| Vagal
influences on upper cervical neurons
The specific
anatomical connections that allow vagal afferents to affect upper
cervical neurons are not established (Figure
5)
The vast majority
of vagal afferents terminates in the region of the nucleus of
the solitary tract (NTS). Projections from NTS to the upper cervical
spinal segments have not been described.
There are
some direct vagal projections to the upper cervical segments;
these could be either branches of vagal afferents that terminate
in NTS or sensory fibers that terminate in the spinal cord rather
than NTS (McNeil
et al., 1991).
Another possibility
is a relay through the parabrachial and subcoeruleus regions in
the dorsolateral pons, because vagal stimulation excites neurons
in these pontine regions, and the dorsolateral pons has known
spinal projections (Ren
et al., 1990).
Any or all
of these projections could account for vagal influences on upper
cervical spinothalamic tract neurons.
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