Second-order Nucleus Research Articles

The dorsal lateral geniculate nucleus and the pulvinar as essential partners for visual cortical functions.

In most neuroscience textbooks, the thalamus is presented as a structure that relays sensory signals from visual, auditory, somatosensory, and gustatory receptors to the cerebral cortex. But the function of the thalamic nuclei goes beyond the simple transfer of information. This is especially true for the second-order nuclei, but also applies to first-order nuclei. First order thalamic nuclei receive information from the periphery, like the dorsal lateral geniculate nucleus (dLGN), which receives a direct input from the retina. In contrast, second order thalamic nuclei, like the pulvinar, receive minor or no input from the periphery, with the bulk of their input derived from cortical areas. The dLGN refines the information received from the retina by temporal decorrelation, thereby transmitting the most "relevant" signals to the visual cortex. The pulvinar is closely linked to virtually all visual cortical areas, and there is growing evidence that it is necessary for normal cortical processing and for aspects of visual cognition. In this article, we will discuss what we know and do not know about these structures and propose some thoughts based on the knowledge gained during the course of our careers. We hope that these thoughts will arouse curiosity about the visual thalamus and its important role, especially for the next generation of neuroscientists.

Control of Energy Expenditure by AgRP Neurons of the Arcuate Nucleus: Neurocircuitry, Signaling Pathways, and Angiotensin

Here, we review the current understanding of the functional neuroanatomy of neurons expressing Agouti-related peptide (AgRP) and the angiotensin 1A receptor (AT1A) within the arcuate nucleus (ARC) in the control of energy balance. The development and maintenance of obesity involves suppression of resting metabolic rate (RMR). RMR control is integrated via AgRP and proopiomelanocortin neurons within the ARC. Their projections to other hypothalamic and extrahypothalamic nuclei contribute to RMR control, though relatively little is known about the contributions of individual projections and the neurotransmitters involved. Recent studies highlight a role for AT1A, localized to AgRP neurons, but the specific function of AT1A within these cells remains unclear. AT1A functions within AgRP neurons to control RMR, but additional work is required to clarify its role within subpopulations of AgRP neurons projecting to distinct second-order nuclei, and the molecular mediators of its signaling within these cells.

Organization of the auditory brainstem in a lizard, Gekko gecko. I. Auditory nerve, cochlear nuclei, and superior olivary nuclei.

We used tract tracing to reveal the connections of the auditory brainstem in the Tokay gecko (Gekko gecko). The auditory nerve has two divisions, a rostroventrally directed projection of mid- to high best-frequency fibers to the nucleus angularis (NA) and a more dorsal and caudal projection of low to middle best-frequency fibers that bifurcate to project to both the NA and the nucleus magnocellularis (NM). The projection to NM formed large somatic terminals and bouton terminals. NM projected bilaterally to the second-order nucleus laminaris (NL), such that the ipsilateral projection innervated the dorsal NL neuropil, whereas the contralateral projection crossed the midline and innervated the ventral dendrites of NL neurons. Neurons in NL were generally bitufted, with dorsoventrally oriented dendrites. NL projected to the contralateral torus semicircularis and to the contralateral ventral superior olive (SOv). NA projected to ipsilateral dorsal superior olive (SOd), sent a major projection to the contralateral SOv, and projected to torus semicircularis. The SOd projected to the contralateral SOv, which projected back to the ipsilateral NM, NL, and NA. These results suggest homologous patterns of auditory connections in lizards and archosaurs but also different processing of low- and high-frequency information in the brainstem.

Vanilloid-Sensitive Afferents Activate Neurons with Prominent A-Type Potassium Currents in Nucleus Tractus Solitarius

Cranial visceral afferents innervate second-order nucleus tractus solitarius (NTS) neurons via myelinated (A-type) and unmyelinated (C-type) axons in the solitary tract (ST). A- and C-type afferents often evoke reflexes with distinct performance differences, especially with regard to their frequency-dependent properties. In horizontal brainstem slices, we used the vanilloid receptor 1 agonist capsaicin (CAP; 100 nm) to identify CAP-sensitive and CAP-resistant ST afferent pathways to second-order NTS neurons and tested whether these two groups of neurons had similar intrinsic potassium currents. ST stimulation evoked monosynaptic EPSCs identified by minimal synaptic jitter (<150 microsec) and divided into two groups: CAP-sensitive (n = 37) and CAP-resistant (n = 22). EPSCs in CAP-sensitive neurons had longer latencies (5.1 +/- 0.3 vs 3.6 +/- 0.3 msec; p = 0.001) but similar jitter (p = 0.57) compared with CAP-resistant neurons, respectively. Transient outward currents (TOCs) were significantly greater in CAP-sensitive than in CAP-resistant neurons. Steady-state currents were similar in both groups. 4-Aminopyridine or depolarized conditioning blocked the TOC, but tetraethylammonium had no effect. Voltage-dependent activation and inactivation of TOC were consistent with an A-type K+ current, I(KA). In current clamp, the activation of I(KA) reduced neuronal excitability and action potential responses to ST transmission. Our results suggest that the potassium-channel differences of second-order NTS neurons contribute to the differential processing of A- and C-type cranial visceral afferents beginning as early as this first central neuron. I(KA) can act as a frequency transmission filter and may represent a key target for the modulation of temporal processing of reflex responsiveness such as within the baroreflex arc.

