Basal Ganglia

We continue our journey deeper into the human brain with our next destination: the basal ganglia. The basal ganglia are an assemblage of structures positioned within the cerebral hemispheres. Within the basal ganglia are even more fun things!

Firstly, note that the basal ganglia extend over more than just the cerebrum. They envelop structures in the cerebrum, midbrain, and the diencephalon (which is located right above the brainstem).

Within the basal ganglia are the caudate, putamen, and globus pallidus (in the cerebrum), substantia nigra (in the midbrain), and the subthalamic nucleus (diencephalon).

Functions of the Basal Ganglia

You can begin to infer the relative location of the basal ganglia from its name: “basal” means “base,” or “bottom.” The basal ganglia are located in the inferior (lower/-est or underneath) portion of the brain. Where you should exercise caution, however, is with the term “ganglia.”

Ganglia is said to be a bit of a misnomer in that this term refers to an assemblage of neurons specifically in the peripheral nervous system. Clusters found in the central nervous system are referred to as a “nucleus.” So, technically, the basal ganglia should really be called the “basal nuclei,” but that’s not as fun to say.

Each of the nuclei in the basal ganglia has their own roles in the brain and are collectively responsible for major networks that control cognition, emotions, and movement. They have also been noted for their role in muscle memory and learning from new situations.

Movement

The basal ganglia are best known for their role in controlling movement. They work in tandem with a system called the pyramidal motor pathway. This pathway functions by conducting signals for action (movement) to the nerves that connect the cerebral cortex to the motor neurons, which then activate the skeletal muscles.

Action signals from the basal ganglia are meticulously “planned,” in a way, such that the signals are specific to the movement desired. Information is passed in a loop: a stimulus is received by the cortex, through the basal ganglia, passed through the thalamus, and returned to the cortex.

By this process, the basal ganglia manage the precise timing and amounts of activity that are directed to the nerves from the cortex. Still, there is something even more interesting about how this system functions:

The basal ganglia play their role in muscle memory because they can directly control the intensification and suppression of activities based on their outcome.

When certain actions result in positive outcomes, such as reaching an intended goal or is rewarded in some way, that action is intensified and stored as information for future use. (This is what is meant by learning from new situations.)

For instance, imagine the quick physical responses Robert Irwin needs to work with the crocodiles at Australia Zoo. The first time he worked with the crocs, a novel (new) situation, his actions in dodging the crocodile’s advances were rewarded by not having his arm ripped off. This information was stored by the basal ganglia, and such actions were encouraged for the future.

Alternatively, any action that may have endangered Robert Irwin in his interactions with the crocodiles would be suppressed. This result would be called “deleterious,” meaning negative in nature.

This could arise, for example, if there is a certain physical condition and specific movements result in pain or discomfort. The deleterious outcome would lead the basal ganglia to suppress the actions that caused it.

Movements orchestrated by the basal ganglia are executed in “motor plans.” Specified actions are carried out by the number of neurons involved in the signal transmission, and the way said neurons are arranged.

Those networks that are not involved in the desired movement must be suppressed in the meantime (this is called “signal inhibition.” Those neurons involved in the signal transmission for the movement are activated by “signal promotion”). The basal ganglia filter the signal, such that it becomes increasingly specific as it passes through.

Cognition

Cognition is defined as… well, there is no agreed-upon definition for cognition as of yet. For now, in the case of the human brain, let’s define it as the ability to form thoughts, develop plans, and brain functions related to spatial and temporal awareness, learning, and memory.

Regarding cognition, the basal ganglia are involved in the selection and activation of various cognitive, executive, and emotional programs that are stored in the prefrontal association cortex and the limbic cortex. The prefrontal association cortex (PFC) is a portion of the brain located in the frontal lobe and is referred to as the “center of higher cortical functions.”

The PFC and its critical role in the human brain were famously discovered when a railroad construction staff known as Phineas Gage was pierced through the skull by an iron rod. Miraculously, he survived, but his personality was completely changed thereafter.

