Researchers have figured out how marijuana use affects the nerve cells that control our muscles, and say it could explain why it's had positive effects on people with neurodegenerative diseases such as multiple sclerosis (MS).

A team led by Bernardo Moreno from the NeuroDegeneration and NeuroRepair Group of the University of Cadiz in Spain wanted to figure out why side-effects of marijuana use sometimes include difficulty speaking and forming words, difficulty breathing, and - despite the intense need for munchies - difficulty swallowing food. Many previous studies have focussed on psychotropic effects of the drug, such as anxiety, depression, cognition, learning, and memory, but until now, little has been done to figure out the cellular and molecular mechanisms that underlie the impairment of motor skills in some users.

To investigate, the team used synthetic analogues of the psychoactive compounds of marijuana and observed their effects on the motor neurons - or nerve cells - that control our muscle movement. Using lab mice, they focussed on the nerve cells that control the tongue, because it’s responsible for speaking, breathing and swallowing.

Publishing in the journal Neuropharmacology, the team reports that the psychoactive compounds in marijuana actually inhibit the transmission of information between these neurons via the synapses. The result of this is muscular weakness. “The motor neuron - that is, the one that gives the order to the muscle to contract - sees its activity reduced which, as a consequence, would weaken the strength of the muscle contraction,” Moreno said in a press release.

And while the mechanism behind making it harder for us to speak and swallow might not sound like a helpful one, the team thinks it’s at play when people with neurodegenerative diseases such as multiple sclerosis report beneficial affects from using marijuana. "In pathological processes associated with muscular hyperactivity phenomena, the reduction in motor neuron activity induced by cannabis could lead to a symptomatological improvement,” says Moreno.

Interestingly, just last year, a separate study also looked into how marijuana affected the connectivity of the brain, and found that when it comes to long-term use, it appears to shrink a certain part of the brain in heavy users, but their brains will actively compensate for this by increasing connectivity - especially if the user started young.