Changes in rat plantaris motor unit profiles with advanced age

Area of Science:

• Physiology
• Aging Research
• Motor Control

Background:

• Muscle force generation declines with age.
• Motor unit alterations are implicated in age-related muscle weakness.

Purpose of the Study:

• To investigate the physiological changes in single motor units in rat plantaris muscles during aging.
• To compare motor unit characteristics between young and very old rats.

Main Methods:

• In situ determination of physiological characteristics of single motor units.
• Classification of motor units based on fatigue resistance and 'sag' during unfused tetani.
• Estimation of motor unit numbers and force production (P0).

Main Results:

• Old muscles exhibited 43% less tetanic force per gram compared to young muscles.
• Slow motor units in old rats had 255% greater mean P0 than in young rats; fast motor units were similar.
• Motor unit numbers decreased in old rats (29 vs. 48), with a higher proportion of slow motor units (11 vs. 5-6).

Conclusions:

• Aging leads to a loss of muscle tetanic force and motor units, with a reorganization of the remaining motor unit profile.
• Increased slow motor unit number and size occur with age, independent of histochemical fiber type changes.
• Motor units maintain their physiological distinctiveness into very old age and do not dedifferentiate.