Electrical polarization and axon outgrowth

Bayliss, in Principles of General Physiology (1915), described neurone protoplasm as full of fine granules, behaving as a viscous fluid and responding to reagents as an electronegative colloid.¹ He noted that observers using dark-ground illumination found neither Nissl bodies nor neuro-fibrils in the living cell.¹ Here electronegativity is a property of the colloid, not yet a field acting on growth. Crile, quoting Wilson, gave a related axial picture: the oral region of hydroids is electronegative to the basal region.⁷ Hyman and Bellamy reported that electrical gradients closely follow metabolic ones, with high metabolic rate being electronegative.⁷

Lillie's 1923 book reports that Kappers read the direction of outgrowth of nerve-tracts in the central nervous system as the expression of a directive electrical influence, analogous to galvanotropism, and named it neurobiotaxis.⁴ Lillie sends readers to Child's discussion of the subject.⁴ Child's 1921 bibliography lists a paper that compares neurobiotaxis with other phenomena of taxis and tropism and treats the dynamic polarization of the neuron.³ Child's own diagram places a gradient in electronegativity along the physiological gradient and has excitation raise electronegativity at a point.² The held passages do not state where Child and Kappers agree or differ.

Lillie wrote that the permeability of many plasma membranes, especially of irritable cells, is very sensitive to changes of electrical condition.⁵ He also called the application of membrane principles to the plasma membrane uncertain, since its structure differs from that of macroscopic porous membranes.⁵ Child's 1924 book summarises Lillie's view: the membrane is electropositive outside and electronegative inside, and excitation raises permeability to negative ions, so the excited region becomes more electronegative externally.⁶ The currents between excited and unexcited regions were taken as the means of transmission.⁶ Child's 1921 diagram adds that such a current is compensated by the current of the physiological gradient.²

Sundelacruz, Levin and Kaplan (2008) concluded that differentiated hMSCs are hyperpolarized compared with undifferentiated hMSCs, and that polarization is associated with differentiation toward osteogenic or adipogenic lineages.⁸ Funk (2015) discussed endogenous electric fields as guiding cues for migration. In wounded epithelia the transepithelial potential is enhanced at the cathode at the wound center, and the field is probably the earliest signal prompting directional migration into the wound bed.⁹ Neither of these passages addresses axon outgrowth.