The nervous system of Loligo , III. Higher motor centres: the basal supraoesophageal lobes

Abstract

The three main basal lobes are orientated in different planes set approximately at right angles. The components of each are similar and based on an organization reminiscent of a cerebellum, like that of the peduncle lobes. They all have large cells and fibres ventrally and numerous small cells dorsally. Each of the two parts of the anterior basal lobe contains a region with numerous very fine parallel fibres, similar to the ‘spine’ of the peduncle lobes. The dorsal basal lobes contain a similar system, but less regular. The three main parts of the basal lobe system all send fibres to the oculomotor centres of the lateral pedal lobes. The two parts of the anterior basal lobe also send fibres to the centres controlling arm movements in the anterior pedal lobe. The median basal lobe sends large tracts to the posterior pedal lobe, controlling movement of the funnel and fins. It also sends a large tract to the region of the first order giant cell, initiating the jet. There is a further massive system of descending fibres from all the basal lobes (and the precommissural lobe) sending branches to all parts of the magnocellular and palliovisceral lobes. The functional organization of the basal and peduncle lobes can be understood as follows. They all receive visual and static inputs and send large outputs to the oculomotor centre and back to the optic lobes. The oculomotor centre also receives direct inputs from the statocyst. The control of eye movements is thus organized in cephalopods in the same way as it is in vertebrates: there is a direct static input to the oculomotor centre, which also receives indirect static influences combined with visual ones, by way of the cerebellum in vertebrates, or basal and peduncle lobes in cephalopods. The basal and peduncle lobes show further similarity to the cerebellum in the presence of the numerous parallel fibres of various diameters, some very fine. These lobes receive dorsal and ventral sets of visual fibres and a set from the magnocellular lobes. The dorsal visual fibres follow the topology of the optic lobes and it is suggested that they provide a tracking system. The numerous fibres passing back from the basal lobes to the optic lobes, allow for a ‘corollary discharge’ (‘efference copy’). The ventral set of large fibres from the optic lobes to the basal lobes may serve to promote the final attack, after the smaller fibres have produced the preliminary tracking movements. The system of large fibres from the magnocellular to the basal lobes are perhaps concerned with avoiding reactions. Each of the large fibres of both of these sets gives branches to all parts of the basal lobes. All the basal lobes contain many microneurons with trunks limited to the lobes. Some of these are very short amacrines. The median basal and dorsal basal lobes contain especially numerous small cells, perhaps neurosecretory and related to reproduction by way of the optic gland, whose nerve arises nearby.