Video: Basal ganglia

Basal ganglia - two words that have been filling anatomy students with dread since the beginning of time. This group of gray matter structures that control so many of our conscious actions also seem to have an unreasonable influence on our stress levels. But fear not, we're going to overcome this basal ganglia angst, and by the end of this video, you'll be a BG expert. Buckle up, it's time to tackle the basal ganglia.

So, what exactly will we be learning about in this tutorial? Let's take a look. First up, we'll define the basal ganglia and describe their general location. We’ll then discuss what structures are actually included in this group and discuss their individual functions. We’ll then look at some structures which are considered part of the basal ganglia based on their location and anatomy.

Next, we'll briefly discuss some other structures which are sometimes considered part of the basal ganglia based on their function. We’ll then see how the basal ganglia interacted with other structures of the brain through a number of well-defined pathways, and finally, as always, we'll finish up with a clinical note to put the importance of the basal ganglia into perspective. That's a lot to cover, so let's waste no time at all and ask the question – what are the basal ganglia?

So, the word ganglion means a gray matter mass which as we know is formed by collections of neuronal cell bodies. These masses of gray matter are located deep in the central nervous system in the inferior part of the cerebral hemispheres lateral to the thalamus. Now on to the individual elements of the basal ganglia. First up, let's talk about the corpus striatum.

The corpus striatum is a bilateral collection of three gray matter nuclei found in the inferior cerebral cortex. It is formed by the putamen and the globus pallidus which together form the lentiform nucleus and the caudate nucleus. The corpus striatum literally translates to stripy body, and if you look at a horizontal section through the brain, you can see why. All three nuclei forming the corpus striatum have a striated appearance due to the strands of gray matter passing through the internal capsule from the caudate nucleus to the putamen.

Now let's look at some of the finer details of each individual nucleus beginning with the caudate nucleus. This long and lanky C-shaped structure can be divided into three parts – a head, body, and tail – which terminates at the amygdaloid body which is not part of the caudate nucleus. You can quite easily orientate yourself because the head is the anterior most portion of the structure.

The caudate nucleus is perhaps one of the most critical members of the basal ganglia due to the long list of critical functions it holds. Firstly, it plays an important role in controlling the speed and accuracy of directed or voluntary movements. It's also involved in what's known as executive function, which means that it helps guide our brain through decision-making processes related to focus and attention. This structure has an important role in reward and reinforcement. It is a system involved in associated learning where an action becomes linked to a certain response. Studies have also linked the caudate nucleus to our emotions, notably our responses to visual beauty and attraction.

Procedural learning is another area controlled by the caudate nucleus. It refers to a group of tasks that you learn by doing in which you can then repeat without a conscious effort or remembering other times you've done that task – for example, tying your shoes. Finally, the caudate nucleus also plays a part in inhibition of action based on previous experience, meaning that it allows you to make a conscious decision not to carry out an action that you feel compelled to do. For example, putting your hands close to a fire might seem like a good idea to warm up, but once you've been burned, your caudate nucleus will remind you in the future that it's not such a good idea.

The caudate nucleus is closely related to the lateral ventricle of the brain. Its head lies in the floor and the lateral wall of the anterior horn of the lateral ventricle. The body of the caudate nucleus sits in the floor of the body of the ventricle and the tail curves following the contour of the inferior horn of the lateral ventricle occupying a space in the ventricular roof.

The nucleus accumbens is a small region in the forebrain located close to the head of the caudate nucleus. In reality, however, it's not a defined separate structure and hence cannot be identified in isolation in gross anatomical sections. It is often colloquially known as the pleasure center, which makes reference to its role in the perception of pleasure after consuming certain stimuli. For example, that feeling when you stuff a piece of chocolate into your mouth or feeling good vibes when you hear your favorite song. Just like its closest neighbor – the caudate nucleus – it plays a role in the reward and reinforcement system and is often linked to impulse control disorders.

Moving on, we're looking at the lentiform nucleus or lenticular nucleus, which is a term used to collectively refer to the putamen and the globus pallidus. Let's talk about each of them in more detail.

The putamen is closely related to the caudate nucleus. In fact, it's inferior most part is continuous with the head of the caudate nucleus right about here. The putamen is separated from the globus pallidus medially by the external medullary lamina. Laterally, it’s separated from the claustrum by the external capsule. The putamen and the globus pallidus are separated from the thalamus by the internal capsule medially. Fibers connect the putamen to the caudate nucleus along most of its length, but are the most pronounced in the anterior region.

