Initiation of motor responses in developing Xenopus laevis tadpoles

Abstract

The neuronal basis for some locomotor responses is starting to be understood. However, relatively little attention has been addressed to their initiation. Here, initiation of motor responses is studied in hatchling Xenopus laevis mainly by electrophysiology in tadpoles immobilised with a neuromuscular blocker, but also by behavioural observations including high-speed video analysis. Responses to touching the trunk skin typically start with a rapid (~10 ms) reflex bend to the opposite side followed later (~44 ms), and with a variable delay, by swimming whose first bend is on the stimulated side. This pattern is established by the time coordinated swimming has developed 53 hrs post fertilisation at 20 - 24°C. Absence of an initial reflex is associated with a loss of sidedness of subsequent swimming. Some neurons in segments (rhombomeres) throughout the length of both sides of the hindbrain fire impulses very soon (~10 ms) after a trunk skin stimulus. This is followed by sustained firing in hindbrain neurons that precedes and overlaps the start of swimming, consistent with playing a role in the initiation of swimming. This sustained firing occurs even at stimuli too weak to evoke swimming. Skin cells in the tadpole are electrically excitable. Skin impulses evoked by poking the skin can directly excite neurons in the spinal cord to evoke a motor response. This direct access of skin impulse signals to the spinal cord is lost over the short period of development during which the skin is becoming fully innervated by the free nerve endings of mechanosensory Rohon-Beard neurons. Evidence suggests that access of skin impulse signals to the spinal cord in young tadpoles is through these Rohon-Beard neurons, probably while they are immature and before they respond to touch stimuli.

Date of Award	17 Jun 2009
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Stephen R Soffe (Supervisor)