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the buildup of the pattern of movement-related responses in the primary motor
and parietal cortex and results in a loss of directional specificity of reciprocal
and bidirectional cells in the motor cortex as well as in a reduction in their ac-
tivities and their rates of change. These changes result in delays in recruiting the
appropriate level of muscle force sufficiently fast and in an inappropriate scaling
of the dynamic muscle force to the movement parameters. A repetitive triphasic
pattern of muscle activation is sometimes needed to complete the movement. All
of these disruptions result in an increase of mean reaction time and a slowness of
movement.
11.3 Empirical Signatures
The validity of the model's hypothesis is based on the existence of a widespread
dopaminergic innervation in not only the basal ganglia, but also in cortex and spinal
cord as well as on its effects on movement, muscular, and neuronal parameters of
Parkinson's disease patients and MPTP-lesioned animals.
11.4 Is There Dopaminergic Innervation of the Cortex
and Spinal Cord?
A widespread dopaminergic innervation from the substantia nigra (SN), the VTA,
and the retrorubral area (RRA) to the cortex and spinal cord exists [6, 54]. A
schematic diagram of the dopaminergic innervation of the neocortex is depicted
in Fig. 11.2. DA afferents are densest in cortical areas 24, 4, 6, and SMA, where
they display a trilaminar pattern of distribution, predominating in layers I, IIIa,
and V-VI [5, 54, 25, 27, 28]. In the granular prefrontal (areas 46, 9, 10, 11, 12),
parietal (areas 1, 2, 3, 5, 7), temporal (areas 21, 22), and posterior cingulate
(area 23) cortices, DA afferents are less dense and show a bilaminar pattern of
distribution in the depth of layers I and V-VI [5, 42, 43, 27, 28, 46]. Area 17
has the lowest DA density, where the DA afferents are mostly restricted to
layer I [5].
In addition to the DAergic innervation of the neocortex, the presence of dopamin-
ergic fibers in the dorsal and ventral horns of the spinal cord has been observed by
several groups [7, 8]. In the dorsal horn, DA fibers are localized in the superficial
layers and in the laminae III-V and X. In ventral horn, DA fibers are found in layers
VII, VIII, and laminae IX [51]. The sources of the dorsal DAergic innervation are
the posterior and dorsal hypothalamic areas and the periventricular gray matter of
the caudal thalamus, whereas of the ventral dopaminergic innervation is the cau-
dal hypothalamus A11 cell group [47]. Finally, an uncrossed nigrospinal DAergic
pathway has been documented by anatomical methods [15].
