Distributed Processing of Load and Movement Feedback in the Premotor Network Controlling an Insect Leg Joint

In legged animals integration of information from various proprioceptors in and on the appendages by local premotor networks in the central nervous system is crucial for controlling motor output. To ensure posture maintenance and precise active movements, information about limb loading and movement is required. In insects, various groups of campaniform sensilla (CS) measure forces and loads acting in different directions on the leg, and the femoral chordotonal organ (fCO) provides information about movement of the femur-tibia (FTi) joint. In this study, we used extra- and intracellular recordings of extensor tibiae (ExtTi) and retractor coxae (RetCx) motor neurons (MNs) and identified local premotor nonspiking interneurons (NSIs), and mechanical stimulation of the fCO and tibial or trochanterofemoral CS (tiCS, tr/fCS), to investigate the premotor network architecture underlying multimodal proprioceptive integration. We found that load feedback from tiCS altered the strength of movement-elicited resistance reflexes and determined the specificity of ExtTi and RetCx MN responses to various load and movement stimuli. These responses were mediated by a common population of identified NSIs into which synaptic inputs from the fCO, tiCS, and tr/fCS are distributed, and whose effects onto ExtTi MNs can be antagonistic for both stimulus modalities. Multimodal sensory signal interaction was found at the level of single NSIs and MNs. The results provide evidence that load and movement feedback are integrated in a multimodal, distributed local premotor network consisting of antagonistic elements controlling movements of the FTi joint, thus substantially extending current knowledge on how legged motor systems achieve fine-tuned motor control.

Five types of nonspiking interneurons in local pattern-generating circuits of the crayfish swimmeret system

We conducted a quantitative analysis of the different nonspiking interneurons in the local pattern-generating circuits of the crayfish swimmeret system. Within each local circuit, these interneurons control the firing of the power-stroke and return-stroke motor neurons that drive swimmeret movements. Fifty-four of these interneurons were identified during physiological experiments with sharp microelectrodes and filled with dextran Texas red, Neurobiotin, or both. Five types of neurons were identified on the basis of combinations of physiological and anatomical characteristics. Anatomical categories were based on 16 anatomical parameters measured from stacks of confocal images obtained from each neuron. The results support the recognition of two functional classes: inhibitors of power stroke (IPS) and inhibitors of return stroke (IRS). The IPS class of interneuron has three morphological types with similar physiological properties. The IRS class has two morphological types with physiological properties and anatomical features different from the IPS neurons but similar within the class. Three of these five types have not been previously identified. Reviewing the evidence for dye coupling within each type, we conclude that each type of IPS neuron and one type of IRS neuron occur as a single copy in each local pattern-generating circuit. The last IRS type includes neurons that might occur as a dye-coupled pair in each local circuit. Recognition of these different interneurons in the swimmeret pattern-generating circuits leads to a refined model of the local pattern-generating circuit that includes synaptic connections that encode and decode information required for intersegmental coordination of swimmeret movements.

Premotor Interneurons in the Local Control of Stepping Motor Output for the Stick Insect Single Middle Leg

In insect walking systems, nonspiking interneurons (NSIs) play an important role in the control of posture and movement. As such NSIs are known to contribute to state-dependent modifications in processing of proprioceptive signals from the legs. For example, NSIs process a flexion of the femur-tibia (FTi) joint signaled by the femoral chordotonal organ (fCO) such that the stance phase motor output is reinforced in the active locomotor system. This phenomenon representing a reflex reversal is the first part of the “active reaction” (AR) and was hypothesized to functionally represent a major control feature by which sensory feedback supports stance generation. As NSIs are known to contribute to the AR, the question arises, whether they serve similar functions during stepping and whether the AR is generally part of the control system for walking. We studied these issues in vivo, in a single leg preparation of Carausius morosus with the leg kinematics being confined to changes in one plane, along the coxa-trochanteral and the FTi-joint. Following kinematic analysis, identified NSIs (E1-E8, I1, I2, and I4) were recorded intracellularly during single leg stepping at different velocities. We detected clear similarities between the activity pattern of NSIs during single leg stepping and their responses to fCO-stimulation during the generation of the AR. This strongly supports the notion that the motor output generated during the AR reflects part of the control regime for stepping. Furthermore, our experiments revealed that alterations in stepping velocity result from modifications in the activity of the premotor NSIs involved in stance phase generation.

