Nell B. Cant

Parallel auditory pathways: projection patterns of the different neuronal populations in the dorsal and ventral cochlear nuclei

The cochlear nuclear complex gives rise to widespread projections to nuclei throughout the brainstem. The projections arise from separate, well-defined populations of cells. None of the cell populations in the cochlear nucleus projects to all brainstem targets, and none of the targets receives inputs from all cell types. The projections of nine distinguishable cell types in the cochlear nucleus-seven in the ventral cochlear nucleus and two in the dorsal cochlear nucleus-are described in this review. Globular bushy cells and two types of spherical bushy cells project to nuclei in the superior olivary complex that play roles in sound localization based on binaural cues. Octopus cells convey precisely timed information to nuclei in the superior olivary complex and lateral lemniscus that, in turn, send inhibitory input to the inferior colliculus. Cochlear root neurons send widespread projections to areas of the reticular formation involved in startle reflexes and autonomic functions. Type I multipolar cells may encode complex features of natural stimuli and send excitatory projections directly to the inferior colliculus. Type II multipolar cells send inhibitory projections to the contralateral cochlear nuclei. Fusiform cells in the dorsal cochlear nucleus appear to be important for the localization of sounds based on spectral cues and send direct excitatory projections to the inferior colliculus. Giant cells in the dorsal cochlear nucleus also project directly to the inferior colliculus; some of them may convey inhibitory inputs to the contralateral cochlear nucleus as well.

The bushy cells in the anteroventral cochlear nucleus of the cat. A study with the electron microscope

Abstract The bushy cells in the anterior division of the anteroventral cochlear nucleus of the cat were studied with the electron microscope. In the anterior part of the anterior division, profiles of bushy cells and their processes are easily identified, since few cells of other types are found in this region. In the posterior and posterodorsal parts of the anterior division, the bushy cells are intermingled with stellate and small cells but can be identified on the basis of light-microscopic descriptions and comparisons with the results from the anterior part. Bushy cells are large, spherical cells with a centrally located nucleus enveloped by sheets of rough endoplasmic reticulum. Thin proximal dendrites jut abruptly from the cell body and contain a relatively pale cytoplasm. The distal dendrites contain few organelles other than numerous, very large mitochondria. The cell soma and proximal dendrites, as well as the axon hillock, receive numerous synaptic terminals, but the distal dendritic processes are contacted by relatively few endings. At least four types of terminals form synaptic contacts with the bushy cells. Very large terminals, containing large, spherical synaptic vesicles and forming multiple asymmetrical contacts, correspond to the end-bulbs of Held from the cochlea. These terminals disappear after cochlear ablations, but the other three types remain. The most numerous of these is a large terminal that contains flattened synaptic vesicles and forms long, nearly symmetrical contacts with the soma and dendrites of bushy cells. The second type of non-cochlear terminal is smaller and contains small, pleiomorphic synaptic vesicles that are not flattened. The third type occurs mainly on bushy cell dendrites, contains small, spherical synaptic vesicles, and forms moderately asymmetrical contacts. The bushy cells probably correspond to the primarylike units described in electrophysiological studies of the anterior division. Primarylike units respond to activity in auditory nerve fibers in a one-to-one manner, a finding compatible with the observation that much of the surface of the soma and dendrites of the bushy cells is contacted by auditory nerve terminals (end-bulbs of Held). Neither the origins nor the functions of the several types of non-cochlear inputs to the bushy cells are known. Further analysis of these inputs and of the other neuronal types in the anterior division, when correlated with physiological and biochemical data from the same cell types, could clarify the functional significance of the observed patterns of synaptic organization.

