The number of neuronal cell bodies has been counted in a narrow strip (30 micrometers) through the depth of the neocortex in several different functional areas (motor, somatic sensory, area 17, frontal, parietal and temporal and in many species (mouse, rat, cat, monkey and man). With the exception of area 17 of the visual cortex in a number of primates the same absolute number (congruent to 110) of neurons has been found in all areas and in all species. In the binocular part of area 17 of the primates there are approximately 2.5 times more neurons. Thus in mammalian evolution the area of the neocortex increases in larger brains but the number of neurons through the depth remains constant, except in area 17 of primates. From these and other findings it is suggested that the intrinsic structure of the neocortex is basically more uniform than has been thought and that differences in cytoarchitecture and function reflect differences in connections.
It has been assumed that the molecular weight (MW) cut-off of a newly fabricated polysulfone capillary dialyzer (F60, Fresenius, FRG) is similar to that of the human glomerulus. We recently tested the device in vivo and found this not to be so, based on the device's ability to eliminate substances of a MW of 10,000 to 60,000 daltons. One of the reasons for this discrepancy was found to be the influence of secondary membrane formation on solute permeability. Endogenous marker substances of a defined MW (beta 2-microglobulin, myoglobin, RBP, alpha 1-microglobulin, acid alpha 1-glycoprotein, alpha 1-antitrypsin, prealbumin, and albumin were measured by laser nephelometry or radioimmune assay; sieving coefficients (SC) and protein eliminations were calculated for each low MW protein.
The intrinsic organization of the central nucleus of the inferior colliculus has been studied in the adult cat by means of the Golgi and Nauta techniques. On the basis of cytoarchitecture and fiber connections, the central nucleus can be divided into: (i) a smaller, dorsomedial division. consisting mainly of large cells and receiving fibers from the auditory cortex, lateral lemniscus and probably the central nucleus of the other side; (ii) a larger, ventrolatenzl division consisting mainly of medium and small cells but with some intermingled larger types. This part receives fibers from the lateral lemniscus only.The ventrolateral division has a pronounced laminar arrangement of cells, dendrites and axons. The laminae, although overlapping extensively, form an onion-like series of concentric, curved shells, most of which are incomplete except for those closest to the center of curvature in the dorsolateral part of the nucleus. It is probable that these laminae provide a basis for the pronounced tonotopic organization of neurons in the nucleus. The thickness of the laminae is determined by the dendritic ramifications of two principal cell types which are fusiform or bi-tufted. Other, multipolar and large cells have dendrites which lie across one or several laminae and may form a basis for interaction between laminae.Four axon types can be distinguished: T y p e I (lateral lemniscal) and type II (corticofugal) axons run parallel to the laminae but in opposite directions, though type I1 axons are confined to the parts of the laminae projecting into the large-celled, dorsomedial division. T y p e 111 axons are widely ramifying and may arise within the central nucleus or be another form of afferent fiber. Type I and I11 axons end in dense clusters of terminals on principal cells but as single e n passant terminals on multipolar cells. Type IV axons are the efferents of the inferior colIiculus and run a recurrent course through the central nucleus giving off collaterals which sometimes end on the parent cell.
The central nucleus of the inferior colliculus has been examined electron microscopically in the adult cat, particular attention being paid to its ventrolateral division in which the majority of lateral lemniscal afferents end.Two cell types can be recognized: one, which is the equivalent of the spiny principal cell of light microscopy, is distinguished by having its soma and large proximal dendrites covered by axon terminals. More distally situated dendrites have fewer terminals ending mainly on dendritic spines. These spines resemble those of the cerebral cortex and have a modified spine apparatus. The second type of cell is t h e cquivalent of the small multipolar cell of light microscopy. It has spineless dendrites and these and the soma receive relatively fewer axon terminals. It has a very thin axon.Several types of axon terminal can be recognized. Type I terminals are large, contain spherical synaptic vesicles and end by means of asymmetrical synaptic contacts, on the somata and proximal dendrites of principal cells and on the smaller dendrites of multipolar cells. The extracellular space on one or both sides of the synaptic complex is often characteristically widened. Correlative experimental degeneration studies show that these are the terminals of lateral lemniscal fibers. Various forms of smaller terminal also make asymmetrical contacts and have spherical vesicles. One type has a high concentration of large, densecored vesicles as well.Two types of axon terminal end in symmetrical synaptic contacts. One contains large, homogeneously flattened synaptic vesicles; the other, which is more common, contains smaller, irregularly flattened or pleomorphic synaptic vesicles.In the dorsomedial division of the central nucleus, the terminals of corticofugal fibers have spherical synaptic vesicles and end in asymmetrical synaptic contacts upon large dendritic spines.
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