THz and mid-IR spectroscopy of interstellar ice analogs: methyl and carboxylic acid groups.

A fundamental problem in astrochemistry concerns the synthesis and survival of complex organic molecules (COMs) throughout the process of star and planet formation. While it is generally accepted that most complex molecules and prebiotic species form in the solid phase on icy grain particles, a complete understanding of the formation pathways is still largely lacking. To take full advantage of the enormous number of available THz observations (e.g., Herschel Space Observatory, SOFIA, and ALMA), laboratory analogs must be studied systematically. Here, we present the THz (0.3-7.5 THz; 10-250 cm(-1)) and mid-IR (400-4000 cm(-1)) spectra of astrophysically-relevant species that share the same functional groups, including formic acid (HCOOH) and acetic acid (CH3COOH), and acetaldehyde (CH3CHO) and acetone ((CH3)2CO), compared to more abundant interstellar molecules such as water (H2O), methanol (CH3OH), and carbon monoxide (CO). A suite of pure and mixed binary ices are discussed. The effects on the spectra due to the composition and the structure of the ice at different temperatures are shown. Our results demonstrate that THz spectra are sensitive to reversible and irreversible transformations within the ice caused by thermal processing, suggesting that THz spectra can be used to study the composition, structure, and thermal history of interstellar ices. Moreover, the THz spectrum of an individual species depends on the functional group(s) within that molecule. Thus, future THz studies of different functional groups will help in characterizing the chemistry and physics of the interstellar medium (ISM).

Targets of horizontal connections in macaque primary visual cortex.

Pyramidal neurons within the cerebral cortex are known to make long-range horizontal connections via an extensive axonal collateral system. The synaptic characteristics and specificities of these connections were studied at the ultrastructural level. Two superficial layer pyramidal cells in the primate striate cortex were labeled by intracellular injections with horseradish peroxidase (HRP) and their axon terminals were subsequently examined with the technique of electron microscopic (EM) serial reconstruction. At the light microscopic level both cells showed the characteristic pattern of widespread, clustered axon collaterals. We examined collateral clusters located near the dendritic field (proximal) and approximately 0.5 mm away (distal). The synapses were of the asymmetric/round vesicle variety (type I), and were therefore presumably excitatory. Three-quarters of the postsynaptic targets were the dendritic spines of other pyramidal cells. A few of the axodendritic synapses were with the shafts of pyramidal cells, bringing the proportion of pyramidal cell targets to 80%. The remaining labeled endings were made with the dendritic shafts of smooth stellate cells, which are presumed to be (GABA)ergic inhibitory cells. On the basis of serial reconstruction of a few of these cells and their dendrites, a likely candidate for one target inhibitory cell is the small-medium basket cell. Taken together, this pattern of outputs suggests a mixture of postsynaptic effects mediated by consequence the horizontal connections may well be the substrate for the variety of influences observed between the receptive field center and its surround.

Microcircuitry of beta ganglion cells in cat retina.

We reconstructed from electron micrographs of 189 serial ultrathin sections a major portion of the dendritic tree of an on-beta ganglion cell through its sixth order of branching. One hundred three contacts from three cone bipolar cells were identified. Forty-seven contacts were from a single CBb1 cone bipolar. These were distributed widely over the dendritic tree but were frequently found on the slender "basal tuft" dendrites. Twenty-two additional contacts from a second CBb1 cell were found but not studied in detail. Thirty-four contacts were from a single CBb2 cone bipolar; these also were distributed widely but were primarily on the branches of the main dendritic arborization. A major portion of the dendritic tree of an off-beta cell was also reconstructed through its seventh order of branching. Thirty-five contacts from two cone bipolar cells were identified. Twenty-three contacts were from a single CBa1 cone bipolar and 12 widely distributed over the off-beta cell dendritic tree. We propose that the photopic receptive field center of a beta cell corresponds to the envelope of the receptive fields of the bipolar cells that connect it to the cones. The center response of a beta cell may be generated by a "push-pull" mechanism. For the on-beta cell there would be excitation at light on from CBb1 and disinhibition from CBb2 and the reverse at light off. For the off-beta cell there would be inhibition at light on from CBa2 and withdrawal of excitation from CBa1. Should the bipolars have antagonistic surrounds (so far reported only for CBb1), the beta cell surrounds as well as their centers might be generated by this push-pull mechanism.

