Several have asked about how the destroying of portions of thalamus or
pallidum can make things better. I am not an expert, but I will attempt an "overview".

first; a few paragraphs quoted from THE CONTENTS OF CONSCIOUSNESS:A
NEUROPSYCHOLOGICAL CONJECTURE Jeffrey A. Gray - that should give you the
flavor of the complexity that is "a brain".

<<<At the information-processing level, the key proposal (Gray et al. 1991 a)
is that a particular set of neurons firing at a particular time in the basal
ganglia: (1) represents a step in a goal-directed motor program; and (2) is
selected for this function by instrumental reinforcement mediated by the
connectivity of the neurons that make up the set (Rolls &amp;Williams 1987).
Within this overall function, the role played by the caudate subsystem is that
of encoding the specific content(in terms of relationships between stimuli,
responses and reinforcement) of successive steps in the program.  The
complementary role played by the accumbens subsystem is that of(1) switching
between steps in the motor program; and (2), in interaction with the SHS,
monitoring the smooth running of the motor program in terms of progress
towards the intended goal. In more detail, the specific hypotheses are as
follows.    1.  The caudate system, by way of its connections with sensory and
motor cortices, encodes the specific sensorimotor content of each step in a
motor program (e.g. for a rat, turn left at a junction in a maze).  2.  The
accumbens system operates in tandem with the caudate system so as to permit
switching from one step to the next in a program.  In addition, there are
outputs to exploratory behavioural systems (via the superior colliculus and
mesencephalic locomotor region) that are activated in response to novel
stimuli.  3.  Both the establishment of the sequence of steps that makes up a
given motor program, and the subsequent orderly (i.e.,goal-directed) running
of the program, are guided by the projection to n. accumbens from the
amygdala; this projection conveys information concerning cue-reinforcement
associations(Rolls &amp; Williams 1987).  4.  The septohippocampal system is
responsible for checking whether the outcome of a particular motor step
matches the expected outcome; this information is transmitted to n.accumbens
by the projection from the subiculum.  For 'match' topographically localised
signal is sent to the accumbens, permitting switching to the next step in the
motor program.  For 'mismatch' a generalised subicular input to accumbens
interrupts all accumbens function relating to motor steps, enabling instead
the activation of exploratory behaviour outputs.  5.  The activities of the
caudate, accumbens, and septohippocampal systems are coordinated and kept in
step with one another by the prefrontal cortex, acting by way of its
interconnections, respectively, with (a) the cortical components of the
caudate system, (b) n. accumbens, dorsomedial thalamus and amygdala, and (c)
the entorhinal and cingulate cortices.  6.  Timing is coordinated between the
septohippocampal monitoring system and the basal ganglia motor programming
system;given the assumption that time is quantized in the SHS by the theta
rhythm (Gray, 1982 a), corresponding to an 'instant' of about a tenth of a
second, this must also be the duration of a step in the motor program.>>>

also:<<<One is conscious of the outputs of motor programs, not of the program
itself.  It is a fact of common experience that we are not normally aware of
either the planning or the execution of movements as such, but only of the
end-points(which may include kinaesthetic and proprioceptive feedback making
up the 'feel' of the movement) that constitute the successive sub-goals of
these movements (cf. Lashley 1956;O'Keefe 1985, p. 69; Jackendoff 1987, p.
45).  Consider, for example, running towards and kicking a football; or
articulating and speaking a sentence.  As Velmans (in Marsh 1993, p. 121)
points out, we are aware even of what we are saying only after we have said
it; and the sub-vocal speech that constitutes much of the process we call
'thinking'consists (for me, at least) of the hearing of words --
i.e.,outputs, clearly, of a linguistic motor program -- in my head. Since the
postulated comparator is designed precisely to compare actual with expected
outputs of motor programs, it follows naturally from the hypothesis that such
outputs constitute the contents of consciousness.  5.3 Consciousness occurs
too late to affect the outcomes of the processes to which it is apparently
linked.  Velmans(1991) has reviewed a range of human information processes
that are normally accompanied by conscious awareness (the analysis and
selection of stimuli, learning and memory, and the production of voluntary
responses, including those requiring planning and creativity).  He has
marshalled evidence that, in each of these cases, the relevant conscious
events follow the information processes to which they are related.>>>

The nigro-striatal pathways consist of dopaminergic neurons which originate in
the substantia nigra (black substance) in the midbrain and project to the
striatum (basal ganglia), indirectly controlling involuntary motor movement.
Parkinson's disease results from degeneration of these neurons with depletion
of dopamine from the neuron terminals and a loss of control of involuntary
motor control (tremor, rigidity, etc) mediated by the extra-pyramidal
pathways. This forms the basis for the administration of the dopamine
precursor, l-dopa as replacement therapy, dopamine agonists, and drugs which
indirectly facilitate dopamine activity.

The dopaminergic meso-limbic and meso-cortical pathways arise in the midbrain
(meso) and project to parts of the limbic system and cortex.

Anatomy of the Basal Ganglia by John B. Penney, Jr., M.D. in UPF 1994
newsletter #1 part 1 is an excellent article to read to better understand PD.

The outputs of the basal ganglia come from the internal segment of the globus
pallidus and the substancia nigra pars reticulata. They send their outputs to
the thalamus and to a few small nuclei in the brain stem.  The largest pathway
goes to the thalamus and then back to the cortex to modify the activity of
nerve cells in the cortex.

The actions of the dopamine that is used as the neurotransmitter by the nerve
cells of the substancia nigra pars compacta are more complex than those of
glutamate, acetylcholine, nitric oxide, or GABA.  Dopamine can either excite
or inhibit the cells to which the nigra pars compacta is connected, depending
on the nature of the cell that is receiving the dopamine input.  The dopamine
is received through specialized molecules called receptors and there are at
least five types of dopamine receptors.

Only two of the dopamine receptors have important roles in the function of the basal
ganglia.  These are the D1 receptor, which is thought to cause excitation, and
the D2 receptor which is thought to cause inhibition of nerve cells.

There is the analogy of a thermostat operating a control circuit. It seems
that the signals from the thalamus and pallidum must be controlled by the
neurons of the substancia nigra that connect to the thalamus and pallidum. If
they do not inhibit the over-signalling that causes tremor or rigidity, we
tremor or move with difficulty.  The dopamine shortage is helped by the
medication.

The surgical lesioning stops the circuits that are sending "static" (too much signalling).

The implanted electrical stimulators are countering the static by signal wave interference.

The reader is encouraged to obtain the United Parkinson Foundation 1994
newsletter #1, part 1 to read the entire article. For those interested - who
can access web pages, an excerpt derived from Dr. Penney's article with two
diagrams can be seen:

http://www.ridgecrest.ca.us/~rfvetter/basaglia.html
--
Ron Vetter 1936, 1984 PD dz   'paradise is where you make it, not a place to go'
e-mail: [log in to unmask]                  janet paterson
http://www.ridgecrest.ca.us/~rfvetter