Physiological and anatomical development of glycinergic inhibition in the mouse superior paraolivary nucleus following hearing onset

Neuronal activity regulates development and maturation of neural circuits. This activity can include spontaneous burst firing or firing elicited by sensory input during early development. For example, auditory brainstem circuits involved in sound localization require acoustically evoked activity to form properly. Here we show, that an inhibitory circuit, involved in processing sound offsets, gaps, and rhythmically modulated vocal communication signals, matures before the onset of acoustically evoked activity.

The Superior Paraolivary Nucleus

This chapter summarizes the current concepts of the superior paraolivary nucleus (SPON)—a structure embedded in the superior olivary complex in the mammalian auditory brainstem. SPON is driven by input pathways from two of the most temporally secure neurons in the brain: the octopus cells in the cochlear nucleus and the neurons of the medial nucleus of the trapezoid body. These inputs activate spiking activity that marks the onset and offset of sound, the latter based on a rebound depolarization mechanism. This makes the SPON an excellent detector of transient sound energy. Robust detection of the coarse sound pattern over time further gives SPON the capacity to track the temporal envelope of complex sounds with supreme precision. Since the SPON circuitry is constant in mammals and resilient to sensory perturbation, it indicates its high survival value. A possible neuroevolutionary role of SPON in the processing of vocalizations is discussed.

ON and OFF inhibition as mechanisms for forward masking in the inferior colliculus: a modeling study

Masking effects of a preceding stimulus on the detection or perception of a signal have been found in several sensory systems in mammals, including humans and rodents. In the auditory system, it has been hypothesized that a central “OFF-inhibitory” mechanism, which is generated by neurons that respond after a sound is terminated, may contribute to the observed psychophysics. The present study constructed a systems model for the inferior colliculus that includes major ascending monaural and binaural auditory pathways. The fundamental characteristics of several neuron types along the pathways were captured by Hodgkin-Huxley models with specific membrane and synaptic properties. OFF responses were reproduced with a model of the superior paraolivary nucleus containing a hyperpolarization-activated h current and a T-type calcium current. When the gap between the end of the masker and the onset of the signal was large, e.g., >5 ms, OFF inhibition generated strong suppressive effects on the signal response. For smaller gaps, an additional inhibitory source, which was modeled as ON inhibition from the contralateral dorsal nucleus of the lateral lemniscus, showed the potential of explaining the psychophysics. Meanwhile, the effect of a forward masker on the binaural sensitivity to a low-frequency signal was examined, which was consistent with previous psychophysical findings related to sound localization.

The function of offset neurons in auditory information processing

Offset neurons which respond to the termination of the sound stimulation may play important roles in auditory temporal information processing, sound signal recognition, and complex distinction. Two additional possible mechanisms were reviewed: neural inhibition and the intrinsic conductance property of offset neuron membranes. The underlying offset response was postulated to be located in the superior paraolivary nucleus of mice. The biological significance of the offset neurons was discussed as well.