NEUROMOTOR BASES OF THE ESCAPE BEHAVIOUR OF NASSA MUTABILIS

The complex sequence of movements in the escape behaviour of the snail Nassa mutabilis (L.) was described in detail and the neuromotor activity underlying the behaviour was investigated by extra- and intracellular recording. The escape reaction is triggered by a chemical stimulus to the animal's foot, in these experiments either application of KCl solution or contact with a starfish. It consists of a preliminary phase in which the shell tilts to its side, the actual locomotor phase, and a final righting movement. The snail performs leaps, in which the foot and the shell are repeatedly rotated with respect to one another. EMGs recorded from the columellar muscle during the escape reaction showed that bursts of potentials are coupled to the shell rotations. In the intact animal this burst activity ordinarily began 0.6 ± 0.3 s after stimulation with KCl. In an animal dissected for recording from the columellar nerve (which supplies the columellar muscle), KCl stimulation of the dorsum of the foot induced burstlike neuronal activity with a latency of 0.5 ± 0.3 s. The dorsal foot region, the site at which the escape reaction can be triggered, was found to be supplied by the posterior pedal nerves; electrical stimulation of these nerves elicited bursts in the columellar nerve. The left pleural ganglion, which is known to contain neurones that project into the columellar nerve, was also found to contain neurones responsive to KCl stimulation of the foot. These findings suggest that the left pleural ganglion contains a motor centre which is involved in control of activity of the columellar nerve, and is also active during the escape reaction.

The nervous system of loligo ii. suboesophageal centres

A well-marked hierarchy of centres can be recognized within the suboesophageal lobes and ganglia of the arms. The inputs and outputs of each lobe are described. There are sets of motoneurons and intermediate motor centres, which can be activated either from the periphery or from above. They mostly do not send fibres up to the optic or higher motor centres. However, there is a large set of fibres running from the magnocellular lobe to all the basal supraoesophageal lobes. The centre for control of the four eye-muscle nerves in the anterior lateral pedal lobe receives many fibres direct from the statocyst and from the peduncle and basal lobes, but none direct from the optic lobe. The posterior lateral pedal is a backward continuation of the oculomotor centre, containing large cells that may be concerned in initiating attacks by the tentacles. An intermediate motor centre in the posterior pedal lobe probably controls steering. It sends fibres to the funnel and head retractors, and by both direct and interrupted pathways to the fin lobe. It receives fibres from the crista nerve and basal lobes, but none direct from the optic lobe. The jet control centre of the ventral magnocellular lobe receives fibres from the statocyst and skin and also from the optic and basal lobes. Some of these last also give extensive branches throughout the palliovisceral lobes. The branching patterns of the dendritic collaterals differ in the various lobes. Some estimates are given of the numbers of synaptic points. The dendritic collaterals of the motoneurons spread through large volumes of neuropil and they overlap. The incoming fibres spread widely and each presumably activates many motoneurons either together or serially. Many of the lobes contain numerous microneurons with short trunks restricted to the lobe, but there are none of these cells in the chromatophore lobes or fin lobes. The microneurons have only few dendritic collaterals, in contrast to the numerous ones on the nearby motoneurons.

