NO NEED TO BE A CHEMIST TO KNOW YOU'RE IN LOVE.

Byline: Jeff Kramer Orange County Register

Somewhere deep in your brain, memory mates with imagination and gives birth to anticipation.

With all the subtlety of a car crash, your limbic system - the most ancient region of the brain - converts the happy thought into raw emotion. Hypothalamus: check. Pituitary: check. Thyroid and adrenal glands: check. Your heartbeat spikes to that of a jogger's. Electrical impulses skitter across a veneer of sweat. Perhaps you feel breathless or sick to your stomach.

You may even suffer from piloerection - i.e., goose bumps.

Congratulations. You're in love.

Either that, or you're being chased by a wild animal. From a physiological perspective, the two states don't differ that much - or so say scientists.

``Love is an imbalance, but it's part of the normal continuum,'' says James Fallon, professor of anatomy and neurology at the University of California, Irvine. ``This may take some of the romance out of it. But something is happening.''

What in the central nervous system is going on?

Quite a bit, it turns out.

As difficult as love is to define, its first flickers apparently begin in the prefrontal cortex, the section of your brain that enables you to anticipate the joy of being with a particular person - even one you've never met. If it's powerful enough, this so-called memory of the future engages the ancestral ``fight or flight'' response of the lower brain, which is responsible for such involuntary functions as stammering, tripping, drooling, exchanging astrological signs and laughing too loudly at someone else's joke.

Endorphins fuel the chemical cocktail. Similar in structure to morphine, endorphins are perhaps best known for creating a blissful sense of calm in long-distance runners. They leave lovers feeling similarly tranquil - but not in the early going.

During the initial stages of attraction, endorphins serve as a catalyst by triggering special cells in the midbrain to produce dopamine - a powerful natural amphetamine. In the boot camp of romance, dopamine is the drill sergeant. It barks at the brain to select a plan of action - any plan.

Against so powerful a force, the amygdala - home of the brain's inhibition center - gamely attempts to introduce a note of caution.

But unless the risk of romantic entanglement is sufficiently dire (i.e., ``You could go to prison for a long, long time!'') the amygdala is swept asunder by the hormonal tsunami.

In simplest terms, you've been reduced to a slave of your brain chemistry - but brain chemistry alone doesn't explain why you're behaving so strangely.

Experts say previous romantic experiences - or lack of them - play a major role in determining who you fall for and to what degree. Culturally imprinted expectations are part of the mix as well.

And long before your heart goes pitter-patter, evolutionary imperatives have conspired to define the field of ideal mates.

Researchers have found that males throughout the world, regardless of culture, exhibit universally strong responses to women 18-28 and to women whose waist circumference is 70 percent that of their hips. Both the age and the build, scientists believe, suggest fertility to males.

Similarly, computer tests show that symmetry in the human face and form is highly coveted by both sexes - a preference clearly rooted in genetics. ``When we see asymmetrical features, it's a suggestion of a genetic defect or a developmental problem, so we tend to avoid those people as love objects,'' says Michael Mills, a psychology professor at Loyola Marymount University in Los Angeles.

All of which suggests that love is anything but whimsical in an evolutionary sense. To the contrary, it's adaptive, a tool to help the species survive. ``The reason we fall in love is because our ancestors who didn't fall in love didn't leave many descendants behind,'' Mills says.

Which in turn helps explain why you break out in nervous sweats and go through a quart of cologne a week when you're in love.

This is serious stuff. Your long-dead relatives nag at you through your genes.

The very existence of your unborn descendants depends on your next move. One misstep and you risk derailing the DNA train.

Logic dictates you take a break from this madness and have something to eat, maybe one of those party-sized submarine sandwiches or a 16-inch stuffed crust pizza, extra cheese.

But all those hormones have turned the densely innervated lining of your digestive tract to jelly.

You're so keyed up that merely bumping into your prospective partner unexpectedly is enough to trigger the startle reflex.

Ever remember being awakened by a loud noise? ``You have the same sort of response if you're in love and your loved one comes across the corner,'' says UCI's Ralph E. Purdy, who specializes in cardiovascular pharmacology.

Romantics tend to gloss over the startle reflex with phrases such as ``My heart lept with joy'' - but actually it's more complicated than that. Supernormal levels of norepinephrine enter the bloodstream, causing your pupils to dilate, your blood pressure to climb and your artery walls to relax to keep from bursting. If your heart normally pumps 5 liters of blood per minute, the volume might climb as high as 15 liters.

How long you go on living like a frightened animal depends largely on whether your amorous ambitions are realized. In unrequited love, the yearnings of the prefrontal cortex fail to get reinforced by actual experience, short circuiting the chemical loop and causing a build-up of tension and frustration. The elevated stress levels can hobble the immune system. Illness, depression or malaise may follow.

Obviously, the outlook is happier when love runs both ways.

The release of oxytocin, a peptide linked to mother-child bonding and sometimes referred to as ``the cuddle chemical,'' may produce feelings of relaxed satisfaction and attachment between lovers.

Endorphin-enforced calm may improve your digestion and overall temperament. Your levels of testosterone - the libidinal hormone for both sexes - probably will increase.

All those swirling hormones even may cause you to lead a more active dream life - in which case you might start dreaming about being with someone else. In that case, congratulations one more time. You get to go through this all over again.