Viscerosensory Pathways

Viscerosensory Receptors

Viscerosensory receptors may be categorized as either nociceptors or physiologic receptors ( Table 19.1 ). Nociceptors in the viscera are the free nerve endings of Aδ and C fibers located in the heart, respiratory structures, gastrointestinal tract, and urogenital tract ( Table 19.1 ). It is well established that stimuli adequate for activation of somatic (cutaneous) nociceptors differ substantially from stimuli that activate visceral nociceptors. For example, visceral nociceptors do not respond to cutting, crushing, or burning. Instead, these receptors respond to intense mechanical stimuli (such as overdistention or traction), ischemia, and endogenous compounds (including bradykinin, prostaglandins, and hydrogen and potassium ions) that accompany inflammatory responses. Activation of visceral nociceptors produces consciously perceived pain that may signify either a pathologic process, such as myocardial ischemia or appendicitis, or a benign condition, such as gastrointestinal cramping or bloating. Visceral pain is often described as being diffuse and difficult to localize and is frequently referred to an overlying somatic body location (discussed later in this chapter).

Physiologic receptors are responsive to innocuous stimuli, and they monitor the functions of visceral structures on a continuing basis. These receptors mediate 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 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 ) also operate within the viscerosensory system. These include baroreceptors, chemoreceptors, osmoreceptors, and internal thermal receptors. Baroreceptors ( Fig. 19.1 A ) 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. 19.1 B ) 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.

Additional specialized chemoreceptors, osmoreceptors, and internal thermal receptors reside in the hypothalamus. 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. There are two important exceptions to this general rule: (1) in the gastrointestinal tract, visceral receptors that are distal to the midway point of the sigmoid colon convey both physiologic and nociceptive signals back to the central nervous system via parasympathetic pelvic nerves; and (2) in the remainder of the abdominopelvic cavity, visceral receptors that are inferior to the pelvic pain line (delineated by the inferior margin of the peritoneum) also convey both physiologic and nociceptive signals via parasympathetic pelvic nerves.

The separation of pathways conveying visceral nociception is fundamental for a variety of analgesic procedures. For example, injection of an agent that blocks action potentials in nerve fibers passing through the celiac plexus can, in some cases, relieve intractable pain arising from terminal cancer of foregut viscera. Likewise, delivery of obstetric anesthetics via a caudal epidural block will anesthetize the uterine cervix and the vagina (inferior to the pelvic pain line) but will have little effect on pain signals from the uterine body (superior to the pelvic pain line), allowing the mother to be aware of her uterine contractions during participatory childbirth. Injection to the lumbar epidural space will block pain signals from both the uterus and the vagina and is the most common approach for neuraxial labor analgesia.

Ascending Pathway for Sympathetic Afferents

Afferent fibers conveying nociceptive information from thoracic and abdominal viscera travel via the cardiac and pulmonary nerves (thoracic viscera) and splanchnic nerves (abdominal viscera) ( Fig. 19.2 ). For example, nociceptive input from the stomach is conveyed via primary afferent fibers that 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 proximal to the midway point of the sigmoid colon or superior to the pelvic pain line, such as the ureters and ascending colon, is conveyed by viscerosensory fibers traveling through the hypogastric plexus and lumbar splanchnic nerves.

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 ( tract of Lissauer ) before terminating in laminae I and V or laminae VII and VIII. Cells in laminae I and V project mainly to the contralateral side of the spinal cord 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. This relationship between viscerosensory fibers and the VE cell groups to which they project forms the basis for spinal autonomic reflexes.

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 heart enter the spinal cord over the posterior roots of T1 to T5 and terminate in the same spinal segments that convey visceral efferent outflow to the heart .

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 ).