Growth in culture of the peripheral axons of the spiral neurons in response to displacement of the receptors

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Abstract
Observations were made on the growth of the peripheral axons of the spiral neurons in injured explants of the cochlea of the mouse. Opening of the cochlear duct during explantation usually results in a displacement of the hair cells into the outgrowth zone if the basilar membrane of the excised organ of Corti faces the substrate. Spiral neurons respond with vigorous growth to the stress created by the displacement of their receptors. Two different growth reactions occur in succession: 1) growth by elongation of the synaptically engaged fibre 2) free growth. The response of the neuron to an incipient displacement of the hair cells is an elongation of the fibre while its contacts with the receptors are, at least partly, preserved. Most or all fibres that are connected with the displaced cells elongate. The growth is well organized, limited to the region of the displaced hair cells, and restricted in length by the position of the receptors. It is inferred that tension on the synaptically engaged fibre may be a stimulus for its growth and that the growth ceases when the tension is relieved. Continuous stress eventually leads to a rupture of the fibre. The break usually occurs near or at the terminals, leaving the terminals attached to the hair cells. The proximal end of the fibre — now free — starts to grow. The growth is independent of the receptors, unrestricted in length, uncontrolled in amount, and continues for at least three weeks after explantation. The path of the fibre is tortuous, and collaterals are often emitted. Free growth proceeds at a markedly faster rate than growth by elongation. Maintenance of the synapse and growth of the fibre seem to be mutually dependent events. A peripheral axon of the spiral neuron can sustain in culture a normal innervation pattern, without any obvious signs of abnormal growth, if its connections with the hair cells are undisturbed; it elongates in an organized manner, if these connections come under stress; it responds with an uninhibited growth if the connections are broken.