The basal ganglia are a group of nerves cell clusters, called nuclei, located deep in the lobes of the brain collectively referred to as subcortical nuclei. The different ganglion (nuclei) are labeled in the picture below and include the: caudate nucleus, putamen, globus pallidus, subthalamic nuclei, substantia nigra and nucleus accumbens, which is not included in this picture. The general outline of these subcortical nuclei looks similar to the outline of the limbic system because both systems surround the lateral and third ventricles found in the core of the brain.
The basal ganglia are important to the execution of smooth movements from start to finish. They also play a role in learning routine movements, including bad habits, such as obsessive compulsive disorders, as well as cognition and emotion. As far as movement is concerned, one of their functions is to inhibit excitation of muscles to prevent overreactions. The ganglia get their signals from many different areas in the lobes of the brain, as well as from the midbrain which is the top of the brainstem.
Glutamate, is an excitatory neurotransmitter and the most abundant used by the basal ganglia. Acetylcholine is also used and is, likewise, excitatory. In addition to the excitatory glutamate and acetylcholine neurotransmitters, the nuclei of the system also use GABA and dopamine which are inhibitory. Although they are less abundant compared to the excitatory transmitters, the input from the inhibitory neurotransmitters plays a key role in smooth execution of movement.
The basal ganglia are most noted for their involvement in Parkinson’s and Huntington’s disease. While the two conditions share certain signs and symptoms, they are notably different. For example, Parkinson’s disease primarily affects the substantia nigra resulting in decreased production of the inhibitory neurotransmitter dopamine. One of the characteristic signs of Parkinson’s disease is that patients have a hard time initiating movement to get going.
Huntington’s disease primarily affects the caudate nucleus and putamen. In contrast to decreased dopamine seen in Parkinson's, Huntington's disease is associated with excess glutamate and subsequent excitotoxicity. I will cover Huntington's disease more as this site develops. As for movement, patients with Huntington’s disease have a hard time preventing unwanted movements. What these conditions share in common, is that in advanced stages they can both significantly impair cognition.
More recently, the basal ganglia have been implicated in multiple sclerosis (MS). Instead of the characteristic supratentorial periventricular and perivenular hyperintensity signals associated with the white matter of myelinated nerves seen in MS, they show up as hypointensity signals found in the grey matter nuclei, such as the basal ganglia, that are located deep within the lobes of the brain along the lateral ventricles.
The difference between hyper and hypointensity signals seen on MRI in MS has to do with fluid content and the ability of fluids to diffuse through different densities and types of tissues that surround them. Thicker fluids don’t diffuse as well as thinner fluids. The differences in thickness and diffusibility affect appearances on MRI scans. Hyper, hypo and isointensity signals will be covered further in future pages. They are the focus of current research and rapidly adding to our understanding of neurodegenerative processes because radiologists have the technology now to see things they couldn't before.
The signs and symptoms of basal ganglia involvement include tremors, involuntary movements, weakness and spasticity. These signs and symptoms are common in Parkinson's disease and multiple sclerosis.
The Nuclei of the Basal Ganglia
The nuclei of the basal ganglia are found on both sides of the brain. They are located across from the walls of the thalamus to the outside, and above the nuclei and inner portion of the lobes of the brain that comprise the limbic system. The overall shape and layout of the basal ganglia is similar to the limbic system. The nuclei of the basal ganglia are listed in the first paragraph of this page.
The caudate nucleus and putamen are often referred to collectively as the corpus striatum because they are structurally and functionally related but they are separated from one another by a network of myelinated white fibers that look like stripes. The myelinated fibers are part of an important and major communication pathway called the internal capsule. The putamen and globus pallidus are similarly related and sometimes collectively referred to as the lenticular nucleus because they are shaped like a lens. The striatum receives information from the many different areas in the cortex of the brain and sends them out to the globus pallidus and other nuclei of the system, which send inhibitory signals to motor related areas of the brain.
