Neurology of Methamphetamine Use
When most people think about the nervous system, they imagine the form and shape of the human brain. However, the nervous system is actually much more than that. Together with the spinal cord, the brain comprises the central nervous system. The peripheral nervous system is all other nerve tissue in the body, including nerves leading to and from the skin, muscles, blood vessels, and internal organs, brain, and spinal cord. Methamphetamine use has many effects on both the central and peripheral nervous systems.
In humans, a thin ring of tissue surrounding the central core of the brain governs much of behavior and emotion. Within this central ring, several collections of neurons and the connections between them are known together as the limbic system.
Eating a piece of frybread feels good, making it highly likely that you will eat some frybread again. Sex also creates great pleasure. The limbic system evolved over millions of years to become stimulated by activities (such as eating, drinking fluids, and having sex) that are important for survival of the individual and important for survival of the species. However, methamphetamine use also stimulates the limbic system.
Once activated by a particular behavior (such as eating, sex, or methamphetamine use), the limbic system uses pleasure as a reward, convincing the mind that whatever the body just did was very good and should be repeated. In this way, the limbic system motivates behaviors. To accomplish this feat, unique networks of cells in the limbic system manipulate the rest of the brain.
Your limbic system is composed of several networks of cellular connections called Diffuse Modulatory Systems (DMS). When stimulated, a DMS creates a broad (diffuse) signal throughout the brain, which controls (modulates) levels of brain activity.
Classification of a DMS is based on the type of neurotransmitter released by that systemís neurons. In methamphetamine users, the dopamine system is the DMS most affected by the drug and may lead to addiction [Link to Jace]. The norepinephrine and serotonin systems also have a role during use of methamphetamine [Link to Jace].
Concentrate for a moment on your heartbeat. Imagine if you had to actively force, or "tell" your heart to beat, much like you have to force your eyes to read this sentence or tell your finger to click the mouse button. You wouldn't accomplish much of anything if you had to perpetually concentrate on your heart rate, breathing rate, or metabolism.
Your Autonomic Nervous System (ANS) allows you to live each day without sacrificing intellectual firepower on a task such as maintaining a steady cardiac rhythm. The ANS works as a sort of cruise control for life-sustaining bodily functions like heart rate and metabolism. Two different divisions of the ANS, the sympathetic and parasympathetic divisions, determine whether your cruise control speeds up or slows down.
When your bodily functions have to speed up or become more active, like when confronted with a physical threat or a compromising situation, the sympathetic division of the ANS activates. Direct consequences of sympathetic stimulation include increase in heart rate, inactivation of digestion, release of glucose from the liver for immediate energy, constriction of blood vessels to increase blood pressure and flow, sweating to eliminate excess water, relaxation of bladder muscles to store urine, and stimulation of ejaculation. These changes prepare the body for immediate action.
In humans, the sympathetic division of the ANS uses only one neurotransmitter, called norepinephrine. In comparison with norepinephrine, methamphetamine is an analog, a molecule with a similar structure and function as another molecule. Each time a user smokes, injects, or snorts methamphetamine, the drug fools the organs controlled by the sympathetic division of the ANS into activation. Heavy and frequent use of the drug leads to the weight loss, dilated pupils, increased heart rate and increased blood pressure common in many methamphetamine addicts.
Slowing down or relaxing bodily functions is the responsibility of the parasympathetic division of the ANS. Inducing the opposite effects of the sympathetic division, the parasympathetic division activates digestion of food, lowers heart rate, dilates blood vessels, and inhibits metabolism. Since acetylcholine is the neurotransmitter used by the parasympathetic division, methamphetamine has no direct effect on parasympathetic activation or inhibition.
Your nervous system is crucial to survival. It produces your thoughts, feelings, and emotions. The nervous system also allows you to react to critical situations and controls your involuntary activities, such as digesting your food and maintaining your heart rate. Methamphetamine use interferes with all of these important aspects of nervous system function, making the user very likely to one day appear in a treatment center, emergency room, or even, a morgue.