VASOMOTOR REACTIONS, ESPECIALLY IN VIEW OF PHYSIOLOGY AND METHODS
Authors: FOG MOGENS 
Published: August 2007

Our present knowledge of the autonomic functions of the body has so far, not been utilized in the diagnostic system of neurology. This depends primarily‐ on the primitive and undifferentiated nature of these functions. A localized lesion of the autonomic system is more easily compensated for and an irritative state more often generalized than is the case within the somatic system. Further, the autonomic functions are usually not open to observation by simple means of examination as are the somatic ones. It seems that at present disturbances of the vasomotor reactions are most easily evaluated in the daily practice of neurology, and the aim of the present report is to give a survey of the physiological statements, on which a vasomotor diagnostic might be based.

Vasomotor reactions are defined as active variations of the vascular diameter. This is a property of both arteries, arterioles, capillaries and veins. Universal reactions, which comprise all parts of the circulatory system must be distinguished from local reactions, which are elicited within a single organ. In both cases, the vasomotor responses play a part in the regulation of circulation: Whereas the heart is the predominant factor in determining the cardiac output, the peripheral resistance and the capacity of the vascular bed is regulated through changes in the diametres of the vessels. Variation in resistance particularly depends on reactions of the arterioles, while the capacity varies after the actual state of dilatation of the capillaries and the small veins. The various segments of the vascular bed are to a large extent able to react independently of one another.‐ It is worth to keep in mind that regulation of the circulation only plays a part in more generalized functions, which also comprise other areas of the autonomic and of the somatic system, viz. metabolism, regulation of temperature and regulation of blood pressure.

The vasomotor reactions are elicited by either a physical, a chemical or a nervous stimulant; in the last named case both a reflex mechanism and a change in central tone (produced by changes of the composition of the blood) may come into question. The neurogenous reactions are described, following an anatomical line. The centres of the bulb, the spinal chord and the lateral sympathetic chain are reviewed, and it is stressed that any of these centres are able to re‐establish a cnstrictory tone of the vessels after section of their superior connections. Even after a total exstirpation of the lateral sympathetic chain a vascular tone is regained, but the vasomotor adaptation to a variation in the functional demands is then impaired. Also the dilatatory centres of the cranial and the sacral parasympathetic system are mentioned and the possibility of dilatatory nerves passing the posterior nerve roots is discussed. It has been convincingly demonstrated that segmentated dilatatory responses of the axon reflex type can be evoked through the posterior roots, but the presence of independent vasomotor pathways through these roots is impropable.–The higher centres of the hypothalamus, the thalamus and the pre‐motor cortiral areas are considered ant1 related to clinical experience. As for the cortical representations the hypothesis is advanced that their function might be related to voluntary muscular action: At the same moment as an impulse to contraction of a special muscle group is started, there might be started a dilatatory impulse to the arterioles of the same region. This hypothesis would explain the ready circulatory changes of a muscle which conies into action. The hypothesis assumes a regional cortical innervation of the vessels, which has not yet been demonstrated.

Vasomotor reflexes can be established through the centres of the bulb, the spinal chord and the sympathetic chain, and further through peripheral mechanisms of the afferent and vasomotor nerves. The effect will be the more generalized, the higher a centre is the “top point” of the reflex.

A survey was given of the reflex regulation of blood pressure through the vasomotor centres of the bulb; of the general constrictory and the segmentated dilatatory reflexes on afferent stimulation of the sensory nerves; of the long‐pathed viscerocutaneous and the short‐pathed local reflexes of the axon type. Some clinical observations on this subject were drawn into consideration.

At last the responses of the arteries and capillaries on direct physical stimulation were mentioned and the main features of the theory of neuro‐humoral transmission were summarized. It was emphasised that there is a difference in principle between the sympathetic nervous system, which exerts a more generalized action, and the parasympathetic system, in which the reactions are usually of a localized nature.

Normally the diametres of the vessels are expressive of an equilibrium between constrictory and dilatatory impulses. Any change of this equilibrium is a vasomotor reaction. It may be of a merely local character, elicited through physical, chemici11 or neurogenous (axon reflex) mechanisms; or segmental through the spinal chord; or finally universal, in which case a cerebral centre is involved. Here it seems that the bulbar centres serve the regulation of blood pressure, the hypothalamic centres primarily the balance of heat, whereas the cortical ones possibly react in the function of voluntary muscular action. It is stated that phenomena of interference often will come into play, and that local reactions within a certain organ often will dominate the universal impulses.

The available clinical methods of determining the responses of the arteries, the capillaries and veins were briefly mentioned, and it was stressed that standardized tests of the vasomotor functions in men are highly necessary.