The control of hindgut motility in the lobster,Homarus gammarus(L.)

The sixth abdominal ganglion (6 A.G.) of the lobster Homarus gammarus (L.) innervates the rectum via the paired posterior intestinal nerves (P.I.N.’s) and the paired anal nerves. The anterior branches of the P.I.N.’s supply the anterior hindgut, the main faecal expulsion region, whilst the posterior branches (P.I.N.p.’s) supply the posterior region and the 5 extrinsic radial muscle groups around the anus. 2. Stimulation of the ventral nerve cord (V.N.C.) or the oesophageal connectives initiates co‐ordinated hindgut movements, the defaecatory response. The nervous activity eliciting these movements passes down the P.I.N.’s. The anal nerves are devoid of motor function with respect to the hindgut. 3. In addition to neurogenic movements the rectum also undergoes non‐coordinated, low amplitude longitudinal and circular muscle contractions. These are thought to be due to independently acting endogenous oscillators within the muscles themselves. The radial muscles of the anus also exhibit rhythmical contractions after an initial maximal contraction following stimulation of the P.I.N.p.’s. 4. Receptors responding to anal dilation and closure have been shown to exist in the anal nerves. They are non‐specific soft cuticle receptors which are, apparently, positionally sensitive. These receptors are not thought to modulate motor output from the 6 A.G. to the rectum at the level of the 6 A.G. 5. Bifurcating motor axons are thought to exist in the P.I.N.’s. 6. It is concluded that the defaecatory response of the lobster is a centrally programmed phenomenon.