Enhancement of the quadrupole interaction of an atom with guided light of an ultrathin optical fiber

We investigate the electric quadrupole interaction of an alkali-metal atom with guided light in the fundamental and higher-order modes of a vacuum-clad ultrathin optical fiber. We calculate the quadrupole Rabi frequency, the quadrupole oscillator strength, and their enhancement factors. In the example of a rubidium-87 atom, we study the dependencies of the quadrupole Rabi frequency on the quantum numbers of the transition, the mode type, the phase circulation direction, the propagation direction, the orientation of the quantization axis, the position of the atom, and the fiber radius. We find that the root-mean-square (rms) quadrupole Rabi frequency reduces quickly but the quadrupole oscillator strength varies slowly with increasing radial distance. We show that the enhancement factors of the rms Rabi frequency and the oscillator strength do not depend on any characteristics of the internal atomic states except for the atomic transition frequency. The enhancement factor of the oscillator strength can be significant even when the atom is far away from the fiber. We show that, in the case where the atom is positioned on the fiber surface, the oscillator strength for the quasicircularly polarized fundamental mode HE$_{11}$ has a local minimum at the fiber radius $a\simeq 107$ nm, and is larger than that for quasicircularly polarized higher-order hybrid modes, TE modes, and TM modes in the region $a<498.2$ nm

Efferent controls in crustacean mechanoreceptors

International audienceSince the 1960s it has been known that central neural networks can elaborate motor patterns in the absence of any sensory feedback. However, sensory and neuromodulatory inputs allow the animal to adapt the motor command to the actual mechanical configuration or changing needs. Many studies in invertebrates, particularly in crustacea, have described several mechanisms of sensory-motor integration and have shown that part of this integration was supported by the efferent control of the mechanosensory neurons themselves. In this article, we review the findings that support such an efferent control of mechanosensory neurons in crustacea. Various types of crustacean proprioceptors feeding information about joint movements and strains to central neural networks are considered, together with evidence of efferent controls exerted on their sensory neurons. These efferent controls comprise (1) the neurohormonal modulation of the coding properties of sensory neurons by bioamines and peptides; (2) the presynaptic inhibition of sensory neurons by GABA, glutamate and histamine; and (3) the long-term potentiation of sensory-motor synapses by glutamate. Several of these mechanisms can coexist on the same sensory neuron, and the functional significance of such multiple modulations is discussed

Direct glutamate-mediated presynaptic inhibition of sensory afferents by the postsynaptic motor neurons

International audienceAn in vitro preparation of the crayfish central nervous system was used to study a negative feedback control exerted by the glutamatergic motor neurons (MNs) on to their presynaptic cholinergic sensory afferents. This negative control consists in small amplitude, slowly developing depolarizations of the primary afferents (sdPADs) strictly timed with MN bursts. They were not blocked by picrotoxin, but were sensitive to glutamate non-N-methyl-D-aspartate (NMDA) antagonists. Intracellular recordings were performed within thin branches of sensory terminals while electrical antidromic stimulation were applied to the motor nerves, or while glutamate (the MN neurotransmitter) was pressure-applied close to the recording site. Electrical motor nerve stimulations and glutamate pressure application had similar effects on to sensory terminals issued from the coxo-basipodite chordotonal organ (CBTs): like sdPADs, both stimulation-induced depolarizations were picrotoxin-resistant and were dramatically reduced by non-NMDA antagonist bath application. These results indicate that sdPADs are likely directly produced by MNs during locomotor activity. A functional scheme is proposed

Adaptive motor control in crayfish

International audienceThis article reviews the principles that rule the organization of motor commands that have been described over the past ®ve decades in cray®sh. The adaptation of motor behaviors requires the integration of sensory cues into the motor command. The respective roles of central neural networks and sensory feedback are presented in the order of increasing complexity. The simplest circuits described are those involved in the control of a single joint during posture (negative feedback±resistance re¯ex) and movement (modulation of sensory feedback and reversal of the re¯ex into an assistance re¯ex). More complex integration is required to solve problems of coordination of joint movements in a pluri-segmental appendage, and coordination of dierent limbs and dierent motor systems. In addition, beyond the question of mechanical ®tting, the motor command must be appropriate to the behavioral context. Therefore, sensory information is used also to select adequate motor programs. A last aspect of adaptability concerns the possibility of neural networks to change their properties either temporarily (such on-line modulation exerted, for example, by presynaptic mechanisms) or more permanently (such as plastic changes that modify the synaptic ecacy). Finally, the question of how``automatic'' local component networks are controlled by descending pathways, in order to achieve behaviors, is discussed.