The fine structure of two types of stellate cells in the anterior division of the anteroventral cochlear nucleus of the cat

Abstract The stellate cells in the anterior division of the anteroventral cochlear nucleus of the cat were studied with the electron microscope. Although only one type of stellate cell has been identified at the light-microscopic level, two types can be recognized in electron micrographs. Both can be distinguished from the bushy cells that are also present in the anterior division, since they lack the nuclear cap of granular endoplasmic reticulum characteristic of the bushy cells. The somas of the type I stellate cells receive very few synaptic contacts, but the number of synaptic terminals increases markedly along the proximal dendrites. In contrast, both the soma and proximal dendrites of the type II stellate cells receive numerous synaptic contacts. Both neuronal types receive synaptic endings that contain large, spherical vesicles and that disappear after cochlear ablation. Both types of stellate cells are also contacted by synaptic terminals with small vesicles similar to those that contact bushy cells. In addition, the type II stellate cells receive a type of synaptic ending unlike those previously described. This is a relatively large terminal, containing large, flattened or disk-shaped vesicles, and forming slightly asymmetric synaptic complexes with the postsynaptic cell. These terminals as well as those with small synaptic vesicles survive cochlear ablation. The sources of the non-cochlear terminals are not known. The results indicate that the anterior division of the anteroventral cochlear nucleus of the cat contains at least three types of large neurons, each of which receives synaptic input from the cochlea as well as from other sources. The organization of the synaptic endings on the surface of each type is different. Since distinctive arrangements of cochlear and non-cochlear synaptic terminals could result in different response patterns to acoustic stimuli, each of these neuronal types may correspond to a different type of single unit, defined physiologically.

Projections from the lateral nucleus of the trapezoid body to the medial superior olivary nucleus in the gerbil

We made small injections of horseradish peroxidase into the medial superior olivary nucleus (MSO) of gerbils in order to examine the sources of input into that nucleus. As previously described, the MSO receives inputs from neurons in the rostral part of both anteroventral cochlear nuclei. In addition, we found evidence for a projection from the ipsilateral lateral nucleus of the trapezoid body (LNTB). Our results are also compatible with previous reports that the medical nucleus of the trapezoid body (NMTB) projects to the MSO. It is likely that these projections into the MSO from the LNTB and MNTB are sources of inhibitory synaptic inputs.

Organization of the neurons in the anterior division of the anteroventral cochlear nucleus of the cat. Light-microscopic observations

Abstract The anterior division of the anteroventral cochlear nucleus of the cat was studied in the light microscope. Criteria were developed to distinguish neurons in the Nissl-stained anteroventral cochlear nucleus which could then be correlated with those types found in Golgi preparations. Based on the patterns of distribution of the neuronal types, their size and shape, and the number of primary dendrites, the bushy cells (Golgi) are shown to correspond to the spherical cells (Nissl), whereas the stellate and small cells (Golgi) correspond to the ovoid cells (Nissl). The present results provide a background for a detailed study of the synaptic organization of the different cell types of the anterior division with the electron microscope and electrophysiological methods.

Organization of the inferior colliculus of the gerbil (Meriones unguiculatus): Differences in distribution of projections from the cochlear nuclei and the superior olivary complex

The inferior colliculus (IC) receives its major ascending input from the cochlear nuclei, the superior olivary complex, and the nuclei of the lateral lemniscus. To understand better the terminal distribution of the inputs from these sources relative to one another, we made focal injections of a retrograde tracer, biotinylated dextran amine, in different parts of the IC in 74 gerbils (Meriones unguiculatus). The cases could be divided into three groups based on counts of labeled cells in brainstem auditory nuclei. Group 1 cases had labeled cells in both the cochlear nuclei and the lateral and medial superior olivary nuclei. Group 2 cases had labeled cells in the cochlear nuclei but few or none in the lateral and medial superior olivary nuclei. Both groups had labeled cells in the nuclei of the lateral lemniscus and the superior paraolivary nucleus. Group 3 cases had few labeled cells in any of the ascending auditory pathways. The group to which a case belonged was strongly related to the location of the injection site in the IC. The injection sites for both group 1 and group 2 were located in the central nucleus, but those for group 1 tended to be located laterally relative to those for group 2, which were located more medially and caudally. The injection sites for group 3 cases lay outside the central nucleus of the IC. The two regions of the central nucleus of the IC, distinguished on the basis of connectivity, are likely to subserve different functions. J. Comp. Neurol. 495:511–528, 2006.

