The nervous system ofLoligoIV. The peduncle and olfactory lobes

Abstract

The paired peduncle lobes lie at the sides of the brain behind the optic tracts, partly embedded in the optic lobes. Each lobe comprises two regions: the basal zone, with medium to large cell bodies and fibres, and the spine, with small cells and fibres. The spine is remarkable for the presence of two banks of numerous, fine parallel fibres. Some of these are the processes of cells intrinsic to the spine but others derive from cells in the ipsilateral optic lobe and statocyst and from the contralateral peduncle lobe. The array of longitudinally running fibres, with efferents passing across them at right angles, gives the lobe a cerebellum-like appearance. The peduncle lobe receives a visual input from second and third order cells in the optic lobe, and a ‘labyrinthine’ input from cells in the statocyst. The opticopeduncular fibres preserve precise topological relations. The lobe projects widely to motor areas, including the basal lobes and the oculomotor centre in the lateral pedal lobe. It also sends fibres back to the ipsilateral optic lobe. There is a conspicuous peduncle commissure. The organization of the peduncle lobes is similar to that in the three main basal lobes, and must be considered part of the basal lobe system. The functional interpretation of the lobe is that it provides a region where visual and ‘labyrinthine’ information can interact to regulate motor programs controlled by the optic lobe. This involves fine adjustments particularly of the mobile eyes but also of the fins, funnel, head and arms, so that the animal can track visually and smoothly follow moving targets. Such regulation could be achieved by precise timing in a feed forward situation and this may be the function of the parallel fibre system. The olfactory lobe lies close to the peduncle lobe but is distinct in its connections and cyto-architecture. Its input derives from the olfactory organ and the optic lobe and its main projection is to an area of the posterior basal region that lies close to the optic gland. The cells are large and some are almost certainly secretory. The neuropil lacks any obvious spatial regularities. The nature of the olfactory system remains enigmatic but it may be involved in reproduction.