Interoception is the perception of sensation originating from within the body, especially the visceral organs. Together with exteroception, it comprises the senses of the body for animals and humans. Most of interoception comes from sensory receptors within visceral organs. However, there are a large number of receptors that reside in the blood vessels called baroreceptors. These baroreceptors help with monitoring levels of chemicals, oxygen, carbon dioxide, and noxious stimuli within the body. These signals are then translated into an appropriate response to help maintain homeostasis (a constant internal state at which the organism functions the most efficiently) within the body.
Stretch receptors are one of the largest types of receptors that originate from visceral organs. When these stretch receptors are activated, the receptors will send signals to the brain for an action to take place. For instance, stretch receptors help play a role in the sensations of hunger and the need for defecation and diuresis. When these receptors are stretched tightly in the urinary bladder, the brain interprets the signal as needing to relieve the pressure and as such you urinate. Similarly in the rectum and anus, when stretch receptors are under high stretch, they indicate that the bowels need to be voided. However, in the stomach, low stretch in these receptors plays a role in letting the brain know that the body needs to eat more food. Hunger is one of the biggest sensations that arises from the visceral organs, but it is also strongly affected by some of the external senses such as sight, smell, and taste.
Other examples of stretch receptors are those that exist in the lungs, esophagus, and pharynx. Stretch receptors in the lungs help to control and regulate breathing rates. The level of stretch, either high or low, indicates how full of air the lungs are and if a deep breath is required. These receptors work with baroreceptors (a type of chemoreceptor) to help keep the oxygen (O2) to carbon dioxide (CO2) ratio stable. The esophagus and pharynx both contain stretch receptors that send signals to the brain that there are objects within the space. Within the esophagus, the receptors play a role in the sensation of swallowing and vomiting. The stretch receptors within the pharynx alert the brain that there is a foreign object within the pharynx. This creates the response of gagging to clear the foreign object from the pharynx.
Chemoreceptors are another type of interoceptor and are found in many of the visceral organs to help maintain proper function. Chemoreceptors within the small intestine help the body determine how much pancreatic amylase (a key digestive enzyme) needs to be released to create the correct pH within the small intestine for proper absorption of nutrients. Additionally, chemoreceptors within the brain determine if noxious chemicals have breached the blood-brain barrier and will tell the body to vomit to remove them from itself. These receptors reside in the chemoreceptor trigger zone in the medulla. Other chemoreceptors reside within the brain and play a key role in determining carbon dioxide levels in the blood. When carbon dioxide levels reach a high concentration, the brain feels starvation of oxygen and tells the lungs to breathe more to increase the amount of oxygen within the blood.
Chemoreceptors also exist within blood vessels and measure levels of sugars and salts. If the level of sugar or salt is too low, the response to these receptors is often a craving for the foods that will provide more of the sugar or salt. High concentrations of salts in the bloodstream prompt thirst in the individual to help reduce the concentration to an acceptable level. Receptors in the esophagus and pharynx respond to noxious chemicals and produce a vomiting response to help rid the body of the harmful chemical.
Riannon C. Atwater
See also: Autonomic Nervous System; Baroreceptors; Exteroception; Sensory Receptors
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