The functional organization of the brain of the cuttlefish Sepia officinalis

The functional organization of the brain of Sepia has been investigated by electrical stimulation. As a result several new divisions of the brain have been made. The pedal ganglion has been shown to consist of four parts: (1) the anterior chromatophore lobes innervating the skin and muscles of the anterior part of the head and arm s; (2) the anterior pedal lobe innervating the arms and tentacles; (3) the posterior pedal lobe innervating the funnel, collar and retractor muscles of the head; (4) the lateral pedal lobes innervating the muscles of the eyes and tissues of the orbits. The palliovisceral (or visceral) ganglion, apart from the magnocellular lobe demonstrated by Young (1939), is shown here to consist of (1) a central palliovisceral lobe innervating the mantle, funnel and viscera ; (2) a pair of lobes innervating the muscles of the fins; (3) a pair of posterior chromatophore lobes innervating the muscles of the chromatophores and skin of the mantle, fin and back of the head; (4) a pair of vasomotor lobes. Because of these new divisions the three main groupings of the suboesophageal neural tissue are now referred to as the anterior, middle and posterior suboesophageal masses corresponding to the old brachial, pedal and palliovisceral divisions. The suboesophageal centres are classified as lower motor centres and intermediate motor centres, depending on the kind of response they give to electrical stimulation and their peripheral connexions. In the supraoesophageal lobes, higher motor centres and silent areas are recognized. The silent areas include the vertical, superior frontal, subvertical, precommissural and dorsal basal lobes. Of the higher motor centres the anterior basal lobe is primarily concerned with the positioning of the head, arms and eyes, particularly during movements involving changes in direction while swimming. Such manoeuvres are brought about by the anterior basal lobe control over the fins and position of the funnel. The posterior basal lobe is here shown to consist of six main divisions: (1) the sub vertical lobe; (2) the dorsal basal lobes; (3) the precommissural lobe; (4) the medial basal lobe; (5) the lateral basal lobe; (6) the interbasal lobe. The medial, lateral and interbasal lobes are higher motor centres. The lateral and medial basal lobes control movements of the chromatophores and skin; the medial basal lobe controls swimming, breathing, fin movements and various visceral functions. The interbasal lobe controls the movements of the tentacles. The optic nerves and the optic lobes, at their periphery, are electrically inexcitable. Electrical stimulation of the centre of the optic lobes evokes all the responses that can be obtained from the other higher m otor centres. The results are discussed in term s of Sanders & Young’s (1940) physiological classification of the brain. A further category intermediate motor centre is recognized. Summary lists of the responses of each lobe are given on pages 516, 520, 525.

Nervous Control of Movement in Cephalopods

1. Nerve muscle preparations have been made of the mantle and stellar nerves of octopuses and squids. 2. Two motor innervation systems have been found in each. Both have been observed as unit preparations. The possibility of double innervation of the same muscle cells exists but has not been directly checked. 3. The fast innervations produce electrical responses which are maximal to the first stimulus and which have little or no absolute refractory period. They appear to be local rather than spike potentials. Fatigue is very rapid. The mechanical response sums in Octopus, but not in Loligo. 4. The slow innervations produce electrical and mechanical responses which facilitate with repetition. The fast system of Loligo does likewise after fatigue to a low level of response. 5. No evidence was found for a functional nerve net in the mantle. 6. Organizational features of the stellate ganglion have been identified physiologically in MOctopus. The ganglion acts both as an integrating motor centre and as a reflex centre.

Reflex fractionation of a muscle

Studying spinal reflexes Camis (1) (1910) from observations on M. semitendinosus (cat) reached the conclusions that “ the cells of a spinal motor centre ca be regarded from a functional point of view as divided into several independent groups,” but that “such independence is however not absolute.” The present experiments pursue a like inquiry. That in a reflex evoked by weak excitation of the afferent nerve the resulting contraction of the muscle may involve a portion only of the muscle has common acceptance. Camis’s observations, however, employed maximal stimuli and yet the muscle evidenced fractional responses ; whereas later (3) Dreyer and one of us found, contrary to previous (4) experience, that reflex tetani in some instances activated the sum-total of the muscle. Both of these observations are confirmed by the present experiments. Method . The sample muscles taken have been a hip-flexor, tensor fasciœ femoris , a knee flexor, semitendinosus , and an ankle flexor, tibialis anticus . The preparation (cat) has been spinal, with cord transection, performed under deep anæsthesia, in the anterior lumbar region, prior to intereollicular decerebration, the anæsthesia being later relaxed. All other muscles of the limb except that one attached to the myograph have been paralysed by resection or nervesection. The limb has been securely fixed by steel drills clamped to the table. For exciting reflex contraction various afferent nerves, as cited below, of the ipsilateral limb have been stimulated by faradisation. The inductorium has been coreless and its primary circuit fed by a current of less than 0·2 amp. An optically recording myograph with isometric registration and of the pattern described in a previous paper (2) has been used.