Notes: The lamprey normally swims with the dorsal side up. Illumination of one eye shifts the set-point of the vestibular roll control system, however, so that the animal swims with a roll tilt towards the source of light (the dorsal light response). A tilted orientation is often maintained for up to 1 min after the stimulation. In the present study, the basis for this behaviour was investigated at the neuronal level. The middle rhombencephalic reticular nucleus (MRRN) is considered a main nucleus for the control of roll orientation in lampreys. Practically all MRRN neurons receive vestibular and visual input and project to the spinal cord. Earlier extracellular experiments had shown that optic nerve stimulation potentiates the response to vestibular stimulation in the ipsilateral MRRN. This most likely represents a neural correlate of the dorsal light response. Experiments were carried out in vitro on the isolated brainstem of the silver lamprey (Ichthyomyzon unicuspis). MRRN cells were recorded intracellularly, and the overall activity of descending systems was monitored with bilateral extracellular electrodes. The responses to 10 Hz optic nerve stimulation and 1 Hz vestibular nerve stimulation, and the influence of optic nerve stimulation on the vestibular responses, were investigated. In most preparations, optic nerve stimulation excited practically all ipsilateral MRRN cells. After stimulation, the cell was typically depolarized and showed an increased level of synaptic noise for up to 80 s. In contralateral MRRN neurons, optic nerve stimulation usually evoked hyperpolarization or no response. Vestibular nerve stimulation evoked compound excitatory postsynaptic potentials (EPSPs) or spikes in -90% of the cells, both ipsilaterally and contralaterally. A smaller subpopulation of MRRN cells (-10%) received vestibular inhibition. In 26 of 48 recorded MRRN cells, the response to vestibular stimulation was potentiated after ipsilateral optic nerve stimulation. The potentiation was seen in cells receiving either excitatory or inhibitory vestibular input as an increase in EPSP amplitudelspiking (85%) and a decrease in inhibitory postsynaptic potential amplitude (15%) respectively. In most cases the vestibular responses did not return to control levels during the testing period (10–30 min), and thus the visual stimulation most likely induced long-lasting changes in the functional connectivity of the roll control network, in addition to the short-lasting afteractivity. In four of the 11 cells recorded contralateral to the stimulated optic nerve, a depression of the vestibular response could be seen. In potentiated cells, single vestibular pulses often evoked longer episodes of large synaptic noise and sometimes spiking. In the latter case, the action potentials appeared with highly variable latency after each stimulation pulse. This indicates that an important mechanism underlying the potentiation may be a long-lasting increase in excitability in a pool of unidentified interneurons located either upstream of the MRRN cells, relaying vestibular and visual inputs, or downstream, providing positive feedback.

Notes: An electromyographical study has been made of carp swimming muscles at various points along the body length during sustained and burst locomotion. During sustained swimming, red muscles show a constant time lag between activation of anterior and posterior segments which is practically independent of the speed of locomotion. In contrast, anterior and posterior homolateral segments are activated simultaneously during bursts of rapid movement. This pattern of co-ordination between body segments differs from that reported for other species.

Notes: Summary (1) The buccal mass of the freshwater snail Planorbis corneus, dissected together with the buccal ganglia, performs rhythmic feeding movements. Radula movements and the electrical activity in various nerves of buccal ganglia were recorded in such a preparation. The cycle of radula movements consisted of three phases: quiescence (Q), protraction (P) and retraction (R). The activity in the radular nerve was observed mainly in the P-phase and that in the dorsobuccal nerve, largely in the R-phase. (2) Isolated buccal ganglia were capable of generating a feeding rhythm, the activity in buccal nerves being similar to that observed in the buccal mass-buccal ganglion preparation, i.e., a burst in the radular nerve preceded a burst in the dorsobuccal nerve. The activity of neurons in isolated buccal ganglia during generation of the feeding rhythm has been studied with intracellular microelectrodes. About 10% of ganglion neurons exhibited periodic activity related to the feeding rhythm (“rhythmic” neurons). (3) Rhythmic neurons have been divided into 7 groups according to the phase of their activity and to the characteristics of slow oscillations of the membrane potential during the feeding cycle. Group 1 neurons revealed a gradual increase of depolarization during the Q- and P-phases. In subgroup le neurons, spike discharges began in the Q-phase, while in subgroup 1d neurons activity started in the P-phase. During the R-phase, group 1 neurons were strongly hyperpolarized, and their discharges terminated. In group 2 neurons, small depolarization gradually increased during the Q- and P-phases. Then, in the R-phase, a large (20–50 mV) rectangular wave of depolarization arose with superimposed high-frequency oscillations. Group 3 neurons exhibited an excitatory postsynaptic potential (EPSP) in the P-phase and inhibitory postsynaptic potential (IPSP) in the R-phase. The neurons of group 4 revealed two EPSPs: a small one in the P-phase and a larger one in the R-phase. Group 5 neurons exhibited an EPSP in the P-phase, those of group 7 — an IPSP in the R-phase, and those of group 9 — IPSPs in the P- and R-phases. Neurons within each of the groups 1, 2 and 4 were electrically coupled, and in addition, there were also electrical connections between neurons of groups 2 and 4. (4) Data are presented showing that neurons of groups 1 and 2 are the main source of postsynaptic potentials in rhythmic neurons in the P-phase and in the R-phase of the cycle, respectively.

