Most people with a healthy curiosity about the mind and brain know that nerve cells transmit information in the form of electrical impulses. When an electrical impulse reaches the end of the axon of one neuron, packets of molecules (called neurotransmitters) are released from that neuron into the narrow space between that neuron and the next neuron.
This space is called a ‘synapse.’ The neurotransmitters, after their release, have actions at molecules called ‘receptors’ that are imbedded in the lining of neurons. Neurotransmitters attach to receptors and change the shape of the receptors,, which triggers a range of actions depending on the part of the brain and the specific neurons involved.
Endorphins and enkephalins are molecules that function as neurotransmitters in a number of brain regions, including regions that regulate pain, that affect mood, and that alter the body’s response to trauma.
The receptors in these brain regions are activated not only by these neurotransmitters, but also by molecules that are found in the sap of certain poppy flowers. The sap is harvested and concentrated to make a substance called ‘opium.’ Receptors that are activated by this substance, and also by endorphins and enkephalins, are called ‘opioid receptors.’ There are a number of types and subtypes of opioid receptor; the mu type is the receptor activated by most opioid pain relievers, and also by most of the abused opioid substances, including morphine, heroin, oxycodone, and fentanyl.