Effects of static bending of sensory hairs on sound reception in the goldfish

The Mauthner-cell circuit of fish as a model system for startle plasticity

The Mauthner-cell (M-cell) system of teleost fish has a long history as an experimental model for addressing a wide range of neurobiological questions. Principles derived from studies on this system have contributed significantly to our understanding at multiple levels, from mechanisms of synaptic transmission and synaptic plasticity to the concepts of a decision neuron that initiates key aspects of the startle behavior. Here we will review recent work that focuses on the neurophysiological and neuropharmacological basis for modifications in the M-cell circuit. After summarizing the main excitatory and inhibitory inputs to the M-cell, we review experiments showing startle response modulation by temperature, social status, and sensory filtering. Although very different in nature, actions of these three sources of modulation converge in the M-cell network. Mechanisms of modulation include altering the excitability of the M-cell itself as well as changes in excitatory and inhibitor drive, highlighting the role of balanced excitation and inhibition for escape decisions. One of the most extensively studied forms of startle plasticity in vertebrates is prepulse inhibition (PPI), a sensorimotor gating phenomenon, which is impaired in several information processing disorders. Finally, we review recent work in the M-cell system which focuses on the cellular mechanisms of PPI and its modulation by serotonin and dopamine.

A mathematical analysis of the peripheral auditory system mechanics in the goldfish (Carassius auratus)

The dynamic response of the goldfish peripheral auditory system has been analyzed using lumped-parameter mechanical and fluid system models for the swimbladder, Weberian apparatus, and saccule. The swimbladder is treated as a two degree-of-freedom mechanical system consisting of two coupled mass-spring-damper arrangements. The swimbladder is coupled to the Weberian ossicles using a phenomenological analysis of the anterior swimbladder tunica externa which permits both stretching and sliding. Analysis of the saccule features only a single degree of freedom, corresponding to the direction of orientation of the ciliary bundles. Inputs to the saccule consist of the transverse canal fluid motion and the motion of the animal's head (assumed to match the local acoustic particle motion). Mechanical properties required for the system equations were estimated from published literature, direct measurements, and curve fits to experimental data for the motions of the swimbladders. The results indicate that the Weberian apparatus has a significant impact on hearing ability over the entire auditory bandwidth, not just at higher frequencies, and that the saccule functions as a displacement sensor above approximately 300 Hz.

Structure and function in the saccule of the goldfish (Carassius auratus): a model of diversity in the non-amniote ear

The vertebrate inner ear is comprised of a remarkable diversity of cell types, including several types of sensory hair cells. In amniotes (reptiles, birds, and mammals), the morphological and physiological characteristics that distinguish these cell types have been well documented, while cellular variation in the ears of non-amniotes (all other vertebrate groups) has remained underrecognized. Since non-amniotes have become increasingly popular models for developmental and genetic research, a more comprehensive understanding of structure and function in the inner ears of these species is warranted. This paper first reviews the large body of data describing the morphology and physiology of hair cells and afferent neurons in the inner ear of the goldfish (Carassius auratus). In particular, we examine the structure of the goldfish saccule, an endorgan that has been the subject of numerous investigations on audition. New data on the structural variation of synaptic bodies in saccular hair cells are also presented, and the functional implications of these data are discussed. Finally, we conclude that hair cell structure varies along the length of the goldfish saccule in a manner consistent with known physiological characteristics of the endorgan. The saccule provides an excellent model for investigating structure-function relationships in the vertebrate inner ear, as well as the development of auditory and vestibular sensory epithelia.

A connectionist model of left-right sound discrimination by the Mauthner system

Artificial neural networks were used to explore the auditory function of the Mauthner system, the brainstem circuit in teleost fishes that initiates fast-start escape responses. The artificial neural networks were trained with backpropagation to assign connectivity and receptive fields in an architecture consistent with the known anatomy of the Mauthner system. Our first goal was to develop neurally specific hypotheses for how the Mauthner system discriminates right from left in the onset of a sound. Our model was consistent with the phase model for directional hearing underwater, the prevalent theory for sound source localization by fishes. Our second goal was to demonstrate how the neural mechanisms that permit sound localization according to the phase model can coexist with the mechanisms that permit the Mauthner system to discriminate between stimuli based on amplitude. Our results indicate possible computational roles for elements of the Mauthner system, which has provided us a theoretical context within which to consider past and future experiments on the cellular physiology. Thus, these findings demonstrate the potential significance of this approach in generating experimentally testable hypotheses for small systems of identified cells.

An efferent inhibition of auditory afferents mediated by the goldfish Mauthner cell

Intracellular recordings from goldfish auditory afferents revealed a hyperpolarization triggered by a single impulse in the Mauthner cell. The firing of either one of the two Mauthner cells alone was sufficient to evoke this potential change. The all or none hyperpolarization, which could only be recorded in some auditory fibers, presumably was an inhibitory postsynaptic potential. The inhibitory postsynaptic potential typically had a latency of 6 ms, an amplitude of 1 mV and a half-decay time of 6.8 ms; it could block or delay impulses evoked by direct current injection and could attenuate the amplitude of excitatory postsynaptic potentials evoked by sound pulses. However, this inhibitory postsynaptic potential did not reduce the amplitudes of electrotonic coupling potentials produced by antidromic impulses in the Mauthner cell. We propose that the inhibitory postsynaptic potential is generated at the dendrites of the auditory fibers, i.e. in the ear, rather than at the central terminals of the afferent, where the antidromic coupling potentials originate. The possibility that the inhibitory postsynaptic potential actually represented dis-facilitation due to an efferent inhibition of the hair cells, which tonically depolarize the saccular fibers, was ruled out because depolarization of these fibers increased the inhibitory postsynaptic potential amplitude. Possible morphological substrates for the efferent inhibition and the behavioral significance of this inhibition are discussed.

