Pre-existing Connections Research Articles

Stability of new electrical connections between adult Helisoma neurons is influenced by preexisting neuronal interactions.

The factors responsible for the selective stability of new electrical connections between regenerating, adult Helisoma neurons (5, 6) were examined. In particular the alternative hypotheses that the normal reduction in strength of a specific connection is dependent on the formation of a new stable connection or the maintenance of a preexisting connection were tested. To this end, single neurons were eliminated either by microdissection or photoinactivation. Formation of stable 5-5 [i.e., left neuron 5 (L5) to right neuron 5 (R5)] coupling was prevented by removal of a single neuron 5 soma. The time course of the normally transient neuron 5-4 connection in these ganglia, however, was the same as that of control preparations. Thus the 5-4 connection is transient even when stable 5-5 coupling is unable to develop. After removal of a single neuron 4 soma, however, the 5-4 connection was stabilized. This was despite the concurrent development of stable 5-5 coupling in these preparations. Similar results were obtained when neurons were eliminated by photoinactivation, a procedure that kills axonal processes in addition to the neuronal cell body. Morphological examination of untreated neurons by Lucifer Yellow CH stain showed that neurite outgrowth was not prevented by either of the techniques employed to eliminate single neurons. It is concluded that the development of the new, stable 5-5 electrical connection does not influence the transient nature of the 5-4 connection. The presence of both neurons 4, however, which have a preexisting connection, is required for reduction of 5-4 coupling. Thus preexisting neurons and/or their connections appear to play a role in the stability of new connections in this adult nervous system.

Introduction to Special Series

This introduces a special four-part series about neural plasticity. Plasticity is an important property of developing and mature neural tissue and is currently a very popular topic for scientific investigation. The term plasticity is used here to mean the adaptive phenomena occurring in neural tissue when structural changes coincide with functional modifications. The developing nervous system is capable of a high degree of plasticity. It has the capacity to alter its pattern of pre-existing connections. Changes in the cellular, chemical, or “experiential” environment of the developing neuron may produce new neuronal pathways while previously established connections are lost. In the adult, plastic responses to experiential manipulation occur as well, but the responses in the adult nervous system usually differ qualitatively and quantitatively from those of the developing nervous system.…

Functional changes in ventrobasal thalamic neurones responsive to noxious and non-noxious cutaneous stimuli after chloralose treatment: New evidence for the presence of pre-existing “silent connections” in the adult nervous system?

Twenty-five neurones of the thalamic VB complex of the rat, first characterized under moderate volatile anaesthesia ( 1 3 oxygen− 2 3 nitrous oxide , 0.5% halothane) were subsequently studied under deep chloralose anaesthesia. As described in a previous study, neurones were classified during the control period as “noxious” (N), “non-noxious” (Nn), or “convergent” (NnN) according to their responsiveness to cutaneous mechanical stimulations. For all the N (n = 15) and NnN (n = 4) neurones, the responses induced by noxious stimuli (mechanical electrical or thermal) progressively disappeared after chloralose administration. Simultaneous with disappearance of the responses elicited by noxious stimuli, responses to non-noxious stimuli appeared for 12 N neurones: 5 responded to a brisk tap applied to any part of the body, and 7 to brushing of light touch from a new receptive field (RF), at a variable distance from the initial RF, but always contralateral to the recording site. The Nn neurones (n = 6) continued to be activated by light touch or brushing but there was a consistent concentric enlargement of their RF. The NnN neurones also presented an enlarged RF to the non-noxious stimuli. The depressive and “unmasking” effects of chloralose are discussed and compared to the various indications of pre-existing connections in the CNS.