The peduncle lobe: a visuo-motor centre in octopus

Abstract

The peduncle lobe, the olfactory lobe and the optic gland constitute the peduncle complex. The two former structures are nervous, with a central neuropil surrounded by a cell layer, but the latter is an endocrine organ. The peduncle lobe contains two regions, the spine and the basal zone. In the spine the cells are ‘small’ (less than 5 µ m), and the neuropil comprises a parallel array of axons, oriented with their long axes horizontal and parallel to the antero-posterior axis of the body. The axons form at least three efferent tracts, which leave the lobe via the basal zone, and run to higher motor centres in the central brain. In the basal zone the majority of cells are ‘large’ (greater than 5 µ m) but about one third are ‘small’. The large cells send axons to higher motor centres in the central brain. The small cells in the basal zone project into the peduncle spine. The basal zone receives afferent fibres from the ipsilateral optic lobe, the contralateral peduncle lobe and the ipsilateral basal lobes and is presumed to provide an area for interaction between information from these areas. The organization of the neuropil is less obvious here than it is in the spine, but it is possible to discernoptico-pedunculo afferents, redistributing in a regular way in the horizontal plane. Both regions of the peduncle lobe project to motor areas in the central brain. The olfactory lobe, however, does not project to motor areas. It is composed of three distinct lobules: the posterior two lobules receive the olfactory nerve from the olfactory organ. The lobe is connected with the contralateral olfactory lobe and the ipsilateral dorsal basal lobe, a ‘silent’ area. Its neuropil is confluent with that of the peduncle lobe and they are assumed to influence one another closely. Stimulation of the nervous parts of the peduncle complex produces—in acute preparations —motor responses including colour changes, arm movements, and movements of the whole animal. After surgical removal of the nervous parts of both complexes there is locomotor dysfunction. Removal of the olfactory organs alone has no apparent effect on the behaviour of blind Octopus or Sepia and these organs are no more sensitive to the chemical stimuli used than other parts of the body. Anatomical, physiological and behavioural evidence suggest that the nervous part of the peduncle complex, probably the peduncle lobe alone, is part of a motor-control system. Evidence is presented that it regulates motor activity on the basis of visual information derived from the optic lobe.