Crossed receptive field components and crossed dendrites in cat sacrocaudal dorsal horn.

The hypothesis that sacrocaudal dorsal horn neurons with crossed receptive field components on the tail have dendrites which cross to the contralateral dorsal horn was tested in a combined electrophysiological and morphological study. Dorsal horn cells in the sacrocaudal spinal cord of anesthetized cats were penetrated with horseradish peroxidase-filled microelectrodes. After mapping their low threshold mechanoreceptive fields, cells were iontophoretically injected with horseradish peroxidase. A sample of 16 well-stained cells was obtained in laminae III and IV. Cells with receptive fields crossing the dorsal midline of the tail (n = 8) had somata in the lateral ipsilateral dorsal horn, and some of these cells (5/8) had dendrites which crossed to the lateral contralateral dorsal horn. Cells with receptive fields spanning the ventral midline (n = 2) were located near the center of the fused dorsal horn, and one of these had bilateral dendrites in this region. Cells with receptive fields on the lateral tail, crossing neither the dorsal nor the ventral midline (n = 6), had cell bodies in the middle of the ipsilateral dorsal horn; half had only ipsilateral dendrites, and half had crossed dendritic branches. Although the relationship between cell receptive field (RF) location (RF center, expressed as distance from tips of toes) and mediolateral location of the cell body was statistically significant, the correlation between crossed RF components and crossed dendritic branches was not significant.

Using the Gompertz-Strehler model of aging and mortality to explain mortality trends in industrialized countries.

Mortality trends in industrialized countries are characterized by declines in vascular disease (ischemic heart disease and stroke) and rises in cancers and degenerative diseases. These trends are typically analyzed by examining each disorder in isolation using the perspective of genetic and environmental influences. However, longitudinal Gompertzian analysis and the Gompertz-Strehler model of aging and mortality as modified by Lestienne suggest that age-specific mortality rates, for both general and disease-specific mortality, are an interrelated deterministic function of aggregate genetic, environmental and competitive influences. Consequently, evolving mortality trends and patterns appear to be influenced by three factors (with deterministic competition being the third factor), rather than just two factors (genetic and environmental) as commonly depicted.

Mortality among the elderly in the U.S., 1956-1987: demonstration of the upper boundary to Gompertzian mortality.

Mortality in the United States among individuals aged 85 years and older between the years 1956 and 1987 was analyzed using a cumulative summation technique. This analysis demonstrates that general mortality conforms to Gompertzian dynamics through age 96 years for both men and women. Of the 33 320 985 deaths in men in the U.S. between 1956 and 1987, only 123,643 (0.37%) occurred in men aged 97 years and older. Of the corresponding 26,946,599 deaths in women, only 327,291 (1.21%) occurred in women aged 97 years and older. These results suggest that the competitive and deterministic features of the Gompertzian model of human aging and mortality remain valid through age 96 years and cover the vast majority of human mortality.

Influence of map scale on primary afferent terminal field geometry in cat dorsal horn.

1. Thirty-one physiologically identified primary afferent fibers were labeled intracellularly with horseradish peroxidase (HRP). 2. A computer analysis was used to determine whether the distribution of cutaneous mechanoreceptive afferent terminals varies as a function of location within the dorsal horn somatotopic map. 3. An analysis of the geometry of the projections of these afferents has shown that 1) terminal arbors have a greater mediolateral width within the region of the foot representation than lateral to it, 2) terminal arbors have larger length-to-width ratios outside the foot representation than within it, and 3) the orientation of terminal arbors near the boundary of the foot representation reflects the angle of the boundary. Previous attribution of mediolateral width variations to primary afferent type are probably in error, although there appear to be genuine variations of longitudinal extent as a function of primary afferent type. 4. Nonuniform terminal distributions represent the first of a three-component process underlying assembly of the monosynaptic portions of cell receptive fields (RFs) and the somatotopic map. The other two components consist of the elaboration of cell dendritic trees and the establishment of selective connections. 5. The variation of primary afferent terminal distributions with map location is not an absolute requirement for development of the map; for example, the RFs of postsynaptic cells could be assembled with the use of a uniform terminal distribution for all afferents, everywhere in the map, as long as cell dendrites penetrate the appropriate portions of the presynaptic neuropil and receive connections only from afferent axons contributing to their RFs.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatotopic organization of single primary afferent axon projections to cat spinal cord dorsal horn.

Horseradish peroxidase injection of identified low threshold cutaneous mechanoreceptor (LTCM) primary afferent axons was used to assess the somatotopic organization of hindlimb projections to laminae III and IV of cat dorsal horn. Multiple injections in the same animals were used to assess bilateral symmetry and precision. Thirty-one axons were injected, with more than 1 axon injected in each of 8 animals (25 axons). Somatotopic relations between their receptive field (RF) centers and the centers of their dorsal horn projections were similar to the somatotopic relations between dorsal horn cell RF centers and cell locations. Very few reversals of mediolateral somatotopic gradients (proximodistal RF location as a function of mediolateral projection center) were observed. Two afferents with nearly identical RFs in 1 animal had nearly identical projections. These observations held for many different combinations of receptor types. A simple mathematical model was used to demonstrate that assembly of dorsal horn cell RFs via passive sampling of the presynaptic neuropil by dorsal horn cell dendrites cannot account for the sizes of dorsal horn cell LTCM RFs. Hypothesized mechanisms for assembly of dorsal horn cell RFs must take into account the functional selectivity of connections required to produce RFs smaller than those predicted by the passive assembly model.