Notes: Summary (1) Neurons of different groups (for group classification, see Arshavsky et al. 1988a) have been polarized through an intracellular recording microelectrode in Planorbis corneus buccal ganglia during feeding rhythm generation. Group 1 neurons, active in the quiescence (Q) and in the protractor (P) phases of the cycle, and also group 2 and 4 neurons, active in the retractor (R) phase, have proved to be “influential”, i.e., altering the rhythm generator operation. (2) Injection of a depolarizing current into group 1 neurons caused an increase of the rate of depolarization that neurons of this group exhibit in the Q- and P-phases of the feeding cycle. As a result, Q-phase shortened, the P-phase became longer, and the feeding rhythm accelerated. Opposite effects occured when a hyperpolarizing current was injected into group 1 neurons. In some of the experiments, the hyperpolarization of group 1 neurons resulted in cessation of both their activity and the activity of all other protractor neurons. As a result, the P-phase of the cycle disappeared, i.e., the rhythm generator transited from A mode of operation to B mode. (3) With hyperpolarization of individual group 2 or 4 neurons, excitation of the R-phase neurons was delayed and the feeding rhythm phase shifted. This delay was accompanied by the enhanced activity of protractor neurons. (4) A generator model is considered in which two groups (1 and 2) of endogeneously active neurons are coordinated by the excitatory effect of group 1 on group 2 and the inhibitory action of group 2 on group 1. (5) Evidence is given that the different modes of rhythm generator operation (A, B and C, see Arshavsky et al. 1988a) are determined by different tonic inflow to group 1 neurons.

Notes: Summary Efferent neurons in isolated pedal ganglia of the pteropodial mollusc Clione limacina were filled with Lucifer Yellow through the wing nerves. Then the ganglia were illuminated with intense blue light which resulted in the complete inactivation of these neurons. After inactivation of efferent neurons, interneurons of the pedal ganglia continued to generate the locomotor rhythm.

Notes: Summary Isolated buccal ganglia of Planorbis corneus are capable of generating a feeding rhythm. In the present work, “rhythmic” neurons of different groups (see Arshavsky et al. 1988a) have been extracted, by means of an intracellular microelectrode, from the buccal ganglia. (1) After extraction, efferent neurons of groups 3, 5, 7, 9 and most group 4 neurons generated repeated spikes at a frequency controlled by a polarizing current. Any periodic oscillations, similar to those during feeding rhythm generation, were absent in these isolated neurons. It is concluded, therefore, that these neurons are “followers”, that is, their rhythmic activity before extraction is determined by synaptic inputs from other neurons of the ganglia. (2) Isolated interneurons of groups 1 and 2 generated slow periodic oscillations similar to those observed in these neurons before their extraction. Subgroup 1e neurons generated smoothly growing depolarization accompanied by increasing spike activity; this depolarization was periodically interrupted by abrupt hyperpolarization, after which a new cycle started. Subgroup 1d neurons periodically generated short series of spikes. Group 2 neurons periodically generated a rectangular wave of depolarization with spike-like oscillations on its top. These results suggest that feeding rhythm generation in Planorbis is based on the endogenous rhythmic activity of group 1 and 2 neurons. (3) A pulse of hyperpolarizing current injected into an isolated neuron of subgroup 1e stopped the growth of depolarization in the neuron and reinitiated the process. This property as well as the character of the synaptic interactions of the interneurons (group 1 neurons excite those of group 2, while those of group 2 inhibit group 1 neurons; Arshavsky et al. 1988b) determine the alternating activity of groups 1 and 2.

Notes: Summary A method has been developed for recording the response of single neurons in the lamprey brainstem in vitro to natural stimulation of vestibular receptors. The brainstem dissected together with the intact vestibular apparatus could be rotated in space, in two perpendicular planes (transverse, the roll tilt, and sagittal, the pitch tilt), in one of them up to 360°, and in the other one up to ± 30°. The responses of single reticulospinal (RS) neurons, in all four reticular nuclei of the brainstem, to roll and pitch were recorded extracellularly and, with small inclinations (up to ±45°) also intracellularly. Two types of preparations were used, with and without the rostral part of the spinal cord. In the brainstem preparations, most RS neurons responded both to a definite brain orientation in space and to a change of the orientation (static and dynamic reactions). Responses to roll tilt were similar in all reticular nuclei: all cells were excited with roll tilt towards the contralateral side, this reaction was qualitatively preserved when the roll was performed in combination with different pitch inclinations. Responses to pitch tilt were less clearcut; some neurons were activated with noseup deflection while others responded to nose-down tilt. In preparations including the spinal cord, responses of RS neurons to roll and pitch tilt differed from those in the isolated brainstem in that they were much less specific and sfable. Roll and pitch tilts could trigger the spinal locomotor CPG, which, by sending “efference copy” signals back to the brainstem, produced modulation of RS neurons in relation to the locomotor rhythm.

