In order to clarify relationship between muscle typing and mode of innervation of the intrinsic laryngeal muscles, following three morphological studies were made on the posterior cricoarytenoid muscle of dog; 1) analysis of muscle fiber composition by means of histochemical techniques, 2) light microscopic observation on the terminal endings of motor neuron stained by silver inpregnation method, 3) scanning electron microscopic observation on the subneural apparatus. Three types of muscle fiber, type 1, 2A and 2C were differentiated by histochemical stainings; myosin ATPase, succinic dehydrogenase and reduced nicotinamide dinucleotide dehydrogenase staining. The muscle consisted of 45. 7% of type 1, 51. 3% of type 2A and 3. 0% of type 2C.
All muscle fibers were focally innervated with the plate-like (en plaque) end plates. No multifocal innervation was found. There were three types of subneural apparatus namely A, B and C types, which were characterized by the following findings.
A type: The area of subsynaptic apparatus (ASA) was the smallest. Primary synaptic clefts (PSC) formed grooves with sparse, pit-like openings of the secondary synaptic clefts (SSC).
B type: The ASA was larger than that of A type. PSC formed grooves with dense, slit-like and pit-like openings of SSC.
C type: The ASA was the largest and muscle fiber the finest. PSC formed a large number of small oval depressions and SSC were much sparse.
The histochemical study and morphometrical analysis indicated that subneural apparatus of the 1, 2A and 2C muscle fiber corresponded to A, B and C type respectively.
