BARORECEPTORS: FUNCTIONS AND CLASSIFICATION - BIOLOGY - 2025

The baroreceptor consist of sets of nerve endings that are able to perceive the related detente with changes in blood pressure. In other words, these are pressure receptors. They are abundant in the carotid sinus and the aortic arch.

Baroreceptors are responsible for providing useful information to the brain related to blood volume and blood pressure. When the volume of blood increases, the vessels expand and activity in the baroreceptors is triggered. The reverse process occurs when blood levels drop.

The main function of the baroreceptors is the perception of pressure.

Source: Bryan Brandenburg, via Wikimedia Commons

When the distention of the blood vessels occurs due to the increase in pressure, the activity of the vagus nerve increases. This causes inhibition of the sympathetic output of the RVLM (rostral ventromedial bulb, from English rostral ventromedial medulla), which ultimately leads to a decrease in heart rate and blood pressure.

In contrast, the decrease in blood pressure produces a decrease in the output signal of the baroreceptors, leading to disinhibition of central sympathetic control sites and a decrease in parasympathetic activity. The end effect is an increase in blood pressure.

What are baroreceptors?

Baroreceptors are mechanoreceptors (sensory receptor that detects mechanical pressure, related to the sense of touch) located at different points in the blood circulation.

In this circulation system, the baroreceptors are found on the walls of the arteries and on the atrial walls, as arborescent nerve endings.

Among the baroreceptors, the most important from the physiological point of view is the carotid baroreceptor. This receptor's main function is to correct marked and sudden changes in blood pressure.

Features

These mechanoreceptors are responsible for maintaining systemic blood pressure at a relatively constant level, especially when changes occur in the position of the individual's body.

Baroreceptors are particularly efficient in preventing violent pressure changes in time intervals between an hour and two days (the time interval in which the baroreceptors act will be discussed later).

Classification

High and low pressure baroreceptors

There are two types of baroreceptors: arterial or high pressure and atrial or low pressure.

Those of high pressure are located in really abundant quantities in the internal carotid arteries (carotid sinuses), in the aorta (aortic arch) and also in the kidney (juxtaglomerular apparatus).

These play an indispensable role in detecting blood pressure - the pressure that blood exerts against the walls of the arteries, helping blood circulation.

On the other hand, low pressure baroreceptors are found in the walls of the atria. They are related to the detection of atrial volume.

Type I and II baroreceptors

Other authors prefer to call them type I and II baroreceptors and classify them according to their discharge properties and degree of myelination.

The type I group consists of neurons with large myelinated afferent fibers. These baroreceptors have low activation thresholds and are activated more quickly after stimulation.

The other group, those of type II, is made up of neurons with non-myelinated or small afferent fibers with little myelination. These baroreceptors tend to have higher activation thresholds and discharge at lower frequencies.

It is speculated that the two types of receptors may have a differential role in the regulation of blood pressure. Type II baroreceptors are believed to show fewer readjustments than type I baroreceptors and consequently may be more important in long-term control of blood pressure.

How do baroreceptors work?

The baroreceptors work in the following way: the signals that originate in the carotid sinuses are transmitted through a nerve known as Hering's nerve. From here the signal goes to another nerve, the glossopharyngeal nerve, and from this it reaches the solitary bundle located in the bulbar region of the brain stem.

The signals that come from the aortic arch area and also from the atria are transmitted to the solitary bundle of the spinal cord thanks to the vagus nerves.

From the solitary bundle, signals are directed to the reticular formation, the brainstem, and the hypothalamus. This last region, occurs the modulation, integration and production of brain tonic inhibition.

In the event of a reduction in effective circulating volume, the activity of the high and low pressure baroreceptors also decreases. This phenomenon produces a reduction in brain tonic inhibition.

Causes of reduced effective circulating volume

The effective circulating volume can be negatively affected by various circumstances, such as bleeding, loss of blood plasma caused by dehydration, burns or formation of the third space, or by circulatory impairment caused by a tamponade in the heart or by an embolism in the lung.

Relationship with chemoreceptors

Chemoreceptors are cells of the chemosensitive type, which have the property of being stimulated by the reduction in oxygen concentration, the increase in carbon dioxide or excess hydrogen ions.

These receptors are closely related to the previously described blood pressure control system, orchestrated by the baroreceptors.

In certain critical conditions, a stimulus occurs in the chemoreceptor system thanks to a decrease in blood flow and oxygen supply, in addition to an increase in carbon dioxide and hydrogen ions. It is worth noting that they are not considered a fundamental system of blood pressure control.

Long-term temporary pressure control

Historically, arterial baroreceptors have been linked to vital functions of controlling mean arterial pressure in the short term - on a time scale of minutes to seconds. However, the role of these receptors in the long-term response has been ignored.

Recent studies using intact animals suggest that the action of the baroreceptors is not as short as previously thought.

This evidence proposes a reconsideration of the traditional function of baroreceptors, and they should be associated with the long-term response (more information in Thrasher, 2004).

References

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3. Lohmeier, TE, & Drummond, HA (2007). The baroreflex in the pathogenesis of hypertension. Comprehensive Hypertension. Philadelphia, PA: Elsevier, 265-279.
4. Pfaff, DW, & Joels, M. (2016). Hormones, brain and behavior. Academic Press.
5. Robertson, D., Low, PA, & Polinsky, RJ (Eds.). (2011). Primer on the autonomic nervous system. Academic Press.
6. Thrasher, TN (2004). Baroreceptors and the long-term control of blood pressure. Experimental physiology, 89 (4), 331-335.