Nervous Systems 815 



Prosser^^^ recorded action potentials from various segmental nerves and 

 from the central nerve cord of the crayfish. By stimulating different append- 

 ages and making appropriate cuts, he obtained a functional map of the nerve 

 cord. The available connections are multiple; which ones are used in a given 

 behavior pattern must depend on the strength of stimulus and on the ease 

 of passage through a particular reflex arc. 



Locomotor rhythms in insects and Crustacea, as in annelids, are reflex in 

 nature, and a variety of patterns of locomotion may be used, depending on 

 external requirements. 



Molluscs. In gastropod and pelecypod molluscs, locomotor reflexes are 

 mediated by three pairs of ganglia— cerebral, pedal, and visceral. 



Pavlov in 1885 experimented with the mussel Anodonta. Spontaneous ac- 

 tivity of the foot stopped when the visceral ganglion was removed, but tonus 

 could still be maintained in the adductor muscles after they were denerv- 

 ated. The posterior adductor muscle received motor (exciting) impulses from 

 the visceral ganglion, the anterior adductor from the cerebral ganglion; both 

 adductors recei\'ed inhibitory fibers from the cerebral ganglion. In the razor- 

 shell clam Ensis^^^ local stimulation of the foot failed to elicit general re- 

 sponses if the sensorv connections to the cerebral ganglia were eliminated. 

 Crossed responses go from one cerebral via the visceral back to the other 

 cerebral ganglion."^ In Mytilus'^^^ all of the ganglia act somewhat independ- 

 ently; e.g., if the cerebral ganglion is removed the foot still spins and creeps, 

 by pedal control. If the visceral ganglion is removed, opening and closing 

 reactions to changes in freshness of water are lost. All of these observations 

 argue for fairly restricted control of local areas by specific ganglia in pelecy- 

 pods. 



In gastropods the pedal ganglion gives off a series of nerves which appear 

 to discharge successively to elicit peristalsis along the foot."*^ When the pedal 

 ganglion of Aplysia was removed'---' the foot showed more resistance to stretch 

 (tonus) and contractions occurred, particularly at the foot margins. When 

 the cerebral ganglion was removed there was increased locomotor activity 

 and increased excitability of the "wings" (foot). It has been suggested 

 therefore, that the cerebral ganglion inhibits locomotor functions of the 

 pedal ganglion and that the pedal ganglion inhibits tonic mechanisms in the 

 foot. Each nerve from the "wings" contains sensory fibers which reflexly 

 through the pedal ganglion elicit contractions in certain areas of the oppo- 

 site side of the foot. The cerebral ganglion is not necessary for this response. 

 If the interpedal commissure is cut, only the homolateral responses persist. 

 In HeliXy foot potentials indicate that the cerebral ganglion inhibits motor 

 cells of the pedal ganglion. ^'-"^ 



Ganglionic reflexes control the muscles and chromatophores of the mantle 

 and fin of cephalopods. The mande nerve goes from the brain mass to the 

 mantle or stellate ganghon and from this the stellar nerves pass to the man- 

 tle. The mantle ganglia maintain tonus of the muscles and chromatophores 

 of the mantle.^^- Stimulation of the mantle nerve at low intensity inhibits 

 the stellate ganglion, at higher intensities excites it. Ganglionic delay of 10 

 msec, has been reported and summation indicated. Fatigue of a stellar nerve 

 did not affect the mantle nerve. When the mantle nerve was cut, a weak 

 reflex response could be elicited through the stellate ganglion alone. Reflex 