Microcircuitry of bipolar cells in cat retina.

We have studied 15 bipolar neurons from a small patch (14 X 120 micron) of adult cat retina located within the area centralis. From electron micrographs of 189 serial ultrathin sections, the axon of each bipolar cell was substantially reconstructed with its synaptic inputs and outputs by means of a computer-controlled reconstruction system. Based on differences in stratification, cytology, and synaptic connections, we identified eight different cell types among the group of 15 neurons: one type of rod bipolar and seven types of cone bipolar neurons. These types correspond to those identified by the Golgi method and by intracellular recording. Those bipolar cell types for which we reconstructed three or four examples were extremely regular in form, size, and cytology, and also in the quantitative details of their synaptic connections. They appeared quite as specific in these respects as invertebrate "identified" neurons. The synaptic patterns observed for each type of bipolar neuron were complex but may be summarized as follows: the rod bipolar axon ended in sublamina b of the inner plexiform layer and provided major input to the AII amacrine cell. The axons of three types of cone bipolar cells also terminated in sublamina b and provided contacts to dendrites of on-beta and other ganglion cells. All three types, but especially the Cb1, received gap junction contacts from the AII amacrine cell. Axons of four types of cone bipolar cells terminated in sublamina a of the inner plexiform layer and contacted dendrites of off-beta and other ganglion cells. One of these cone bipolar cell types, CBa1, made reciprocal chemical contacts with the lobular appendage of the AII amacrine cell. These results show that the pattern of cone bipolar cell input to beta (X) and probably alpha (Y) ganglion cells is substantially more complex than had been suspected. At least two types of cone bipolar contribute to each type of ganglion cell where only a single type had been anticipated. In addition, many of the cone bipolar cell pathways in the inner plexiform layer are available to the rod system, since at least four types of cone bipolar receive electrical or chemical inputs from the AII amacrine cell. This may help to explain why, in a retina where rods far outnumber the cones, there should be so many types of cone bipolar cells.

Patterns of synaptic input to layer 4 of cat striate cortex.

Although cells in layer 4 of cat striate cortex represent the first stage in the cortical processing of visual information, they have considerably more complicated receptive field properties than the afferents to the layer from the lateral geniculate nucleus. In considering how these properties are generated, we have focused on the intrinsic cortical circuitry, and particularly on the projection to layer 4 from layer 6. Layer 6 pyramidal cells were injected with horseradish peroxidase and examined at the light and electron microscopic level. The labeled axon terminals were found to form asymmetric synapses and to show a strong preference for contacting dendritic shafts. Serial reconstruction of dendrites postsynaptic to labeled layer 6 cell axon terminals showed that a large proportion of the postsynaptic dendrites belonged to smooth and sparsely spiny stellate cells, suggesting a selective innervation of these cell types. In contrast, the geniculate projection to layer 4 made synapses primarily with dendritic spines and, as a result, the large majority of terminals ended on spiny cells. Since smooth and sparsely spiny stellate cells are thought to mediate inhibition within the cortex, we suggest that one effect of the layer 6 to layer 4 projection could be to contribute to inhibitory features of the receptive fields of layer 4 cells.

Interplexiform cell in cat retina: identification by uptake of gamma-[3H]aminobutyric acid and serial reconstruction.

After intravitreal injection of gamma-[3H] aminobutyric acid (GAB), 2% of the neurons at the outer margin of the inner plexiform layer were intensely labeled. Reconstructions of these neurons from serial electron microscope autoradiograms showed that they are interplexiform cells, which synapse on bipolar processes in the outer plexiform layer and on amacrine and bipolar processes in the inner plexiform layer.

Toward a functional architecture of the retina: serial reconstruction of adjacent ganglion cells.

Twenty adjacent ganglion cells in cat retina were partially reconstructed from electron micrographs of serial thin sections. Cells were classified by size and by dendritic branching patterns as alpha, beta, or gamma cells. The alpha and beta cells were further subdivided by differences in the laminar distribution of their dendrites in the inner plexiform layer. The distribution of synaptic contacts on the cells was distinctive for each of the five major classes. Contacts on the alpha and beta cells were mainly on the dendrites in the sublamina in which a cell's major dendritic arborization was contained.