Parkinson's Disease
Neurodegenerative Disease
Side view
Midline view
A progressive movement disorder caused by the loss of dopamine-producing neurons in the midbrain, leading to tremor, stiffness, and slowed movement.
Common Signs & Symptoms
- Resting tremor, often starting in one hand
- Muscle stiffness (rigidity)
- Slowed movement (bradykinesia)
- Balance and postural instability
- Non-motor symptoms including sleep disruption and mood changes
Why These Symptoms Happen
Each symptom can be traced, at least in part, to the job a particular part of the brain normally does. These links are simplified: real symptoms usually come from several connected regions.
| Symptom | Brain region | Why it happens |
|---|---|---|
| Slowness and difficulty starting movements | Brain StemBasal Ganglia | Dopamine-producing neurons in the substantia nigra (in the midbrain) feed the basal ganglia, which act as a gate for selecting and starting movements. With less dopamine, that gate is harder to open, so movements become small, slow, and hard to begin. |
| Resting tremor | Basal GangliaCerebellum | The tremor is thought to arise from abnormal rhythmic activity in circuits linking the basal ganglia with the cerebellum and motor cortex, set off by the loss of dopamine. |
| Muscle stiffness | Basal Ganglia | Without normal dopamine signalling, the basal ganglia's control over muscle tone is disrupted, so opposing muscles stay partly contracted at the same time. |
| Poor balance and falls | Basal GangliaBrain StemCerebellum | Balance depends on the basal ganglia, brain-stem posture centres, and the cerebellum working together; the disease affects several parts of this system, especially in later stages. |
| Loss of smell and acting out dreams in sleep | Brain Stem | These "non-motor" signs often appear years before the tremor. They may reflect early changes in the lower brain stem and smell pathways, where Parkinson's-related protein clumps are often found first. |
Overview
Parkinson's disease is a progressive movement disorder caused primarily by the loss of dopamine-producing neurons in the substantia nigra, a structure within the midbrain portion of the brain stem. Dopamine from this region is essential for smooth, well-timed voluntary movement, and its depletion produces the condition's hallmark motor symptoms: tremor, rigidity, and slowness of movement. By the time these movement symptoms appear, a large proportion of these neurons - often estimated at half or more - have already been lost.
Affected Brain Regions
The core pathology centres on the substantia nigra within the brain stem's midbrain, which projects dopamine to the basal ganglia - deep structures involved in initiating and smoothing movement. Because the cerebellum works closely with these circuits to fine-tune movement and balance, gait and coordination problems in Parkinson's often involve altered cerebellar activity as well, even though the cerebellum isn't the primary site of degeneration.
Inside affected neurons, a protein called alpha-synuclein clumps into deposits known as Lewy bodies. In 2003, Heiko Braak and colleagues proposed that these deposits typically appear first in the lower brain stem and the smell pathways, and only later reach the midbrain and the cortex. That would explain why loss of smell and sleep disturbances can come years before any tremor - though not every patient follows this pattern.
How It's Studied
Parkinson's is usually diagnosed clinically: a neurologist looks for slowness of movement together with tremor or stiffness, and watches how symptoms respond to dopamine-replacing medication. There's no single definitive blood test in routine use. In uncertain cases, a DaT scan - a type of nuclear imaging that shows dopamine-releasing nerve endings in the basal ganglia - can reveal the characteristic loss of dopamine signalling. Researchers are also developing tests that detect abnormal alpha-synuclein in spinal fluid or skin samples, aiming to identify the disease earlier.
Common Myths
"Parkinson's is just a tremor."
Tremor is the best-known symptom, but not everyone with Parkinson's has one. Slowness, stiffness, balance problems, and non-motor symptoms - including loss of smell, constipation, sleep disorders, low mood, and anxiety - can affect quality of life as much as or more than the tremor.
"Only elderly people get Parkinson's."
Risk rises with age and most people are diagnosed after 60, but "young-onset" Parkinson's can begin before 50.
"Every tremor means Parkinson's."
Many tremors have other causes. The most common, essential tremor, typically appears when the hands are being used (for example, holding a cup), whereas the classic Parkinson's tremor is most obvious when the hand is at rest.
Current Research Directions
Current treatment mainly aims to replace or supplement dopamine signalling, most commonly with the medication levodopa, though its effectiveness can change over years of use. Deep brain stimulation - implanting electrodes that modulate abnormal activity in movement-related circuits - has become an established option for suitable patients with advanced symptoms. Research into earlier diagnosis (including via imaging and biomarkers), and into therapies that might slow or halt neuron loss rather than only treating symptoms, is ongoing.
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Key Terms
Related Brain Regions
Explore the regions involved in parkinson's disease in more depth:
This article is for general education and isn't a substitute for professional medical advice or diagnosis.
Sources & Further Reading
- Parkinson's Disease - National Institute of Neurological Disorders and Stroke
- What Is Parkinson's? - Parkinson's Foundation
- Parkinson's disease - NHS
- Staging of brain pathology related to sporadic Parkinson's disease - Braak et al. (2003), Neurobiology of Aging