After several decades of research, scientists studying the effects of marijuana made several important discoveries. Not only did they identify the active ingredient in marijuana, they also discovered where and how it works in the brain—via a new system they called the endocannabinoid (EC) system. The EC system—named after the marijuana plant Cannabis sativa and its active ingredient delta-9-tetrahydrocannabinol (THC)—is a unique communications system in the brain and body that affects many important functions, including how a person feels, moves, and reacts.

The natural chemicals produced by the body that interact within the EC system are called cannabinoids, and like THC, they interact with receptors to regulate these important body functions. So what makes the EC system unique and how does THC’s impact on this system affect a person’s memory, risk for accidents, and even addiction?

Review Figure 1 and the steps below to take a closer look at the components of the EC system, how it works, and the effects of THC.

Figure 1: The Endocannabinoid (EC) System and THC

How Cannabinoids Work Differently From Other Neurotransmitters

Brain cells (neurons) communicate with each other and with the rest of the body by sending chemical “messages.” These messages help coordinate and regulate everything we feel, think, and do. Typically, the chemicals (called neurotransmitters) are released from a neuron (a presynaptic cell), travel across a small gap (the synapse), and then attach to specific receptors located on a nearby neuron (postsynaptic cell). This spurs the receiving neuron into action, triggering a set of events that allows the message to be passed along.

But the EC system communicates its messages in a different way because it works “backward.” When the postsynaptic neuron is activated, cannabinoids (chemical messengers of the EC system) are made “on demand” from lipid precursors (fat cells) already present in the neuron. Then they are released from that cell and travel backward to the presynaptic neuron, where they attach to cannabinoid receptors.

So why is this important? Since cannabinoids act on presynaptic cells, they can control what happens next when these cells are activated. In general, cannabinoids function like a “dimmer switch” for presynaptic neurons, limiting the amount of neurotransmitter (e.g., dopamine) that gets released, which in turn affects how messages are sent, received, and processed by the cell.

How Does THC Affect the EC System and Behavior?

When a person smokes marijuana, THC overwhelms the EC system, quickly attaching to cannabinoid receptors throughout the brain and body. This interferes with the ability of natural cannabinoids to do their job of fine-tuning communication between neurons, which can throw the entire system off balance.

Because cannabinoid receptors are in so many parts of the brain and body, the effects of THC are wide-ranging: It can slow down a person’s reaction time (which can impair driving or athletic skills), disrupt the ability to remember things that just happened, cause anxiety, and affect judgment. THC also affects parts of the brain that make a person feel good—this is what gives people the feeling of being “high.” But over time THC can change how the EC system works in these brain areas, which can lead to problems with memory, addiction, and mental health.

Refer to Figure 2 to see areas of the brain with cannabinoid receptors, then locate those areas on the chart to study some of the different effects of THC on the user.

Figure 2: Locations of Cannabinoid Receptors in the Brain