Nervous Control of Chromatophores in Teleost Fishes I. Electrical Stimulation in the Minnow

It has been firmly established that in many teleosts the sympathetic nervous system contains cells whose activity causes aggregation of the pigment within the melanophores and therefore paling of the skin. In the minnow these W (whitening) fibres were shown by von Frisch (1911) to follow the pathways shown in Fig. iA; that is they pass from a centre in the hind-brain via the spinal cord to the level of the 15th vertebra, through rami to the sympathetic chain and then anteriorly and posteriorly to emerge through all the spinal and the trigeminal (fifth cranial) nerves to the skin. Nerve section at any level in these tracts first produces intense pigment dispersion in the paralysed melanophores, later followed by slower adaptive responses under humoral influences. Electrical stimulation of the intact tracts causes rapid melanophore aggregation, as do adrenalin and other sympathomimetic agents. Parker (1948) claimed to have demonstrated the presence of a system of opposing, melanophore-dispersing or B (blackening) fibres in Fundulus and Ameiurus. His arguments are still controversial and at best provide only indirect evidence of double innervation, but two experiments seem to be critical. These are, first, that a second nerve-section within a previously paralysed region produces redispersion both orthodromically and antidromically (1934a, 1936,1937); and, secondly, that the application of a small cold block just peripheral to a fresh nerve-section limits the spread of the response and prevents dispersion from appearing distally (19346). From these results Parker argued that the initial dispersion is an active process produced by repetitive discharges in B fibres and originating from a permanently depolarized region at the site of the nerve-section. This view has met with considerable opposition from a number of sources, although no satisfactory alternative explanation has been advanced. Parker & Rosenblueth (1941) claimed to have stimulated B fibres by electrical means in Ameiurus. They applied rectangular pulses, 6-8 V. in amplitude and 300-500 msec, in duration, at a rate of 1-2 pulses/sec, to the skin through bipolar electrodes and obtained local darkening in about 10 min. Pulses of 8 V., 4-8 msec, in duration and delivered at 15-25 pulses/sec, produced local paling in 15-25 min. These experiments may be criticized on three points: first, the mark/space ratio was very high for the pulse trains used to obtain dispersion and the d.c. component may have blocked normal