Physiologic receptors are responsive to innocuous stimuli, and they monitor the functions of visceral structures on a continuing basis. These receptors also mediate normal visceral reflexes, such as the baroreceptor reflex. Examples of physiologic receptors are (1) rapidly adapting mechanoreceptors, (2) slowly adapting mechanoreceptors, and (3) various types of specialized receptors.
Rapidly adapting mechanoreceptors (Table 19-1) signal the occurrence of dynamic events, such as movement or sudden changes in pressure. This class of receptor is present in organs of the thoracic, abdominal, and pelvic cavities. In the thoracic cavity, it is represented by free nerve endings that exist in the epithelia of pulmonary airways. Because these nerve endings are sensitive to the presence of inhaled particles, they have been referred to as cough receptors. Rapidly adapting mechanoreceptors in the abdominal and pelvic cavities vary greatly in size and location and may be either unencapsulated or encapsulated. The largest example of a rapidly adapting mechanoreceptor is the Pacinian corpuscle.
Slowly adapting mechanoreceptors (Table 19-1) signal the presence of stretch or tension within a visceral structure. These typically unencapsulated receptors are located in the smooth muscle layer of the pulmonary airways and in the smooth muscle layers of hollow abdominal and pelvic viscera. They provide the afferent limbs of some visceral reflexes, for example, the emptying reflexes of the rectum or bladder. Slowly adapting mechanoreceptors are also essential for the perception of a sense of fullness in certain viscera, such as the stomach or bladder.
Certain specialized receptors (Table 19-1) are unique to the viscerosensory system. These include baroreceptors, chemoreceptors, osmoreceptors, and internal thermal receptors. Baroreceptors (Fig. 19-1A) are found in the walls of the aortic arch and carotid sinus and respond to rapid increases or decreases in blood pressure. For baroreceptors to effectively perform this task, blood pressure must be in the range of 30 to 150 mm Hg. Chemoreceptors (Fig. 19-1B) are found in structures called carotid bodies (located at the bifurcation of the common carotid artery) and aortic bodies (located in the aortic arch) and are activated by changes in the composition of arterial blood. These changes include alterations in oxygen and carbon dioxide tension and in acidity.
Figure 19-1. Diagrammatic representation of a baroreceptor (A) and a chemoreceptor (B). The baroreceptor is located in the adventitia in apposition to collagen and elastic fibrils. This receptor alters its firing rate in response to changes in blood pressure. The chemoreceptor is composed of a receptor cell and the numerous afferent endings it contacts. Changes in blood chemistry trigger responses in the receptor cell that result in an action potential in the subjacent afferent ending.
Additional specialized visceroreceptors also reside in the hypothalamus as chemoreceptors, osmoreceptors, and internal thermal receptors. These viscerosensory receptors are activated by changes in blood chemistry or osmolarity or by changes in the temperature of blood circulating through the hypothalamus. Hypothalamic neurons that respond to these changes by altering their firing rates are considered to be the “receptor” cells.
VISCEROSENSORY FIBERS
Sympathetic and parasympathetic divisions of the autonomic (visceral motor) nervous system (see Chapter 29) have traditionally been considered to consist of only visceromotor (visceral efferent [VE]) fibers. These fibers travel through sympathetic nerves (such as splanchnic and cardiac nerves) or through parasympathetic nerves (such as vagus and pelvic nerves). However, these sympathetic and parasympathetic nerves also contain viscerosensory (visceral afferent [VA]) fibers that serve many important functions. In this chapter, the terms “sympathetic afferent” and “parasympathetic afferent” are used to describe viscerosensory fibers contained in sympathetic and parasympathetic nerves, respectively. In addition to its conciseness, this usage complies with the terminology introduced by Langley, a pioneer in studies on the autonomic nervous system.
Visceral afferents tend to predominate in parasympathetic nerves but are comparatively sparse in sympathetic nerves. For example, more than 80% of the fibers in the vagus nerve (a parasympathetic nerve) are viscerosensory, whereas less than 20% of the fibers in the greater splanchnic nerve (a sympathetic nerve) are visceral afferents. Most visceral afferents (90%; both sympathetic and parasympathetic) are either unmyelinated or thinly myelinated and therefore are slowly conducting fibers.
