This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. A path.

n. Physiology A course usually followed by a body part or process.

n. Physiology A chain of nerve fibers along which impulses normally travel.

n. Physiology A sequence of enzymatic or other reactions by which one biological material is converted to another.

n. A footpath or other path or track

n. A sequence of biochemical compounds, and the reactions linking them, that describe a process in metabolism or catabolism

n. A footpath; a beaten track; any path or course. Also used figuratively.

n. A path; usually, a narrow way to be passed on foot; also, a way or a course of life.

n. a trodden path

n. a bundle of myelinated nerve fibers following a path through the brain

"And even more than the qualifiers it already gives, the page reminds us that "The carbohydrate - tryptophan - serotonin pathway is simply a hypothesis at this point.""

path   

footpath   

mechanism    path    receptors    channel    route    activation    avenue    response    molecule    lane