Basal Ganglia: Structure

In “Little Dorrit”, a book  by Charles Dickens, there was an agency that was engaged in a peculiar type of work. They were receiving various complaints, requests, and reports – but never did anything about them. If one believes Dickens, they were inventing various ways “how not to do it”. This agency was entirely useless. But sometimes not doing something can be beneficial – and a group of structures called basal ganglia can prove it!

Basal ganglia are structures most famous for their ability to block movements. You would naturally ask – why is that useful? But stop and think for a moment. Imagine that you need to take a plate from the cupboard. You need to make a lot of movements in order to do this properly:

• you need to get your arm up,
• the arm has to be oriented towards the plate of your choice;
• You have to use your hand to take the plate – and also prevent it from falling;
• You need to carefully place the plate on the table.

One wrong movement – and the plate would fall on the floor and break. But if all possible wrong movements are blocked – it can successfully be carried to the table. And in order to do that – you need your basal ganglia to assist you.

Similar to a sculptor, they cut of all the unnecessary movements in order to produce the perfect sequence of actions. Moreover – they can do the same for other areas – such as learning or forming memories! Let us get acquainted with this part of the brain better!

What are the Basal Ganglia?

Basal ganglia are structures located in the base of the forebrain [1]. There is a lot of confusion and complex terminology surrounding them. We will try to untangle the mystery a bit in order to understand the crucial function they play.

In order to understand how these structures are related to other brain areas, imagine that you hold a model of the brain in your hands. There is that familiar folded cerebral cortex we usually imagine when we are speaking about brain. This part is called the forebrain, or the cerebrum.

It is divided into two hemispheres. If we take the upper parts of the latter apart and look into the base of the cerebrum, we would see several structures that are distinctly different from the tissue around it. Those are the basal ganglia we will be speaking about.

What are the main things we need to know first?

First of all, they are not truly ganglia – the correct term would be “nuclei” [2]. There are several reasons for this: 

• Ganglia are clusters of neurons located in the autonomous nervous system. Our structures are located in the CNS – the brain;
• Ganglia are organized differently from the brain structures;

The more correct term is “basal nuclei”. You would see both terms used in literature and also in this article.

There are several basal nuclei and they basically form a kind of “underground railroad” in the brain base:

• Corpus striatum (a complex structure composed of several smaller nuclei) is located in the cerebrum;
• Substantia nigra – located in the upper portion of the brainstem;
• Subthalamic nucleus is located between the brainstem and the cerebrum –the area termed diencephalon.

Here is the picture to help you imagine what do they look like:

All the basal nuclei are connected into a system and each of the nuclei perform their own function based on the signals they receive from various brain areas and other nuclei. They are like a railway with certain neurons transporting cargo. In their case, the cargo is called neuromediators and they are needed in various stations.

Corpus striatum – the main railway hub.

Let not the strange Latin name confuse you. “Corpus striatum” means “striped body” – because of the way these structures look when brain preparations are made. There are several nuclei that compose it:

• Caudate nucleus;
• Putamen;
• Nucleus accumbens
• Globus pallidus

The first three structures collect information from the various brain regions, while the globus pallidus is the brave structure that actually sends out the instructions outside of the main hub.

Corpus striatum is the most well-connected area in the base of the brain. Look at this simple schematic of how it is all connected:

Let us look at the three substations of the striatum hub in detail.

What do you need to know about caudate nucleus:

• In translation from Latin, this is the nucleus “with the tail”.
• If you look at this structure from the above, it would resemble a comma with a very big head and a long tail.
• The tail of the caudate nucleus ends near the amygdala  – the structure responsible for a variety of functions, including emotions and reaction to stress.
• There are two caudate nuclei: one in each hemisphere.
• The caudate nucleus receives the information from the cerebral cortex:
  • The frontal lobe sends the information to the head of the caudate [4];
  • The lobes at the temples and upper portion of the head (temporal and parietal)  are sending information to the body of the caudate nucleus [4];
  • The lobes at the temples send the information to the tail of the caudate [4];
• Caudate nucleus is important for creation and storing of memories [5];
• Caudate nucleus helps in using  information we have received in the past in deciding what we need to do at present [5];
• Left side of the caudate is thought to control communication skills [5]
• Malfunction in the head of the caudate nucleus was recently linked to the development of obsessive – compulsive disorder [6].
• The caudate nucleus is also responsible for motivation and planning of our behavior [7].