There is a division of responsibility between parasympathetic and sympathetic nerves in terms of viscerosensory input. Information originating from physiologic receptors (innocuous input) is conveyed primarily by fibers contained in parasympathetic nerves. In contrast, input from nociceptors is conducted almost exclusively by sympathetic nerves. This feature can be exploited in treatment of intractable pain arising from disease of the abdominal viscera. For example, injection of an agent that blocks action potentials on nerve fibers passing through the celiac plexus can, in some cases, relieve the pain associated with terminal cancer of the pancreas.
ASCENDING PATHWAY FOR SYMPATHETIC AFFERENTS
Afferent fibers conveying nociceptive information from thoracic and abdominal viscera travel via the cardiac and splanchnic nerves (Fig. 19-2). For example, primary sensory fibers that originate from the stomach join the greater splanchnic nerve, enter the sympathetic trunk, and pass through a white ramus to join the spinal nerve. Nociceptive input from pelvic viscera such as the prostate and sigmoid colon is conveyed by viscerosensory fibers traveling through the hypogastric plexus and lumbar splanchnic nerves.
Figure 19-2. Primary sensory sympathetic afferent fibers (red) shown in relation to posterior horn tract cells (green) conveying visceral information to the thalamus and to general visceral efferent neurons (blue).
The cell bodies of origin of sympathetic afferent fibers are located in posterior root ganglia at about levels T1 to L2 (Fig. 19-2). The central processes of these fibers enter the spinal cord via the lateral division of the posterior root. They may ascend or descend one or two spinal levels in the posterolateral fasciculus before terminating in laminae I and V or laminae VII and VIII. Cells in laminae I and V project mainly to the contralateral side as part of the anterolateral system (ALS), whereas the neurons in laminae VII and VIII project bilaterally as spinoreticular fibers. In addition, some primary viscerosensory fibers terminate on preganglionic sympathetic cell bodies located in the intermediolateral cell column at spinal levels T1 to L2 (Fig. 19-2). The axons of these cells in turn exit through the anterior root as VE preganglionic sympathetic fibers.
In general, viscerosensory fibers that enter the spinal cord at a particular level originate from structures that receive VE input from the same spinal level (Fig. 19-2). For example, visceral afferent fibers from the stomach enter the spinal cord over the posterior roots of T5 to T9 and terminate in the same spinal segments that convey visceral efferent outflow to the stomach.
Projections to Thalamus
Some neurons located in laminae I and V receive nociceptive input from sympathetic afferent fibers and send their axons rostrally via two routes in the ALS (Fig. 19-3). Some fibers cross in the anterior white commissure and ascend in the ALS, whereas others ascend in this bundle on the ipsilateral side. These ALS fibers terminate in the ventral posterolateral nucleus (VPL) of the thalamus, which in turn projects to the inferolateral part of the postcentral gyrus (the parietal operculum) and to the insular cortex (Fig. 19-3). The location from which this visceral nociceptive information originated is encoded in these particular regions of the cerebral cortex. However, visceral pain is poorly localized (lacks detailed point-to-point representation) because receptor density is low and receptive fields are correspondingly large and because this input converges in the pathway. Consequently, it is not possible to distinguish, for example, whether perceived pain is coming from the stomach or the duodenum; rather, it can be determined only that the pain is coming from the general area of the upper abdomen (epigastric pain).
Figure 19-3. Ascending visceral afferent input travels in the anterolateral system (green) and through multisynaptic circuits via the reticular formation of the brainstem (gray). These fibers influence specific and diverse areas of the cerebral cortex.
Projections to Reticular Formation
In addition to the direct path to the thalamus and sensory cortex via the ALS and VPL, there are indirect routes via the reticular formation through which visceral nociceptive information can reach the cortex. The reticular formation receives spinoreticular inputs (mainly from laminae VII and VIII) and collaterals from the ALS (Fig. 19-3
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