Intra-axonal labeling of saccular afferents in the goldfish, Carassius auratus: correlations between morphological and physiological characteristics

Lucifer yellow was used to label axons of 32 physiologically defined S1 afferent fibers and 23 physiologically defined S2 afferent fibers of the goldfish sacculus. Analysis of these labeled fibers allowed us to study the relationship between electrical activity in these primary neurons and the morphology of their peripheral arborizations in the sensory macula. Morphological characteristics were highly indicative of response type: The peripheral arborizations of individual S1 fibers occupied a relatively small region (approximately 40 microns in diameter) in the rostral one-fourth of the saccular macula, whereas those of individual S2 fibers covered a larger area (roughly 80 microns across) in the caudal part of the macula. In addition to this rostrocaudal dimension, a ventral projection was related to a rarefaction response, a dorsal projection was related to a compression response, and a two-sided innervation was related to fibers having both responses. S1 fibers had either large or small terminals; the S2 fibers had only small terminals. On average, S1 fibers gave rise to approximately four terminals (e.g., seven small terminals or one to two large terminals) and S2 fibers approximately ten terminals. Spontaneous discharges were absent in all S1 fibers but present in some S2 fibers. Such S2 fibers showed spontaneous activity of either an irregular type or a burst type. In these, there was a tendency for fibers having more extensive arborizations to exhibit a burst type of spontaneous discharge. We conclude that structure-function relationships can be determined for these primary neurons.

A model for signal transmission in an ear having hair cells with free-standing stereocilia. IV. Mechanoelectric transduction stage

A model is described for mechanoelectric transduction in hair cells with free-standing stereocilia in the alligator lizard cochlea. The model relates the angular displacement of the stereocilia to the receptor potential in the absence of the hair cell's membrane capacitance whose effect is considered elsewhere (Leong and Weiss, 1985, in preparation). The model consists of two parts: a population of membrane ionic channels and an electric network that relates the channel conductance to the equivalent resistance of the hair cell. The membrane ionic channels tend to open when the stereociliary tuft is displaced toward the kinocilium (or basal body) and tend to close when the tuft is oppositely displaced. The fraction of channels that is open for a given tuft displacement is governed by Boltzmann statistics and the energies of open and closed configurations of the channels are separated by a single energy barrier whose height depends on the angular displacement of the stereociliary tuft. The resulting channel conductance is a hyperbolic-tangent type function of the angular displacement of the stereociliary tuft. The channel conductance is coupled to the rest of the hair cell by an equivalent electric network containing constant resistance and a capacitance. The Thévenin equivalent resistance change across the basolateral membrane of the hair cell, called the transducer function, is also a hyperbolic-tangent type function of angular displacement. The parameters of the channel conductance and the values of resistances in the hair-cell electric model determine the scale factors and the location of the operating point of this hyperbolic-tangent type function. The hyperbolic-tangent type function is a specific example of a class of monotonically decreasing and saturating, or sigmoidal, transducer functions. The spectral properties of sigmoidal transducer functions are examined for sinusoidal angular displacements of amplitude theta. General results are obtained for arbitrary sigmoidal transducer functions; particular results are obtained for the hyperbolic-tangent type function. General conclusions concerning spectral components of the resistance change include: all spectral components are independent of the frequency of the angular displacement; the constant or DC component can be positive or negative; the fundamental component is 180 degrees out of phase with the angular displacement, i.e. the resistance decreases when the stereocilia are displaced towards the kinocilium; for small values of theta, the magnitude of the nth harmonic tends to grow as theta n for n greater than 0; the zeroth harmonic or DC component grows as theta 2.(ABSTRACT TRUNCATED AT 400 WORDS)

Gravity and the cells of gravity receptors in mammals

Two new findings, that crystals located in the inner ear gravity receptors of mammals have the internal organization requisite for the piezoelectric property, and that sensory hair cells of these same receptors possess contractile-appearing striated organelles, have prompted the author to model mammalian gravity receptors in the ear on the principles of piezoelectricity and bioenergetics. This model is presented and a brief discussion of its implications for the possible effects of weightlessness follows.

Receptor potentials from hair cells of the lateral line

Intracellular recordings from hair cells in the tail lateral line of mudpuppy Necturus maculosus show receptor potentials less than 800 microvolts, peak to peak, from stimuli that are considered large compared to natural stimuli. The hair cells are in neuromasts that are sensitive at the time of recording and are identified by both in vivo and in vitro examination of intracellular staining.

Ouabain and streptomycin: their different loci of action on saccular hair cells in goldfish

Ouabain, when applied to the periotic spaces, that is, to the basal end of the saccular hair cells, suppressed microphonic potentials of the inner ear in goldfish. In contrast, streptomycin produced such an effect only when it was applied directly into the endolymph, that is, to the hair-bearing ends of the hair cells.