The diameters of the fibres of the peripheral nerves of Octopus

The diameters and numbers of fibres have been measured throughout the peripheral nervous system. The nerves of the muscles that act upon the outside world contain few fibres, having very large and medium-sized fibres but no very small ones. Thus the muscles of the head receive 6000 fibres, the largest of 30 μ m diameter. The eye muscles receive 3300 fibres, reaching 24 μ m. The stellar nerve fibres are more numerous (150 000), but smaller (< 20 μ m). The preganglionic fibres joining the c. n. s. to the stellate ganglia are fewer than the postganglionic ones (4000, < 16 μ m). In some of the somatic motor nerves the longest bundles contain the largest fibres. However these are accompanied by a distinct group of smaller fibres, whose significance is uncertain. It is not clear that there is a distinct proprioceptor group. The fibres to the chromatophores are numerous (30000) and of medium diameter (< 12 μ m). The visceral motor and vasomotor nerves, however, contain hundreds of thousands of minute fibres (< 3 μ m). The significance of these numerous small fibres can hardly be to obtain great resolution of movement in such acts as secretion of saliva. Presumably the great number has a special significance. The fibres to the muscles of the buccal mass are more numerous and smaller than the somatic motor fibres, but fewer and larger than those for the viscera. The muscles of the arms and suckers have 3.0 x 10 6 fibres, all < 6 μ m, originating from neurons within the arms. They are controlled from the brain by relatively few but large fibres (32000, < 26 μ m). There is also a reduction of 100 times on the afferent pathway of the arms, from some 18 x 10 6 receptors at the periphery of the suckers to 140000 fibres entering the brain. The brain thus serves for major decisions, whose detailed execution is left to peripheral reflex centres in the arms. The optic nerve fibres are very numerous, and all small (20 x 10 6 , < 1.3 μ m) presumably for economy of space and material. By contrast the static nerves, although they are short, contain a small number of large fibres (1400 between 6 and 22 μ m) as well as several thousand smaller ones. The presence of sets of fibres with their distinctive diameters, conduction velocities and other properties is evidently a fundamental feature of the design of the nervous system of cephalopods as it is of vertebrates, although the significance of many features remains to be explored.