract neurons in the C1-C2
segments (Chandler et al.,
1995, 1996).
Nerve ablation
experiments demonstrate that interruption of ipsilateral vagal
fibers significantly reduces the mean increase in neuronal activity
of a majority of the C1-C2 spinothalamic tract cells in response
to intrapericardiac algogenic chemicals. These results support
the idea that cardiac vagal afferents transmit a majority of the
nociceptive information to C1-C2 spinothalamic tract neurons.
In addition,
noxious somatic input from the neck, jaw, ear, and upper arm regions
converges onto these spinothalamic tract cells (Chandler
et al., 1995, 1996).
These results are consistent with the suggestion that activation
of cardiac vagal fibers might lead to production of the sensation
of referred pain, particularly in the neck and jaw regions. These
results also indicate that nociceptive information from the heart
could modulate the activity of neurons in the upper cervical segments.
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