Modulation of Membrane Potential in Mesothoracic Moto- and Interneurons During Stick Insect Front-Leg Walking

During walking, maintenance and coordination of activity in leg motoneurons requires intersegmental signal transfer. In a semi-intact preparation of the stick insect, we studied membrane potential modulations in mesothoracic (middle leg) motoneurons and local premotor nonspiking interneurons that were induced by stepping of a front leg on a treadmill. The activity in motoneurons ipsi- and contralateral to the stepping front leg was recorded from neuropilar processes. Motoneurons usually exhibited a tonic depolarization of ≤5 mV throughout stepping sequences. This tonic depolarization depended on membrane potential and was found to reverse in the range of −32 to −47 mV. It was accompanied by a mean membrane resistance decrease of ∼12%. During front-leg stepping, an increased spike activity to depolarizing current pulses was observed in 73% of contralateral flexor motoneurons that were tested. Motoneurons ipsilateral to the walking front leg exhibited phasic membrane potential modulations coupled to steps in accordance with previously published results. Coupling patterns were typical for a given motoneuron pool. Local nonspiking mesothoracic interneurons that provide synaptic drive to tibial motoneurons also contribute to the modulation of membrane potential of tibial motoneurons during front-leg walking. We hypothesize that the tonic depolarization of motoneurons during walking is a cellular correlate of arousal that usually increases effectiveness of phasic excitation in supporting motoneuron firing.

GABA-like immunoreactivity in nonspiking interneurons of the locust metathoracic ganglion

SUMMARYNonspiking interneurons are important components of the premotor circuitry in the thoracic ganglia of insects. Their action on postsynaptic neurons appears to be predominantly inhibitory, but it is not known which transmitter(s) they use. Here, we demonstrate that many but not all nonspiking local interneurons in the locust metathoracic ganglion are immunopositive for GABA (γ-aminobutyric acid). Interneurons were impaled with intracellular microelectrodes and were shown physiologically to be nonspiking. They were further characterized by defining their effects on known leg motor neurons when their membrane potential was manipulated by current injection. Lucifer Yellow was then injected into these interneurons to reveal their cell bodies and the morphology of their branches. Some could be recognised as individuals by comparison with previous detailed descriptions. Ganglia were then processed for GABA immunohistochemistry. Fifteen of the 17 nonspiking interneurons studied were immunopositive for GABA, but two were not. The results suggest that the majority of these interneurons might exert their well-characterized effects on other neurons through the release of GABA but that some appear to use a transmitter other than GABA. These nonspiking interneurons are therefore not an homogeneous population with regard to their putative transmitter.

Cooperative Mechanisms Between Leg Joints of Carausius morosusI. Nonspiking Interneurons That Contribute to Interjoint Coordination

Brunn, Dennis E. Cooperative mechanisms between leg joints of Carausius morosus. I. Nonspiking interneurons that contribute to interjoint coordination. J. Neurophysiol. 79: 2964–2976, 1998. Three nonspiking interneurons are described in this paper that influence the activity of the motor neurons of three muscles of the proximal leg joints of the stick insect. Interneurons were recorded and stained intracellularly by glass microelectrodes; motor neurons were recorded extracellularly with oil-hook electrodes. The motor neurons innervate the two subcoxal muscles, the protractor and retractor coxae, and the thoracic part of the depressor trochanteris muscle. The latter spans the subcoxal joint before inserting the trochanter, thus coupling the two proximal joints mechanically. The three interneurons are briefly described here. First, interneuron NS 1 was known to become more excited during the swing phase of the rear and the stance phase of the middle leg. When depolarized it excited several motor neurons of the retractor coxae. This investigation revealed that it inhibits the activity of protractor and thoracic depressor motor neurons when depolarized as well. In a pilocarpine-activated animal, the membrane potential showed oscillations in phase with the activity of protractor motor neurons, suggesting that NS 1 might contribute to the transition from swing to stance movement. Second, interneuron NS 2 inhibits motor neurons of protractor and thoracic depressor when depolarized. In both a quiescent and a pilocarpine-activated animal, hyperpolarizing stimuli excite motor neurons of both muscles via disinhibition. In one active animal the disinhibiting stimuli were sufficient to generate swing-like movements of the leg. In pilocarpine-activated preparations the membrane potential oscillated in correlation with the motor neuronal activity of the protractor coxae and thoracic depressor muscle. Hyperpolarizing stimuli induced or reinforced the protractor and thoracic depressor bursts and inhibited the activity of the motor neurons of the retractor coxae muscle, the antagonistic muscle of the protractor. Therefore interneuron NS 2 can be regarded as an important premotor interneuron for the switching from stance to swing and from swing to stance. Finally, interneuron NS 3 inhibits the spontaneously active motor neurons of both motor neuron pools in the quiescent animal. During pilocarpine-induced protractor bursts, depolarizing stimuli applied to the interneuron excited several protractor motor neurons with large action potentials and one motor neuron of the thoracic depressor. No oscillations of the membrane potentials were observed. Therefore this interneuron might contribute to the generation of rapid leg movements. The results demonstrated that the two proximal joints are coupled not only mechanically but also neurally and that the thoracic part of the depressor appears to function as a part of the swing-generating system.