Glossary

A small presynaptic protein encoded by the SNCA gene. Normally involved in vesicle trafficking and synaptic plasticity. In Parkinson's disease, it misfolds, aggregates into oligomers and fibrils, and accumulates as the primary component of Lewy bodies. Alpha-synuclein can spread between neurons in a prion-like manner, which underlies Braak staging theory.

A group of subcortical nuclei - including the striatum (caudate + putamen), globus pallidus (internal and external), subthalamic nucleus, and substantia nigra - that form a loop with the cortex and thalamus. The basal ganglia plays a central role in action selection, motor learning, and habit formation. Dopamine depletion in PD disrupts basal ganglia circuit balance.

Synchronized electrical oscillations in the 13–30 Hz frequency range, recorded in the subthalamic nucleus and globus pallidus. In Parkinson's disease, beta oscillations become pathologically exaggerated due to dopamine loss. High beta power correlates with bradykinesia and rigidity severity. Deep brain stimulation at 130 Hz disrupts these oscillations, and adaptive DBS uses beta power as a real-time biomarker to control stimulation.

A selective semi-permeable barrier formed by specialized endothelial cells, astrocyte end-feet, and pericytes that separates the brain's extracellular fluid from circulating blood. It allows oxygen, glucose, and lipid-soluble molecules to pass while blocking most large or hydrophilic molecules, pathogens, and many drugs. The BBB is a major obstacle for drug delivery in PD and neurodegeneration - one reason levodopa must be taken as a prodrug that crosses the barrier and is then converted to dopamine.

A six-stage neuropathological classification of Parkinson's disease proposed by Heiko Braak in 2003, based on the anatomical distribution of Lewy body pathology in post-mortem brains. The staging follows a caudal-to-rostral pattern: Stages 1–2 affect the dorsal motor nucleus of the vagus and olfactory bulb; Stage 3–4 involve the substantia nigra and limbic system; Stages 5–6 reach the neocortex. Approximately 70–80% of PD cases match this pattern.

A membrane protein expressed on dopaminergic nerve terminals that reuptakes dopamine from the synaptic cleft back into the presynaptic neuron. DAT is essential for terminating dopaminergic signaling. In PD, progressive loss of SNc neurons leads to loss of DAT. DAT-SPECT (DaTscan) imaging measures striatal DAT density and is used clinically to confirm dopaminergic degeneration and differentiate PD from essential tremor.

A neurosurgical procedure in which a thin electrode is implanted into a specific deep brain structure - most commonly the subthalamic nucleus (STN) or globus pallidus interna (GPi) - and connected to a pulse generator in the chest. High-frequency stimulation (~130 Hz) disrupts pathological beta oscillations and significantly improves motor symptoms in appropriate PD candidates. FDA-approved since 2002.

One of two major routes through the basal ganglia. The direct pathway runs from D1-receptor-bearing striatal neurons to the globus pallidus interna (GPi) and substantia nigra pars reticulata (SNr), inhibiting them and thereby releasing the thalamus from inhibition - facilitating movement. Dopamine acting on D1 receptors enhances this pathway. In PD, dopamine loss weakens the direct pathway, making movement harder to initiate.

A brainstem nucleus in the medulla that provides parasympathetic innervation to thoracic and abdominal organs via the vagus nerve. According to Braak staging, the DMV is among the first sites affected by Lewy body pathology in PD (Stage 1), which explains early non-motor symptoms such as constipation, reduced heart rate variability, and other autonomic dysfunctions.

A catecholamine neurotransmitter synthesized from L-DOPA by DOPA decarboxylase in dopaminergic neurons. In the nigrostriatal pathway, dopamine modulates the balance between direct and indirect basal ganglia pathways to facilitate smooth, voluntary movement. In PD, 50–70% of SNc dopamine neurons must be lost before motor symptoms appear, due to compensatory mechanisms. Dopamine also plays key roles in reward, motivation, working memory, and mood via the mesolimbic and mesocortical pathways.

