Control of swimming in the hydrozoan jellyfish Aequorea victoria: subumbrellar organization and local inhibition.

The subumbrella of the hydrozoan jellyfish Aequorea victoria (previously classified as Aequorea aequorea) is divided by numerous radial canals and attached gonads, so the subumbrellar musculature is partitioned into subumbrellar segments. The ectoderm of each segment includes two types of muscle: smooth muscle with a radial orientation, used for local (feeding and righting) and widespread (protective) radial responses, and striated muscle with a circular orientation which produces swim contractions. Two subumbrellar nerve nets were found, one of which stained with a commercial antibody produced against the bioactive peptide FMRFamide. Circular muscle cells produce a single, long-duration action potential with each swim, triggered by a single junctional potential. In addition, the circular cells are electrically coupled so full contractions require both electrotonic depolarization from adjacent cells and synaptic input from a subumbrellar nerve net. The radial cells, which form a layer superficial to the circular cells, are also activated by a subumbrellar nerve net, and produce short-duration action potentials. The radial muscle cells are electrically coupled to one another. No coupling exists between the two muscle layers. Spread of excitation between adjacent segments is decremental, and nerve net-activated junctional potentials disappear during local inhibition of swimming (such as with a radial response). Variable swim contractions are controlled by a combination of synaptic input from the motor network of the inner nerve ring, synaptic input from a subumbrellar nerve net, and electrotonic depolarization from adjacent, active muscle cells.