What do you need to know about the putamen:

• There are two putamen nuclei – one in each hemisphere.
• Putamen nuclei are round and large and are located near the caudate nucleus in the forebrain;
• The putamen together with the caudate nucleus forms the so-called dorsal striatum.
• The putamen controls coordination and movements based on the information that comes from the so-called motor cortex (an area in the cerebrum that controls movement) and sensory cortex (the area that controls the information received from our sensory organs) [4, 5].
• Сerebral strokes often happen in this area. If the putamen is damaged due to a stroke, there can be various consequences [8]:
• Movement problems;
• Problems with the sensory organs;
• Problems with both movement and ability to sense the environment;
• Problems with speech;
• General weakness, problems with coordination and inability to move a leg and/or arm – so-called ataxic hemiparesis;
• Flailing or jerky movements.

Based on the observations in the stroke patients and other of studies, it was concluded that the putamen is responsible for movement control, control of the speech, as well as coordination.

In course of a recent study of a patient with damage to the putamen, it was discovered that such trauma results in problems with working memory and ability to learn sequences of complex movements [9].

What do you need to know about the nucleus accumbens:

• The nucleus accumbens is a special area where the caudate nucleus and the putamen join together;
• This structure is located in the region called ventral striatum;
• The nucleus accumbens also receives information from the limbic areas – the structures responsible for feelings, memory, and certain behavior responses such as the “fight or flight instinct”;
• The nucleus accumbens is a part of the reward circuit – the system that is responsible for us feeling well if we do something good [4.5];
• Nucleus accumbens also helps us in choosing of appropriate behavior in different situations.

What do you need to know about the globus pallidus:

• In translation from Latin it means “a pale orb”;
• This structure is located near the caudate nucleus and the putamen;
• Unlike the three previous nuclei that mainly accept information, the globus pallidus is an output nucleus – it sends nervous impulses to various targets outside the basal ganglia chain of command;
• The neurons of the globus pallidus contain a neuromediator called GABA – gamma-aminobutyric acid.
• This neuromediator is a blocker – if it is sent to a certain cell (for example, a muscle cell), the cell in question would cease activity (i.e., the muscle cell would stop contracting).
• The main role of the globus pallidus is to block the unnecessary movements or reactions.
• There are two major parts of the globus pallidus (GP):
• external globus pallidus (GPext);
• internal globus pallidus (GPint);
• The external GP is connected to another structure – subthalamic nucleus.

There are other basal nuclei that do not belong to the striatum – the subthalamic nucleus and the substantia nigra.

What do you need to know about the subthalamic nucleus:

• There are two small subthalamic nuclei in each of the hemispheres;
• The subthalamic nuclei are located just below the thalamus;
• The subthalamic nuclei are constanly receiving and sending impulses [10]:
• Subthalamic nuclei get information from the:cerebral cortex;
  • Thalamus
  • External part of the globus pallidus;
  • brainstem;

•  The subthalamic nucleus also sends impulses outward:
  • globus pallidus;
  • substantia nigra;
  • brainstem;
  • striatum;

• The subthalamic nucleus contains neurons that send out neuromediator dopamine. They are activators, not blockers;
• If the subthalamic nucleus is overstimulated in mice, the animals start moving abnormally;
• If this structure is damaged, it may lead to jerky, uncontrolled movements;
• Subthalamic nucleus often works abnormally in Parkinson’s disease [10];
• An interesting study on rats has shown that a relatively small damage to the subthalamic nucleus can decrease the level of addiction to cocaine in rats [11];

The subthalamic nucleus often works together with another structure that is located outside of the striatum – substantia nigra.

