Vol.97 No.3 contentsPRACTICA OTO-RHINO-LARYNGOLOGICA
Vol. 97 No. 3 March 2004
Age-Related Morphological Changes of the Neuromuscular
Junction of the Hypopharyngeal Constrictor Muscles in Rats;
A Scanning Electron Microscopic Study
Aki Taguchi
(Ehime University)
Age-related functional deterioration of deglutition is a serious problem in the elderly population since it increases the risk of aspiration pneumonia. In the present study, aging effects in the subneural apparatus (SNA) were histologically investigated in the inferior pharyngeal muscle of a rat using scanning electron microscopy. In young-adult group, the primary synaptic clefts (PSCs) of the SNAs in the thyropharyngeal muscle (TP) were predominantly labyrinthine type, whereas, those in the cricopharyngeal muscle (CP) were mostly depression type. The ratio of length to width (L/W ratio) of the secondary synaptic clefts (SSCs) and the ratio of areas of SSCs to those of PSCs (ASC/APC ratio) were larger in the TP than in the CP.
In the TP, the labyrinthine type of PSC decreased in percentage with age, while the depression type increased instead. However, the CP showed no significant change of PSC typing. The L/W ratio and ASC/APC ratio of the SSC decreased in the TP with age, and those in the CP increased. In the aged group, thin muscle fibers which contained immature SNA were occasionally identified in both muscles. These fibers were considered to be undergoing a regenerating process. Myosin ATPase staining showed age-related decrease of type II muscle fiber and increase of type I fiber in the TP. Similar findings were noted in the CP, although the change was not as great as in the TP.
These age-related changes of the TP might suggest weakening and slowing of the muscle contraction, and therefore cause a disturbance of food transport into the esophagus during deglutition. In contrast, the CP is less affected by aging. Disturbed function of the TP together with maintained tonic contraction of the CP may work disadvantageously for deglutition, thereby resulting in increased risk of aspiration.
Key words : hypopharyngeal constrictor muscles, rat, scanning electron microscopy, aging, subneural apparatus