EMG-responses to sudden onset free fall.
Recordings of axial and limb muscles in reaction to a free fall induced startle were performed in subjects while they were lying on a tilting couch with their eyes closed. Young normals (n = 24, aged 31.1 +/- 6.6 years) showed an activation sequence consisting of sternomastoid (N.XI: 57 ms), abdominal muscles (T10: 65 ms), quadriceps (L3: 75 ms) and deltoid (C5: 78 ms) and tibialis anterior (L4: 80 ms). The sequence of activation is not compatible with the current hypothesis of the startle being produced by a single volley spreading rostrally and caudally from the lower brainstem. Instead it is suggested that the startle is a patterned response organized by a putative reticular generator capable of spatio-temporal sequencing. Two avestibular patients had responses at mildly delayed latencies, showing that these can be elicited by non-vestibular inputs. Similar testing of 11 subjects aged 70-80 years showed a latency delay of ca. 26% in the EMG response but a similar activation sequence. The amount of delay in the elderly can only partially be attributed to age-dependent motor conduction slowing and suggests a prolongation of central processing time. In patients with advanced stages of akinetic-rigid syndromes abnormalities were seen in cases with an involvement of the brainstem reticular formation.
Bisdorff
A R
AR
MRC Human Movement and Balance Unit, National Hospital for Neurology and Neurosurgery, London, UK.
Bronstein
A M
AM
Gresty
M A
MA
Wolsley
C J
CJ
Davies
A
A
Young
A
A
D000058:Accidental Falls; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001143:Arousal / Q000502:physiology; D001933:Brain Stem / Q000503:physiopathology; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation*; D004856:Postural Balance / Q000502:physiology*; D012016:Reference Values; D013216:Reflex, Startle / Q000502:physiology*; D012154:Reticular Formation / Q000503:physiopathology; D012683:Sensory Deprivation / Q000502:physiology; D018667:Tilt-Table Test; D015837:Vestibular Diseases / Q000175:diagnosis / Q000503:physiopathology*; D014725:Vestibular Nerve / Q000503:physiopathology