Basal ganglia: Anatomy & Function

Basal gangliaAnatomy and function of the basal ganglia. Direct and indirect pathways
~ 12 min

In this note, we are going to discuss the anatomy and function of the basal ganglia.

Basal ganglia
Basal ganglia

Terminology

The basal ganglia, like the cerebral cortex, consist of gray matter. It is represented by the neuronal cell bodies that produce nerve impulses.

The term “basal” means that these structures can be found not on the surface, but in the deeper regions of the brain.

Thalamus

Well, first of all, the basal ganglia system is connected to the thalamus, which is an egg-shaped structure in the diencephalon. Basically, the thalamic nuclei act as a relay station for the numerous pathways, which run through the thalamus.

Thalamus
Thalamus
Thalamus
Thalamus

It allows some signals to get into the cerebral cortex, while other signals are blocked or slowed down. In general, the thalamus acts as a kind of “bouncer” in the brain.

Function

The main function of the basal ganglia is to control our voluntary movements, our motor activity. They help us to start or initiate movements, then to stop or inhibit them, and also to modulate in different ways. Like preventing unwanted motor activity, regulating muscle tone, providing motor fluency, and so on.

Moreover, the basal ganglia take part in the so-called reward system, decision-making processes, memory processes, and eye movements.

Anatomical organization

This system includes a fairly large number of components, and in various scientific sources, the set of these components is always somewhat different. But the main ones are the caudate nucleus

Caudate nucleus
Caudate nucleus

and the lentiform nucleus,

Lentiform nucleus
Lentiform nucleus

which together form the corpus striatum.

Corpus striatum
Corpus striatum

The caudate nucleus has three simple parts:

  • Head
Head
Head
Head
Head
  • Body
Body
Body
Body
Body
  • Tail
Tail
Tail
Tail
Tail

The lentiform nucleus is a bit more complicated, it consists of two divisions:

  • Globus pallidus
Globus pallidus
Globus pallidus
  • Putamen
Putamen
Putamen

In turn, the globus pallidus has two its own components – internal

Internal segment of the globus pallidus - GPi
Internal segment of the globus pallidus - GPi
Internal segment of the globus pallidus - GPi
Internal segment of the globus pallidus - GPi

and external.

External segment of the globus pallidus - GPe
External segment of the globus pallidus - GPe
External segment of the globus pallidus - GPe
External segment of the globus pallidus - GPe

In addition to the corpus striatum, there are several more structures – the claustrum,

Claustrum
Claustrum
Claustrum
Claustrum

the subthalamic nucleus, and of course the substantia nigra.

Subthalamic nucleus
Subthalamic nucleus
Substantia nigra
Substantia nigra

Substantia nigra

The substantia nigra is located a little lower, in the midbrain, and it consists of two parts: the pars (or zona) reticulata and pars (or zona) compacta.

Substantia nigra
Substantia nigra

The pars reticulata contains inhibitory GABAergic neurons, and the pars compacta contains dopaminergic neurons and also the pigment called neuromelanin, which, in fact, gives the substantia nigra a dark color.

Functional organization

It was an anatomical organization, but functionally, based on how these nuclei interact with each other, they are classified in a slightly different way and form the so-called striatopallidal system, which is a part of a larger extrapyramidal system. Once again, this is not an anatomical system, but a certain functional principle of interaction between the structures.

The striatopallidal system consists of the striatum and the pallidum. The striatum is made up of a caudate nucleus and the putamen of a lentiform nucleus.

Once again – a caudate nucleus and only the putamen of a lentiform nucleus. The pallidum, obviously, is a globus pallidus, in particular its internal component.

And here’s the thing: the “striatum” is a functional unit, and the “corpus striatum” is an anatomical structure. So, actually, they are not the same thing, don’t get confused.

Pathways

And now let’s take a closer look at the connections and interactions between the basal ganglia. As I said, they control our voluntary movements.

  • Some connections between the basal ganglia increase our motor activity (they form the direct pathway)
  • Some decrease our motor activity (they form the indirect pathway)
  • Some modulate or modify our motor activity (they form the nigrostriatal pathway).

And in all these interactions, the striatum acts as an input – it receives and processes information, and the pallidum acts as an output – it redirects signals to other structures, including the thalamus.

In the context of this topic, the key point is quite simple. If the thalamus allows a signal to pass to the motor cortex, we start to move, and if it does not, we stop.

Neurons and neurotransmitters

We will talk about the excitatory and inhibitory neurons. In the context of this topic, a neurotransmitter of the excitatory or stimulatory neurons is glutamate, and a neurotransmitter of the inhibitory neu rans is GABA (gamma-aminobutyric acid).

