THE LOCALIZATION of high-frequency sinusoids depends on differences in the intensity of the sounds arriving at the two ears, that of low-frequency sinusoids on differences in phase (30). The present paper considers how the introduction of these two cues affects the discharge of individual binaural neurons of the superior olivary complex. Also described is the response of these cells to variations in the average intensity of binaural stimuli. Most of the neurons were located in the medial superior olive (MSO) or the medial preolivary nucleus (MPO).

Response of binaural neurons of dog superior olivary complex to dichotic tonal stimuli: some physiological mechanisms of sound localization.

The Auditory Brainstem

The inferior colliculus receives excitatory and inhibitory input from parallel auditory pathways that differ in discharge patterns, latencies, and binaural properties. Processing in the inferior colliculus may depend on the temporal sequence in which excitatory and inhibitory synaptic inputs are activated and on the resulting balance between excitation and inhibition. To explore this issue at the cellular level, we used the novel approach of whole-cell patch-clamp recording in the midbrain of awake bats ( Eptesicus fuscus ) to record EPSCs or IPSCs. Sound-evoked EPSCs were recorded in most neurons. These EPSCs were frequently preceded by an IPSC, followed by an IPSC, or both. These findings help explain the large latency range and transient responses that characterize inferior colliculus neurons. The EPSC was sometimes followed by long-lasting oscillatory currents, suggesting that a single brief sound sets up a pattern of altered excitability that persists far beyond the duration of the initial sound. In three binaural neurons, ipsilateral sound evoked a large IPSC that partially or totally canceled the EPSC evoked by contralateral sound. In one binaural neuron with ipsilaterally evoked IPSCs, contralaterally evoked IPSCs occurred in response to frequencies above and below the neuron’s best frequency. Thus, both monaural and binaural interactions can occur at single inferior colliculus neurons. These results show that whole-cell patch-clamp recording offers a powerful means of understanding how subthreshold processes determine the responses of auditory neurons.

https://www.jneurosci.org/content/jneuro/16/9/3009.full.pdf

Whole-Cell Patch-Clamp Recording Reveals Subthreshold Sound-Evoked Postsynaptic Currents in the Inferior Colliculus of Awake Bats

This study examines the dorsal nucleus of the lateral lemniscus (DNLL) and its afferent and efferent connections. In Nissl-stained material, DNLL has three parts: dorsal, ventral, and lateral. Although each part contains neurons with similar Nissl patterns, the subdivisions may be distinguished by the size, shape, and orientation of the cells. The lateral DNLL contains a mixture of DNLL neurons and cells from the sagulum. Afferent connections to DNLL were investigated with anterograde axonal transport techniques. Bilateral inputs to DNLL arise from the anteroventral cochlear nucleus and lateral superior olive, while unilateral inputs are provided by the ipsilateral medial superior olive and the contralateral DNLL. The inputs appear to have a tonotopic organization. Afferent fibers to DNLL form horizontal bands that are continuous both mediolaterally and rostrocaudally. All parts of DNLL do not share the same inputs, and a medial-to-lateral gradient in the labeling of some pathways is evident. To study the efferent connections of DNLL, both retrograde and anterograde axonal transport techniques were used. The DNLL projects to the inferior colliculus and the contralateral DNLL. The topography of these projections suggests that areas of similar tonotopic organization are connected. In the inferior colliculus, the projection is heaviest to the central nucleus and extends to the adjacent dorsal and caudal cortex, the rostral pole nucleus, and the ventrolateral nucleus. Axons from DNLL terminate along the fibrodendritic laminae of the central nucleus as bands that are prominent on the contralateral side, whereas those on the ipsilateral colliculus are more diffuse. The afferent and efferent connections of DNLL constitute a multisynaptic pathway, parallel to the other ascending pathways to the inferior colliculus. The other ascending pathways include the direct pathways from the cochlear nucleus to the inferior colliculus and the indirect pathways via the superior olivary complex. Ascending pathways are discussed as to their relationship to the subdivisions of the inferior colliculus, the laterality of their projections, and their banding patterns in the central nucleus. In contrast to the excitatory pathways to the inferior colliculus, the neurons in DNLL may use GABA as a neurotransmitter. Axons from the DNLL terminate in the inferior colliculus as bands that could have a unique inhibitory function. Thus, the multisynaptic, DNLL pathway may provide feed-forward inhibitory inputs to the inferior colliculus, bilaterally, and to the contralateral DNLL.

Connections of the dorsal nucleus of the lateral lemniscus: an inhibitory parallel pathway in the ascending auditory system?

In the classical neuroanatomical literature, the inferior and superior colliculi, located on the roof of the midbrain, are grouped together as the corpora quadrigemina However, modern research indicates that these structures have quite different functions The inferior colliculus is part of the auditory pathways and is involved in processing both ascending and descending information about sound This information may concern the frequency and the monaural or binaural properties of sound In contrast, the superficial portions of the superior colliculus are devoted to visual processing and are organized retinotopically The deeper portions of the superior colliculus are related to initiation of eye and head movements, and it is there that information from multiple sensory systems converge, presumably to assist in the control of these movements

P. G. Nelson

Papers

Synaptic mechanisms of excitation and inhibition in the central auditory pathway.