Jeremy has parkinson's disease, a progressive neurodegenerative disease that affects motor skills. in addition to motor symptoms, jeremy has noticed changes in his mood, and he feels the need to smoke more cigarettes than he used to. it is likely that jeremy's is producing less than it needs to.

jeremy's SUBSTANCIA NIGRA  is producing less DOPAMINE than what it needs to.

Substancia nigra is a "black substance" located in the midbrain. When there is the death of dopaminergic neurons in the substantia nigra, the quantity of dopamine decreases, causing Parkinson symptoms. With time passing, the fewer neurons Jeramy will have and those symptoms will become more severe, that is the reason why is smoking more. The nicotine from cigarettes help modulate dopamine transmission and so, to reduce some symptoms.

Reproduction i think