The responses to topical application of neurotransmitters to the mantle and fin muscles of the spear squid, Loligo bleekeri, were examined. In the mantle, the circular fibres contract in response to L-glutamate and the radial and longitudinal fibres contract in response to acetylcholine. 5-hydroxytryptamine (5-HT) did not affect contractions of any of the mantle muscle fibres. The structure of the fin is similar to that of the mantle, with muscles arranged in three orthogonal planes. Topically applied L-glutamate causes all three muscle types to contract. Acetylcholine does not affect them. Pre-treatment with 5-HT blocks the L-glutamate response of the transverse and dorso-ventral muscles but has no effect on the longitudinal fibres. These results suggest that a secondary innervation pathway exists in musculature responsible for producing complex movements, such as the fin, but not in those with a simpler mode of action, like the mantle.
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