The carotid sinus baroreceptors are innervated by the sinus nerve of Hering, which is a branch of cranial nerve IX (glossopharyngeal nerve)

So, you mean that this is not the receptors that are stimulated by pressure but directly the nerve is stimulated?

Mudhur is right. Massaging the carotid sinus stimulates an area in the artery wall that contains nerve endings. These nerves slow the heart rate directly, not via baroreceptors, which are 'strain gauges' on the wall of the aorta.

Before Hering came around, the view was that this massaging high up in the neck would stimulate the vagus nerve to the heart directly. He showed convincingly that the carotid sinus wall was deformed by the manipulation, thereby activating stretch receptors, which are innervated by a branch of the IX-nerve. These ‘baroreceptors’ are found on more places, like in the aortic arch, they serve together the ‘baroreflex’ which tends to stabilize blood pressure, primarily at the cost of an unstable heart rate.

Baroreceptors are stimulated by wall tension which is created by arterial pressure or by carotid message.

Fernando, if carotid wall contains nerve endings, hence these nerves can’t spread to the heart directly. I believe that we should accept the explanation given above by John Karemaker.

The course of vagus nerve is such that it passes by the carotid sinus, thereby massaging the area stimulates vagus innervating atrial region and sinus node. This decreases the heart rate. Stimulation of right side of vagus produces greater bradycardia.

You may find below articles interesting to go through in this regard.

STIMULATION OF THE VAGUS NERVE AND CAROTID SINUS

IN MAN. By IAN G. W. HILL. From the Surgical Clinic of

Professor D. P. D. Wilkie, and the Clinical Laboratory, the

Royal Infirmary, Edinburgh. (With six figures in the text.)

(Received for publication 8th April 1932.)

Am J Physiol. 1986 Oct;251(4 Pt 2):H764-73.

Selective vagal innervation of sinoatrial and atrioventricular nodes in canine heart.

Ardell JL, Randall WC.

Ojashwi, thank you for your thoughts and for these references. In particular Hill’s publication from 1932 is an interesting and careful study. He pinched the vagus nerves and the carotid sinuses directly in humans under anesthesia (ether or chloroform, tricky stuff when looks for autonomic effects, but he is aware of that).

It is his conclusion that most of the time in normal, unanesthetized cases, when performing carotid massage (what still was called ‘caroto-vagal pressure’ in his time) one stimulates the receptors in the wall of the carotid sinuses, eliciting a baroreflex response. When the manoeuvre is performed so forcefully as to stimulate the vagus nerves mechanically, the effects on heart rate have a long delay and a long duration, which led him to conclude that the effect was, again, reflexly elicited, by unknown vagal reflexes; the vagus nerve here acting both as afferent and efferent limb.

Thank you Ojashwi for your interesting reference….histomorphologically, receptors (nerves)  are localized in the outer part of the adventitial layer and periadventitial tissue in the region of carotid tripod…..lightmicroscopically never seen such receptors in the tunica media or intima….best wishes Jan

I agree with John Karemaker. The massage of the carotid sinus stimulates receptors located in the adventitial layer of the carotid sinus. From this receptors the impulses travel along the Hering's and the glossopharingeal nerves to the nucleus tracti solitarii  (NTS). From this nucleus other fibers go to stmulate the nucleus ambiguus and the caudal ventrolateral medulla (CVLM). From the nucleus ambiguus the efferent  vagal fibers arise to innervate the sinu-atrial node and to reduce heart rate. The fibers which arise from  the CVLM are inhibitory fibers who reduce the activity of the rostro ventrolateral medulla thus reducing the sympathetic discharge and attenuating the total periferal resistance.

I would like to ask John why the direct stimulation of vagal fibers has a long delay. If they are efferent fibers a synaptic delay must be excluded, unless the problem resides in their higher threshold with respect to the baroreceptors. 

Hi Gianni,

A direct effect of mechanical vagal stimulation should have resulted in an almost instantaneous, but short-lasting effect on heart rate (if any response at all). However, it did not come out this way; below I will cite the text from Hill’s original paper. But I should add to this that the clinical experience of some of my colleagues in applying what they call ‘carotid sinus massage’ effectively might be something as described by Hill when they provoke cardiac slowing by forceful pressing the whole carotid sinus area against the spine, resulting in heart rate slowing only after a few seconds, lasting for much longer than the duration of the maneuver.

I consider this not to be stimulation of the carotid sinuses; that would have an effect ultimo on the heartbeat after the massaging started and should not outlast it for more than a few beats. The longer lasting choking maneuver is, looking at the timing of the response, probably of reflex nature, but not from the carotid sinuses. Deformation of the sinus wall, sufficient to stimulate the receptors, should not require that kind of force.

The long delay which was noticed by I.G.W. Hill (1932) is described as follows in the results:

Case XV "W. L." Male, 64 years…. Pinching the vagus nerve for five seconds produced slowing of the heart-rate from 100 to 60 per minute: the effect was apparent, about three seconds after the start of the stimulation and persisted for over half a minute after cessation of the pressure.

Later, in the discussion he elaborates on this:

In cases where vagus stimulation is effective, there is a certain amount of evidence for the view that it is reflex in nature. When during a surgical operation a motor nerve is pinched lightly with forceps, the muscle supplied responds with a single twitch. Tetanus does not develop in the muscle no matter how long the pressure be maintained. By analogy one would expect, therefore, that pinching such a nerve as the vagus would hardly give rise to a train of peripheral impulses of sufficient duration to produce a direct effect upon the heart. But possibly the short train of impulses liberated by such a pinch might, travelling centrally, provoke discharge from the centre and so induce cardiac slowing.

Thank you Jan for the fact of precise location of baroreceptors. This is quite an interesting fact to educate undergraduates and clinicians alike. Moreover, the carotid massage bilaterally may be utilized clinically and maneuver is just the location dependent. I mean of course with the precaution of not pressing both carotid sinus simultaneously in the same time.