EMBRYONIC DETERMINATION OF
NEURAL CONNECTIONS
GEORGE SZÉKELY
Department of Anatomy, University of Pecs, Pecs, Hungary
I. Introduction
181
II. Inherent Specificities
185
A. Visual System
185
B. Determination of the Limb-Moving Segmental
Apparatus in the Spinal Cord
196
C. The Functional Specificity of Cranial Sensory Ganglia
....
198
III. Acquired Specificities
202
A. Corneal Specificity
203
B. Limb Specificity
206
IV. Conclusion
209
V. A Final Remark
216
References
217
i. Introduction
The infinite complexity of the fabric in which neurons of the nervous
system are interconnected has from the very beginning puzzled neurologists, especially in view of the mechanisms by which these connections
are established during embryonic development. Since biologists at the
turn of the century were impressed by a number of newly discovered
mechanisms of various "tropisms," it is only too obvious that they considered neurotropic and neurotactic influences decisive for the formation
of orderly connections in the nervous system. Paul Weiss (1929, 1955),
however, in an impressive series of experiments was able to show the
whole concept to be erroneous. The general picture emerging from these
ingeniously devised experiments was that there appeared to be no such
thing as specific attraction of nerve growing tips by their eventual
peripheral terminals. Rather, the developing fibers appeared to be guided
by forces acting at close range, for example, micellar orientation of the
surrounding tissue matrix and contact surfaces of cells and fibers that
had developed earlier. Although the explanation given by Weiss is less
ambitious than words such as neurotropism, it gives a sound account of
experimentally proven factors in the orientation of nerve growth.
It was not much later, however, that these parsimonious explanations
181
NEURAL CONNECTIONS
GEORGE SZÉKELY
Department of Anatomy, University of Pecs, Pecs, Hungary
I. Introduction
181
II. Inherent Specificities
185
A. Visual System
185
B. Determination of the Limb-Moving Segmental
Apparatus in the Spinal Cord
196
C. The Functional Specificity of Cranial Sensory Ganglia
....
198
III. Acquired Specificities
202
A. Corneal Specificity
203
B. Limb Specificity
206
IV. Conclusion
209
V. A Final Remark
216
References
217
i. Introduction
The infinite complexity of the fabric in which neurons of the nervous
system are interconnected has from the very beginning puzzled neurologists, especially in view of the mechanisms by which these connections
are established during embryonic development. Since biologists at the
turn of the century were impressed by a number of newly discovered
mechanisms of various "tropisms," it is only too obvious that they considered neurotropic and neurotactic influences decisive for the formation
of orderly connections in the nervous system. Paul Weiss (1929, 1955),
however, in an impressive series of experiments was able to show the
whole concept to be erroneous. The general picture emerging from these
ingeniously devised experiments was that there appeared to be no such
thing as specific attraction of nerve growing tips by their eventual
peripheral terminals. Rather, the developing fibers appeared to be guided
by forces acting at close range, for example, micellar orientation of the
surrounding tissue matrix and contact surfaces of cells and fibers that
had developed earlier. Although the explanation given by Weiss is less
ambitious than words such as neurotropism, it gives a sound account of
experimentally proven factors in the orientation of nerve growth.
It was not much later, however, that these parsimonious explanations
181