The Neurons that Control Axial Movements in a Frog Embryo

Abstract

SYNOPSIS. This paper reviews nineteen different classes of neuron present in the nervous system of late embryos of the amphibian Xenopus laevis to see how far the behaviour of these animals can be explained in terms of the properties of these neurons. Movements can be initiated by light sensitive neurons in the pineal vesicle and touch sensitive neurons innervating head and trunk skin. Swimming can be stopped by activity in neurons innervating head skin and the cement gland. A trigeminal pathway allows the skin impulse access to the nervous system to initiate movement. Central pathways exist in the hindbrain and spinal cord to carry excitation and inhibition to the opposite side following sensory stimulation. Two classes of spinal neuron appear sufficient to coordinate motor neuron activity in simple reflexes and the basic alternation in swimming. However, the longitudinal coordination in swimming and struggling movements is not understood. For some of the cell classes described there is no evidence on function. I conclude that the Xenopus embryo nervous system and its relation to behaviour is better understood than any other but still leaves us with many questions to answer!