The putamen is involved in regulation of movements and stores information about previously learned movements, which we'll learn more about later in the tutorial when we look at the pathways of the basal ganglia.

The smaller part of the lentiform nucleus is known as the globus pallidus. It is located medial to the putamen and lateral to the internal capsule. The globus pallidus is made up of two parts – the medial globus pallidus segment and the more lateral globus pallidus segment.

You can now see the globus pallidus in the coronal and horizontal sections through the brain. You'll notice that the globus pallidus appears lighter than the rest of the corpus striatum due to the presence of myelinated fibers. In this view, you can easily identify the internal and external segments and you can also see that they are separated by an internal medullary lamina. Though they are right beside each other and have similar inputs, their functions are quite different due to the fact that the internal part promotes motor activity while the external part suppresses or inhibits motor activity.

The structures of the corpus striatum can also be grouped differently based on their function. It is divided into the neostriatum and paleostriatum. The neostriatum, more often simply referred to as the striatum, contains the caudate nucleus and the putamen which have the same connections and have a similar cellular structure. They form the afferent or receiving part of the basal ganglia. The paleostriatum only includes the globus pallidus, which is functionally different from the rest of the basal ganglia because it functions as the efferent portion of this gray matter nuclei cluster.

Now, I realize the terminology here is just a touch confusing, so let's take a moment to make sure you're familiar with all the terms.

As we've already mentioned in some of the sources, other structures are also considered parts of the basal ganglia. Let's see what they are.

First up, we're looking at the subthalamic nuclei, and as the name suggests, the subthalamic nuclei are located inferior to the thalamus. Although technically part of the diencephalon, they have numerous connections to the globus pallidus and are similar to it in structure. So, functionally, they are often grouped with the basal ganglia. It plays an important role in suppression of unwanted movements, and when damaged, results in uncontrollable flailing of one or more body parts on the opposite side of the body.

Also often grouped with the basal ganglia due to its function is the substantia nigra. The substantia nigra is located in the midbrain of the brainstem. It is located inferior to the corpus striatum and has extensive connections to it. The substantia nigra sends signals to the basal ganglia to increase or decrease movement. For this reason, we call it a modulatory structure.

Now we're moving into structures which look like they should be grouped into the basal ganglia but are not. The most obvious of these structures is the amygdaloid body. It's connected to the tail of the caudate nucleus after all. It's sometimes known as the amygdaloid nucleus or simply the amygdala. Traditionally, it was considered a part of the basal ganglia due to its location, but more recently, it has been suggested that the amygdaloid body is instead part of the limbic system based on its role in emotion-related memory.

Another structure sometimes considered part of the basal ganglia is the claustrum. This thin gray matter sheet is bordered medially by the capsula externa, and laterally, by the capsula extrema. The function of the claustrum is currently unknown.

Although not part of the basal ganglia, the thalamus lies medial to the corpus striatum. The thalamus receives efferent signals from the basal ganglia. This is the last stop that messages from the basal ganglia take on their way up to the cortex. You might also know that sensory information from the periphery of the body as well as subcortical motor signals travel through here on their way to the cortex.

Let's take our anatomical knowledge of the basal ganglia now and have a look at their related pathways which will help us understand their functions.

The functions of the basal ganglia are carried out through a series of functional loops as well as excitatory and inhibitory pathways. Today, we're going to look at four major loops which the basal ganglia are associated with – a motor loop concerned with the learned movements, a learning loop associated with motor intentions or learning a movement, a limbic loop which is concerned with emotional aspects of movement, and finally, an oculomotor loop which deals with gaze and eye movements.

We're going to begin with the motor loop, but first, let me show you the basic connections that exist here.

Afferent signals are sent from the motor cortex to the striatum which in turn sends signals to both the internal and external segments of the globus pallidus. From here, the motor loop splits into two pathways. The first is a direct pathway which involves communication directly between the internal segment of the globus pallidus and the second is an indirect pathway which involves communication between the external segment of the globus pallidus and the subthalamic nucleus which then loops back to the internal segment of the globus pallidus and thalamus. From here, the thalamus communicates back to the motor cortex completing our loop.

Let's take a closer look at each pathway to see what they're all about.

