Basal Ganglia Overview

Basal Ganglia Overview

  • The term basal ganglia (BG) refers to a group of nuclei (masses of gray matter) located in the forebrain and the upper part of the brainstem.

  • These nuclei are critically involved in motor activities and influence the pyramidal system's activity.

  • The basal ganglia differ from the cerebellum in that they do not receive inputs from the spinal cord; instead, they receive direct input from the cerebral cortex and primarily affect the motor area of the cortex via the thalamus.

Functional Relationship with the Cerebral Cortex

  • The basal ganglia work in close association with the cerebral cortex and the corticospinal motor control system.

  • They receive a majority of their inputs from the cerebral cortex and return nearly all output signals back to it.

Contributions Beyond Motor Functions

  • In addition to motor control, the basal ganglia also contribute to affective and cognitive functions.

  • The basal ganglia comprise five interactive structures on each side of the brain:

    • Caudate nucleus

    • Putamen

    • Globus pallidus

    • Subthalamic nucleus

    • Substantia nigra

  • The caudate nucleus and putamen collectively form the striatum; whereas the putamen and globus pallidus collectively form the lenticular nucleus.

  • Lesions in the basal ganglia can lead to abnormal movements and postures.

Structural Overview and Organization

  • Almost all motor and sensory nerve fibers connecting the cerebral cortex and spinal cord pass through the internal capsule, situated between the major masses of the basal ganglia (caudate nucleus and putamen).

Divisions of the Basal Ganglia

  1. Lenticular nucleus:

    • Caudate nucleus

    • Putamen

    • Globus pallidus (pallidum)

  2. Striatum:

    • Subthalamic nucleus

    • Substantia nigra

Spatial Relationships within the Brain

Correlation with Ventricles and Thalamus

  • The basal ganglia have specific anatomical relationships with:

    • Cerebral cortex

    • Cerebral white matter

    • Corpus callosum

  • Key structures include the anterior and posterior horns of the lateral ventricle, the thalamus, and the third ventricle.

Motor Activity Modification and Pathways

The Role of Basal Ganglia in Motor Control

  • The basal ganglia play a significant role in controlling complex patterns of motor activity, collaborating with the corticospinal system.

  • Example: Writing letters. Damage to the basal ganglia may result in motor control being impaired, leading to crude attempts at writing akin to a child learning to write.

Other Motor Activities

  • Other essential patterns include:

    • Cutting paper using scissors

    • Hammering nails

    • Shooting a basketball

    • Throwing a football

    • Shoveling dirt

  • Many skilled movements, vocalization, and eye control, requiring basal ganglia interaction, happen subconsciously.

Neural Pathways of the Putamen Circuit

  1. The pathway initiates in the premotor and supplementary areas of the motor cortex and the somatosensory areas of the sensory cortex.

  2. Signals travel to the putamen (bypassing the caudate nucleus) and progress to the internal portion of the globus pallidus.

  3. From there, signals move to the ventro-anterior and ventrolateral relay nuclei of the thalamus and eventually back to the cerebral primary motor cortex and adjacent areas (premotor and supplementary).

Neural Pathways Illustration

  • Key components: prefrontal area, ventroanterior and ventrolateral nuclei of the thalamus, subthalamus, substantia nigra, premotor, and supplemental areas, alongside somatosensory pathways.

Caudate Circuit through the Basal Ganglia

  • The caudate nucleus has a head and tail, and is crucial for cognitive planning and sequential motor pattern achievements.

  • This circuit operates to modify and achieve conscious motor objectives.

Chemical Pathways and Neurotransmitters in the Basal Ganglia

Distinct Pathways

  1. Nigrostriatal dopaminergic system

  2. Intrastriatal cholinergic system

  3. GABAergic system

    • High oxygen consumption and copper content characterize it.

Afferent Connections

  • Main afferent connections primarily terminate in the neostriatum and are predominantly excitatory (cholinergic):

    1. Corticostriate projection: From all lobes, especially frontal and parietal to the neostriatum.

    2. Thalamostriate fibers: From thalamus to neostriatum.

Internuclear Connections

  • Dopaminergic nigrostriatal tract: Cell bodies in the snc release dopamine (inhibitory), and degeneration leads to Parkinsonism.

  • GABAergic inhibitory projections:

    • Neostriatum projects to the pars reticularis (substantia nigra) and both segments of the globus pallidus.

  • The external segment of the globus pallidus projects to the subthalamic nucleus.

Efferent Connections

  • Main outputs occur through:

    • Internal segment of globus pallidus: Projects via thalamic fasciculus to the thalamic nuclei and onwards to prefrontal and premotor cortex.

    • Substantia nigra: Projects to ventrolateral and ventroanterior thalamic nuclei and some additional projections to other areas (e.g., habenula).

Role of Neurotransmitters

Key Neurotransmitters in Functioning

  1. Dopamine: Inhibitory from substantia nigra to corpus striatum.

  2. GABA: Inhibitory intrinsic fibers within the basal ganglia.

  3. Acetylcholine: Excitatory from cerebral cortex to caudate nucleus and putamen.

  4. Substance P: Excitatory from globus pallidus reaching substantia nigra.

  5. Enkephalins: Excitatory with various roles.

  6. Noradrenaline: Exerts a stimulating role between basal ganglia and the reticular formation.

  7. Glutamic acid: Excitatory from the subthalamic nucleus.

Functions of Basal Ganglia

  • Motor activity planning and programming: Transition from involuntary movement ideas to precise actions.