What do you need to know about the substantia nigra [12]:

• The name in translation from the Latin means “black substance”
• The substantia nigra looks like a “black streak” on the brainstem;
• The substantia nigra has two distinct parts:
• Pars compacta;
• Pars reticulata;
• Pars compacta carries neurons that are sending the neuromediator dopamine;
• This region is very densely packed and looks dark – that is why the nucleus is called “black substance”;
• The pars compacta connects to the nuclei of the striatum – putamen and caudate nucleus;
• The pars compacta cells contain neuromelanin – the substance that gives them their typical black color.
• The neurons of the pars compacta send out the neuromediators dopamine that “switches the cells on”
• The pars reticulata is different: it contains neurons that send the neuromediator GABA, a blocker;
• If the substantia nigra is damaged, the person can have problems with initiating movements. This happens due to the damage to the dopaminergic, activating neurons.
• Such situation is observed in a debilitating syndrome – Parkinson’s disease.
• Parkinson’s disease is thought to be connected to the malfunction of the substantia nigra or its connections to the putamen [10].

We have established now that basal nuclei are very important. Unlike offices from Dickens books, they do very important work. But how do these structures actually do it? It turns out, these nuclei have two intricate chains that help us move and behave appropriately.

The first chain is called the direct pathway [4].

As you can see from the picture, there is a railroad that connects the striatum, globus pallidus, the thalamus and the cortex (including motor cortex, responsible for initiating movements). But the trick is not in the connections themselves, but in the neurons that form them. Do you remember – the globus pallidus contains blocker neurons that carry GABA? Now, follow me closely:

1. Globus pallidus blocks the activity of the thalamus through its GABA neurons;
2. Thalamus is responsible for activating the nerve cells in the cortex;
3. Striatum also contains GABA neurons;
4. The striatum sends blocking impulses to a certain area of globus pallidus – internal GP;
5. As a result of the signals sent by the striatum, globus pallidus stops sending blocking neuromediators;
6. The block from the thalamus is lifted – movement (or any other activity) can now happen!

But there can also be a need to block certain cells. In order to do that, the indirect pathway has to be switched on.

Indirect pathway [4]

The indirect pathway also begins in the striatum – but the neurons are different, and connect to another area – external globus pallidus.

Now, let us follow closely the flow of events in this case:

1. The striatum blocks the activity of the neurons in the external globus pallidus.
2. The external globus pallidus is connected to the subthalamic nucleus and blocks the activity of the subthalamic nucleus.
3. The subthalamic nucleus, in its turn, has dopaminergic neurons that send activating impulses to the internal globus pallidus.
4. The internal globus pallidus, as you already know, is connected to the thalamus;
5. As striatum neurons switch the external GP off, the subthalamic nucleus is switched on – and turns on another part of the GP.
6. Now, as GP is active again, it begins sending stop signals to the thalamus and the unnecessary activity is blocked.

The balance between those two pathways is the secret behind our fine-tuned movements and ability to speak politely. But sometimes this balance is upturned. This results in disorders. We have mentioned one of them – Parkinson’s disease. In case of the Parkinson’s the basal ganglia block too much – and it is hard to initiate movement or to have a quick reaction to something.

There is another syndrome connected to the basal ganglia function. It is called Huntington’s disease [14]. This is a genetic syndrome. In patients with this syndrome, basal ganglia – especially the striatum – are slowly getting destroyed. As a result, the person increasingly loses control of his/her movements. The hands begin to tremble, the movements become more and more jerky. The patients also begin to have trouble walking, speaking, eating, etc. With time, even their ability to think declines, too.

There are other disorders that are caused by basal ganglia malfunction, but they are less well studied.

As you can see, sometimes not doing it – is the right thing to do. And basal ganglia have indeed made “not doing” into something of an art that allows us to be who we are. Do not forget about them. Just because they are located that deeply in the brain does not mean they are not important.

References

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11.  Baunez, C. et al. (April, 2005) The subthalamic nucleus exerts opposite control on cocaine and ‘natural’ rewards. Nature Neuroscience, V. 8. № 7, p. 484-9.
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