S. Onodera et Tp. Hicks, PROJECTIONS FROM SUBSTANTIA-NIGRA AND ZONA INCERTA TO THE CATS NUCLEUS OF DARKSCHEWITSCH, Journal of comparative neurology, 396(4), 1998, pp. 461-482
The goal of the present experiments was to examine the relationships o
f the nucleus of Darkschewitsch (ND) with the substantia nigra pars re
ticulata (SNr), the zona incerta (ZI), and the oculomotor nuclei by us
ing wheat germ agglutinin-horseradish peroxidase (WGA-HRP) as a retrog
rade and anterograde neuronal tracer injected into various sites of th
e cat's brain. To eliminate the possibility that fibres of passage fro
m the motor cortex passing through the SNr and ZI were responsible for
the ND label, WGA-HRP also was injected into the SNr or the ZI after
a large area of the frontal cortex, including the motor area, was dest
royed. Retrograde axonal transport demonstrated that many cells of the
rostromedial part of the ZI project to the ND, with the ipsilateral p
rojections being dominant. Some cells of the caudomedial part of the S
Nr project to the ND, again, with the ipsilateral projection being dom
inant. A few small cells in the ND project bilaterally to the oculomot
or nucleus. Anterograde tracer demonstrated that the SNr-ND terminal f
ields are less dense than the ZI-ND terminal fields. A few fine termin
al fibres were observed bilaterally in the oculomotor, trochlear, and
abducens nuclei. Electron microscopic examination demonstrated that th
ese fine, labelled terminals contain pleomorphic vesicles and have sym
metrical synaptic contacts with dendrites. These results indicate that
the ND, a structure that is known to be important for the control of
axial muscles (i.e., eye, head, and body muscles), is the target of pr
ojections from restricted areas of the SNr and ZI: areas that, during
saccadic eye movement, may lead to disinhibition of the ND-oculomotor
projection. Accordingly, the ND may function to inhibit the activity o
f extraocular muscles during saccades. (C) 1998 Wiley-Liss, Inc.