visceral sensation

visceral sensation Most of the time we go through our daily lives without being consciously aware of the events occurring continuously in our internal organs, or viscera — the heart, lungs, stomach, intestines, kidney, etc. These organs contain sensory nerve endings, but the messages that they transmit to the central nervous system rarely enter into consciousness. Despite the continuous beating of the heart, and the movements of the stomach and intestines, we are, thankfully, usually unaware of these events. Occasionally we may become conscious of the world inside our bodies, experiencing palpitations of the heart, a throbbing headache, racing pulse, abdominal cramps, or colic. And on occasions, such as during stress, we may experience dyspepsia, or vague sensations referred to as ‘butterflies in the stomach’. The term ‘gut feelings’ indicates the overall, vague, unfamiliar, or affective connotations of abdominal sensations.

Visceral sensations and visceral pain are often regarded as synonymous; however, not all visceral sensations are painful. Stretch of the walls of the stomach or the bladder can yield two qualities of sensation: an awareness of fullness — which in the case of the stomach may be associated with a pleasant feeling of satiety — or, if the degree of stretch is severe, pain. During childbirth, the midwife asks the mother to report when she feels ‘contractions’ — the spasmodic pains that wax and wane, associated with contractions of the uterus. But some visceral sensations are intensely pleasurable: those associated with contractions of parts of the reproductive organs of both sexes during orgasm — the climax of sexual sensations.

From the medical perspective, visceral pain is an important symptom of disorders in internal organs. It may arise through a variety of pathological causes: over-distension of a hollow organ; excessive contraction of the muscular walls of hollow organs — particularly against an obstruction; stretch of a region of mesentery (the membranous tissue that wraps and holds in place the organs within the abdomen); inflammation of an organ or inadequate blood supply to it; movement of the roughened, inflamed surfaces of viscera over one another; spillage of intestinal contents into the peritoneal cavity through a perforation of the wall of the gut; and internal bleeding.

In skin or muscle, pain occurs as a consequence of events that injure or threaten to injure these tissues. In contrast, considerable damage can occur within the viscera without a conscious sensation. The classic investigations of Sir Thomas Lewis in London in the 1930s showed that parts of the intestine may be clamped without eliciting any sensation, whereas the application of similar forces to normal skin would always be painful — and stretching a mesentery always elicits severe pain.

The afferent nerves that innervate the viscera and mesenteries consist of nerve fibres of small diameter, which conduct impulses at relatively slow speeds, because they lack a fatty myelin sheath. They have their cell bodies in the dorsal root ganglia — swellings on the bundles of nerves, called ‘dorsal roots’, that enter the spinal cord at each segment along its length. Attempts to analyse the nature of the signals, caused by mechanical and chemical events in internal organs, which give rise to visceral pain, has yielded some controversy. In situations in which pain is the only sensation that arises from an organ, the intensity of that sensation appears to depend on the total number of nerve impulses conducted to the brain along all the sensory nerves from that organ. However, it is less clear how more than one quality of sensation can arise from a single organ.

Many of the small sensory nerve fibres in viscera are polymodal: they respond to a variety of forms of stimulation (mechanical, chemical, and possibly thermal). They also respond to — or become more sensitive to other stimuli in the presence of — a cocktail of chemicals in the local environment of their nerve endings. These sensitizing chemicals are generally thought to be inflammatory chemical mediators, released from cells of the immune system or from cells of the tissue itself when injured. Examples of such chemical mediators are peptides (such as bradykinin, released as a result of the liberation of cellular enzymes); fatty molecules (such as eicosanoids, the breakdown products of disrupted cell membranes); intracellular chemicals (such as potassium, and ATP); and growth factors (such as the neurotrophins, which are released from inflamed tissues).

The pain of a heart attack (myocardial infarction), caused by blockage of a main artery in the heart, is often described as a sensation of tightness or heaviness constricting the chest, and may be accompanied by tightness in the throat, and tingling and numbness in the left arm. Why should ‘cardiac pain’ be associated with such diverse sensory symptoms covering such a wide area of the body? The underlying mechanism that gives rise to such scattered symptoms can be understood if one considers the distribution of the spinal nerves that innervate all of these areas. The segments of the spinal cord that receive sensory fibres from the heart also receive nerves from the throat and parts of the arm and hand. The cells that give rise to these parts of the body all lie close together at early embryonic stages, and they retain nerve connections to the same region of the spinal cord as they migrate away to their ultimate locations in the body.

The common segmental origin of the sensory nerves serving all of the regions involved in the sensation of pain derived from one particular organ is stated in Ruch's Convergence– Projection Theory of Referred Pain. There are many examples of such ‘referred pain’. There appear to be no highly specific pathways for visceral sensation in the central nervous system. Rather, information concerning events in the viscera is carried from the spinal cord to the brain by nerve fibres that also carry information from areas of the skin and body musculature innervated by the same segments of spinal cord. Hence there is a potential confusion about the origin of the sensation resulting from the activity of such nerve fibres. Information about the location and nature of abdominal pains is transmitted along busy lines of communication in the spinal cord, usually used for some other purpose. The messages received by the brain are ambiguous, in the sense that the events giving rise to them might have arisen in the viscera, in the skin, or in muscle, and the message can be misinterpreted. No wonder diagnosis of an abdominal pain on the basis of a patient's descriptions can be so unreliable, and so fraught with difficulty.

John Morrison

See also pain; sensation; sensory receptors.