Neural Mechanisms of Reflex Reversal in Coxo-Basipodite Depressor Motor Neurons of the Crayfish

International audienceNeural mechanisms of reflex reversal in coxo-basipodite depressor motor neurons of the crayfish. J. Neurophysiol. 77: 1963–1978, 1997. The in vitro preparation of the fifth thoracic ganglion of the crayfish was used to investigate the mechanisms underlying the reflex reversal in a sensory-motor pathway. Sensory afferent neurons from the coxo-basipodite chordotonal organ (CBCO), which senses vertical movements of the limb, connect monosynaptically with basal limb motor neurons (MNs). In tonically active preparation, stretching the CBCO (corresponding to downward movements of the leg) stimulates the levator MNs, whereas releasing the CBCO activates the depressor (Dep) MNs. These reflexes, opposed to the imposed movement, are termed resistance reflexes. By contrast, during fictive locomotion, the reflexes are reversed and termed assistance reflexes. Intracellular recordings from all 12 Dep MNs were performed in single experiments. It allowed us to characterize three types of Dep MNs according to their response to CBCO imposed step-and-ramp movements: 8 of the 12 Dep MNs are resistance MNs that are depolarized during release of the CBCO and are connected monosynaptically to release-sensitive CBCO neurons; 1 Dep MN is an assistance MN that is depolarized during stretching of the CBCO and is connected monosynaptically to exclusively velocity-coding stretch-sensitive CBCO neurons; in our experimental conditions, 3 Dep MNs do not display any response to CBCO stimulation. Assistance reflex interneurons (ARINs), involved in polysynaptic assistance reflexes recorded from depressor MNs, are presented. During low-velocity (0.05 mm/s) stretching ramps imposed on the CBCO, ARINs display compound excitatory postsynaptic potentials (EPSPs), whereas during high-velocity (0.25 mm/s) ramps, they display a mixed excitatory and inhibitory response. Whereas a single MN generally receives monosynaptic EPSPs from three to six CBCO neurons, ARINs receive monosynaptic EPSPs from up to eight velocity-coding stretch-sensitive CBCO neurons. In addition, ARINs receive disynaptic inhibitory phasic inputs from stretch-sensitive CBCO afferents. Injection of a depolarizing current pulse into ARINs elicits a fast transient voltage-dependent depolarization. Its time to peak decreases, and its peak amplitude increases with increasing current intensity. ARINs likely are to be connected directly to Dep MNs. The synaptic delay between these nonspiking ARINs and Dep MNs is short (<2 ms) and constant. The postsynaptic EPSP amplitude increases with increasing current pulse intensity injected into ARIN. The dual sensory control (excitatory and inhibitory) makes it likely that ARIN represents a key element in reflex reversal control

Active Motor Neurons Potentiate Their Own Sensory Inputs via Glutamate-Induced Long-Term Potentiation

International audienceAdaptive motor control is based mainly on the processing and integration of proprioceptive feedback information. In crayfish walking leg, many of these operations are performed directly by the motor neurons (MNs), which are connected monosynapti-cally by sensory afferents (CBTs) originating from a chordotonal organ that encodes vertical limb movements. An in vitro preparation of the crayfish CNS was used to investigate a new control mechanism exerted directly by motor neurons on the sensory inputs themselves. Paired intracellular recordings demonstrated that, in the absence of any presynaptic sensory firing, the spiking activity of a leg MN is able long-lastingly to enhance the efficacy of the CBT-MN synapses. Moreover, this effect is specific to the activated MN because no changes were induced at the afferent synapses of a neighboring silent MN. We report evidence that long-term potentiation (LTP) of the monosynaptic EPSP involves a retrograde system of glutamate transmission from the postsynaptic MN, which induces the activation of a metabotropic glutamate receptor located presynaptically on the CBTs. We demonstrate that LTP at crayfish sensory-motor synapses results exclusively from the long-lasting enhancement of release of acetylcholine from presynaptic sensory af-ferent terminals, without inducing any modifications in postsyn-aptic MN properties. Our data indicate that this positive feedback control represents a functional mechanism that may play a key role in the auto-organization of sensory-motor networks