To elucidate innervation in the upper esophageal sphincter (UES) muscle of the 
eel, a key muscle in swallowing, repetitive electrical field stimulation (EFS; 
30 mA, 40 V, 300 micros, 10 Hz, 10 trains) was employed. Anatomically, the eel 
UES muscle consists of striated fibers. The EFS-induced contraction of the UES 
was completely blocked by tetrodotoxin and curare, and abolished in Ca2+ -free 
Ringer solution. These results suggest that the EFS stimulates nerve fibers 
specifically and releases acetylcholine as a neurotransmitter. In fact, 
acetylcholine and carbachol constricted the UES in a concentration-dependent 
manner. Even after blocking neuronal firing with tetrodotoxin, acetylcholine 
constricted the UES muscle, suggesting the existence of acetylcholine receptors 
on the UES muscle cells. Both EFS- and carbachol-evoked contractions of the UES 
were blocked by curare at a lower concentration than by atropine or 
hexamethonium, suggesting that the acetylcholine receptor is nicotinic. Even in 
Ca2+ -free Ringer solution, a direct current stimulus (2 s duration) constricted 
the UES muscle to an extent similar to that in the presence of Ca2+, indicating 
that the muscle contraction itself does not need extracellular Ca2+, i.e., the 
muscle can be constricted by a release of Ca2+ from the sarcoplasmic reticulum.