A brainstem nucleus in the midbrain tegmentum that is the primary source of serotonergic projections throughout the brain. It projects to the striatum, cortex, limbic system, and hypothalamus. In PD, the dorsal raphe is affected by Lewy body pathology at Braak Stages 1–2, contributing to depression, anxiety, and sleep disturbances that precede motor diagnosis.

The gene encoding glucocerebrosidase (GCase), a lysosomal enzyme that breaks down glucocerebroside. Heterozygous mutations in GBA1 are the most common genetic risk factor for Parkinson's disease, present in 5–15% of PD patients and conferring 5–30 fold increased risk depending on the variant. GBA1 mutations impair lysosomal function, leading to alpha-synuclein accumulation and accelerated pathology. Biallelic GBA1 mutations cause Gaucher disease.

The internal segment of the globus pallidus, which serves as the primary output nucleus of the basal ganglia along with the substantia nigra pars reticulata. GPi neurons are GABAergic and tonically inhibit thalamic motor nuclei. In the parkinsonian state, GPi becomes pathologically overactive (rate model), excessively suppressing thalamo-cortical circuits. GPi is a target for both DBS and pallidotomy in PD treatment.

A monosynaptic projection from the cortex directly to the subthalamic nucleus, bypassing the striatum. It is the fastest route through the basal ganglia and is thought to allow the cortex to rapidly suppress competing motor programs just before a movement is executed. In PD, the hyperdirect pathway may contribute to pathological beta oscillations by providing a fast re-entrant excitatory drive to the STN–GPe circuit.

One of two major routes through the basal ganglia. The indirect pathway runs from D2-receptor-bearing striatal neurons through the globus pallidus externa (GPe) to the subthalamic nucleus (STN), which then excites GPi/SNr and ultimately suppresses thalamic activity - inhibiting movement. Dopamine acting on D2 receptors normally suppresses this pathway. In PD, dopamine loss disinhibits and strengthens the indirect pathway, contributing to hypokinesia.

A spherical eosinophilic intracytoplasmic inclusion found in neurons affected by Parkinson's disease and related disorders. Lewy bodies are primarily composed of aggregated alpha-synuclein, along with ubiquitin, neurofilaments, and other proteins. The appearance of Lewy bodies is the neuropathological hallmark of PD diagnosis at autopsy. Their presence in a neuron is associated with cell stress and eventual death, though whether they are toxic or protective remains debated.

A small nucleus in the pontine brainstem that is the primary source of norepinephrine in the brain. It projects widely to cortex, limbic system, hypothalamus, and spinal cord, modulating arousal, attention, and stress responses. The locus coeruleus is affected by Lewy body pathology at Braak Stage 2, before the substantia nigra. LC degeneration contributes to PD symptoms including dysautonomia, sleep disruption, depression, and cognitive impairment.

Leucine-rich repeat kinase 2 - a large, multidomain kinase encoded by the LRRK2 gene (also called PARK8). The G2019S variant is the most common dominant mutation causing Parkinson's disease, with prevalence up to 40% in North African Arab and Ashkenazi Jewish populations. Penetrance is incomplete at 25–42.5% by age 80. Mutant LRRK2 has increased kinase activity; LRRK2 inhibitors are in clinical trials.

The selective autophagy of damaged or dysfunctional mitochondria. In neurons, mitophagy is the primary mechanism for removing mitochondria that have lost their membrane potential and would otherwise generate toxic reactive oxygen species. The PINK1/Parkin pathway is the best-characterized mitophagy mechanism: PINK1 accumulates on damaged mitochondria and phosphorylates Parkin, which ubiquitinates outer membrane proteins to mark them for degradation. Mutations in either gene cause autosomal recessive early-onset PD.

A cluster of large cholinergic neurons in the basal forebrain (substantia innominata) that provides the primary cholinergic innervation of the neocortex. NBM neurons are the primary source of cortical acetylcholine and play critical roles in attention, learning, and memory. In PD, NBM neurodegeneration contributes significantly to cognitive decline and dementia. NBM is targeted in some experimental DBS protocols for PD dementia.