So here we have a schematic of the direct pathway of the motor loop in which were using blue and red arrows to demonstrate connections between various structures. The blue arrows in the diagrams signify excitatory neurons which simulate their target structure and use the neurotransmitter glutamate. The red arrows represent the inhibitory neurons which disable their target structure. The neurotransmitter associated with them is known as GABA for gamma-aminobutyric acid.

So, let's dive in and see what's happening here.

As we can see, a signal is initiated in the motor cortex. This signal acts to stimulate the striatum which, if you remember, is the collective name for the caudate nucleus and the putamen. When stimulated, the neurons of the striatum have an inhibitory effect on the internal globus pallidus and a part of the substantia nigra known as the pars reticulata abbreviated as SNr on our diagram. Since we're talking about inhibition, it's the GABA neurotransmitter that is at work here.

Now the activity of the thalamus in a resting individual is normally inhibited by the internal globus pallidus and the substantia nigra pars reticulata. GABA, again, is the neurotransmitter here. When the activity of the internal globus pallidus is inhibited by the striatum, that removes its inhibitory influence over the thalamus. This process is known as the disinhibition of the thalamus. It allows the thalamus to fire stimulatory impulses at the motor cortex which amplifies its activity. This eventually promotes muscle contractions that make you want to wave your arms about or swing your hips when you hear a good tune.

The indirect pathway contains all the same elements as the direct pathway, with the addition of subthalamic nuclei. The aim of this pathway is the opposite of the direct pathway in that it inhibits the thalamus and prevents it from firing excitatory signals to the motor cortex. The indirect pathway starts with the motor cortex sending an afferent excitatory signal to the striatum. This time, however, the signal received from the motor cortex is from a different area than that involved in the direct pathway and carries directions for unwanted movements, which might compete with the desired movement resulting from the direct pathway.

The putamen and the globus pallidus are somatotopically organized, meaning they have different regions related to different parts of the body. Therefore, they're able to facilitate signals related to our movements via the direct pathway and simultaneous disfacilitation of unwanted leg movements via the indirect pathway. Pretty clever, right? For example, if you wanted to reach out to grab something with your hand, you don't want your legs to carry you away in the opposite direction. Your basal ganglia recognizes this and inhibits this kind of action.

So, an inhibitory signal is sent to the external globus pallidus rather than the internal globus pallidus like in the direct pathway. Again, the neurotransmitter in action is GABA. Instead of having a direct connection to the thalamus in a resting individual, the external globus pallidus sends a constant inhibitory signal to the subthalamic nucleus. When the external globus pallidus is inhibited by the striatum, it's inhibitory effect on the subthalamic nucleus decreases. That causes the subthalamic nucleus to send an excitatory signal to the internal globus pallidus.

As we know, the internal globus pallidus only has an inhibitory influence over the thalamus which causes its inhibition. That means the thalamus is prevented from sending a signal to competing areas of the motor cortex, and voila! No competing movements are propagated.

I want to mention another pathway known as the nigrostriatal pathway, which plays an important role in modulation of both the direct and indirect pathways of the motor loop. This pathway projects from the substantia nigra pars compacta to the striatum and it utilizes the neurotransmitter dopamine. It affects the motor loop in two ways. It excites the direct pathway and it inhibits the indirect pathway. The different effects of the direct and indirect pathway is explained by the activation of the different dopamine receptors that are located within the neurons of the striatum.

There are two types of dopamine receptors - D1 and D2 – which respond differently when stimulated with dopamine. Stimulation of D1 results in the excitation of the neuron while the stimulation of D2 results in inhibition. D1 receptors are found in the striatal neurons that give rise to the direct pathway. On the other hand, D2 receptors are found on the neurons whose axons form the indirect pathway. So when dopamine is released from the substantia nigra pars compacta, it results in the promotion of the direct pathway, and at the same time, the inhibition of the indirect pathway.

We'll take a closer look at this again when we reach the clinical notes at the end of the tutorial.

Next up, we have the learning or associative loop. In this loop, the afferent signals arise mainly from the dorsolateral prefrontal cortex, travel to the caudate nucleus and nucleus accumbens and the thalamus before sending back an efferent signal to the cortex. The motor and learning loops work in tandem. When we learn a new movement, we plan specific strategies to accomplish the desired outcome, which is when the learning loop is hard at work. Once we've perfected the movement and we can execute it without having to think about it, the activity in this loop will decrease and the motor loop will take over the execution of the function.

