What is visceral pain and what are its specific manifestations?

Visceral pain is a common clinical pain phenomenon, often accompanied by emotional and defensive reactions, mainly caused by mechanical pull, chemical stimulation, ischemic changes, electrical stimulation, etc. The following are some of the latest research findings on visceral pain.

Overview of Visceral Pain

The sense of internal organs is usually divided into three categories: the first category of organs has no sensory characteristics, including solid organs such as the liver, lungs, and kidneys; the second category of organs includes hollow organs such as the cardiovascular, respiratory tract, stomach, small intestine, bile system, ureters, and reproductive organs. Pain is the only sensation that can be induced. Its afferent fibers can not only regulate the physiological functions of the organs, but also sense noxious stimuli and mediate pain; the third category of organs includes the esophagus, colon, rectum, and bladder, which can produce non-painful sensations (such as the feeling of urinating the bladder and defecation in the rectum), as well as pain caused by noxious expansion. This type of organ is easy to stimulate and convenient for experimental operations. Different pain transducer fibers are distributed in the internal organs, and the receptors that mediate visceral pain are generally considered to be some free nerve endings. According to their electrophysiological response characteristics to stimulation and their different locations, the receptors of visceral pain are divided into mucosal receptors, muscle receptors and plasma membrane receptors.

The characteristics of visceral pain are: ① vague sensation and unclear location; ② often accompanied by referred pain; ③ often accompanied by movement and/or autonomic motor reflexes; ④ persistent visceral pain can induce pain hypersensitivity. Pathological pain in FGID or CP is usually accompanied by visceral pain hypersensitivity and a lowered pain threshold. The reason for the lowered pain threshold may be that abnormal noxious reactions cause plastic changes in peripheral or central nerves in primary afferent fibers (peripheral sensitization) and spinal cord and brain (central sensitization); due to the lowered pain threshold, both noxious and non-noxious stimuli can cause long-term visceral pain of varying degrees in FGID or CP patients, seriously affecting the patients' daily work and quality of life.

Central pathways of visceral afferents

1. Visceral information transmission pathways in the brainstem

The vagus nerve has long been proven to be involved in the transmission of noxious information. The nucleus of the solitary tract (NST) located in the brainstem not only receives taste information, but also widely receives primary afferent projections from the glossopharyngeal and vagus nerves. In addition, the NST has been proven to be the location of secondary neurons that receive visceral and cardiovascular noxious information from the glossopharyngeal and vagus nerves. The parabrachial nucleus of the pons is an important relay nucleus in the process of visceral sensory transmission. It is derived from the fiber projections of the NST to the subnuclei of the lateral parabrachial nucleus. The NST and the parabrachial nucleus have extensive fiber connections in anatomy and electrophysiology. Experiments have shown that in addition to receiving visceral information from the NST, the parabrachial nucleus of the pons may also directly receive projections from the spinal cord and trigeminal sensory nuclei.

At present, there are also in-depth studies on the projection relationship between the parabrachial nucleus and the thalamus. The taste afferent fibers from the NST that relay through the parabrachial nucleus mainly terminate in the small cell part of the ventral posterior nucleus of the thalamus, while other general visceral afferent fibers mainly terminate in the lateral part of the ventral basal nucleus of the thalamus. In addition, the parabrachial nucleus also sends fiber connections to several nuclei in the forebrain (such as the hypothalamus and amygdala). Electrical stimulation of the parabrachial nucleus can cause e-Fos expression in these two nuclei, further indicating that the visceral afferent to the central pathway needs to pass through more layers of relay sites than the somatic afferent to the central pathway.

2Insula and visceral information transmission

Electrophysiological studies have confirmed that the insula is involved in the transmission of visceral information. According to the research of Cechetto and Saper, the insula can be divided into three sensory areas: taste afferents mainly terminate in the degranular part of the rostral insula, and slight damage to this area can lead to a certain degree of taste disorder; general visceral afferents mainly terminate in the postgranular part, and the activation of gastrointestinal mechanoreceptors can cause strong electrical activity of neurons in this area; cardiopulmonary reactive neurons are located in the most caudal part of the insula. Horseradish peroxidase tracking technology also confirmed that there is indeed a fiber connection between the insula and the ventral basal nucleus of the thalamus. The above research results show that the insula is connected to visceral information, but whether it is related to the perception of visceral harmful information, whether it has a direct connection with the two types of conduction pathways mentioned above, and whether it represents the highest center for the upward transmission of visceral information still needs further in-depth research.