Activation of the brain's immune cells - primarily microglia and astrocytes - in response to injury, protein aggregates, or cellular damage. In PD, aggregated alpha-synuclein activates microglia, leading to release of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) and reactive oxygen species that can further damage or kill dopamine neurons. Neuroinflammation is considered both a consequence of and a contributor to PD pathology, forming a potential vicious cycle.

The dopaminergic projection from the substantia nigra pars compacta (SNc) to the dorsal striatum (primarily the putamen). It is the primary dopamine pathway for motor control and is the pathway most severely damaged in PD. Each SNc neuron maintains an estimated 1–2.4 million synapses in the striatum via highly arborized axons. Loss of nigrostriatal dopamine is the direct cause of the cardinal motor symptoms of PD.

An E3 ubiquitin ligase encoded by the PARK2 gene. Parkin is recruited to depolarized mitochondria by PINK1, where it ubiquitinates outer membrane proteins to trigger mitophagy. Biallelic loss-of-function mutations in Parkin account for approximately 50% of autosomal recessive early-onset PD (onset < 40 years). Parkin-PD often lacks Lewy bodies at autopsy, suggesting that alpha-synuclein aggregation and mitochondrial failure are separate (though overlapping) pathological processes.

PTEN-induced kinase 1, encoded by the PINK1 gene (also PARK6). PINK1 is a mitochondrial serine/threonine kinase that acts as a damage sensor: it continuously imports into healthy mitochondria and is degraded, but accumulates on the outer membrane of depolarized mitochondria, where it phosphorylates ubiquitin and Parkin to initiate mitophagy. Biallelic PINK1 mutations are the second most common cause of autosomal recessive early-onset PD.

A brainstem nucleus at the junction of the midbrain and pons, containing cholinergic and glutamatergic neurons. The PPN is a key node at the interface of basal ganglia and brainstem locomotor circuits, playing roles in gait initiation, postural control, and arousal. In PD, PPN degeneration is linked to freezing of gait, falls, and sleep disturbances. The PPN is an experimental DBS target for patients with gait-dominant PD refractory to conventional DBS.

The largest structure of the striatum, located lateral to the globus pallidus. The putamen receives dense dopaminergic input from the substantia nigra pars compacta via the nigrostriatal pathway, and is the primary striatal node for motor control within the basal ganglia. DAT-SPECT imaging reveals asymmetric loss of putaminal dopamine terminals as one of the earliest and most reliable markers of PD.

A small, lens-shaped glutamatergic nucleus located below the thalamus. The STN is the only excitatory nucleus in the basal ganglia and receives inputs from both the indirect pathway (via GPe) and the hyperdirect pathway (from cortex). In PD, the STN becomes pathologically overactive and a key generator of beta oscillations via its reciprocal connection with GPe. The STN is the primary target for deep brain stimulation in PD.

The primary input nucleus of the basal ganglia, comprising the caudate nucleus and putamen (together called the neostriatum), plus the ventral striatum (nucleus accumbens). The striatum receives glutamatergic input from the entire cortex and dopaminergic input from the SNc. GABAergic medium spiny neurons (MSNs) constitute ~95% of striatal neurons and are the cellular substrate of both the direct and indirect pathways.

A paired midbrain structure divided into the pars compacta (SNc) - the dopaminergic cell group that is the primary site of neurodegeneration in PD - and the pars reticulata (SNr), which is a GABAergic output nucleus of the basal ganglia. The SNc appears dark (hence 'nigra,' Latin for black) due to neuromelanin pigmentation. In PD, dopaminergic SNc neurons die preferentially, with the ventrolateral tier most severely affected.

A large paired structure in the diencephalon that serves as the principal relay station between subcortical structures and the cortex. In the motor circuit, the ventral anterior and ventral lateral thalamic nuclei receive inhibitory output from the basal ganglia (GPi/SNr) and cerebellar outputs, and relay motor signals to the primary motor and premotor cortex. In PD, excessive GPi inhibition excessively suppresses thalamo-cortical drive, contributing to hypokinesia.