© 2010
  University of Idaho
  All rights reserved.
  Psychology Dept.
  University of Idaho
  Design - P&D  CTI

First, (as we see in slide two) the basal ganglia does not have any direct input or output to the spinal cord.  Instead, it receives information from the cortex, and sends information via the brain stem or the thalamus to the cortex.  So, ultimately, there is no direct link to the spinal cord (although the basal ganglia will be extremely important in sending information down the spinal cord). 

In the past, we had identified specific systems that were involved with movement.  These were called the pyramidal and extrapyramidal systems, the corticospinal tract, and other structures including the basal ganglia.  Today however, we know there are many more parts involved, and movement problems can occur in a variety of these different structures, including the brain stem, red nucleus and also the cerebellum. 

So, let’s talk a little bit about the basal ganglia.  The basal ganglia, as we see in slide five, consists of four major principle nuclei, the striatum, globus, substantia nigra and subthalamic nucleus.  Let’s talk about the striatum first. 

The striatum has three specific and important subdivisions, the caudate nucleus, the putamen, and the ventral striatum, including the nucleus accumbens which is extremely important for addictions processes (which we’ll talk about later).  The striatum is also divided by what is called the internal capsule.  The internal capsule fibers go from the cortex to the thalamus in both directions.  So, as you can see, you’re going to have lots of intervation and pathways going around the basal ganglia structures.

The striatum receives most of its fibers from the cortex, the thalamus, and brain stem.  It also sends fibers to the globus pallidus and substantia nigra.  So, as we’re starting to see here right now, there are a lot of different pathways and structures that are going to be involved when we start to talk about the refinement of movement.  There are a couple of important points to know.  First, all areas of the cortex basically send excitatory or what are called glutaminergic fibers to the striatum.  They also get dopamine fibers or dopamine projections from the midbrain, and finally they get serotonergic input from the raphe nuclei.  However, the striatum primarily contains mostly GABA neurons.  The striatum is essentially separated in the parts that they call patches; the matrix and the striosome.  Both of these are separate and have different types of receptors. 

The second part of the striatum is called the globus pallidus.  The globus pallidus lies medial to the putamen and has two segments, the external segment and the internal segment and it uses GABA as a neurotransmitter. 

Another aspect is a structure is called the substantia nigra.  The substantia nigra is extremely important within the basal ganglia.  It lies in the midbrain on the medial side of the internal capsule.  It has a compact zone called pars compacta and is basically a distinct nucleus.  The substantia nigra contains a lot of dopaminergic neurons (dopamine neurons) and the oxidized pigment from the dopamine creates a darkening color.  This makes the structure dark (dark substance).  Ultimately, more darkness occurs as we continue to age.

Another structure within the basal ganglia is called subthalamic nucleus.  It lies below the thalamus and above the substantia nigra.  It is closely connected to parts of the globus pallidus and the substantia nigra and uses glutaminergic neurons.  These are the only glutaminergic neurons that we have in the basal ganglia.  So, all the other neurons with the exception of neurons coming out of subthalamic nucleus are basically inhibitory.

On slide 12 there is a picture that shows a lot of the different pathways and structures that are within the basal ganglia and where they project to within the cortex.  And as we can see in slide 13, the input to the basal ganglia is from the primary motor cortex and substantia nigra, while output from the basal ganglia is going to a variety of different structures including the primary motor area, precentral gyrus, supplemental motor area, superior frontal gyrus, and premotor and brain stem motor nuclei.  Ultimately, we begin to develop a feedback loop.  This is called the cortical-basal ganglia loop.  As we can see here, there’s a variety of different structures going to different places.  Ultimately, these send information back to various other structures as well.  You can read and kind of walk through it yourself. 

Now in the past, when we talked about the basal ganglia, it had only a motor related functions, and that’s what we thought it did.  Today, we know that it does a lot of other things.  It has motor, cognitive, emotional and motivational components that are also extremely important.

So, we’ve talked now a little bit about structures within the basal ganglia, let’s now talk about some output from the basal ganglia. Output neurons discharge at a very high frequency (rate) and ultimately they go through different structures which are listed here (slide 15).  They are modulated by two parallel pathways, basically an indirect pathway and a direct pathway.  And as we see in slide 16, the indirect pathway is basically gabainergic.  It’s going to first go to external pallidal segment, then subthalamic nucleus, and finally the output nuclei in the subthalamic nucleus.  Remember subthalamic nuclei also contain glutaminergic  neurons.

The direct pathway is more direct (as a good point).  Here the striatum is going to go to the pallidum and when activated, it basically shuts down tonicly active neurons.  Ultimately, this allows the thalamus and the cortex to be activated.  So as we see in slide 18, the direct system provides positive feedback.  Its activity disinhibits the thalamus and basically increases thalamic cortical activity which facilitates movement, where as the indirect system provides negative feedback, inhibits the thalamus, and basically helps to shut down movement.

Now dopamine is an extremely important neurotransmitter in the basal ganglia.  As we can see here (slide 19), there are a couple of different types of receptors. There are D-1 receptors which are excitatory and D-2 receptors which are inhibitory.  Basically, what we have is the synaptic actions are different.  Basically both decrease inhibition of thalmocortical neurons, thus both facilitate movement.  What they also do is have impacts on the striatum.  So basically activity increases output in nuclei, and output nuclei then inhibit thalmocortical neurons and the movement decreases.  So, even though we have different systems, they are going to have different effects; whether you’re going to excite the system or shut down the system in relation to movement.

Now, there’s a variety of different disorders that are associated with the basal ganglia>   I’m not going to list them all here because we’re going to talk about them more in detail when we talk about the disorders in the last part of the class.  But the couple we see here that are majorly important are Parkinson’s syndrome (or Parkinson’s disease) and Huntington’s disease.   Ultimately, both of these involve motor problems within the system.  Parkinsonism basically has the classic shaking when you’re resting your limbs and not even doing anything.  So, if you put your hand straight out in front of you and just let it stay there, you’d see a little bit of shaking.  With Parkinsonism the shaking becomes very, very pronounced.  We’ll talk about more about disorders, how they work a little bit later in the course.

So the basal ganglia is extremely, extremely important for movement.  In essence, it’s one of the first systems that helps us to refine the movement that we have.  So, when you are doing that nice script writing on your paper, the basal ganglia is going to be extremely involved with helping you to do that.

In the next section we continue on talking about motor cortical structures, so until then, we hope that you’ve enjoyed this lecture and that you’re having a great day.