When there are more receptors, more caffeine is needed to block them. This is why regular coffee drinkers build up a tolerance to caffeine and need more coffee for the same effect. By blocking adenosine, caffeine lets those excitatory neurotransmitters that stimulate the brain move about freely. This leads to an increase neuron firing, and the pituitary gland notices the uptick in activity. The pituitary gland, in turn, releases hormones that activate the adrenal glands, which produce adrenaline.
The end result of this long chain of reactions is an increase in adrenaline levels. Dopamine is a neurotransmitter that makes us feel good. Caffeine increases the amount of dopamine in our brain by blocking its reabsorption into our bodies. This leads to elevated dopamine levels for a short time, which make us feel good.
The same goes for phenylethylamine, a substance related to a family of stimulants called amphetamines. For example, chocolate contains less phenylethylamine than goat cheese. Anandamide, a neurotransmitter produced naturally by the brain, has also been isolated in chocolate. The neural receptors for anandamide are the same ones to which THC, the main active ingredient in cannabis, binds.
Be that as it may, many scientists agree that dependency on chocolate could simply be due to its taste, which causes a sensation of intense pleasure that people want to repeat. Dopamine appeared very early in the course of evolution and is involved in many functions that are essential for survival of the organism, such as motricity, attentiveness, motivation, learning, and memorization.
But most of all, dopamine is a key element in identifying natural rewards for the organism. These natural stimuli such as food and water cause individuals to engage in approach behaviours. At the cellular level, caffeine blocks the action of a chemical called phosphodiesterase PDE. Many hormones and neurotransmitters cannot cross the cell membrane, and so they exert their actions indirectly via such second messengers; when they bind to a receptor on the surface of a cell, it initiates a chemical chain reaction called an enzyme cascade that results in the formation of second messenger chemicals.
Historically, cAMP was the first second messenger ever described. Now, however, scientists have identified several major classes of second messengers, which are generally formed in similar ways through a set of molecules called G proteins. The advantage of such a complex system is that an extracellular signal can be greatly amplified in the process, and so have a massive intracellular effect.
Thus, when caffeine stops the breakdown of cAMP, its effects are prolonged, and the response throughout the body is effectively amplified. In the heart, this response prompts norepinephrine--also called noradrenalin--and a related neurotransmitter, epinephrine, to increase the rate and force of the muscle's contractions. Although the two act in concert, norepinephrine is released by sympathetic nerves near the pacemaker tissue of the heart, whereas epinephrine is released primarily by the adrenal glands.
Chemical Compounds. Caffeine wakes you up by fooling adenosine receptors. Adenosine slows down nerve cell activity along neural pathways like these, but caffeine which binds to the same receptors speeds activity up. Your pupils dilate. The airway opens up this is why people suffering from severe asthma attacks are sometimes injected with epinephrine. Your heart beats faster. Blood vessels on the surface constrict to slow blood flow from cuts and increase blood flow to muscles. Blood pressure rises.
Blood flow to the stomach slows. The liver releases sugar into the bloodstream for extra energy. Muscles tighten up, ready for action.
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