I have had a request to write about the physiological basis for emotional reactions to be felt in other parts of the body, namely the sensation of heartache and gut feelings, both of which fall under the category of visceral emotions (perhaps not officially, but within my own individual vernacular they at least do). There are several possible reasons for such sensations, so what I am going to discuss in this post is a great deal of conjecture and, though I expect it is fairly reasonable, might of course be refuted by careful scientific study.
There are three main aspects of our physiology that I think best explain the sensation of visceral emotions. The first is that our body is essentially formed segmentally (an evolutionary throwback to the days of worms) with a series of dermatomes (I've linked to the Wikipedia article on them for those that don't know what I'm talking about). While dermatomes are fairly well defined (though with some overlap) on the surface of our skin, things get somewhat less organized when it comes to our insides. Organs might form embryologically with one segment of skin but then get pushed and shifted around to end up somewhere else. Combined with the fact that many of our somatic sensations are not nearly as well localized as we sometimes assume they are (if you have ever done the test where, while blindfolded or looking away, someone simultaneously pokes you on the back or leg with a pair of pointy sticks separated by only a few centimetres you might know what I mean. It is very difficult (provided the poke comes simultaneously) to tell whether you have been poked in one spot or two), this can sometimes lead to a misallocation of sensation. One common example of this (at least for men) is the horrifying sensation in the pit of one's stomach after a blow to the testacles. Also, while I've never experienced them myself, other examples of misleadingly localized visceral pain include appendicitis and a hernia. Even heartburn is a misallocation of indigestion to pain in the heart.
The second major aspect of our physiology which leads to visceral sensation of emotions is the widespread autonomic responses we experience corresponding to shifts in our mental state. As we enter states of alertness, our sympathetic responses tend to be recruited and our heart-rate increases. The sympathetic nervous system is most famously known for its role as the 'fight or flight' response system, but it is engaged by other stimuli as well. Thus, while you have no intention of fighting a pretty girl or handsome guy whom you would like to go on a date with, the presence of your crush still elevates one's alertness and results in many of the same responses that result from stress and fear. I'm not actually sure the physiological reason, but acute action of the sympathetic nervous system can sometimes lead to vomiting and nausea (if anyone has ever been in an exam room in which a test taker was so nervous he lost his lunch, you have an idea of what I am talking about), which might help explain the (much more pleasant but similar) sensation of 'butterflies in the stomach'. A fairly cute psychology study was done a number of years ago in which an attractive lady stopped men for a survey in two different situations. In the first, she waited on a foot-bridge over a rather severe drop, and in the second she simply stopped men on the street. Part of the survey asked for takers to follow-up with a phone call to the researcher. Significantly more of the men who encountered the attractive lady over the gorge made the follow-up call. This was interpreted (debatably, of course, like a lot of psychology research) to mean that the men who encountered the lady in a dangerous situation found her more enticing due to a conflation of their physiological response to the fear (quickened heart and elevated alertness) with a similar response to an attractive member of the opposite sex. While I don't think the study was in any way conclusive, I bring it up now because I do think it provides some supporting evidence for the fact that there is not a unique set of physiological responses to every emotion. Rather, there is a messy and often confused interplay.
The third aspect of our physiology involves the setup of our reward pathways and their strong connection to our viscera. After all, when you think of people as survival and propagating machines, obtaining and consuming sustenance and having sex are pretty much the two main functions (we are, of course, slightly more complicated than that, but those are integral aspects of our species). One of the favourite "brain facts" espoused by clever people who sometimes like to repeat relatively inane facts in lieu of conversation is that eating chocolate provides some of the same pleasurable sensations as sex. While true, the statement is still rather misleading because virtually all food does (particularly when one is excessively hungry), since food and sex are both largely driven by the reward pathways of the limbic system. Chocolate just happens to be an especially rewarding food, and thus sounds better than, say, broccoli and cheese. Chocolate also seems to be a favourite with women, and thus using chocolate in the sentence has a greater chance of garnering a wry smile and snide response, "Oh, I think it's better," from a lady in the group as she gives her boyfriend obviously coy eyes, at which point everyone gets a good chuckle (except, perhaps, for the poor fellow with the slighted sexual prowess).
Taking these three physiological aspects together, I think we may now make a reasonable conjecture as to the nature of both heartache and gut feelings. I will start with heartache, which I am interpreting for the purposes of this post to mean a deep ache felt in the lower chest following a break-up, loss of a loved one, or some other sense of emotional loss. This sensation, I believe, mainly combines aspects of the first two physiological facts discussed. Emotional loss can be deeply distressing, thereby vastly increasing a person's stress levels and forcing a powerful response from the sympathetic nervous system. Unlike in the case of the attractive lady on the bridge discussed above, there is no positive stimulus upon which one can distract and project their feelings, and thus they are interpreted as wholly unpleasant. With an increase in heart rate and mild nausea from increased stress, I surmise that the brain interprets the sensation as an ache centred upon the heart.
A gut feeling, on the other hand, seems to involve the third aspect more than the other two. For me, at least, gut feelings are not particularly localized in the gut, but are rather a sensation that something feels like the right solution from the core of one's being. One thing which many people do not realise about the brain is that the emotional parts of the brain (which tend to be concentrated around the limbic system) actually do quite a bit of information processing and decision making (it is not all rational thought processing and planning from our frontal cortex). This is actually one of the favourite topics discussed by Jonah Lehrer, particularly since it is the subject of his latest book. I plan to write a post about it myself going into more detail, but I hope the aspects of our physiology I discussed in this post can at least give one a general idea for a possible physiological basis for gut feelings and viscerally felt emotions in general.