We will also mention dopamine here, which can be both an inhibitory and an excitatory neurotransmitter, depending on which dopamine receptors it acts on.

Direct pathway

Here is our starting point: at rest, the thalamus is inhibited by the pallidum. We will consider this state as an initial one.

  1. The motor cortex, in particular, its excitatory glutamatergic neurons send a signal to the striatum, which means it activates
  2. The striatum contains inhibitory GABAergic neurons that send a signal to the pallidum, reducing its activity
  3. The thing is, that pallidum also contains inhibitory GABAergic neurons, which now are deactivated by the striatum. Consequently, inhibitory signals from the pallidum are no longer transmitted to the thalamus, which means it is unblocked now, it starts working
  4. And the thalamus, in particular, its ventral lateral and ventral anterior nuclei, send excitatory or stimulatory signals back up to the motor cortex
  5. The motor cortex, in turn, increases the motor activity of the skeletal muscles, and we move

Indirect pathway

Now let’s move on to the indirect pathway, which is focused on decreasing motor activity, in particular inhibiting unwanted motor movements.

In this case, we will consider another initial state, when the EXTERNAL part of the globus pallidus inhibits the subthalamic nucleus.

  1. he motor cortex, in particular, its excitatory glutamatergic neurons send a signal to the striatum, which means it activates
  2. The striatum contains inhibitory GABAergic neurons, and now they send their inhibitory signals to the external part of the globus pallidus, reducing its activity
  3. And now, the external part of the globus pallidus does not inhibit the subthalamic nucleus (so our initial state is changed). The subthalamic nucleus starts sending excitatory signals to the internal part of the globus pallidus, meaning, to the pallidum.
  4. And we remember, that pallidum contains inhibitory GABAergic neurons, which send the inhibitory signals to the thalamus, in particular, to its ventral lateral and ventral anterior nuclei.
  5. Consequently, the motor cortex doesn’t receive any signals, any input, and finally, the motor activity is inhibited, in particular, the unwanted motor movements.

Nigrostriatal pathway

The first variant of the nigrostriatal pathway is focused on reinforcing or amplifying the effect of the direct pathway.

Исходя из этого, непрямой путь выглядит вот так:

  1. Here we have an additional dopaminergic signal from the pars (or zona) compacta of the substantia nigra to the striatum. In this case, dopamine acts on the D7 receptors, which are the stimulatory receptors.
  2. This dopaminergic signal provides an extra stimulus to the striatum which, consequently, inhibits the pallidum even more, and the thalamic nuclei are activated even more.
  3. The motor activity is increased more than normal by the influence of the substantia nigra.

Here we have an additional dopaminergic signal from the pars (or zona) compacta of the substantia nigra to the striatum. In this case, dopamine acts on the D7 receptors, which are the stimulatory receptors.

  1. It is the same dopaminergic signal from the pars compacta of the substantia nigra to the striatum, but in this case, dopamine acts on the D2 receptors, which are actually the inhibitory receptors.
  2. The signal inhibits the GABAergic neurons of the striatum, so they send less GABA to the external part of the globus pallidus, so it gets kinda unblocked and consequently inhibits the subthalamic nucleus.
  3. So now the subthalamic nucleus sends fewer excitatory signals to the internal part of the globus pallidus.
  4. And the thalamus is now unblocked, partly released from inhibition, and it sends excitatory signals back up to the motor cortex, increasing motor activity. This type of interaction provides more fluidity to our movements.

Ventral striatopallidal complex

In addition to all these structures, there is one more component of the basal ganglia system called the ventral striatopallidal complex. It helps us to initiate movements in response to a certain motivation and emotions.

Basically, it means, when I’m angry – I’m yelling, when I’m happy – I’m smiling.

And vice versa, this complex is part of the “reward system”, which allows you to receive a psychological reward in response to certain actions.

The ventral striatopallidal complex consists of the ventral striatum and the ventral pallidum. The ventral striatum is made up of the nucleus accumbens and the olfactory tubercle. It receives impulses from the cerebral cortex and sends them to the ventral pallidum, which in turn redirects them to the thalamus.

Golosary

Basal ganglia: Anatomy & Function

basal ganglia
nuclei basales
thalamus
thalamus
caudate nucleus
nucleus caudatus
lentiform nucleus
nucleus lentiformis
striatum
corpus striatum
globus pallidus
globus pallidus
putamen
putamen
internal / medial segment of the globus pallidus
globus pallidus internus / medialis
external / lateral segment of the globus pallidus
globus pallidus externus / lateralis
head
caput
body
corpus
claustrum
claustrum
subthalamic nucleus
nucleus subthalamicus
substantia nigra
substantia nigra
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