Rhythmic movements are produced by central pattern-generating networks whose output is shaped by sensory and neuromodulatory inputs to allow the animal to adapt its movements to changing needs. This review discusses cellular, circuit, and computational analyses of the mechanisms underlying the generation of rhythmic movements in both invertebrate and vertebrate nervous systems. Attention is paid to exploring the mechanisms by which synaptic and cellular processes interact to play specific roles in shaping motor patterns and, consequently, movement.

Principles of rhythmic motor pattern generation

Central pattern generators and the control of rhythmic movements

Neurobiologists have long sought to understand how circuits in the nervous system are organized to generate the precise neural outputs that underlie particular behaviours. The motor circuits in the spinal cord that control locomotion, commonly referred to as central pattern generator networks, provide an experimentally tractable model system for investigating how moderately complex ensembles of neurons generate select motor behaviours. The advent of novel molecular and genetic techniques coupled with recent advances in our knowledge of spinal cord development means that a comprehensive understanding of how the motor circuitry is organized and operates may be within our grasp.

Circuits controlling vertebrate locomotion: moving in a new direction

We study pacemaker rhythms generated by two nonoscillatory model cells that are coupled by inhibitory synapses. A minimal ionic model that exhibits postinhibitory rebound (PIR) is presented. When the post-synaptic conductance depends instantaneously on presynaptic potential the classical alternating rhythm is obtained. Using phase-plane analysis we identify two underlying mechanisms, “release” and “escape,” for the out-of-phase oscillation. When the postsynaptic conductance is not instantaneous but decays slowly, the two cells can oscillate synchronously with no phase difference. In each case, different stable activity patterns can coexist over a substantial parameter range.

Alternating and synchronous rhythms in reciprocally inhibitory model neurons

Oocytes and their companion somatic cells maintain a close association throughout oogenesis and this association is essential for normal oocyte and follicular development. This review summarizes current concepts of the role of the somatic cells in the regulation of mammalian oocyte growth, the maintenance of meiotic arrest, the induction of oocyte maturation, and the acquisition of full embryonic developmental competence during oocyte maturation in vitro. Gap junctions appear to mediate these regulatory processes. The regulatory interaction of oocytes and somatic cells, however, is not unidirectional; the oocyte participates in the proliferation, development, and function of the follicular somatic cells. The oocyte secretes factors that enable the cumulus cells to synthesize hyaluronic acid and undergo cumulus expansion in response to hormonal stimulation. In addition, the oocyte produces factors that promote the proliferation of granulosa cells. These interactions in vitro do not appear to require the mediation of gap junctions. The oocyte also promotes the differentiation of granulosa cells into functional cumulus cells, but this function of the oocyte appears to require the continued presence and close association of the oocyte and granulosa cells. Therefore, oocytes and follicular somatic cells are interdependent for development and function.

Interactions between somatic cells and germ cells throughout mammalian oogenesis.

The central pattern generator for swimming in the pteropod mollusk Clione limacina consists of at least four pedal interneurons, two each controlling parapodial upstroke and downstroke. The two sets of antagonistic interneurons are linked by reciprocal monosynaptic inhibitory synapses, and all exhibit apparently strong postinhibitory rebound. This simple neuronal network produces reverberating alternate cyclic activity in the absence of tonic drive or apparent feedback modulation.

https://science.sciencemag.org/content/sci/229/4711/402.full.pdf

Reciprocal Inhibition and Postinhibitory Rebound Produce Reverberation in a Locomotor Pattern Generator

Swim acceleration in Clione limacina can occur via central inputs to pattern generator interneurons and motor neurons and through peripheral inputs to the swim musculature In the previous paper, peripheral modulation of the swim muscles was shown to increase wing contractility In the present paper, central inputs are described that trigger an increase in swim frequency and an increase in motor neuron activity In dissected preparations, spontaneous acceleration from slow to fast swimming included an increase in the cycle frequency, a baseline depolarization in the swim interneurons and an increase in the intensity of motoneuron firing Similar effects could be elicited by bath application of 10(-5) mol l-1 serotonin Two clusters of cerebral serotonin-immunoreactive interneurons were found to produce acceleration of swimming accompanied by changes in neuronal activity Posterior cluster neurons triggered an increase in swim frequency, depolarization of the swim interneurons, an increase in general excitor motoneuron activity and activation of type 12 interneurons and pedal peripheral modulatory neurons Cells from the anterior cerebral cluster also increased swim frequency, increased activity in the swim motoneurons and activated type 12 interneurons, pedal peripheral modulatory neurons and the heart excitor neuron The time course of action of the anterior cluster neurons did not greatly outlast the duration of spike activity, while that of the posterior cluster neurons typically outlasted burst duration It appears that the two discrete clusters of serotonin-immunoreactive neurons have similar, but not identical, effects on swim neurons, raising the possibility that the two serotonergic cell groups modulate the same target cells through different cellular mechanisms

