You can “run out of them” for brief periods of time. Capsaicin cream, made from the same capsaicin found in hot peppers, is used at various strengths to treat chronic and acute pain. You rub the capsaicin on the area of pain (using gloves for the heavy duty stuff) and it first makes the nerves all fire quickly (and painfully) and empty out all of those neurotransmitter chemicals. You then get pain relief during the time it takes the nerves to reabsorb all those chemicals they “lost.” So, the nerves “run out of them,” but your body does not. There is a little bit of oversimplification there, but I hope you get the idea.
The chemicals, neurotransmitter molecules, are released into a small space called the synaptic cleft. After they’re used, the majority will be recycled for the next neuron firing. This process of recycling is called reuptake and some medicines work by acting on this processes such as selective serotonin reuptake inhibitors.
TLDR: Because you eat food.
The short answer is because we keep making more of them.
It’s not like your body has a tank that gets filled up when you are born.
Rather the cells themselves produce the chemicals needed for these operations. You provide them the material they need to synthesis this chemicals when you eat.
As you said, neurons send signals to the neuron next to them via a flood of chemicals. These chemicals, called neurotransmitters, are released into the synapse that connects the two neurons. But, after they’ve done their job of sending the message, they are reabsorbed by the neuron. This process of reabsorption is called reuptake.
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