Developmental changes in innervation of rat extensor digitorum longus muscle

Area of Science:

• Neuroscience
• Developmental Biology
• Motor System Physiology

Background:

• Understanding the developmental trajectory of motor neuron innervation is crucial for comprehending neuromuscular system maturation.
• The extensor digitorum longus (EDL) muscle serves as a model to investigate age-related changes in motor unit properties.

Purpose of the Study:

• To investigate the developmental changes in motor neuron number, size, and functional innervation of the rat EDL muscle.
• To determine the contribution of different ventral roots to EDL muscle innervation during postnatal development.

Main Methods:

• Horseradish peroxidase (HRP)-retrograde labeling was used to identify motor neurons innervating the EDL muscle.
• In vivo muscle tension recordings were performed to assess functional innervation and motor unit properties.
• Quantitative analysis of motor neuron counts and fiber numbers in ventral roots (L3-L5) was conducted.

Main Results:

• EDL muscle nucleus size increased with age, but motor neuron number and size remained unchanged.
• Twitch and tetanus tension increased proportionally with muscle mass, indicating functional maturation.
• The number of motor fibers innervating the EDL muscle significantly increased from 0.5 to 3.5 months of age, with the most rapid changes occurring between 0.5 and 1.5 months.

Conclusions:

• The rat EDL muscle exhibits incomplete functional innervation at 0.5 months, which improves with postnatal development.
• Motor neuron innervation of the EDL muscle primarily originates from the L3, L4, and L5 ventral roots.
• While motor neuron cell bodies do not change, the number of motor fibers innervating the muscle increases substantially during development.