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Old 11-27-2012
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Default Initiating movement by Basal Ganglia

I just can't seem to figure this out.

Let me explain my question by an example. To raise my left hand, I will first stimulate the right motor cortex, which will send axons to the spinal cord, and after synapsing at the ant horn, will give rise to movement.

However, to initiate such a movement, first basal ganglia should be stimulated, isn't it? So in essence, whenever we start a movement(by raising the left hand in my example), I first would stimulate the basal ganglia structures on the same side, which will project back to the motor cortex, and initiate the movement.

So isn't our voluntary control of muscles really not initiated by first going through Basal Ganglia? When I want to lift my left hand, wouldn't my cortex first stimulate (via Glutamate) the striatum, and via the direct pathway stimulate the specific muscle?

If so, then initiating a motor action isn't really a 2 neuron pathway.

I'll be glad for your input on this question.
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Old 11-28-2012
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Default Basal ganglia and movement


I would distinguish two things when talking about movement:

1. pyramidal pathway (1. primary motor area -> capsula interna -> decussatio pyramidum -> alpha motoneuron @ ant. horn [synapse] -> peripheral nerve -> NM junction).
This is two neuron pathway and actually performs the movement.

2. Basal ganglia INITIATES and REGULATES movement.
There is excitatory and inhibitory pathway. Apart from input from cortex to striatum (described here, striatum receives input also from subst. nigra pars compacta (SNc).
D from SNc is released onto D1 (Gs, stimulatory) receptors on striatum neurons of the direct/excitatory pathway. D thus stimulates stimulatory pathway -> movement initiated.
D from SNc is released also onto D2 (Gi, inhibitory) receptors on striatum neurons of the indirect/inhibitory pathway. D thus inhibits inhibitory pathway -> movement initiated.
Thus, when there is no D in SNc in Parkinson, you have trouble initiating movement (and its regulation).

The inhibitory pathways invovles ncl. Subthalamicus and this model if pathways explains why there is hemibalism in lesion of ncls. subthalamicus.

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