80
ARTHUR HUGHES
By far the greater part of the experimental studies of innervation have
been concerned with regenerative processes. Classic studies on the movement of supernumerary transplanted limbs have involved the regeneration either of a whole limb (Detwiler, 1925, 1933) or of the nerves within
a fully developed and transplanted member (Weiss, 1922). These researches clearly showed that in urodeles the homologous function of a
normal and a nearby grafted limb owed nothing to anatomical arrangement of the nerves, which within the transplant were wholly unlike the
normal pattern (Weiss, 1937). The "synonomous response" of the
muscles within each limb was explained in terms of "myotypic specification" (Weiss, 1941) whereby the individual muscle is believed to determine the pattern of synaptic relationships of the motor neurons with
which it is in contact. It has, however, been shown that in the normal
limb of Ambystoma
there is little or no selective distribution of the
motor axons within each nerve of the brachial plexus to individual
muscles. This conclusion has emerged both from stimulation of ventral
roots (Nicholas and Barron, 1935) and also by tracing the distribution
of degenerating fibers after cutting each root in turn (Thompson, 1936).
This feature of the nervous system of the urodele is one among others in
which animals of this group differ sharply from all other tetrapods
(Strauss, 1946). There is, therefore, some doubt whether the results of
experimental studies on the nervous system in this group of animals are
generally applicable.
In recent years, R. W. Sperry and his colleagues have described striking experiments on teleost fishes in which random regeneration of severed
motor nerves have been compared with the effects of cross-suturing of
the nerves to antagonistic muscles. A full interpretation of these results
is not yet possible. When the nerves to the pectoral fins of Sphaeroides
spengleri were transected with the cut ends frayed and roughly apposed,
complete restoration of function was observed within 3 weeks of operation (Sperry, 1950); a similar result was obtained when the oculomotor
nerve was severed in Astronotus
ocellatus (Sperry and Arora, 1965).
In the same species, however, cross-suturing of nerves to antagonistic
muscles of the pectoral fin (Mark, 1965) led to a permanent impairment
of function, with no respecification of the wrongly innervated muscles.
Yet in the same species again, a similar operation performed on the
maxillary and mandibular branches of the trigeminal nerve led to full
recovery of the action of the jaws (Arora and Sperry, 1957). On the
basis of their experiments on the regeneration of the oculomotor nerves
in Astronotus, Sperry and Arora (1965) have recently expressed doubts
ARTHUR HUGHES
By far the greater part of the experimental studies of innervation have
been concerned with regenerative processes. Classic studies on the movement of supernumerary transplanted limbs have involved the regeneration either of a whole limb (Detwiler, 1925, 1933) or of the nerves within
a fully developed and transplanted member (Weiss, 1922). These researches clearly showed that in urodeles the homologous function of a
normal and a nearby grafted limb owed nothing to anatomical arrangement of the nerves, which within the transplant were wholly unlike the
normal pattern (Weiss, 1937). The "synonomous response" of the
muscles within each limb was explained in terms of "myotypic specification" (Weiss, 1941) whereby the individual muscle is believed to determine the pattern of synaptic relationships of the motor neurons with
which it is in contact. It has, however, been shown that in the normal
limb of Ambystoma
there is little or no selective distribution of the
motor axons within each nerve of the brachial plexus to individual
muscles. This conclusion has emerged both from stimulation of ventral
roots (Nicholas and Barron, 1935) and also by tracing the distribution
of degenerating fibers after cutting each root in turn (Thompson, 1936).
This feature of the nervous system of the urodele is one among others in
which animals of this group differ sharply from all other tetrapods
(Strauss, 1946). There is, therefore, some doubt whether the results of
experimental studies on the nervous system in this group of animals are
generally applicable.
In recent years, R. W. Sperry and his colleagues have described striking experiments on teleost fishes in which random regeneration of severed
motor nerves have been compared with the effects of cross-suturing of
the nerves to antagonistic muscles. A full interpretation of these results
is not yet possible. When the nerves to the pectoral fins of Sphaeroides
spengleri were transected with the cut ends frayed and roughly apposed,
complete restoration of function was observed within 3 weeks of operation (Sperry, 1950); a similar result was obtained when the oculomotor
nerve was severed in Astronotus
ocellatus (Sperry and Arora, 1965).
In the same species, however, cross-suturing of nerves to antagonistic
muscles of the pectoral fin (Mark, 1965) led to a permanent impairment
of function, with no respecification of the wrongly innervated muscles.
Yet in the same species again, a similar operation performed on the
maxillary and mandibular branches of the trigeminal nerve led to full
recovery of the action of the jaws (Arora and Sperry, 1957). On the
basis of their experiments on the regeneration of the oculomotor nerves
in Astronotus, Sperry and Arora (1965) have recently expressed doubts