Anatomy, physiology and behavioural rôle of sensory nerve endings in the cement gland of embryonic Xenopus

In newly hatched Xenopus tadpoles touching the cement gland is an effective way to stop swimming. The neuronal basis for this response has been examined. The gland is densely innervated by fine unmyelinated neurites (1 μm or less in diameter) from the trigeminal ganglion. These form unencapsulated, bulbous endings near the secretory surface of the gland and contain many microfilaments. Unit impulse activity can be recorded by suction electrodes on the trigeminal ganglion. Some of these units tend to be spontaneously active and are excited by poking the cement gland or by pulling on the mucus strand secreted by the gland. They cannot maintain high firing frequencies. In behavioural tests, pulling on the mucus strand is found to be effective in stopping swimming and also in depressing the responsiveness of a tadpole to stimuli else­where on its body. It is concluded that the sensory endings in the cement gland are responsible for these effects, whose rôle in normal behaviour is discussed. This appears to be the first place where the anatomy of an unmyelinated skin sensory receptor can be correlated closely with its physiology.