Peripheral synaptic contacts at mechanoreceptors in arachnids and crustaceans: Morphological and immunocytochemical characteristics

Abstract

Two types of sensory organs in crustaceans and arachnids, the various mechanoreceptors of spiders and the crustacean muscle receptor organs (MRO), receive extensive efferent synaptic innervation in the periphery. Although the two sensory systems are quite different-the MRO is a muscle stretch receptor while most spider mechanoreceptors are cuticular sensilla-this innervation exhibits marked similarities. Detailed ultrastructural investigations of the synaptic contacts along the mechanosensitive neurons of a spider slit sense organ reveal four important features, all having remarkable resemblances to the synaptic innervation at the MRO: (1) The mechanosensory neurons are accompanied by several fine fibers of central origin, which are presynaptic upon the mechanoreceptors. Efferent control of sensory function has only recently been confirmed electrophysiologically for the peripheral innervation of spider slit sensilla. (2) Different microcircuit configuration types, identified on the basis of the structural organization of their synapses. (3) Synaptic contacts, not only upon the sensory neurons but also between the efferent fibers themselves. (4) Two identified neurotransmitter candidates, GABA and glutamate. Physiological evidence for GABAergic and glutamatergic transmission is incomplete at spider sensilla. Given that the sensory neurons are quite different in their location and origin, these parallels are most likely convergent. Although their significance is only partially understood, mostly from work on the MRO, the close similarities seem to reflect functional constraints on the organization of efferent pathways in the brain and in the periphery.

Spider Peripheral Mechanosensory Neurons Are Directly Innervated and Modulated by Octopaminergic Efferents

“…The insets in F show single orthogonal 1 m sections from planes X, Y, and Z at the areas indicated. sensory neurons (Fabian-Fine et al, 2002). These neurons do not display tonic activity without mechanical stimulation.…”

Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons

“…Spider, Cupiennius salei, mechanosensory neurons receive extensive efferent innervation in the periphery (Fabian-Fine et al 2002). The efferent fibers contain GABA, glutamate, and acetylcholine, which all inhibit the responses of sensory neurons to step mechanical or electrical stimuli (FabianFine et al 2002;Panek and Torkkeli 2005;Panek et al 2002;Widmer et al 2006).…”

Random Stimulation of Spider Mechanosensory Neurons Reveals Long-Lasting Excitation by GABA and Muscimol

“…A larger value of k indicates a greater sensitivity to high frequencies and more rapid adaptation. An increase in information capacity R was previously observed in spider mechanosensory neurons when the firing rate increased (French et al 2001;Widmer et al 2005). Graphical comparisons of AP rate and amplitude as well as membrane potential (E m ) before and after muscimol application are also shown in Fig.…”

“…2. (French et al 2001). This is a common problem in experimental work involving mechanical stimulation.…”

“…For the electrical stimulation this signal was low-pass filtered at 300 Hz, using a custom-built nine-pole active filter. Action potentials were detected by a threshold-detection algorithm (French et al 2001) and stored as time of occurrence. Examples of experiments with mechanical and electrical stimuli with resulting AP trains are shown in Fig.…”