A new class of lipid mediators: cytochrome P450 arachidonate metabolites

Cranial visceral afferents innervate second-order nucleus tractus solitarius (NTS) neurons via myelinated (A-type) and unmyelinated (C-type) axons in the solitary tract (ST). A- and C-type afferents often evoke reflexes with...

Frequency limits on aortic baroreceptor input to nucleus tractus solitarii.

The frequency of baroreceptor volleys to the central nervous system can influence the fidelity of baroreceptor signal transmission and thus may affect baroreflex regulation of blood pressure. We examined 1) the extent to which frequency-dependent depression of aortic baroreceptor signals was initiated at the first central synapse between primary baroreceptor fibers and second-order nucleus tractus solitarii (NTS) neurons; 2) whether the pattern of baroreceptor input influenced the depression; and 3) the potential relevance to baroreflex sympathoinhibition. In urethan-anesthetized rats, NTS action potential responses of neurons classified as second or higher order and averaged lumbar sympathetic nerve activity responses were simultaneously measured during 100 aortic depressor nerve stimuli delivered in constant or phasic patterns (0.8-48 Hz). Frequency-dependent depression was initiated at second-order neurons, with NTS responses decreasing to a 72% response rate at 48 Hz; the depression was greater at higher-order neurons; responses decreased to a 30% response rate. The depression was slightly but significantly greater with phasic inputs. Curve fitting suggested that synaptic depression may limit baroreflex sympathoinhibition. Thus frequency limits on baroreceptor inputs at NTS synapses may affect baroreflex function.

Aberrant projection induced by otocyst removal maintains normal tonotopic organization in the chick cochlear nucleus

Nucleus magnocellularis (NM), a second-order nucleus in the chick auditory system, is topographically and tonotopically organized. The basilar papilla (cochlea) projects onto the ipsilateral NM via the auditory nerve. The anteromedial region of NM is innervated by the proximal end of the basilar papilla and responds to high-frequency sounds; more posterolateral regions receive input from more distal locations along the papilla and respond to progressively lower frequencies. NM projects exclusively to the third-order neurons of nucleus laminaris (NL). Otocyst removal prevents the formation of the ipsilateral cochlea and cochlear nerve and results in the development of an aberrant functional projection from the contralateral NM to the "deafferented" NM on the operated side of the brain (Jackson and Parks, 1988). In the present experiment, the otocyst was removed unilaterally and the tonotopic organization of the deafferented NM was physiologically mapped at 17-18 d of embryonic age (E17-E18). Quantitative analyses revealed that the frequency organization of the deafferented NM is almost identical to that in normal embryos. Progressively higher characteristic frequencies were recorded at successively more anterior and more medial locations in the nucleus, and the orientation of the tonotopic axis was indistinguishable from normal. Furthermore, the correlation between characteristic frequency and anatomical location is comparable in the deafferented (r = 0.91) and normal (r = 0.87) NM. The only noticeable discrepancy is that characteristic frequencies in NM on both sides of the brain of operated embryos are higher than the frequencies observed previously at comparable regions of the nucleus in unoperated controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Patterns of functional innervation in the auditory nuclei of the chick brainstem following early unilateral removal of the otocyst

Neurons in the third-order auditory nuclei in the brainstem of chicks (nuclei laminares, NL) receive functional innervation from the ipsilateral and contralateral second-order nuclei (nuclei magnocellulares, NM) which is restricted to the dorsal and ventral dendrites respectively. This pattern of innervation in NL is established by embryonic stage 40 (day 15 of incubation). We have examined the distribution of this innervation in both NL at this age or older in embryos from which one otocyst had been removed or damaged on day 3 of incubation. The distribution of functional synapses was determined by analysis of the changes in polarity of field potentials evoked by electrical stimulation of either the ipsilateral or contralateral NM. The distribution of field potential polarity in NL of 40% of recordings in operated embryos and in all sham-operated embryos was the same as that observed in unoperated embryos. However, in the remaining operated embryos, the time course of the field potentials and the changes in the polarity of the responses as the recording electrode penetrated NL were abnormal. The abnormal complexity of responses and the abnormal distribution of field potential amplitude in NL in the operated embryos suggests that loss or damage to the first-order auditory innervation can result in (a) the formation of novel, functional synapses between second-order auditory neurons, and/or (b) disruption of processes that produce segregated innervation of the dendrites of the third-order auditory neurons.

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.