The stopping response of Xenopus laevis embryos: behaviour, development and physiology.

1. When Xenopus laevis embryos swim into an obstruction they usually stop. This stopping response to stimulation on the head is present from stage 28 to 45. At stage 37/38 it is more reliable in restrained than in free-swimming animals, and to stimuli to the cement gland than to the head skin. 'Fictive' swimming also stops reliably after the same stimuli but struggling and 'fictive' struggling do not. 2. Discharge of deformation-sensitive trigeminal sensory neurons in response to pressure on the cement gland or head skin precedes the 'fictive' stopping response. When the embryo hangs from cement gland mucus, trigeminal neurons are active and the embryo is less responsive to stimulation. 3. Lesions of the central nervous system have allowed us to draw the following conclusions about this inhibitory pathway: (a) either the cement gland or the head skin must be intact; (b) one trigeminal ganglion is both sufficient and necessary; (c) the pathway is independent of the forebrain and midbrain; (d) it can take an ipsilateral or contralateral route through the hindbrain; (e) at least two hindbrain interneuron components are involved. 4. A similar stopping response is present in embryos and larvae of the urodele Ambystoma mexicanum.