Ventral nucleus of the lateral lemniscus in guinea pigs: cytoarchitecture and inputs from the cochlear nucleus.

Cytoarchitectonic criteria were used to distinguish three subdivisions of the ventral nucleus of the lateral lemniscus in guinea pigs. Axonal tracing techniques were used to examine the projections from the cochlear nucleus to each subdivision. Based on the cell types they contain and their patterns of input, we distinguished ventral, dorsal, and anterior subdivisions of the ventral nucleus of the lateral lemniscus. All three subdivisions receive bilateral inputs from the cochlear nucleus, with contralateral inputs greatly outnumbering ipsilateral inputs. However, the relative density of the inputs varies: the ventral subdivision receives the densest projection, whereas the anterior subdivision receives the sparsest projection. Further differences are apparent in the morphology of the afferent axons. Following an injection of Phaseolus vulgaris‐leucoagglutinin into the ventral cochlear nucleus, most of the axons on the contralateral side and all of the axons on the ipsilateral side are thin. Thick axons are present only in the ventral subdivision contralateral to the injection site. The evidence from both anterograde and retrograde tracing studies suggests that the thick axons originate from octopus cells, whereas the thin axons arise from multipolar cells and spherical bushy cells. The differences in constituent cell types and in patterns of inputs suggest that each of the three subdivisions of the ventral nucleus of the lateral lemniscus makes a distinct contribution to the analysis of acoustic signals. J. Comp. Neurol. 379:363–385, 1997.

Identification of cell types in the anteroventral cochlear nucleus that project to the inferior colliculus

Abstract Type I stellate cells, identified in electron micrographs of the anterior division of the anteroventral cochlear nucleus of the cat, were labeled following injections of horseradish peroxidase in the inferior colliculus. Two other neuronal types present in this part of the anteroventral cochlear nucleus — the type II stellate cells and the bushy cells — were not labeled in these experiments.

Axons from non-cochlear sources in the anteroventral cochlear nucleus of the cat. A study with the rapid Golgi method.

Abstract Six groups of non-cochlear axons which project to the anteroventral cochlear nucleus of the cat can be identified in rapid Golgi preparations. The axons in three of these groups enter the anteroventral cochlear nucleus from its medial border, most of the fibers coming from the trapezoid body. Group I axons terminate in the anterior part of the anterior division of the anteroventral cochlear nucleus. Group II axons terminate in a portion of the small cell cap and in part of the posteroventral cochlear nucleus; they supply some endings to the dorsal part of the posterior division of the anteroventral nucleus as well. Group III axons end diffusely throughout the anterior division but not in the posterior division. Two groups of axons travel from caudal parts of the cochlear nucleus to the anteroventral part within the small cell cap. Group IV axons end in the dorsal part of the posterior division. Group V axons terminate in the dorsal part of the anterior division. Group VI axons course through the granule cell layer and form endings there but not in the anteroventral cochlear nucleus proper. The axons of each group form characteristic patterns of terminal branches, which give the different parts of the anteroventral cochlear nucleus a distinctive appearance in rapid Golgi preparations. Each subdivision of the anteroventral cochlear nucleus receives cochlear input. However, the present findings demonstrate differential non-cochlear inputs to the various subdivisions, implying that non-cochlear influences on the activity of the neurons may not be the same throughout the nucleus. Moreover, each subdivision contains several types of neurons and the non-cochlear inputs may project to all or to only some of these cell types. Thus, the arrangements of the non-primary inputs to the neurons of the cochlear nuclear complex introduce another level of complexity to its synaptic organization.

Conductive Hearing Loss Results in a Decrease in Central Auditory System Activity in the Young Gerbil

Objectives/Hypothesis: The impact of childhood conductive HL (CHL) on development of auditory function has long been debated. The present study was conducted to define and compare the consequences of CHL and cochlear ablation (CA) in young and adult animals, using 2‐deoxyglucose (2‐DG) uptake as a measure of metabolic activity. It was hypothesized that, for both ages, CHL would result in a decrease in activity in the major ascending central auditory system pathway of the manipulated ear, but that this decrease would be significantly less than that observed with CA.