Progressive Supranuclear Palsy (PSP) is characterized by prominent vulnerability of brainstem circuits, distinguishing it from Parkinson's Disease and other movement disorders. The selective degeneration of specific brainstem nuclei and their connecting circuits underlies the core clinical features of PSP, including vertical supranuclear gaze palsy, postural instability, and progressive akinesia[1].
The brainstem, comprising the midbrain, pons, and medulla, houses critical nuclei that control eye movements, postural tone, gait, and autonomic functions. In PSP, 4-repeat tau pathology preferentially targets these nuclei, leading to characteristic circuit dysfunctions that manifest as the disease's hallmark symptoms.
The Subthalamic Nucleus (STN) is a pivotal node in the basal ganglia motor circuit and shows prominent degeneration in PSP[2]. Located in the diencephalon just above the midbrain, the STN receives excitatory inputs from the cerebral cortex and inhibitory inputs from the external globus pallidus, sending excitatory outputs to the internal globus pallidus and substantia nigra pars reticulata.
In PSP, tau-positive neurofibrillary tangles accumulate extensively in STN neurons, leading to:
The Pedunculopontine Nucleus (PPN) is a major cholinergic center for gait and eye movement control. Located in the pontine tegmentum, the PPN receives input from the basal ganglia and sends cholinergic projections to the thalamus and spinal cord.
In PSP, PPN degeneration is a hallmark finding[3][4]:
PSP produces characteristic eye movement abnormalities due to degeneration of multiple oculomotor structures[6]:
The Globus Pallidus externus (GPe) shows early involvement in PSP, contributing to altered indirect pathway activity.
The hallmark of PSP results from degeneration of:
Brainstem circuit degeneration leads to:
Bradykinesia in PSP results from:
Targets include:
The brainstem contains numerous nuclei that are differentially affected in PSP:
| Nucleus/Circuit | Primary Neurotransmitter | Function | PSP Involvement |
|---|---|---|---|
| Substantia nigra pars compacta | Dopamine | Motor initiation | Severe loss |
| Pedunculopontine nucleus (PPN) | Acetylcholine | Gait/balance | Moderate-severe |
| Laterodorsal tegmental nucleus (LDT) | Acetylcholine | Arousal, gaze | Moderate |
| Oculomotor nucleus (CN III) | Acetylcholine | Eye movements | Severe |
| Interstitial nucleus of Cajal (INC) | GABA | Vertical gaze | Severe |
| Rostral interstitial MLF (riMLF) | GABA | Vertical saccades | Severe |
| Red nucleus | GABA | Motor coordination | Moderate |
| Raphe nuclei | Serotonin | Mood, arousal | Moderate |
| Locus coeruleus | Norepinephrine | Attention, arousal | Moderate |
The most pathognomonic feature of PSP is the vertical supranuclear gaze palsy, which results from degeneration of brainstem structures controlling eye movements[2:1]. Unlike horizontal gaze palsies that often result from pontine lesions, the vertical gaze impairment in PSP reflects pathology in midbrain structures:
The riMLF is located in the midbrain and contains burst neurons that generate vertical saccadic eye movements. In PSP:
The INC is critical for vertical gaze holding and torsional eye movements:
| Feature | Parkinson's Disease | PSP |
|---|---|---|
| Vertical gaze | Preserved early | Impaired early |
| Saccade latency | Normal | Prolonged |
| Antisaccade errors | Mild increase | Severe increase |
| Square wave jerks | Rare | Common |
| Glacial gaze | Absent | Present |
The vestibular system and proprioceptive pathways are crucial for maintaining balance. PSP patients develop early and severe postural instability due to brainstem involvement
Key structures affected:
The PPN is a major cholinergic nucleus in the pontine tegmentum that plays a critical role in gait initiation and postural control.
| Feature | Parkinson's Disease | PSP |
|---|---|---|
| PPN degeneration | Mild-moderate | Severe |
| Cholinergic loss | Less prominent | prominent |
| Gait freezing onset | Later | Early |
| Response to dopaminergics | Good | Poor |
The red nucleus receives input from the cerebellum and motor cortex, sending output to the spinal cord via the rubrospinal tract:
The cerebellum communicates with brainstem nuclei through the superior, middle, and inferior cerebellar peduncles:
The dorsal and median raphe nuclei contain serotonergic neurons that project throughout the brain:
The locus coeruleus is the primary source of norepinephrine:
While both PSP and PD involve brainstem structures, the pattern and severity differ substantially:
| Feature | Parkinson's Disease | PSP |
|---|---|---|
| SNc dopaminergic loss | Severe (80-90%) | Moderate (50-60%) |
| Brainstem spread | Later in disease | Early, prominent |
| Oculomotor involvement | Mild (later) | Severe (early) |
| PPN cholinergic loss | Variable | Severe |
| Autonomic nuclei | Severe | Moderate |
| Raphe/Locus coeruleus | Moderate | Moderate-severe |
Litvan I, Mangone CA, McKee A, et al. 'Natural history of progressive supranuclear palsy (Steele-Richardson-Olszewski syndrome) and clinical predictors of survival: a clinicopathological study'. J Neurol Neurosurg Psychiatry. 1996. ↩︎
Hardman CD, Halliday GM, McRitchie DA, Morris JG. The subthalamic nucleus in Parkinson's disease and progressive supranuclear palsy. J Neuropathol Exp Neurol. 1997. ↩︎ ↩︎
Bhattacharya K, Saeedi MJ, Sweadner K, et al. Tau pathology in the pedunculopontine nucleus correlates with gait and oculomotor deficits in progressive supranuclear palsy. Acta Neuropathol Commun. 2023. ↩︎
Pahapill PA, Lozano AM. 'The pedunculopontine nucleus and movement disorders: anatomy and the role of deep brain stimulation'. Parkinsonism Relat Disord. 2006. ↩︎
Karachi C, Houeto JL, et al. Involvement of the pedunculopontine nucleus in progressive supranuclear palsy. J Neurol Neurosurg Psychiatry. 2010. ↩︎
Gilman S, Koeppe RA, Chervin RD, et al. Pedunculopontine nucleus damage in progressive supranuclear palsy. Ann Neurol. 2010. ↩︎