The anatomy of the spinal cord of the minnow Phoxinus phoxinus L., with particular reference to the nerve fibres controlling colour change

Abstract

According to von Frisch (1911) a melanophore-aggregating centre exists in the medulla of the minnow, Phoxinus phoxinus L. From the medulla spinal pigmento-motor fibres pass caudally and have a spinal outflow localised around vertebra 15. Pigmento-motor fibres are then distributed to the cutaneous melanophores via the sympathetic chain, spinal and trigeminal nerves.The investigation described in this thesis is concerned with the location of the spinal pigmento-motor fibres.Mallory's trichrome, Mallory's eosin and methylene blue and Weigert Pal preparations were used to investigate the anatomy of the spinal cord of the minnow. Comparisons are made with the spinal cord of some other teleosts and literature concerning the functions of teleost spinal tracts is reviewed.Microdissection instruments were used to place lesions at vertebrae 3, 4 and 10 and around the spinal outflow. A study of the effects of such lesions upon the responses to black and white background reversal was coupled with a histological examination of the lesions.Such studies indicate a spinal location of the pigmento-motor fibres in the area of the dorsal horns and corpus commune posterius at vertebrae 3, 4 and 10. Complete spinal lesions indicate that the pigmento-motor outflow levels occur between vertebrae 12 and 14, somewhat cephalic to those in von Frisch's mid-European specimens.Cutting some of the spinal pigmento-motor fibres results in differential behaviour of the melanophores, producing a bilateral, dorsolateral cutaneous mottled pattern in response to background reversal. It would appear that the fibres of any specific region of the spinal pigmento-motor tract are associated with melanophore innervating fibres having a wide cutaneous distribution. The melanophore innervating fibres of any specific segment also appear to be associated with spinal outflow nerves of more than one of the spinal levels 12 to 14. It is suggested that postganglionic pigmento-motor fibres retain their integrity rather than form a continuous diffuse cutaneous nerve net.