ABSTRACT
The adaptive responses of motile chromatophores of the lower vertebrates are controlled by “chromatic” centres (both excitatory and inhibitory) located in the optic tectum and brain stem. Differences between incident and background-reflected light are detected by the eyes and lead (a) to release of circulating hormones (e.g. melanophore stimulating hormone (MSH), melanine-concentrating hormone (MCH)) and (b) in many teleosts and some reptiles, to activation of nerve tracts which pass along the spinal cord. The central, antagonistic centres control the level of excitation in pre-ganglionic, cholinergic fibres which enter the sympathetic chain, increase neuronal activity and typically promote aggregation of chromatophore pigment through activation of sympathetic ganglion cells. The postganglionic nerve fibres are typically adrenergic and pass directly to the skin to innervate discrete “dermatomes”; the chromatophores lie in a “ground plexus” of terminals and each effector receives multiple innervation. Paling of the skin depends on activation of α-adrenoceptors (resembling the α 2-subtype) in most fish, but anomalous cases involving a cholinergic innervation are also known. Skin darkening may involve β-adrenoceptor mediation but adenosine compounds, released as co-transmitters, disperse chromato-phore pigment by both direct and indirect actions. Central control of complex colour changes in agonistic/courting behaviour and in adaptations to variegated backgrounds is not yet understood.
