DETERMINATION OF NEURAL CONNECTIONS
211
the various physiological properties of the impulses by themselves cannot
bring organization into a completely random set of neurons; therefore,
some sort of structural organization must be present in any networklike system.
b. One of the characteristic morphological impressions due to networklike neural structures is that synaptic connections are effected by the
various diffuse-type synapses, and a high convergence of presynaptic
fibers is necessary for effective impulse transmission. Convergent synaptic
connections with diffusely arborizing axons result in greatly overlapping
areas within which each neuron receives impulses from several thousand
presynaptic cells, and each presynaptic neuron makes contact with
several thousand postsynaptic cells. The difficulty in establishing selective
connections on an interneuronal affinity basis in such a system is obvious.
There are, however, data in the literature that a kind of structural organization is attained by directional axonal growth. One piece of evidence
is the remarkable finding of Attardi and Sperry (1963) on the preferential selection of pathways by regenerating fibers in the optic nerve
and tract. Another similar phenomenon is the tendency of Mauthner
fibers growing out from atypically located supernumerary cells to follow
the course of the normal fiber (Stefanelli, 1951; Piatt, 1944). A kind of
chemotactic selectivity is suggested to cope with this hitherto unknown
mechanism (Sperry, 1963). Beautiful examples of mechanical factors in
the orientation of fiber growth have been demonstrated by Weiss (1929,
1955). The phenomenon became know
T
n by the name of "contact guidance." Starting from the old observation that fiber growth can only
occur along interfaces (Harrison, 1914), Weiss showed the indirect orienting effect of tension applied to nervous tissue cultured on a medium containing a fibrous matrix. Similar effects may be responsible for the
deflection of nerve bundles toward growing organs, and the plexus formation of the thoracic nerves achieved by transplanting a limb bud (Detwiler, 1936), eye primordium, or nasal placode (Detwiler and Van
Dyke, 1934) in the thoracic region of salamander embryos. The orientation of the micellar ultrastructure of the developing nervous system
may influence the course of central pathways (Burr, 1932). The primary
orientation of the long axis of the neuroblast determines the direction of
the initial axon growth in spinal ganglia (Szentâgothai and Székely,
1956), which may account for the somatotopic projection of sensory
fibers upon the substantia gelatinosa (Szentâgothai and Kiss, 1949).
It is obvious that these factors can account only for a crude orientation
of the growing axons but cannot determine the selective synapsing patterns in the complicated interneuronal connections assumed by the
functional specificity theory. Nevertheless, the animals behave as if all
211
the various physiological properties of the impulses by themselves cannot
bring organization into a completely random set of neurons; therefore,
some sort of structural organization must be present in any networklike system.
b. One of the characteristic morphological impressions due to networklike neural structures is that synaptic connections are effected by the
various diffuse-type synapses, and a high convergence of presynaptic
fibers is necessary for effective impulse transmission. Convergent synaptic
connections with diffusely arborizing axons result in greatly overlapping
areas within which each neuron receives impulses from several thousand
presynaptic cells, and each presynaptic neuron makes contact with
several thousand postsynaptic cells. The difficulty in establishing selective
connections on an interneuronal affinity basis in such a system is obvious.
There are, however, data in the literature that a kind of structural organization is attained by directional axonal growth. One piece of evidence
is the remarkable finding of Attardi and Sperry (1963) on the preferential selection of pathways by regenerating fibers in the optic nerve
and tract. Another similar phenomenon is the tendency of Mauthner
fibers growing out from atypically located supernumerary cells to follow
the course of the normal fiber (Stefanelli, 1951; Piatt, 1944). A kind of
chemotactic selectivity is suggested to cope with this hitherto unknown
mechanism (Sperry, 1963). Beautiful examples of mechanical factors in
the orientation of fiber growth have been demonstrated by Weiss (1929,
1955). The phenomenon became know
T
n by the name of "contact guidance." Starting from the old observation that fiber growth can only
occur along interfaces (Harrison, 1914), Weiss showed the indirect orienting effect of tension applied to nervous tissue cultured on a medium containing a fibrous matrix. Similar effects may be responsible for the
deflection of nerve bundles toward growing organs, and the plexus formation of the thoracic nerves achieved by transplanting a limb bud (Detwiler, 1936), eye primordium, or nasal placode (Detwiler and Van
Dyke, 1934) in the thoracic region of salamander embryos. The orientation of the micellar ultrastructure of the developing nervous system
may influence the course of central pathways (Burr, 1932). The primary
orientation of the long axis of the neuroblast determines the direction of
the initial axon growth in spinal ganglia (Szentâgothai and Székely,
1956), which may account for the somatotopic projection of sensory
fibers upon the substantia gelatinosa (Szentâgothai and Kiss, 1949).
It is obvious that these factors can account only for a crude orientation
of the growing axons but cannot determine the selective synapsing patterns in the complicated interneuronal connections assumed by the
functional specificity theory. Nevertheless, the animals behave as if all