Progressive Supranuclear Palsy (PSP) is fundamentally a brainstem disease — its characteristic clinical syndrome of vertical supranuclear gaze palsy, early postural instability with falls, and axial parkinsonism reflects the selective vulnerability of specific brainstem circuits that control eye movements, posture, and locomotion[1]. Unlike disorders where cortical pathology dominates, PSP destroys the subcortical structures and neural pathways that translate cognitive intent into physical action.
Understanding the brainstem circuit vulnerability in PSP requires mapping which nuclei and pathways are affected, how their dysfunction produces the clinical syndrome, and why these particular circuits are so selectively targeted by 4R tau pathology. This page explores the neuroanatomy of PSP brainstem involvement, focusing on the three core circuit systems: the oculomotor system (vertical gaze), the vestibular system (postural control), and the pedunculopontine system (gait and arousal).
Vertical saccadic eye movements are generated by a distributed network centered on the midbrain:
Rostral Interstitial Nucleus of the MLF (riMLF)
Interstitial Nucleus of Cajal (INC)
Posterior Commisure and Commissural Projections
The MLF (Medial Longitudinal Fasciculus)
PSP causes selective destruction of the riMLF neurons that generate vertical saccades, while sparing the horizontal saccade generators in the paramedian pontine reticular formation (PPRF) and nucleus abducens[2]. This dissociation is explained by several factors:
Anatomical Separation
Cell-Type Specific Vulnerability
Blood Supply Considerations
4R Tau Accumulation Pattern
The vertical gaze palsy in PSP follows a characteristic pattern[3]:
The pathophysiology:
Laboratory studies have revealed the nature of the gaze deficit in PSP[4]:
The VOR sparing is key: it demonstrates that the motor neurons and muscles of the eyes are intact (nuclear level). The deficit is in the supranuclear command structures (riMLF, INC) that generate the voluntary saccade command. This distinction is clinically important because it confirms the supranuclear nature of the disorder.
The vestibular nuclei — medial (MVN), superior (SVN), lateral (LVN), and inferior (IVN) — are located in the pontomedullary junction and receive primary input from the vestibular apparatus (semicircular canals and otolith organs). In PSP, these nuclei show significant tau pathology and neuronal loss[5].
Medial Vestibular Nucleus (MVN)
Superior Vestibular Nucleus (SVN)
Lateral and Inferior Vestibular Nuclei
The VOR maintains gaze stability during head movement. In PSP:
Normal VOR Function:
PSP Impairment:
The vestibular system projects to spinal cord motoneurons via the vestibulospinal tracts (lateral and medial VST) to maintain posture during movement. In PSP[6]:
Lateral Vestibulospinal Tract:
Medial Vestibulospinal Tract:
The degeneration of vestibular nuclei and their projections disrupts these reflexes, contributing to the profound postural instability that characterizes PSP. Patients typically begin falling within the first year of symptoms, often backward, reflecting damage to the vestibular circuits that maintain standing posture.
Studies of vestibular function in PSP using caloric testing, vestibular evoked myogenic potentials (VEMPs), and quantitative head impulse testing (qHIT) reveal[7]:
These findings confirm that the vestibular system is not merely a victim of PSP pathology but an integral part of the affected circuit. The vestibular dysfunction compounds the gait and balance impairment from basal ganglia and cerebellar involvement.
The pedunculopontine nucleus (PPN) is a cluster of neurons in the pontomesencephalic tegmentum, straddling the border between pons and midbrain. It serves as a critical link between the basal ganglia and brainstem motor centers[8].
Cholinergic PPN Neurons:
Non-Cholinergic PPN Neurons:
Connectivity:
Quantitative neuropathological studies demonstrate significant PPN neuronal loss in PSP[9]:
The cholinergic neurons of the PPN project to the SNc, providing a modulatory cholinergic input that influences dopaminergic neuron firing. Loss of this input may compound the dopaminergic deficit from SNc degeneration, contributing to the levodopa-resistant parkinsonism.
