Age changes in neuromuscular junction morphology and acetylcholine receptor distribution on rat skeletal muscle fibres.

1. Neuromuscular junctions in the sternomastoid muscles of female Lewis rats were examined in animals up to 917 d old. 2. The average number of myelinated branches of terminal axons entering a junction increased with age of the animal, up to 400 d. This change could be described by a simple kinetic model which assumed that there was no influence of age on the ability of motoneurones to produce or maintain terminal branches, but that axons could produce or maintain only a limited number of branches. 3. There was a change in the over-all junctional length with age, but there was a significant increase in the number of discrete regions of high ACh receptor density in junctions from older animals. 4. There was a gradual decrease in the number of ACh receptors per junction with age after about 500 d, and muscles from some rats older than 500 d had detectable numbers of extrajunctional ACh receptors. 5. The changes in the neuromuscular junction with increased age occurred gradually over adult life.