B1 plays a role in the de novo synthesis of neurotransmitters (from glucose) including acetylcholine and glutamate.
The pyruvate dehydrogenase complex (which requires B1) makes acetyl-CoA which can act as a precursor for acetylcholine.
B1 seems to also be involved in ACh release & signalling.
Acetylcholine esterase inhibitors are currently prescribed for Alzheimer's. Interesting stuff.