The system of the basal ganglia has components that are closely related to the limbic system such as the nucleus accumbens, the ventral globus pallidus and the ventral tegmental areas. The tegmental area is located on the ventral (front) side of the brainstem anterior to the ventricles. In addition to nuclei related to the limbic system, the tegmental areas of the midbrain, pons and medulla of the brainstem also contain the cranial nerves.
Evidence suggests that these systems play a central role in motivation and reward learning involving the limbic system. The ventral tegmental area also appears to play a role in schizophrenia. Some highly addictive drugs such as cocaine, amphetamines and nicotine are thought to be related to dopamine, as are food and sex. Injuries to the nucleus accumbens and ventral tegmental area can cause a profound state of torpor. Torpor is an uncontrolled state of loss of consciousness.
The basal ganglia also play an important role in controlling eye movements due to their connection to the superior colliculus. The superior and inferior colliculus form a group of four command and control centers on the backside (roof) of the midbrain, called the tectum, that are related to sight, sound and movement. Collectively, they are referred to as the corpora quadrigemina, which means two twin sets or quadruplets. In addition to the eyes, the superior colliculus is also closely connected to neck muscles.The inferior colliculus is related to the ears and sound. An extensive network of connections from different regions of the brain all converge on the superior colliculus. These converging signals help direct the head and eyes toward a point in space, such as an owl hearing the rustle of prey and turning its head to focus its eyes on the source of the sound.
The head of the caudate nucleus of the striatum lies just behind the frontal lobe and arches back toward the occipital lobe. It sends its signals to the frontal lobe, especially the orbital cortex just above the eyes. An underactive caudate nucleus may play a role in attention deficit disorders, depression, schizophrenia and lethargy. The putamen lies beneath and behind the caudate nucleus. It appears to be involved in the memory and execution of automated, repetitive coordinated type skills like riding a bike, driving a car or typing. The globus pallidus is located just to the inside of the putamen. It receives signals from the caudate nucleus and putamen (striatum) and sends them out to the substantia nigra.
The substantia nigra is located in the tegmentum in the ventral (frontal) portion of the midbrain. The midbrain is the topmost portion of the brainstem, just below the thalamus and hypothalamus of the diencephalon. There are two parts to the substantia nigra. One is called the pars compacta. The pars compacta sends its signals to the striatum (caudate nucleus and putamen). Its function is believed to play a role in reward-type learning. The other part of the substantia nigra is called the pars reticulata. The pars reticulata uses mostly GABA neurons and plays a role in the orientation, control and stabilization of movements of the eyes for focus and scanning vision. The pars compacta is affected in Parkinson’s disease and epilepsy.
The nucleus accumbens is the pleasure center of the brain related to rewards, pleasure and laughter on the positive side, and addiction, fear and aggression on the negative side. The nucleus accumbens sits below the level of the caudate nucleus, putamen and globus pallidus underneath the frontal lobes. It gets its signals from the frontal cortex by way of the ventral tegmental area and sends them out to the globus pallidus. The signals in this loop use dopamine.
While the role of the basal ganglia in smooth, intentional and purposeful movement is understood, far less is known about its role in behavior and motivation. It appears that it isn't just the ability to execute movement that is affected in conditions such as Parkinson's as previously thought. In some cases it appears to be related to the motivation to move and do something. For example, some patients with Parkinson’s who typically have decreased ability to initiate movement called bradykinesia, can respond quickly and in a coordinated way to an emergency. Once the situation passes however, they revert back to immobility. It may have to do with other neurotransmitters and circuits within the system that are unaffected by the condition.
Because of their close proximity to the ventricles and close connection to the nuclei of the limbic system, the nuclei of the basal ganglia can be affected by faulty cranial hydrodynamics (fluid mechanics) in the brain. Cranial hydrodynamics can be affected by numerous problems inside the skull, such as missing and narrow pathways or structural design issues in the cranial vault or the brain.
Fluid mechanics in the brain and cord can be further affected by design flaws, injuries and aging of the spine. Malformations and misalignments of the upper cervical spine, and spondylosis, scoliosis and stenosis in the lower spine, can affect blood and CSF flow in the spinal canal. Blood and CSF flow in the spinal canal can in turn affect blood and CSF flow in the cranial vault.
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