Anatomy of the basal ganglia

Generally speaking, our central nervous system is made up of three parts; the cerebrum, the cerebellum, and the brainstem. The cerebrum consists of two nearly symmetrical halves called the cerebral hemispheres, and deep within these hemispheres lie the basal ganglia.

On a coronal or axial section of the brain, the outermost area represents the cerebral cortex, which is made up of gray matter that consists of billions of neuronal cell bodies. The axons that are connected to these cell bodies create the white matter of the brain, which is the innermost area.

One prominent white matter fiber bundle is the internal capsule, which is like a highway that allows signals, and thus information, to flow to and from the cerebral cortex. On both sides of the internal capsule, we can see areas of subcortical gray matter that form the basal ganglia.

The basal ganglia are very important for providing a feedback mechanism to motor cortices for initiation and control of voluntary movements. So, for example, you want to write your name on a piece of paper.

First, you plan the movements using your prefrontal cortex, and that sends a signal to the motor cortex as well as the basal ganglia. The basal ganglia now help the motor cortex prepare for, and initiate the action. The result is that you position your arm so that you can start writing.

Then, the basal ganglia ensure that your hand movements are as precise and executed as planned, and while doing so, they also maintain your posture. In addition to this, the basal ganglia can help you learn new procedural motor skills, like riding a bicycle.

The basal ganglia are a collection of nuclei that include the caudate nucleus, the putamen, the globus pallidus externus and the globus pallidus internus. These nuclei have highly complex connections with other parts of the central nervous system, like the cerebral cortex, the thalamus and the brainstem. But the two most important structures that are closely related to the basal ganglia are the substantia nigra of the midbrain and the subthalamic nuclei of the diencephalon.

Now, let’s look at each of these structures in detail. First up is the caudate nucleus, which is shaped like the letter C. The caudate nucleus can be divided into three parts: the head, the body and the tail. It follows the lateral ventricle of the brain, which is medial to it, and curves around the thalamus. Laterally, the caudate nucleus is bounded by the internal capsule.

On a transverse section of the brain, the head can be seen forming the lateral wall of the anterior horn of the lateral ventricle. On a coronal section, the body of the caudate can be seen along the wall of the body of the lateral ventricle. The tail of the caudate curves around the posterior end of the thalamus and follows the inferior horn of the lateral ventricle, forming its roof.

The putamen and the globus pallidus externus and internus are sometimes collectively referred to as the lentiform, or lenticular, nucleus. This is a wedge-shaped nucleus with the wide base pointing laterally.

On transverse and coronal sections of the brain, we can see that the internal capsule is medial to the lentiform nucleus and separates it from the caudate nucleus and the thalamus. Lateral to the lentiform nucleus is the external capsule. Thin sheets of white matter pass vertically through the lentiform nucleus, dividing it into its three components.

First, the most medially positioned nucleus is the globus pallidus, which has a lighter appearance because it contains more myelinated nerve fibers. The globus pallidus is further divided by a sheet of white matter into the globus pallidus internus, lateral to the internal capsule, and the globus pallidus externus, which is medial to the putamen.

The putamen is the most lateral of the lentiform nuclei and has a darker appearance. It is positioned lateral to the globus pallidus externus and medial to the external capsule.

The putamen and the caudate nucleus are sometimes referred to as the corpus striatum, or just the striatum. These two nuclei are separated by the internal capsule, except anteriorly where the head of the caudate nucleus is continuous with the putamen.

Superior to this connection, small bridges of gray matter between these nuclei pass through the internal capsule, giving it the striated appearance, hence the name corpus striatum.

Let’s take a short break and see if you can identify the function of the basal ganglia, as well as the basal nuclei that comprise the basal ganglia.

Let’s switch gears now and talk about other nuclei that are closely related to the basal ganglia. Inside the midbrain, ventrally, between the tegmentum and the crus cerebri, there is a darkly pigmented area called the substantia nigra, which is Latin for “black substance”. It contains cells that store granules in their cytoplasm, filled with a pigment called neuromelanin, making this region look darker in color.

The substantia nigra can be divided into two parts: the substantia nigra pars compacta, or SNpc, and the substantia nigra pars reticulata, or SNpr. SNpc contains dopaminergic neurons that project their axons to the corpus striatum, forming the nigrostriatal pathway, where they release dopamine.