This article began as a simple homemade valentine to my wife, Lori, a few years ago. I was so deep in the doghouse that my pen seemed the only avenue back to her heart (her brain, actually, but that's the point of the story). I needed something that conveyed the sense that I had spent some time and effort on the endeavor, something that showed the love and appreciation I have for a wonderful woman who has inexplicably forgiven most of my libertine behavior for 25 years.

The ancient Greeks, Aristotle included, linked the processes controlling intellectual capacity to the liver and heart. Moreover, the heart was thought to govern most emotions, especially the one defying any strict definition, love. Hence the reference to the heart in all matters, big and small, regarding love: I gave you my heart. My heart belongs to you. I love you with all my heart. You broke my heart.

Anatomically speaking, it just isn't so.

The heart, to be sure, is no anatomical slouch. Tirelessly pumping oxygen-rich blood -- the elixir of life -- to the entire body, it deserves its share of accolades.

PG graphic: Limbic System
(Click image for larger version)

But emotions are products of a primitive region of the vertebrate brain called the limbic system. It's not like the digestive system (the true source of "heartburn," by the way), in which all of the constituent structures work toward one goal. Instead, the limbic system comprises several structures deep in the center of the brain, each with primary functions associated with hormonal control, memory or sensory processing. They co-evolved to form a loose association with each other, contributing to emotional responses.

The amygdala, a small, almond-shaped structure, is the chief of emotional operations. It receives input from the other structures and, research suggests, has the final say as to which emotions are brought to the fore, inhibited or exaggerated. The amygdala seems to have developed to regulate fight-or-flight responses and experiences associated with pleasure and pain. Complex emotions, such as love, are an outgrowth of survival instincts. Fear and anxiety keep you out of harm's way, whereas love and a sense of well-being are the fruits of a certain cultivated security.

Some of the oldest members of the limbic system are the olfactory bulbs, the structures responsible for our sense of smell. This sense is so ancient that it has direct input to the amygdala and the hippocampus, the primary region for laying down memories. The thalamus relays other senses (vision, hearing and touch) to the cortex, but it has only indirect access (through he cingulate gyrus) to the hippocampus. That's why smell is more quickly and strongly tied to memory than the other senses.

Another structure, the hypothalamus, regulates the secretion of hormones, including oxytocin, from the pituitary gland. Oxytocin, which shapes feelings of emotional and physical connection, is a major player in the biological basis for romantic love.

It all works like this:

When you spend time with a prospective romantic partner, the olfactory bulbs pick up scents you notice and even some you don't. The thalamus records visual, auditory and tactile senses; that is, you also see, hear and feel a would-be mate. Then the hypothalamus contributes hormones that all at once control pleasure, aggression, sexual desire and even anger. This cocktail, channeled through the amygdala, conjures the exhilarating, seemingly illogical emotion we call love.

The hippocampus lays down all of these experiences as memories. The more you experience these sensations with your partner, the more they become embedded in your memory, exciting the amygdala yet again.

Why all of this occurs when a person is exposed to some prospective partners and not others is a poorly understood phenomenon we call "chemistry." The term is apt, given the connection with pheromones and the levels of neurotransmitters such as serotonin and dopamine.

The ancient Greeks weren't totally off base. Hormonal-driven romantic excitement tends to increase heart rate. This is actually a manifestation of the fight-or-flight response: Do you run, or do you stay for what's in store? Hank Williams, Paul McCartney and Taylor Swift all have earned tidy sums singing about this. Romantic excitement also can produce sweaty palms, a touch of nausea and something that feels like a slight kick to the back of the knee. Seems that love affects every inch of your body. But it is all set in motion in your brain.

Knowledge of the limbic system, especially the power of the olfactory bulbs, would have served my parents well one Valentine's Day in the 1970s. My mom, a brunette, dyed her hair blond that day, and my dad failed to notice when he came home for dinner. In his defense, had she changed her perfume instead of her hair color, his brain would have registered the change much more quickly. My mom still doesn't buy it and maintains that my propensity for doghouse-lodging is genetic.

On the front of Lori's valentine, I put two pictures, one of the heart for tradition and one of the brain for scientific accuracy. This is how I ended the card: "You stole my amygdala, hippocampus, hypothalamus, cingulate gyrus and thalamus years ago. I know it lacks the glamour of 'You stole my heart,' but it is anatomically more accurate. And that really is the heart of the matter."

Chuck Welsh, an assistant professor of biology at Duquesne University, teaches anatomy and physiology to students pursuing health-related careers (welshc@duq.edu). He is the author of a human biology textbook and has a doctorate in biological sciences from University of Pittsburgh.

First Published: February 10, 2013, 10:00 a.m.