Table 1.

Characteristics of various classes of neuronal signalling molecules. See main text for references.

type of signalling molecule	diffusion coefficient	synthesis	evolvability	diversity
ions, gases (NO)	∼1–2 × 10−5 cm2 s−1	n.a. or by NO synthase	none	limited
small molecules	∼2 × 10−5 cm2 s−1	synthesis requires several specific enzymes	limited, requires the evolution of new enzymatic activities, can co-evolve with receptor	theoretically unlimited, limited by evolutionary constraints
neuropeptides	∼2–5 × 10−6 cm2 s−1	on ribosomes, followed by proteolytic cleavage and modification	highly evolvable, by divergence within a multi-copy precursor, by gene duplication, often co-evolves with receptor	unlimited, 20n, where n is sequence length (limited by solubility, stability)
globular proteins	∼2–10 × 10−7 cm2 s−1	on ribosomes	highly evolvable, by gene duplication and divergence	unlimited, 20n, where n is sequence length (limited by folding, stability)