The anatomy and function of 'free' nerve endings in an amphibian skin sensory system

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Abstract
The skin of the body and tail in embryonic and young larval Xenopus is innervated by sensory neurones with cell bodies lying along the dorsal midline of the spinal cord. These Rohon-Beard cells have naked peripheral neurites, usually under 1 $\mu $m in diameter, which form a loose network under the skin. In the electron microscope narrower neurites from this network are found running into the skin between the cells. The neurites are periodically varicose and, between the superficial skin cells, these bulges are invaginated into the skin cell membranes where some terminate. Responses of Rohon-Beard cells to skin stimulation can be recorded extracellularly using a suction electrode. Impulses were evoked by light touch or stroking of the skin with fine hairs. These discharges adapted rapidly and quickly fatigued when stimulated repeatedly. Receptive fields (mean area 0.16 mm$^{2}$) were mapped and related to the total number of Rohon-Beard cells and the area of skin to be innervated. This showed that there were enough Rohon-Beard cells to innervate the skin from 1.5 to 2.5 times. The anatomy of the fine, varicose 'free' nerve endings of the Rohon-Beard cell neurites is discussed in relation to their function in detecting transient, touch stimuli to the skin and their role in evoking swimming of the embryo.