Visceral Sensations

Visceral sensations involve structures within the body cavities. Most visceral afferent infor­mation is not available to consciousness but is instead important in directing the autonomic activity in viscera.

Remarkably, the viscera tend to be rela­tively insensitive to stimuli such as crush­ing, cutting, and thermal injury. Surgical manipulation, therefore, tends to produce very little activity in sensory systems. Vis­ceral afferents do respond vigorously to stretch, dilation, tension, and ischemia (reduced blood flow), however. For this reason, cramping (increased muscular tension in the wall of a viscus) and stretch­ing due to gas accumulation are quite painful. The cramping, stretching, and/or ischemia that occur when the equine large intestine twists or is displaced can produce severe abdominal pain, called colic, in horses.

Referred Pain

In humans and presumably in animals, most visceral sensations are ill defined and poorly localized. Noxious stimuli originating in viscera, such as might be caused excessive dilation of a viscus, are often perceived as originating instead from a somatic region (body wall or skin), a phenomenon called referred pain. This per­ception is thought to result from the fact that information from that region of viscera con­verges on spinal cord neurons and pathways that also convey information from somatic structures. since we are much more familiar with sensations arising from our skin and body wall, the pain is interpreted to originate in these somatic structures.

A well-known example of referred pain comes from human medicine; pain due to ischemia of heart muscle is usually perceived as radiating across the left shoulder and down the left arm. This is angina pectoris. A veterinary example may be the high degree of sensitivity in the region of the sternum that some cows exhibit with traumatic peri­tonitis caused by a wire or nail perforating the wall of the forestomach.