Lastly, the limbic loop is involved in giving motor expressions to our emotions – for example, through smiling when you're happy or taking on aggressive positions when you're angry. In this loop, information is gathered from a number of structures in the limbic system, notably, the amygdaloid body and hippocampus as well as the orbitofrontal, cingulate, or temporal cortices. That information is transmitted to the caudate nucleus, and again, the nucleus accumbens where it gets passed into the thalamus via the direct or indirect pathways. The thalamus then sends a signal back to the limbic areas of the cortex.

Let's take a look at our final loop or circuit related to the basal ganglia, which is known as the oculomotor loop. Now before we look at this pathway, I first want to introduce an important term known as the saccade. A saccade is one of the four types of eye movements and describes the rapid movement of the eyes from one point of fixation to another. This is in contrast to other eye movements like smooth pursuit movements which are much slower tracking movements.

Voluntary saccades are generated via two cortical areas – the frontal eye fields and the parietal eye fields – which based on information received from the primary visual cortex help identify and select targets to fixate on. Both of these areas contain neurons that send projections to the superior colliculi from where command signals are sent to the oculomotor, trochlear, and abducens nuclei of the brainstem.

Most of the time, the reticular part of the substantia nigra tonically suppresses the superior colliculus, meaning that it generally holds it in an inhibited state. This inhibition prevents distracting visual stimuli from triggering unwanted saccadic eye movements from the cortical eye fields allowing the eyes to fixate on a desired point of view. When the eyes are required to move from one point of fixation to another in what we now know is called a saccade, the oculomotor loop is engaged.

Selection of which saccadic signals is thought to largely be controlled by the dorsolateral prefrontal cortex. It sends excitatory signals to the caudate nucleus which in turn causes an inhibitory signal to be transmitted to the substantia nigra pars reticularis.

Now we know that the substantia nigra normally holds the superior colliculus in an inhibited state; however, if it is inhibited by the striatum, this removes the proverbial breaks on the superior colliculus allowing it to receive signals from the cortical eye fields telling it to flick the eyes towards a desired target. This is known as the disinhibition of the superior colliculus.

And with that, we've discussed our four loops of pathways of the basal ganglia.

Before we wrap up, let’s take a moment to look at some clinical notes.

Parkinson's disease is a neurodegenerative disorder characterized by a number of motor symptoms. A common symptom is a tremor, an involuntary movement which causes shaking and trembling for the patient. Another prevalent symptom is bradykinesia or slowness of movement. We know that the substantia nigra is affected in Parkinson's disease as there's a reduction in the number of dopamine-producing neurons. Let's have a look at how that affects our direct pathway first.

In the direct pathway, the substantia nigra promotes the neurons stimulatory action resulting in voluntary movement. If we experience a reduction in dopamine being released from the substantia nigra to neurons at the striatum, then we reduce the ability of the basal ganglia to promote wanted movements, which results in bradykinesia.

The indirect pathway is also affected by the functional loss of the substantia nigra. This time, degeneration of the substantia nigra means a reduction in the amount of inhibition being communicated to the thalamus and instead an excessive amount of unwanted excitation is sent to the cortex. This causes unwanted movement to occur presenting as a characteristic tremor.

And that's all you need to know about the basal ganglia.

Before we finish up, let's recap what we learned today.

We started by looking at the corpus striatum which contained the caudate nucleus, putamen, and globus pallidus. The caudate nucleus encircles most of the structures within the basal ganglia in this C-shaped formation. It's made up of the head, body, and tail. We saw that, anatomically, the putamen and the globus pallidus are grouped together to form the lentiform nucleus, however, functionally, the caudate nucleus and the putamen are more similar and form the neostriatum or just striatum, and the globus pallidus is known as the paleostriatum. The globus pallidus is further subdivided into functionally different internal and external parts.

We also quickly had a look at some structures closely associated to the basal ganglia. Attached to the head of the caudate is the nucleus accumbens and attached to the tail is the amygdala. We also looked at the claustrum, subthalamic nuclei, substantia nigra, and the thalamus.

We then had a quick run through the motor loop learning about the direct and indirect pathways contained within it. We also looked briefly at the learning loop which was related to the learning of movements, the limbic loops associated with the emotional expressions of movement, and the oculomotor loop which deals with saccadic movements of the eyes.

In our clinical notes, we saw how the degeneration of the substantia nigra in Parkinson's disease can affect the modulation of these pathways and lead to both reduced speed of movement and unwanted tremors.

Whoa! Talk about a jam-packed tutorial. You definitely deserve a well-earned break. Great work! See you next time and happy studying!