Notes: Abstract A body orientation with the dorsal side up is usually maintained by lampreys during locomotion. Of crucial importance for this is the vestibular-driven control system. A visual input can affect the body orientation: illumination of one eye during swimming evokes roll tilt towards the source of light. The aim of the present study was to investigate the interaction of visual and vestibular inputs in reticulospinal (RS) neurons of the brainstem. The RS system is the main descending system transmitting information from the brainstem to the spinal cord. The response of neurons in the middle rhombencephalic reticular nucleus to a unilateral nonpatterned optic input was investigated, as well as the influence of this input on the response of RS neurons to vestibular stimulation (roll tilt). Experiments were carried out on a brainstem preparation with intact labyrinths and, in some cases, intact eyes. Illumination of one eye or electrical stimulation of the optic nerve (10 Hz) resulted in an activation of RS neurons preferentially on the ipsilateral side of the brainstem. The same result was obtained after ablation of the optic tectum, demonstrating that there are asymmetrical visual projections to the lower brainstem which do not involve the tectum. Stimulation of the optic nerve strongly affected the vestibular response in RS neurons. As a rule RS neurons are silent at the normal (dorsal-side-up) orientation of the brainstem and become active with contralateral roll tilt. During continuous optic nerve stimulation, however, the RS neurons on the side of stimulation fire during normal orientation of the brainstem, and the response to contralateral roll tilt increases considerably in many neurons. The effects of the optic input in contralateral RS neurons were less consistent. Any asymmetry in the signals transmitted to the spinal cord by the two (left and right) sub-populations of RS neurons can be expected to evoke a correcting motor response aimed at turning the body around its longitudinal axis to a position at which the symmetry between left and right RS neurons is restored. Normally, the symmetry will occur when the dorsal side is upwards, but with a unilateral visual input it will occur instead at some degree of ipsilateral roll.

Notes: Summary 1. Type 12 interneurons in pedal ganglia of Clione limacina exerted a strong influence upon the locomotor generator during “intense” swimming. These neurons generated “plateau” potentials, i.e. their membrane potential had two stable states: the “upper” one when a neuron was depolarized, and the “down” one, separated by 30–40 mV. The interneurons could remain in each state for a long time. Short depolarizing and hyperpolarizing current pulses, as well as excitatory and inhibitory postsynaptic potentials, could transfer the interneurons from one state to another. 2. When the pedal ganglia generated the locomotory rhythm, type 12 neurons received an EPSP and passed to the “upper” state in the V2-phase of a locomotor cycle. They remained at this state until the beginning of the D1-phase when they received an IPSP and passed to the “down” state. The EPSP in type 12 neurons was produced by type 8d neurons, and the IPSP by type 7 neurons. 3. Type 12 neurons exerted inhibitory influences upon many neurons active in the V1 and V2 phases, and excitatory influences upon the D-phase interneurons (type 7). 4. The functional role of type 12 neurons was to limit the activity of neurons discharging in the V-phase of a locomotory cycle. In addition, they enhanced the excitation of the D-phase neurons and promoted, thus, the transition from the V-phase to the D-phase.

Notes: Summary (1)Pinna stimulation evoked rhythmic oscillations in the spinal cord of the decerebrate curarized cat (“fictitious” scratch reflex). The role of different spinal segments in generation of these oscillations was studied. For this purpose, destruction of the grey matter of one or of several spinal segments was performed. Besides, different numbers of caudal segments were disconnected from the rest of the cord by cooling the lateral surface of the cord. ENGs of muscle nerves and activity of spinal neurons were recorded. (2) Different parts of the lumbosacral spinal cord, i.e. the L3 and L4 segments disconnected from the caudal part of the cord as well as the isolated L5 segment, are capable of generating rhythmic oscillations with a frequency (3–4 Hz) typical of the scratch reflex. (3) Rhythmic activity of the more caudal segments (L6-S1) usually appears only provided the rostral segments (L3–L5) generate rhythmic oscillations. However, when the dorsal surface of the L6-S1 segments is cooled, pinna stimulation evokes rhythmic activity in these segments earlier than in the L3–L5 segments. (4) The hypothesis is advanced that the L3–L5 segments are the “leading” ones, i.e., they determine the rhythm of activity in the whole spinal hindlimb centre.