| Cochlear nuclei | |
| Latin: | nuclei cochleares |
| Ispartof: | Brainstem |
| System: | Auditory system |
| Artery: | AICA |
The cochlear nucleus (CN) or cochlear nuclear complex comprises two cranial nerve nuclei in the human brainstem, the ventral cochlear nucleus (VCN) and the dorsal cochlear nucleus (DCN). The ventral cochlear nucleus is unlayered whereas the dorsal cochlear nucleus is layered. Auditory nerve fibers, fibers that travel through the auditory nerve (also known as the cochlear nerve or eighth cranial nerve) carry information from the inner ear, the cochlea, on the same side of the head, to the nerve root in the ventral cochlear nucleus. At the nerve root the fibers branch to innervate the ventral cochlear nucleus and the deep layer of the dorsal cochlear nucleus. All acoustic information thus enters the brain through the cochlear nuclei, where the processing of acoustic information begins. The outputs from the cochlear nuclei are received in higher regions of the auditory brainstem.
The cochlear nuclei (CN) are located at the dorso-lateral side of the brainstem, spanning the junction of the pons and medulla.
The major input to the cochlear nucleus is from the auditory nerve, a part of cranial nerve VIII (the vestibulocochlear nerve). The auditory nerve fibers form a highly organized system of connections according to their peripheral innervation of the cochlea. Axons from the spiral ganglion cells of the lower frequencies innervate the ventrolateral portions of the ventral cochlear nucleus and lateral-ventral portions of the dorsal cochlear nucleus. The axons from the higher frequency organ of corti hair cells project to the dorsal portion of the ventral cochlear nucleus and the dorsal-medial portions of the dorsal cochlear nucleus. The mid frequency projections end up in between the two extremes; in this way the tonotopic organization that is established in the cochlea is preserved in the cochlear nuclei. This tonotopic organization is preserved because only a few inner hair cells synapse on the dendrites of a nerve cell in the spiral ganglion, and the axon from that nerve cell synapses on only a very few dendrites in the cochlear nucleus. In contrast with the VCN that receives all acoustic input from the auditory nerve, the DCN receives input not only from the auditory nerve but it also receives acoustic input from neurons in the VCN (T stellate cells). The DCN is therefore in a sense a second order sensory nucleus.
The cochlear nuclei have long been thought to receive input only from the ipsilateral ear. There is evidence, however, for stimulation from the contralateral ear via the contralateral CN,[2] and also the somatosensory parts of the brain.[3]
There are three major fiber bundles, axons of cochlear nuclear neurons, that carry information from the cochlear nuclei to targets that are mainly on the opposite side of the brain. Through the medulla, one projection goes to the contralateral superior olivary complex (SOC) via the trapezoid body, whilst the other half shoots to the ipsilateral SOC. This pathway is called the ventral acoustic stria (VAS or, more commonly, the trapezoid body). Another pathway, called the dorsal acoustic stria (DAS, also known as the stria of von Monakow), rises above the medulla into the pons where it hits the nuclei of the lateral lemniscus along with its kin, the intermediate acoustic stria (IAS, also known as the stria of Held). The IAS decussates across the medulla, before joining the ascending fibers in the contralateral lateral lemniscus. The lateral lemniscus contains cells of the nuclei of the lateral lemniscus, and in turn projects to the inferior colliculus. The inferior colliculus receives direct, monosynaptic projections from the superior olivary complex, the contralateral dorsal acoustic stria, some classes of stellate neurons of the VCN, as well as from the different nuclei of the lateral lemniscus.
Most of these inputs terminate in the inferior colliculus, although there are a few small projections that bypass the inferior colliculus and project to the medial geniculate, or other forebrain structures.
Three types of principal cells convey information out of the ventral cochlear nucleus: Bushy cells, stellate cells, and octopus cells.
Two types of principal cells convey information out of the dorsal cochlear nucleus (DCN) to the contralateral inferior colliculus. The principal cells receive two systems of inputs. Acoustic input comes to the deep layer through several paths. Excitatory acoustic input comes from auditory nerve fibers and also from stellate cells of the VCN. Acoustic input is also conveyed through inhibitory interneurons (tuberculoventral cells of the DCN and "wide band inhibitors" in the VCN). Through the outermost molecular layer, the DCN receives other types of sensory information, most importantly information about the location of the head and ears, through parallel fibers. This information is distributed through a cerebellar like circuit that also includes inhibitory interneurons.
The cochlear nuclear complex is the first integrative, or processing, stage in the auditory system. Information is brought to the nuclei from the ipsilateral cochlea via the cochlear nerve. Several tasks are performed in the cochlear nuclei. By distributing acoustic input to multiple types of principal cells, the auditory pathway is subdivided into parallel ascending pathways, which can simultaneously extract different types of information. The cells of the ventral cochlear nucleus extract information that is carried by the auditory nerve in the timing of firing and in the pattern of activation of the population of auditory nerve fibers. The cells of the dorsal cochlear nucleus perform a non-linear spectral analysis and place that spectral analysis into the context of the location of the head, ears and shoulders and that separate expected, self-generated spectral cues from more interesting, unexpected spectral cues using input from the auditory cortex, pontine nuclei, trigeminal ganglion and nucleus, dorsal column nuclei and the second dorsal root ganglion. It is likely that these neurons help mammals to use spectral cues for orienting toward those sounds. The information is used by higher brainstem regions to achieve further computational objectives (such as sound source location or improvement in signal-to-noise ratio). The inputs from these other areas of the brain probably play a role in sound localization.
In order to understand in more detail the specific functions of the cochlear nuclei it is first necessary to understand the way sound information is represented by the fibers of the auditory nerve. Briefly, there are around 30,000 auditory nerve fibres in each of the two auditory nerves. Each fiber is an axon of a spiral ganglion cell that represents a particular frequency of sound, and a particular range of loudness. Information in each nerve fibre is represented by the rate of action potentials as well as the particular timing of individual action potentials. The particular physiology and morphology of each cochlear nucleus cell type enhances different aspects of sound information.
Young ED, Spirou GA, Rice JJ, Voigt HF . Neural organization and responses to complex stimuli in the dorsal cochlear nucleus . Philos. Trans. R. Soc. Lond. B Biol. Sci. . 336 . 1278 . 407–13 . June 1992 . 1354382 . 10.1098/rstb.1992.0076 .