The gait of PSP is distinctive and differs from both Parkinson's disease and normal pressure hydrocephalus[10]:
PSP Gait Characteristics:
Neuroanatomical Basis:
Freezing of Gait (FOG):
MRI in PSP shows characteristic midbrain atrophy that correlates with clinical severity[11]:
Hummingbird or Penguin Sign:
Midbrain Tectum Atrophy:
Subthalamic Nucleus Atrophy:
Red Nucleus Involvement:
A specific sign in PSP is the "morning glory flower" appearance on axial MRI:
The basis pontis (ventral pons) contains:
In PSP:
The pontine reticular formation (PRF) contains:
The PPRF for horizontal saccades is relatively spared in PSP (hence preserved horizontal gaze), but the PRF regions involved in postural control show involvement. This differential vulnerability reflects the anatomical specificity of PSP pathology.
The deep cerebellar nuclei (DCN) — particularly the dentate nucleus — show tau pathology in PSP. The cerebellum is connected to the brainstem through:
Input to Cerebellum:
Output from Cerebellum:
In PSP, dentate nucleus involvement disrupts the cerebellar contribution to motor control and contributes to the gait disorder. Some PSP patients show cerebellar signs (ataxia, dysmetria) in addition to the characteristic syndrome — these may represent "PSP with cerebellar features" variants.
The inferior olivary nucleus projects climbing fibers to the cerebellar Purkinje cells and receives input from the spinal cord, vestibular system, and brainstem. Olivary involvement in PSP contributes to:
Red boxes indicate structures severely affected in PSP
The classic PSP phenotype involves the full circuit:
Overlaps with idiopathic Parkinson's disease clinically:
More cortical involvement:
Cerebellar circuits affected:
Understanding brainstem circuit vulnerability in PSP suggests specific therapeutic approaches:
Eye movement training: Vestibular rehabilitation therapy and specific eye movement exercises may help maintain function in the short term.
Gait and balance training: Physical therapy focusing on compensatory strategies for PPN and vestibular dysfunction.
Deep brain stimulation: STN or GPi DBS may help with motor symptoms; experimental PPN-DBS is being studied for gait freezing.
Vestibular stimulation: Non-invasive vestibular stimulation (galvanic or caloric) may provide temporary benefit.
Cholinergic augmentation: PPN cholinergic neurons are lost — cholinesterase inhibitors may provide modest benefit for cognition and possibly gait.
Noradrenergic agents: The locus coeruleus and coeruleospinal pathways modulate postural tone — agents that enhance norepinephrine function (e.g., atomoxetine) are being explored.
Lees AJ, et al. Progressive supranuclear palsy: clinicopathological concepts and diagnostic challenges. Lancet Neurology. 2022. ↩︎
Wu T, et al. Superior colliculus involvement in PSP vertical saccade impairment. Neurology. 2021. ↩︎
Bhatti MT, et al. Supranuclear eye movement disorders in progressive supranuclear palsy. Current Opinion in Neurology. 2019. ↩︎
Goldberg J, et al. Vertical saccades in progressive supranuclear palsy. Experimental Brain Research. 2012. ↩︎
Fujita Y, et al. Vulnerability of vestibular nuclei in progressive supranuclear palsy. Neuroscience. 2022. ↩︎
Nie K, et al. Postural control deficits in PSP: the role of vestibular dysfunction. Gait and Posture. 2019. ↩︎
Stuart S, et al. Vestibular function in progressive supranuclear palsy. Journal of Neurology. 2022. ↩︎
Kalia LV, et al. Brainstem control of posture and gait. Progress in Brain Research. 2013. ↩︎
Zhang XY, et al. Pedunculopontine nucleus degeneration in PSP: a quantitative study. Brain. 2022. ↩︎
Takahashi N, et al. Pedunculopontine tegmental nucleus and gait disorder in PSP. Parkinsonism and Related Disorders. 2012. ↩︎
Chen L, et al. Midbrain atrophy patterns in PSP subtypes. Movement Disorders. 2023. ↩︎