  • Control of muscle tone: Through basal ganglia inhibition of stretch reflexes, leading to rigidity when damaged.

  • Cognitive functions: The caudate nucleus is crucial in processing sensory input with stored memory information, impacting speech (with dysarthria when affected).

  • Subconscious movement execution: Controls normal automatic movements e.g. swinging arms while walking, requiring the motor cortex activation.

  • Support for skilled movements: The globus pallidus maintains muscle tone, while substantia nigra coordinates impulses necessary for these movements.

Movement Disorders Associated with Basal Ganglia Dysfunction

Types of Movement Disorders

  1. Athetosis: Slow, continuous, involuntary writhing movements.

  2. Ballism: High-amplitude random flinging movements, often unilateral, caused typically by infarct in the contralateral subthalamic nucleus.

  3. Akathisia: Characterized by purposeless activity and restlessness, leading to fidgeting or pacing.

  4. Parkinson's Disease: A neurodegenerative condition primarily affecting dopaminergic neurons.

Localization of Movement Disorders

  • Dysfunction in the basal ganglia leads to distinct movement disorders:

    • Substantia nigra: Parkinson's disease

    • Subthalamic nucleus: Ballism

    • Caudate nucleus: Chorea

    • Putamen: Dystonia

Parkinson's Disease (PD)

Description and Causes

  • PD occurs due to degeneration of the substantia nigra and/or globus pallidus, typically affecting individuals in late middle age.

  • Characterized by reduced dopamine concentration in the nigrostriatal system, causing a disrupted balance between excitatory and inhibitory inputs.

Symptoms and Diagnostic Criteria

  • Diagnosis requires bradykinesia and over one of the following symptoms:

    • Rigidity

    • Tremor

    • Postural instability

  • Postural issues include neck and trunk flexion, balance disturbances, and gait abnormalities (e.g., shuffling).

  • Symptoms manifest when 60-70% of the dopaminergic neurons are depleted.

Pathology

  • Pathological features include misfolding and aggregation of α-synuclein, often leading to Lewy bodies seen in surviving neurons.

  • Degeneration may spread from brainstem to cerebrum.

Etiologic Classification of PD

  1. Primary Parkinson's: Idiopathic, influenced by genetic/environmental factors.

  2. Secondary Parkinson's: Includes drug-induced, vascular, infectious, or traumatic causes.

Environmental Risk Factors for PD

  • Increased Risk: Pesticides/herbicides, rural living, metabolic syndrome, repeated head trauma, low vitamin D levels.

  • Protective Factors: Regular exercise, caffeine consumption, ibuprofen use, and cigarette smoking.

Clinical Features of PD

Cardinal Motor Symptoms

  • Bradykinesia

  • Rest tremor

  • Rigidity

  • Gait disturbance/postural instability.

Other Motor Features

  • Micrographia

  • Reduced eye blinking

  • Soft voice (hypophonia)

  • Dysphagia

  • Freezing episodes

Non-Motor Features

  • Anosmia (loss of smell)

  • Mood disorders (e.g., depression)

  • Sleep disturbances (e.g., REM sleep behavior disorder)

  • Autonomic disturbances (e.g., orthostatic hypotension, gastrointestinal issues, sexual dysfunction).

Medications for Managing PD

Overview of Anti-Parkinson Drugs

Classifications & Mechanisms of Action

  1. Dopamine precursors:

    • Levodopa: Converted to dopamine, mainly alleviates bradykinetic symptoms.

    • Decarboxylase inhibitors: (e.g., Carbidopa) prevent levodopa's peripheral conversion.

  2. MAO-B Inhibitors: (e.g., Rasagiline, Selegiline)

    • Mechanism: Reduce dopamine breakdown, prolong dopamine availability.

  3. Dopamine Agonists: (e.g., Ropinirole, Apomorphine)

    • Directly act on striatal dopamine receptors.

  4. Other Drugs: NMDA inhibitors (e.g., Amantadine), COMT inhibitors (e.g., Entacapone), and anticholinergic drugs (e.g., Benztropine).

Common Side Effects of Medications

  • Include nausea, vomiting, orthostatic hypotension, somnolence, hallucinations, and dyskinesias.

Device and Surgical Treatment for PD

  • Transplantation of dopamine secreting cells (from fetuses) into the caudate nucleus.

  • Surgical interventions (e.g., lesions in globus pallidus or subthalamic nucleus) aim to normalize output balance.

  • Deep Brain Stimulation (DBS): A technique for symptom management.

Wilson's Disease/Hepatolenticular Degeneration

  • An autosomal recessive disease characterized by copper accumulation, leading to degeneration of the lenticular nucleus and liver cirrhosis.

  • Symptoms include muscular rigidity, tremor, cirrhosis, and emotional disturbances.

Huntington's Disease

  • An autosomal dominant disorder that usually develops between ages 30 to 50.

  • Caused by damage to GABAergic and cholinergic neurons projecting to the putamen, leading to hyperkinetic features such as chorea, slurred speech, and progressive dementia.

  • The disease is gradually progressive with no effective treatment, ultimately resulting in death.

Conclusion

  • The intricacies of the basal ganglia's role in motor control, cognitive functions, and associated disorders like Parkinson's and Huntington's diseases emphasize its essential contribution to human movement and behavior.