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Interactions
of neurons
The interactions
among sympathetic afferent, vagal afferent, cervical spinal, and
thoracic spinal neurons is much more complex than previous sections
of this chapter may indicate.
Stimulation of cardiac sympathetic afferents excites, in addition
to the thoracic spinothalamic tract cells already mentioned, more
than half of the spinothalamic tract neurons in the C5-C6 segments
(Figure
6) (Hobbs
et al, 1992).
In contrast
to thoracic and mid-cervical spinothalamic tract cells, spinothalamic
tract cells in the cervical enlargement (C7-C8) receive little,
if any, input from the activity transmitted in cardiac sympathetic
afferent fibers; their somatic innervation originates primarily
from the distal forelimb and hand. This minimal activation of
cells by stimulation of the cardiac sympathetic afferents most
likely means that pain would not be referred to the distal forelimb
and head, which fits, in general, with clinical observations (Sampson
and Cheitlin, 1971; Sylven,
1989).
Since cardiac
sympathetic afferent fibers enter the spinal cord primarily in
the upper thoracic segments and they do not excite C7-C8 spinothalamic
tract cells, the afferent input must be dependent on a propriospinal
pathway (e.g. a pathway connecting different spinal segments)
originating in the thoracic segments that makes direct or indirect
synaptic connections with upper cervical spinothalamic tract cells
(Figure
6) (Zhang
et al, 1997).
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