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Sympathetic
Sensory Innervation of the Heart
The heart
and coronary arteries are innervated by sympathetic afferent fibers
(Figure
4).
These sympathetic
afferent fibers have their cell bodies concentrated in the dorsal
root ganglion cell bodies of the T2-T6 spinal segments, but some
cell bodies are located as high as the C8 segment and as low as
the T9 segment (Hopkins
and Armour, 1989; Kuo
et al., 1984; Vance and Bowker,
1983).
Dorsal root
ganglion cells have axons that enter the posterior horn and terminate
in the same spinal segment as the dorsal root ganglion neurons,
or the axons can ascend and descend a few segments before terminating
in the spinal gray matter (Kuo
et al., 1984); this represents a greater rostro-caudal distribution
than somatic sensory fibers. Furthermore, the sympathetic afferent
fibers are more diffusely distributed within the posterior horn
gray matter than the somatic afferent fibers (Sugiura
et al., 1989).
Finally, sympathetic
afferents represent only about 2% of the total number of afferent
fibers to the thoracic spinal cord (Cervero
and Foreman, 1990). The combination of a diffuse and extensive
organization of sympathetic afferent fibers along with a relatively
small number of sensory neurons most likely contributes to the
poorly localized nature of angina pectoris.
Stimulation
of cardiac sympathetic afferent fibers strongly excites a majority
of the spinothalamic tract cells in the T1 to T5 segments (Hobbs
et al., 1992). These same spinothalamic tract cells receive
convergent somatic input from the overlying chest and arms. Furthermore,
cardiac sympathetic afferent fibers are excited by the substances
that are released by the heart during myocardial ischemia (Malliani,
1988; Nerdrum
et al., 1986).
The convergence
of cardiac and somatic input onto a common pool of spinothalamic
tract cells provides a substrate to explain referral of pain to
somatic structures. This type of referral is a common component
of angina pectoris.
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