What Is The Basal Ganglia?
The basal ganglia (BG) is an essential region of the brain located just above the brainstem. It plays key roles in regulating movement, behaviour, executive functions, and reward responses.
Basal Ganglia Neuroanatomy
The basal ganglia is part of the brain’s gray matter and is located deep within the centre of the brain, near the top of the brainstem. Because the BG is situated beneath the external cortical layer, it is classified as a subcortical structure.
It consists of many clusters of neurons, which are specialized to fulfill distinct functions. The word ‘clusters’ might make the BG sound disorganized or messy; however, the basal ganglia is actually highly organized and can be further classified into well-defined substructures. These substructures include [1]:
- Caudate nucleus: a pair of C-shaped structures located just above the thalamus. Each caudate nucleus has an anterior “head,” a middle “body,” and a thin “tail” region. Involved in planning and executing skeletal movement, learning, memory, emotion, and reward [2].
- Putamen: involved in motor learning, motor control, speech, language, reward responses, and addiction [3].
- Nucleus accumbens: located near the “head” of the caudate nucleus, the nucleus accumbens plays a central role in linking reward, motivation, and action [4]. The nucleus accumbens has been linked to a variety of addictive behaviours and disorders [5].
- Globus pallidus: consists of an internal region (globus pallidus internus, GPi) and an external region (globus pallidus externus, GPe). Involved in coordinating conscious movements and regulating proprioception [6].
- Substantia nigra: a primary site of signal input for the rest of the basal ganglia. Consists of dopaminergic (dopamine-producing) neurons and critical for voluntary movement, planning, behavioural motivation, and reward responses [7].
- Subthalamic nucleus: contains glutaminergic (glutamine-producing neurons). Glutamine is an essential precursor for excitatory neurotransmitters, such as glutamate, and inhibitory neurotransmitters, such as GABA. The subthalamic nucleus has connections to the GPi and plays an important role in motor inhibition [8].
What Is The Function of The Basal Ganglia?
The basal ganglia is responsible for coordinating a panoply of physiological functions.
Motor Function
The primary function attributed to the basal ganglia is motor control. The caudate nucleus, putamen, and nucleus accumbens receive signals for voluntary movement from the motor cortex, which are then processed in the globus pallidus, subthalamic nucleus, and substantia nigra [1].
Processed motor signals then pass through the globus pallidus and substantia nigra before being transmitted to the brainstem and spinal cord [1].
Damage to any of these nuclei can cause loss of motor coordination; involuntary movement; tremors; and parkinsonism [9].
Additional Functions of the Basal Ganglia
Apart from its role in motor function, the basal ganglia has also been implicated in several higher cognitive activities.
These include motor learning, behavioural coordination, emotional regulation, and reward responses [1]. Hence, the BG is involved in a wide array of neurological processes and activities.
Why Is The Basal Ganglia Important?
The importance of the basal ganglia in neurological coordination cannot be overstated.
Due to its position within the brain, the basal ganglia serves as a crucial intermediate between lower and higher brain centres. Travelling through the BG, neural messages undergo essential informational processing before continuing along vertical neural pathways to their ultimate destination.
This informational processing and exchange between higher and lower brain regions enables functional integration of basic autonomic processes with more complex cognitive activities, which is required for the coordination of physiological and social-emotional processes.
In sum, the basal ganglia is a critical checkpoint between lower and higher brain centres and helps regulate diverse forms of activity. As such, impairments to any region of the basal ganglia can be catastrophic for physiological functioning and damage to the BG causes various forms of disease.
Diseases Affecting The Basal Ganglia
Dysfunction and damage of the basal ganglia have been implicated in various pathological conditions and diseases:
- Stroke: basal ganglia stroke occurs when there is hemorrhage (rupture) or ischemia (blockage of blood flow) of the small blood vessels supplying this brain region [10]. Such disruptions result in tissue damage, cell death, and loss of neurological function, with specific symptoms depending upon the substructure that is impacted [11].
- Parkinson’s disease: a motor disorder characterized by tremors, muscle rigidity, bradykinesia (slow movement), and loss of motor coordination. Parkinson’s disease has been linked to degeneration of dopaminergic neurons in the substantia nigra [12].
- Huntington’s disease: a genetic disease which is characterized by progressive degeneration of nuclei in the caudate and putamen. Results in cognitive impairment, loss of motor control, agitation, and psychiatric complications [13].
- Fahr’s disease: a rare condition wherein deposits of calcium carbonate and phosphate ions accumulate within basal nuclei. This leads to progressive neurological dysfunction and Parkinson’s-like symptoms. While the exact etiology of Fahr’s disease remains unclear, the disease displays an autosomal dominant mode of inheritance [14]. It is also known as Basal Ganglia Calcification (BGC).
- Tourette’s syndrome: a neurological disorder characterized by the presence of verbal and motor tics. Rapid and repetitive muscle movements result in uncontrollable jerking or jolting behaviours, which can vary in frequency and onset. Tourette’s syndrome has been linked to abnormal functioning of excitatory and inhibitory nuclei within the basal ganglia [15].
- Addiction: dopaminergic neurons located in the nucleus accumbens have been linked with the ‘intoxication phase’ of addiction. Exposure to an addictive substance triggers excitatory neurons in the basal ganglia and activates reward-associated neural pathways [16].