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The postsynaptic function of type II cochlear afferents.

ABSTRACT: The mammalian cochlea is innervated by two classes of sensory neurons. Type I neurons make up 90-95% of the cochlear nerve and contact single inner hair cells to provide acoustic analysis as we know it. In contrast, the far less numerous type II neurons arborize extensively among outer hair cells (OHCs) and supporting cells. Their scarcity and smaller calibre axons have made them the subject of much speculation, but little experimental progress for the past 50 years. Here we record from type II fibres near their terminal arbors under OHCs to show that they receive excitatory glutamatergic synaptic input. The type II peripheral arbor conducts action potentials, but the small and infrequent glutamatergic excitation indicates a requirement for strong acoustic stimulation. Furthermore, we show that type II neurons are excited by ATP. Exogenous ATP depolarized type II neurons, both directly and by evoking glutamatergic synaptic input. These results prove that type II neurons function as cochlear afferents, and can be modulated by ATP. The lesser magnitude of synaptic drive dictates a fundamentally different role in auditory signalling from that of type I afferents.

SUBMITTER: Weisz C

PROVIDER: S-EPMC2785502 | biostudies-literature | 2009 Oct

REPOSITORIES: biostudies-literature

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The postsynaptic function of type II cochlear afferents.

Weisz Catherine C Glowatzki Elisabeth E Fuchs Paul P

Nature 20091001 7267

The mammalian cochlea is innervated by two classes of sensory neurons. Type I neurons make up 90-95% of the cochlear nerve and contact single inner hair cells to provide acoustic analysis as we know it. In contrast, the far less numerous type II neurons arborize extensively among outer hair cells (OHCs) and supporting cells. Their scarcity and smaller calibre axons have made them the subject of much speculation, but little experimental progress for the past 50 years. Here we record from type II ...[more]

PMID: 19847265

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Project description:Slowly-adapting type II (SA-II, Ruffini) mechanoreceptive afferents respond well to pressure and stretch, and are regularly encountered in human microneurography studies. Despite an understanding of SA-II response properties, their role in touch perception remains unclear. Specific roles of different myelinated Aβ mechanoreceptive afferents in tactile perception have been revealed using single unit intraneural microstimulation (INMS), via microneurography, recording from and then electrically stimulating individual afferents. This method directly links single afferent artificial activation to perception, where INMS produces specific 'quantal' touch percepts associated with different mechanoreceptive afferent types. However, SA-II afferent stimulation has been ambiguous, producing inconsistent, vague sensations, or no clear percept. We physiologically characterized hundreds of individual Aβ mechanoreceptive afferents in the glabrous hand skin and examined the subsequent percepts evoked by trains of low amplitude INMS current pulses (<10 μA). We present 18 SA-II afferents where INMS resulted in a clear, electrically evoked sensation of large (∼36 mm2 ) diffuse pressure, which was projected precisely to their physiologically-defined receptive field in the skin. This sensation was felt as natural, distinctive from other afferents, and showed no indications of multi-afferent stimulation. Stimulus frequency modulated sensation intensity and even brief stimuli (4 pulses, 60 ms) were perceived. These results suggest that SA-II afferents contribute to perceived tactile sensations, can signal this rapidly and precisely, and are relevant and important for computational models of touch sensation and artificial prosthetic feedback. KEY POINTS: Slowly adapting type II mechanoreceptors (SA-IIs) are primary sensory neurons in humans that respond to pressure and stretch applied to the skin. To date, no specific conscious correlate of touch has been linked to SA-II activation. Using microneurography and intraneural microstimulation to stimulate single sensory neurons in human subjects, we find a specific sensation linked to the activation of single SA-II afferents. This sensation of touch was reported as gentle pressure and subjects could detect this with a high degree of accuracy. Methods of artificial tactile sensory feedback and computational models of touch should include SA-IIs as meaningful contributors to the conscious sensation of touch.

Project description:More than 3000 type II restriction endonucleases have been discovered. They recognize short, usually palindromic, sequences of 4-8 bp and, in the presence of Mg(2+), cleave the DNA within or in close proximity to the recognition sequence. The orthodox type II enzymes are homodimers which recognize palindromic sites. Depending on particular features subtypes are classified. All structures of restriction enzymes show a common structural core comprising four beta-strands and one alpha-helix. Furthermore, two families of enzymes can be distinguished which are structurally very similar (EcoRI-like enzymes and EcoRV-like enzymes). Like other DNA binding proteins, restriction enzymes are capable of non-specific DNA binding, which is the prerequisite for efficient target site location by facilitated diffusion. Non-specific binding usually does not involve interactions with the bases but only with the DNA backbone. In contrast, specific binding is characterized by an intimate interplay between direct (interaction with the bases) and indirect (interaction with the backbone) readout. Typically approximately 15-20 hydrogen bonds are formed between a dimeric restriction enzyme and the bases of the recognition sequence, in addition to numerous van der Waals contacts to the bases and hydrogen bonds to the backbone, which may also be water mediated. The recognition process triggers large conformational changes of the enzyme and the DNA, which lead to the activation of the catalytic centers. In many restriction enzymes the catalytic centers, one in each subunit, are represented by the PD. D/EXK motif, in which the two carboxylates are responsible for Mg(2+) binding, the essential cofactor for the great majority of enzymes. The precise mechanism of cleavage has not yet been established for any enzyme, the main uncertainty concerns the number of Mg(2+) ions directly involved in cleavage. Cleavage in the two strands usually occurs in a concerted fashion and leads to inversion of configuration at the phosphorus. The products of the reaction are DNA fragments with a 3'-OH and a 5'-phosphate.

Dataset Information

The postsynaptic function of type II cochlear afferents.

Publications

The postsynaptic function of type II cochlear afferents.

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