When a myocardial infarction occurs, what is it characterized by?

is it dilation of the heart chambers, necrosis of a portion of the heart muscle, irritation of the pericardium, or destruction of the heart valve?

It's a feeling of "jeezus ******* christ I'm gonna die oh please god save me".

Myocardial Infarct is typically characterized by numbness or pain in the jaw, radiating pain from the chest, and/or pain that shoots down the left arm. This can be accompanied by problems breathing or breaking out in a cold sweat. In women the MI can present a little differently depending on their size. Instead of pain radiating in the chest the pain can be located in between the shoulder blades and instead of being high in the chest the pain can be located sub-sternally (sometimes this causes a misdiagnoses as stomach upset). If you experience these symptoms chew an aspirin and get to the hospital as fast as you can.

Hope that helps

Necrosis of heart muscle. An MI is a heart attack. Blood flow is cut off to a section of the heart which then causes that area of the heart (Usually the apex) to stop functioning.

Been there done that.

It's blockage in the coronary arteries...when it gets to the point where no blood is reaching the myocardium (heart muscle) the result is a heart attack (MI)

Myocardial Infarction is a heart attack. If you are a woman you may not have the typical symptoms. Mine were fatigue and constantly eating sweets!

Jeez, it would take hours to really answer that. Here are some answers to your direct questions.
Yes an MI will cause necrosis to the affected part of the heart. The longer treatment is delayed, the further the necrosis will spread.
Irritation of the pericardium will cause severe discomfort, but won't neccessarily result in an MI. Fluid build up between the pericardium and the heart is called pericardial tamponade. That will gradually decrease the space the heart has to contract until there is no more room for it to pump.
A faulty heart valve will result in a backwards flow of blood. Sometimes this can be quite serious and sometimes it is just painful. Usually this is seen in what is called a valve prolapse. The most commonly known is mitral valve prolapse or MVP. This presents with a crushing chest pain that may make a person believe they are having an infarction. Valves can be replaced, but the condition is not life threatening and most people just accept it as an inconvienient part of life.
Edema ( excessive fluids ) can cause a backflow to the pulmonary system leaving fluids in the lungs. This is called congestive heart failure or CHF. People with this condition are usually on a regimin of furosemide (Lasix) and potassium.
There are a myriad of reasons for an MI to occur. There are many different kinds of dysrhythmias that lead to MIs. Each dysrhythmia is treated in it's own way. There are several types of tachycardias(over 100 beats per minute), several types of bradycardias(less than 60 beats per minute, although this is normal for more athletic people), atrial flutters and fibrilations, ventricular flutters and fibrilations, regular irregular rhythms, irregularly irregular rhythms, occasionally irregular rhythms, sinoatrial rhythms, atrioventricular rhythms, junctional rhythms, several types of blocks, sinus arrest, premature atrial complexes, premature ventricular contractions and so on and so on. Most of those I listed can be broken down into sub-categories each having their own rhythm. Understand that there are many not listed, I just wanted to give you an idea of what you are asking. Each rhythm presents in it's own way and each dysrhrythmia can lead to an MI.
One important thing a couple of other posters stated was that women often do not have the classic symptoms when presenting with an MI. Any discomfort from the waist up in a woman can be a symptom. When the answer to the discomfort is not quickly determinable, a 12 lead is in order just to be on the safe side. The only way to know what the heart is up to is to look at it with an ecg.
You will never get the complete answer to your question on Yahoo Answers. There is just too much. A series of more specific questions might get you closer. Hope this helped.