Scientific Understanding of Consciousness
Basal Ganglia Model
Both Direct and Indirect Pathways are Activated Simultaneously
In this simplified model, cerebral cortex neurons (red) receive advice from striatal neurons (blue) on two possible courses of action: action A, which has previously had a positive outcome, and action B, which has had a negative outcome.
Traditionally, it has been thought that two groups of striatal neurons activate substantia nigra neurons (green) through either a direct pathway favouring action A or an indirect pathway, with an intermediate step in the globus pallidus, advising against action B.
Researchers now report that both direct and indirect pathways are activated simultaneously to recommend action A and advise against action B. These recommendations are then passed through the thalamus (yellow) to the cerebral cortex.
Research Source — Basal Ganglia Direct and Indirect Pathways
Traditional Car Brake Metaphor No Longer Consistent
A metaphor for the basal ganglia has been a car. To go, you take your foot off the brake and press down on the accelerator; to stop, you do the opposite.
Similarly, the basal ganglia are organized into two parallel pathways, which are anchored by spiny projection neurons (SPNs). It has thought that the SPNs in the direct pathway (think accelerator) promote action, whereas those in the indirect pathway (brake) suppress it.
Much of the evidence for this model comes from clinical observations of patients with Parkinson's disease. Loss of the neurotransmitter dopamine in these individuals makes the direct-pathway SPNs sluggish and the striatal, indirect-pathway SPNs jumpy. Consequently, the indirect-pathway SPNs are likely to win any discussion about what to do, resulting in a constant nay recommendation — as if the brake were constantly being pressed firmly down. This is thought to lead to the difficulty in initiating movement that is characteristic of Parkinson's disease.
Striatum Simultaneously Signals Recommendations of What to Do and Not Do
Experimental evidence that direct- and indirect-pathway SPNs do not simply make sequential yea or nay recommendations kills the car metaphor. It is possible, albeit unlikely, that the activity of these neurons is somehow combined to make a binary decision, but researchers have found no indication of this. It is more likely that the striatum is simultaneously making recommendations about what to do and what not to do. Intuitively, this is appealing. When choosing between two alternative courses of action, it is always best to have a reason for choosing one and for not choosing the other.
An opposing scenario — when a course of action has a bad outcome — is played out in the indirect-pathway SPNs. A drop in dopamine release in the striatum promotes the strengthening of connections between cortical neurons and the indirect-pathway SPNs. So, in the future, when the cortical neurons representing these actions become active, indirect-pathway SPNs follow suit, sending back a warning message. This implies that striatal recommendations are about specific movements and are not generalized stop or go commands.
Parkinson's Disease Implications
In primate models of Parkinson's disease, neurons of the globus pallidus and substantia nigra that forward the recommendations from the striatum seem to be forced into 'group-thinking', spiking in synchrony. Similar behaviour is seen in the neurons of patients with Parkinson's disease. This could be interpreted by the thalamus and cortex as a recommendation to act in two mutually exclusive ways at the same time, which would lead to difficulty in making a decision and initiating movement — a cardinal feature of the disease. Therapies designed to allow these neuronal clusters to pass on the recommendations of both direct- and indirect-pathway SPNs for movement initiation, might restore the ability of patients with Parkinson's disease to move more readily.
[end of paraphrase]
Return to — Basal Ganglia
Nature, 494, 14 February 2013, p.178
Neuroscience: To go or not to go
D. James Surmeier
Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611, USA.