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EMG-responses to sudden onset free fall
A R Bisdorff1, A M Bronstein, M A Gresty
1MRC Human Movement and Balance Unit, National Hospital for Neurology and Neurosurgery, London, UK.
Acta Oto-Laryngologica. Supplementum
|January 1, 1995
Summary
Startle responses in muscles follow a specific sequence, challenging single-volley theories. This patterned response suggests a reticular generator, with age and brainstem conditions affecting timing.
Area of Science:
- Neuroscience
- Human Physiology
- Motor Control
Background:
- The startle reflex is a rapid, involuntary response to a sudden stimulus.
- Current hypotheses suggest startle responses spread from a single neural volley.
- Understanding the neural mechanisms of the startle reflex is crucial for diagnosing neurological disorders.
Purpose of the Study:
- To investigate the muscle activation sequence during a free-fall induced startle response.
- To challenge the single-volley hypothesis of the startle reflex.
- To explore the influence of age and vestibular/brainstem integrity on the startle response.
Main Methods:
- Electromyography (EMG) recordings of axial and limb muscles.
- Induction of startle response via free fall while subjects lay on a tilting couch with eyes closed.
- Testing in young healthy adults, elderly individuals, and patients with vestibular or akinetic-rigid syndromes.
Main Results:
- Young adults exhibited a consistent muscle activation sequence: sternomastoid, abdominal, quadriceps, deltoid, and tibialis anterior.
- This sequence contradicts the single-volley hypothesis, suggesting a patterned, spatio-temporal organization.
- Elderly subjects showed a 26% delayed EMG response, indicating prolonged central processing time beyond motor conduction slowing.
- Avestibular patients had mildly delayed responses, demonstrating non-vestibular input capability.
- Patients with akinetic-rigid syndromes showed abnormalities linked to brainstem reticular formation involvement.
Conclusions:
- The startle reflex is likely organized by a patterned response from a reticular generator, not a single spreading volley.
- Aging significantly prolongs central processing time for the startle reflex.
- Brainstem reticular formation integrity is critical for normal startle response sequencing and timing.