First-order Nuclei Research Articles

Ultrastructural study of the granule cell domain of the cochlear nucleus in rats: mossy fiber endings and their targets.

The principal projection neurons of the cochlear nucleus receive the bulk of their input from the auditory nerve. These projection neurons reside in the core of the nucleus and are surrounded by an external shell, which is called the granule cell domain. Interneurons of the cochlear granule cell domain are the target for nonprimary auditory inputs, including projections from the superior olivary complex, inferior colliculus, and auditory cortex. The granule cell domain also receives projections from the cuneate and trigeminal nuclei, which are first-order nuclei of the somatosensory system. The cellular targets of the nonprimary projections are mostly unknown due to a lack of information regarding postsynaptic profiles in the granule cell areas. In the present paper, we examined the synaptic relationships between a heterogeneous class of large synaptic terminals called mossy fibers and their targets within subdivisions of the granule cell domain known as the lamina and superficial layer. By using light and electron microscopic methods in these subdivisions, we provide evidence for three different neuron classes that receive input from the mossy fibers: granule cells, unipolar brush cells, and a previously undescribed class called chestnut cells. The distinct synaptic relations between mossy fibers and members of each neuron class further imply fundamentally separate roles for processing acoustic signals.

Ultrastructural study of the granule cell domain of the cochlear nucleus in rats: Mossy fiber endings and their targets

The principal projection neurons of the cochlear nucleus receive the bulk of their input from the auditory nerve. These projection neurons reside in the core of the nucleus and are surrounded by an external shell, which is called the granule cell domain. Interneurons of the cochlear granule cell domain are the target for nonprimary auditory inputs, including projections from the superior olivary complex, inferior colliculus, and auditory cortex. The granule cell domain also receives projections from the cuneate and trigeminal nuclei, which are first-order nuclei of the somatosensory system. The cellular targets of the nonprimary projections are mostly unknown due to a lack of information regarding postsynaptic profiles in the granule cell areas. In the present paper, we examined the synaptic relationships between a heterogeneous class of large synaptic terminals called mossy fibers and their targets within subdivisions of the granule cell domain known as the lamina and superficial layer. By using light and electron microscopic methods in these subdivisions, we provide evidence for three different neuron classes that receive input from the mossy fibers: granule cells, unipolar brush cells, and a previously undescribed class called chestnut cells. The distinct synaptic relations between mossy fibers and members of each neuron class further imply fundamentally separate roles for processing acoustic signals. © 1996 Wiley-Liss, Inc.

GABAergic inhibition shapes temporal and spatial response properties of pyramidal cells in the electrosensory lateral line lobe of gymnotiform fish.

1. The amplitude-coding pyramidal neurons of the first-order nucleus in weakly electric gymnotiform fish (Eigenmannia), the electrosensory lateral line lobe (ELL), exhibit 2 major physiological transformations of primary afferent input. Pyramidal cells rapidly adapt to a step change in amplitude, and they have a center/surround receptive-field organization. This study examined the physiological role of GABAergic inhibition on pyramidal cells. GABAergic synapses onto the somata of pyramidal cells primarily originate from granule-cell interneurons along with descending input. 2. Pyramidal cells fall into two physiologically distinct categories: E units, which are excited by a rise in stimulus amplitude, and I units, which are inhibited by a rise in stimulus amplitude. Microiontophoretic application of bicuculline methiodide onto both types of pyramidal cells increased the time constant of adaptation, defined as the time required for the neuron's response to decay to 37% of its maximum value, by 70-90%. The peak firing rate of E units to a step increase in stimulus amplitude increased by 49%, while the firing rate of I units did not change significantly. 3. Bicuculline application demonstrated that GABAergic inhibition may contribute to the strict segregation of E and I response properties. In the presence of bicuculline, many E units (normally excited only by stimulus amplitude increases) became excited by both increases and decreases; many I units (normally excited only by amplitude decreases) also became excited to increases. 4. The size of the excitatory receptive-field of E units was not affected by bicuculline, although response magnitude increased. The inhibitory surround increased in spatial extent by 175% with bicuculline administration. Neither the size of the I unit receptive-field center nor the response magnitude changed in the presence of bicuculline. The antagonistic surround of I units, however, increased by 49%. 5. The anatomy of the ELL is well understood (see Carr and Maler 1986). The physiological results obtained in this study, along with the results of Bastian (1986a, b), further our understanding of the functional role of the ELL circuitry. Our results suggest that spatial and temporal response properties of pyramidal cells are regulated by different but interacting inhibitory interneurons, some of which use GABA as a neurotransmitter. The activity of these interneurons is in turn controlled by descending feedback systems.

On the growth of lenticular martensite

It is proposed that the growth of lenticular martensite typically occurring in the FeNi and FeC systems takes place by the propagation of waves throughout the material. Two different types of waves are postulated: longitudinal transformation waves and transverse transformation waves, propagating at velocities of the order of elastic waves. The longitudinal transformation wave initiates the transformation process, once an embryo has reached a critical size whereupon it becomes a nucleus; it propagates radially along directions contained in the habit plane, forming the mid-rib. The martensitic disc generated by the longitudinal transformation wave acts as a second-order nucleus for the transverse transformation. The term ‘second-order nucleus’ is used to distinguish it from the ‘first-order nucleus’ that gives rise to the start of the transformation. The transverse transformation waves propagate perpendicularly to the habit plane, starting at the mid-rib. Accordingly, different defect generation mechanisms operate along the longitudinal and transverse propagation directions, due to the differences in stress state and substructure at the fronts and propagation velocities. The model allows the determination of the shape of a growing martensite plate, which closely resembles lenticular martensite. If xz is the habit plane, the direction of transverse propagation is oy and the major shear direction is ox, the martensite lens can be described, at time t, by the equation (x 2 + z 2) 3 2 x 2v ed+ z 2v es − 1 k ln(1 − k¦y¦ v es ) = t where v ed and v es are the velocities of longitudinal and shear elastic waves and k is a parameter. The arrest of growth takes place by uncoupling between the transformation front and the plastic waves that precede it. The implication of the pressure rise associated with the wave upon the nucleation is discussed.