Foundations

Anatomy & Physiology

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0:00Postural control draws on three sensory streams: vision, vestibular input, and somatosensation from the feet and ankles.

0:09The central nervous system integrates these streams continuously, weighing each according to how reliable it appears in the current environment.

0:20When one input becomes unreliable — say, vision in the dark, or somatosensation on a moving surface — the system re-weights toward the remaining streams.

0:33Computerised dynamic posturography exploits exactly this re-weighting by selectively making each input unreliable, condition by condition.

0:47Two principal postural strategies handle small and large perturbations.

0:54The ankle strategy rotates the body around the ankle joints. It is used for small perturbations on a stable, wide surface.

1:06The hip strategy rotates the body around the hip joints. It is recruited for larger or faster perturbations, or when the support surface is narrow.

1:20Both strategies depend on long-loop reflexes traversing brainstem and spinal pathways, with response latencies of roughly 120 to 150 milliseconds.

1:36These long-loop latencies are what the Motor Control Test measures, and they are the principal CDP marker of central involvement.

1:52Adaptation — the down-weighting of inappropriate responses over repeated trials — depends critically on the cerebellum, and is the principal job of the Adaptation Test.

2:08With these three foundations — sensory integration, postural strategy, and central adaptation — every CDP finding can be placed in its physiological context.

In this module

  1. The three pillars: visual, vestibular, somatosensoryFoundation · Trainee · Clinician
  2. Vestibular labyrinth — relevant anatomyFoundation · Trainee · Clinician
  3. Central postural-control pathwaysTrainee · Clinician
  4. Long-loop vs short-loop reflexesClinician

The three pillars: visual, vestibular, somatosensory

Quiet standing depends on three sensory streams: vision, vestibular input from the inner ear, and somatosensation from receptors in the feet, ankles, joints, and muscles. The central nervous system integrates all three continuously, weighing each by how reliable it appears moment-to-moment.

When one input becomes unreliable — vision in the dark, somatosensation on a soft mattress, vestibular input after a unilateral lesion — the system re-weights toward the remaining streams. A healthy person can stand comfortably with any single input missing. Losing two simultaneously, or losing the only remaining functional input, is what produces falls.

CDP is built around exactly this re-weighting. By degrading each input in turn under controlled conditions, the test reveals which streams the patient actually relies on, and which are no longer carrying their share.

Vestibular labyrinth — relevant anatomy

Each labyrinth contains three semicircular canals — anterior, posterior, and horizontal — and two otolith organs, the utricle and saccule. The canals sense angular head acceleration in three roughly orthogonal planes; the otoliths sense linear acceleration and head tilt relative to gravity.

For postural control on a fixed surface, the otoliths matter more than the canals. Subtle drifts of the head relative to gravity — what happens when the body sways — are the principal vestibular signal driving the corrective responses CDP measures. Canal input becomes more important during head turns and walking.

The afferents from both labyrinths converge on the vestibular nuclei in the dorsolateral pons and medulla, where they meet input from the cerebellum, the visual system, and the somatosensory pathways. The output goes to the spinal cord (vestibulospinal tracts), to the oculomotor nuclei (vestibulo-ocular reflex), and back up to cortex.

Central postural-control pathways

The descending postural pathways most relevant to CDP are the lateral vestibulospinal tract, originating from the lateral vestibular nucleus (Deiters' nucleus) and projecting bilaterally to anti-gravity muscles down the length of the spinal cord, and the reticulospinal tract, providing a more diffuse modulation of axial and proximal muscle tone.

Cerebellar input modulates both. The flocculus and paraflocculus tune the vestibulo-ocular reflex; the anterior vermis and fastigial nucleus tune postural responses; the posterior vermis handles trunk control. Cerebellar lesions in any of these regions can produce abnormal CDP findings, though the patterns differ in detail.

Cortical input is more sparing but real. Frontal cortex contributes to anticipatory postural adjustments; parietal cortex to the integration of vestibular and somatosensory cues. Cortical lesions rarely produce isolated CDP abnormalities, but they can shape the pattern in patients with multifocal disease.

Long-loop vs short-loop reflexes

Short-loop reflexes — the monosynaptic stretch reflex and its polysynaptic siblings — operate at roughly 30–50 ms latency and are mediated entirely within the spinal cord. They are too fast and too local to account for the responses CDP measures.

Long-loop reflexes traverse brainstem and possibly cerebellar circuitry, and emerge at 100–200 ms after a perturbation. The MCT response, at 120–150 ms in healthy adults, is squarely in this range. The latency reflects the round-trip transit time through the long loop, not the strength of the response.

Prolongation of MCT latency therefore implicates the long loop specifically — meaning the brainstem, spinal cord, or the major descending tracts. Peripheral neuropathy can sometimes prolong latency by slowing the afferent or efferent limb, but more often it reduces amplitude (the response is weak rather than late). A latency over 170 ms in a patient with intact peripheral nerves is a substantial flag for central involvement.

afferentbrainstemvestibular & reticular nucleiefferentround-trip ≈ 120–150 ms · MCT latency

The long-loop postural reflex transits from peripheral somatosensory afferents through the spinal cord to the brainstem, where it is integrated with vestibular input, then descends through the vestibulospinal and reticulospinal tracts to drive corrective muscle activity. The MCT latency measures this round-trip time — typically 120–150 ms in healthy adults.