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Serotonergic modulation of swimming speed in the pteropod mollusc Clione limacina. III. Cerebral neurons.

Metabolic, fluorescent dye and electrical coupling between hamster oocytes and cumulus cells during meiotic maturation in vivo and in vitro.

In the pteropod mollusc Clione limacina (Phipps), swimming is accomplished through alternate dorsal and ventral flexions of a pair of strongly muscularized wing-like parapodia (wings). Wing musculature is arranged in seven muscle groups. The two outer most dorsal and ventralgroups produce the bending movements of swimming. The three innermuscle groups include longitudinal and transverse wing retractors and dorsoventral muscles. The overall muscle arrangement is similar to that of the generalized mollusc foot. During hovering locomotion the wings pronate on downstroke and supinate on upstroke to produce a maximal angle of attack of 42° for both phases. Wing tips nearly touch or overlap in the saggital plane at the extreme of each half-cycle. High speed movie analysis of hovering swimming indicates that upstroke and downstroke are nearly symmetrical. It is suggested that wings produce lift in both wing phases. We estimate from wing dimensions and velocity measurements that the Reynolds number of the wings is approximately 200. A novel lift-generating mechanism, similar to the ‘clap-and-fling’ of insects, may be utilized by the Clione wing to generate lift throughout the wing cycle despite the reversal of wing movement in each half-stroke. Note: A significant portion of this work was conducted at Friday Harbor Laboratories, Friday Harbor, Washington.

/pdf/swimming-in-the-pteropod-mollusc-clione-umacina-i-behaviour-2soh6o0szp.pdf

Swimming in the Pteropod Mollusc, Clione Umacina: I. Behaviour and Morphology

Serotonin-immunoreactive somata in the pteropod mollusc Clione limacina were restricted to the cerebral and pedal ganglia. 10-14 pairs of cells were consistently found in the cerebral ganglia, including one large pair that had soma positions and axon branching patterns reminiscent of those of the metacerebral cells of other molluscs. Two clusters of somata were found on the midline near the cerebral commissure, one on the anterior-lateral margin and one posterior-laterally. A distinct paired cluster of up to nine somata was found on the dorso-lateral margin of the pedal ganglia, near the emergence of the pedal commissure. Up to five of these cells innervated the ipsilateral wing via the wing nerve. Dye-fills of these cells showed that they branch repeatedly in the ipsilateral wing and innervate the swim musculature. Double-labelling experiments indicated that the filled neurons were also serotonin-immunoreactive. Neurobiotin fills that were processed for electron microscopy revealed two types of terminals associated with the swim musculature: direct contacts and reactive terminals adjacent to non-labelled presynaptic terminals. Additional immunoreactive neurons in the pedal ganglia included the asymmetrical heart excitor neuron of the left pedal ganglion and up to nine ventral somata.

/pdf/serotonergic-modulation-of-swimming-speed-in-the-pteropod-1tm8xo38yw.pdf

Serotonergic modulation of swimming speed in the pteropod mollusc Clione limacina. I. Serotonin immunoreactivity in the central nervous system and wings.

Richard A. Satterlie

Papers

Reciprocal Inhibition and Postinhibitory Rebound Produce Reverberation in a Locomotor Pattern Generator

Serotonergic modulation of swimming speed in the pteropod mollusc Clione limacina. III. Cerebral neurons.

Metabolic, fluorescent dye and electrical coupling between hamster oocytes and cumulus cells during meiotic maturation in vivo and in vitro.

Swimming in the Pteropod Mollusc, Clione Umacina: I. Behaviour and Morphology

Serotonergic modulation of swimming speed in the pteropod mollusc Clione limacina. I. Serotonin immunoreactivity in the central nervous system and wings.