His PFC was no longer able to suppress unwanted behaviors and he flipped from being a kind and thoughtful man to someone who was highly disruptive both socially and in the workplace. From this incident on, it has been found that the PFC plays a central role in many psychiatric disorders like depression and obsessive-compulsive disorder (OCD).

The limbic cortex, a major structure of the limbic system, of course, holds within it the cingulate gyrus and the parahippocampal gyrus. These two are responsible for the regulation of your heart rate and blood pressure, in addition to the processing of emotions and spatial memory.

The basal ganglia’s role in motor memory is manifested here as well, on the cognitive side, however. It has been observed that many different kinds of learning require repeated trials and are often subconscious in nature.

With repeated exposure to a certain stimulus, a person is able to refine their reaction and the basal ganglia work to increase the frequency with which this action is performed. Those with basal ganglia disorders experience difficulty with learning in this way.

Pathways of the Basal Ganglia

There are many pathways that interconnect the various structures of the basal ganglia, however, the two we will focus on here are the direct and indirect pathways. These two, as their names suggest, have two opposing effects on the structures of the thalamus they are programmed to target.

The direct pathway excites the thalamic structures which then excite the cortical neurons; whereas the indirect pathway inhibits thalamic neurons, suppressing their ability to excite motor cortex neurons. The standard functioning of the basal ganglia requires the perfect balance between these two pathways.

It is thought that these pathways work simultaneously, with the direct pathway exciting highly specific motor or cognitive actions while the indirect pathway suppresses motor plans that may be specifically oppositional to those excited.

An imbalance in this system would lead to motor dysfunctionality and cause conditions like extrapyramidal syndrome, Parkinson’s disease, and Huntington’s disease.

Disorders of the Basal Ganglia

As you’ve seen, the basal ganglia are heavily involved in many different functions that are vital to survival and human cognition. This is why any damage to the basal ganglia can result in many different types of neurological disorders.

Since the basal ganglia are most widely regarded for their role in movement, some of the most commonly known disorders related to these structures are Parkinson’s disease and Huntington’s disease.

Parkinson’s Disease

Some of the most easily recognizable symptoms of Parkinson’s disease are noticeably slow movements (bradykinesia), the lack of ability to move (akinesia), rigidity, and tremoring when at rest, namely in the hands and fingers.

The disease is caused by a loss of key neurons in a structure of the basal ganglia called the substantia nigra pars compacta. This structure is responsible for exciting the direct pathway and inhibiting the indirect pathway, and with the loss of these important neurons, the balance is damaged between the two pathways and shifts in favor of the indirect pathway.

This results in the inhibition of thalamic neurons, which, in turn, reduces the excitement levels of the motor cortex neurons and reducing the capabilities of the motor system.

Huntington’s Disease

Symptoms of this disease are very much the opposite of those that emerge in Parkinson’s disease. Huntington’s disease is characterized by what are called choreiform movements. These are involuntary, continuous movements of the body and occur most often in the arms and legs, and in the face.

This is caused by the selective loss – meaning not all, but specific losses – of striatal neurons in the indirect pathway. What this means is that there is a severely reduced ability of the basal ganglia to inhibit unwanted motor plans due to the shift in balance favoring the direct pathway.

This enables thalamic neurons to be excited randomly, leading the motor cortex to activate motor programs with no control of the individual.

Significance of Basal Ganglia

The basal ganglia – rather, the basal nuclei, if we want to get fancy – are incredibly important to numerous vital functions within the human nervous system. These clusters are what allow us to move, our hearts to beat, and part of what gives us the cognitive abilities that separate us from all non-human animal species on this earth.

Their role in the human experience is still being intensely studied, but as we currently know, it is certainly a part of the human nervous system anatomy that we could not survive without.

Their role in the human experience is still being intensely studied, but as we currently know, it is certainly a part of the human nervous system anatomy that we could not survive without.