Evidence of self-sustained motoneuron firing in young and older adults.
Motoneurons demonstrate a type of self-sustained firing behavior that seems to be produced by a prolonged period of depolarization caused by intrinsic long-term changes in the motoneuron. Such self-sustained firing behavior has previously been reported in human motor units. The purpose of the present study was to investigate the occurrence of self-sustained firing behavior in older adults. Eight young (mean age 24 yrs) and eight older (mean age 73 yrs) individuals participated in the investigation. While subjects produced light dorsiflexion contractions, a brief vibration stimulus was applied to the tibialis anterior muscle. Motor unit recordings were also obtained from the tibialis anterior muscle. Self-sustained firing behavior was evidenced by the appearance of new motor unit recruitment following vibration, even as the motor units that fired before the vibratory stimulus maintained a steady firing rate. The proportion of motor units exhibiting self-sustained firing activity was similar in both young and older adults (approx. 23% of trials). We conclude that self-sustained firing behavior is a ubiquitous phenomenon that does not seem to be affected by the aging process.
Kamen
Gary
G
Department of Exercise Science, Totman 160A, University of Massachusetts - Amherst, 01003, USA. Kamen@excsci.umass.edu
Sullivan
Ryan
R
Rubinstein
Scott
S
Christie
Anita
A
D000200:Action Potentials; D000328:Adult; D000368:Aged; D000375:Aging / Q000502:physiology; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction; D007866:Leg; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010812:Physical Stimulation; D011999:Recruitment, Neurophysiological; D014732:Vibration