Seconds after marijuana is consumed it begins to affect your brain. The main cannabinoid produced by the cannabis plant is called THC (tetrahydrocannabinol), this cannabinoid binds to cannabinoid receptors within three main places in the brain. The cerebellum, basal ganglia, and hippocampus are responsible for short-term memory, learning, problem-solving, and coordination. Cannabinoid receptors are activated by anandamide, which is a neurotransmitter and natural cannabinoid that the body produces. THC mimics anandamide and binds to the cannabinoid receptors that activate neurons in the brain affecting the mind and body.
To understand the steps that THC take to bind and active neurons, it's best to know the parts of the brain that are involved, "Neurons are the cells that process information in the brain. Chemicals called neurotransmitters allow neurons to communicate with each other. Neurotransmitters fill the gap, or synapse, between two neurons and bind to protein receptors, which allow various functions in the brain and body to be turned on and off. Some neurons have thousands of receptors that are specific to particular neurotransmitters. Foreign chemicals, like THC, can mimic or block actions of neurotransmitters and interfere with normal functions."
Cannabinoids work slightly different from other neurotransmitters; in fact, they work backwards. Neurons work by communicating with each other and the rest of the body by sending chemical messages. These messages are responsible for regulating our motor and cognitive functions. "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)." When this occurs, it sends the receiving neuron into action, allowing the message to travel further throughout the body. However, with the endocannabinoid (EC) system, the message is communicated differently. "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." Once they are released from that cell, they travel backwards to the presynaptic neuron, allowing them to attach to cannabinoid receptors. They can control what happens next when the cells are activated, allowing it to control how messages are sent, received, and processed but the cell.
Scientists have identified the two primary cannabinoid receptors which are CB1 and CB2. The CB1 receptor is found in the brain and nervous system and is the main receptor for THC and anandamide. The receptor found in the immune system and surrounding structures is CB2, which is responsible for modulating antiinflammatory effects. When "THC gets into the brain rapidly it attaches to cannabinoid receptors. The natural EC system is finely tuned to react appropriately to incoming information. But THC overwhelms the EC system. It prevents the natural chemicals from doing their job properly and throws the whole system off balance." This is why scientists have found THC to affect so many systems throughout the body from motor skills to cognitive function.
The following video does a great job of explaining how the brain normally functions without any interference from THC and how it changes once THC in consumed.