Autonomic Functions
Autonomic
mechanisms of the spinal cord take part in the regulation of a number of
visceral functions. There is evidence, for example, of the participation of thermosensitive
neurons of the spinal cord in the maintenance of homeostasis of the organism.
Since maintenance of blood pressure constancy is one of the most important
manifestations of homeostasis of autonomic functions involving the spinal cord,
let us consider this issue in more detail. The functional state of sympathetic
preganglionic neurons (SPNs) of the spinal cord is of crucial importance in
maintaining blood pressure homeostasis and regulation of vascular tone and
cardiac activity. When studying the functional specialization of these neurons,
it was shown that on vasomotor neurons of sympathetic ganglia responding
antidromically to muscle nerve stimulation (sympathetic postganglionic fibers
oriented only to blood vessels pass in muscle nerves), axons of only B2 and
C-SPNs converge. The conclusion that B2 -SPNs are the main vasomotor output
from the spinal cord is confirmed by the data obtained when studying the
background activity of SPNs of the lateral horns [1]. Not only somato-visceral
afferentation but also descending influences converge to vasomotor SPNs.
Bulbo-spinal neurons of the lateral, reticular, small cell, paramedian, ventral
reticular, gigantocellular nuclei of the reticular formation, and the central
nucleus of the medulla oblongata participate in the descending transmission of
suprasegmental influences on the sympathetic mechanisms of the spinal cord.
According to Henry and Calaresu, at least five descending reticulo-spinal
systems of the medulla oblongata, of which two are sympatho-activating and
three sympathoinhibitory, are involved in the suprasynal regulation of
cardioacceleratory SPNs. In studying the influence of bulbospinal monoaminergic
pathways on sympathetic nerve activity, it has been shown that
catecholaminergic pathways pass in the dorsolateral canaliculus of the medulla
oblongata. Descending sympathoactivating pathways are also localized in this
region. According to Coote and McLeod, there are both noradrenergic and
serotonergic sympathoinhibitory pathways. The authors note the existence of
three descending inhibitory systems: the noradrenergic sympathoinhibitory
system of ventrolateral medulla oblongata neurons with axons in the
dorsolateral canaliculus of the spinal cord, the serotoninergic system of
suture nuclei neurons with axons in the dorso- and ventrolateral canaliculi of
the spinal cord, and the reticulo-spinal system of the ventromedial reticular
formation of the medulla oblongata. When the ventromedial reticular formation,
i.e., the classic depressor area of the medulla oblongata, is irritated,
inhibition of the SPN occurs with a latency period of 5-30 ms, whereas the
latency period of baroreflex inhibition of the sympathetic discharge of the
spinal cord when the sinus nerve is irritated is 150-300 ms. Therefore, it is
unlikely that the system of ventromedial reticular formation neurons with
fast-conducting axons participates in the baroreflex inhibition of SPN. When
dorso- and ventrolateral tubules are irritated in the area C3 -C4 of the spinal
cord and sympathetic discharge is registered at the level of thoracic segments
(distance of about 130 mm), inhibition of somatosympathetic discharge occurs
with a latency period of 130 ms. Taking into account the localization of
monoaminergic neurons of the medulla oblongata, whose axons form these
bulbospinal inhibitory pathways, we can think that baroreflex inhibition is
transmitted through descending channels from the ventrolateral reticular
formation of the medulla oblongata and the caudal nucleus of the suture, which
are characterized by a slow rate of conduction of inhibitory bulbospinal
impulsation[2,3]. The question of the level at which baroreflex inhibition of
spinal cord SPN occurs is of interest. There is data on both the bulbar and
spinal level of realization of baroreflex inhibition of SPN. The authors who
adhere to the viewpoint about the bulbar level of closing the reflex arc of
late somatosympathetic reflex responses, taking into account the
barosensitivity of these responses and the absence of a pronounced barosensitivity
of the early spinal response, believe that baroreceptor inhibition of
sympathetic activity occurs at the bulbar level. However, a number of data
contradicts the idea of the bulbar level of realization of baroreflex
inhibition of SPN activity and testifies to the existence of spinal mechanisms
of baroreceptor inhibition of sympathetic discharge. Thus, it was found that
irritation of descending pathways at the level of lateral canaliculi of the
spinal cord causes the same inhibition of sympathetic activity as baroreflex
inhibition. It is shown that not only early but also partially late
barosensitive components of somatosympathetic discharge are formed mainly at
the level of the spinal cord. The data on some barosensitivity of the early,
purely spinal component of somatosympathetic discharge are indicative of the
spinal, not only bulbar, level of realization of baroreflex inhibition.
Recently, convincing data have been obtained showing that baroreflex
sympathoinhibitory influences are probably realized on the interneuronal
apparatus of the spinal cord [4-6]. Multichannel transmission of suprasegmental
influences on SPN is provided not only by the reticulospinal system of the
medulla oblongata, but also by a number of other descending projection pathways
(pontomesencephalic, hypothalamic, corticospinal), which are also important
components of the mechanisms of central control of autonomic functions that
maintain homeostasis of various physiological constants of the organism. The
data on convergence of extero- and interoceptive afferent inflow and
supraspinal modulating influences on SPN indicate some integration of
supraspinal cardiovascular reflexes at the spinal level. This view differs from
the classical concept of the organization of central mechanisms of regulation
of cardiovascular functions, according to which afferent and suprabulbar
activity is integrated in the medulla oblongata and transmitted to the spinal
cord through common excitatory and inhibitory pathways [7,8]. The question of
possible realization by the spinal cord of the reflex control of autonomic
functions has long been disputed by many authors. However, to date, a number of
convincing data on the contribution of spinal cord structures to the formation
of the neurogenic component of vascular tone maintenance, to the realization of
somatosympathetic reflexes and, ultimately, to the vasomotor regulation have
been accumulated. Thus, a number of studies have shown the possibility of
realization of reflex reactions of the cardiovascular system in spinal animals.
In early terms after high atraumatic transection of the spinal cord, performed
by ultrasound scalpel, a high level of blood pressure can be maintained and
pressor reactions can be induced. All these data were the basis for the assertion
that spinal cord mechanisms can maintain arterial pressure at values peculiar
to an animal with an intact central nervous system. A number of
electrophysiological studies have shown the existence of tonically active
sympathetic elements in the spinal cord even after chordotomy [9].