Biomedical subjects

C F Ramos

Publications and source records attributed to C F Ramos.

6 recordsLinked to original sources

PubMed1991

Use of Y-chromosome-specific DNA probes to evaluate an XX male.

1. The function of a Y human chromosomal DNA sequence was evaluated. The Y-5 probe was isolated from a flow-sorted chromosome library and detects Y-specific sequences. 2. The Y-5 probe and other Y-specific probes were used to analyze an XX male patient without ambiguous genitalia. 3. DNA sequences from the short arm of the chromosome Y that were detected with pDP1007 and pDP105 in the patient's genome explain the testis differentiation observed in this case. 4. Failure of the patient's DNA to hybridize to the Y-5 probe shows that the primitive gonads can differentiate into testes even in the absence of this chromosome region. In contrast, a gene controlling spermatogenesis may exist in this region because the patient is azoospermic.

Adolescent↗

PubMed1990

Postural maintenance during movement: simulations of a two joint model.

Voluntary movements of the upper body are accompanied by anticipatory postural adjustments to the lower body in a standing subject. The long-standing hypothesis is that these anticipatory adjustments serve to counteract the perturbation to the body's center of gravity caused by the voluntary arm movement. This paper presents model simulations investigating the possible roles of anticipatory postural activity that accompanies a rapid, upward arm swing. The model incorporates two (idealized) antagonistic muscle pairs controlling the movements of a double-joint system, with a "shoulder joint" between the arm and stiff body links, and an "ankle joint" between the stiff body-leg segment and the ground. Each muscle is represented by a nonlinear viscoelastic element and also includes proprioceptive feedback. Four inputs to the model define the motor control signals for muscle force generation in both the arm and the postural muscle pairs. The neurological component of the model describes consequences of alternate strategies for cocontractions, stretch reflex activity, and anticipatory and synchronous postural activities (or combinations thereof). Simulations with this model show that: (1) none of the postural maintenance schemes considered in these simulations (including varying anticipation) could suppress the initial backward thrust on the body link; (2) the more important destabilizing perturbation is a subsequent forward sway that, left uncountered by postural activity, would eventually leave the body to fall flat on its face; and (3) anticipatory silencing of the postural extensor followed by a brief period of extensor activation (descending control) and synchronous reflex activity (feedback control) appears to be the most likely postural stabilizing strategy that inhibits the continuous forward sway and is consistent with the experimental evidence.

Arm↗

PubMed1990

Postural maintenance during fast forward bending: a model simulation experiment determines the "reduced trajectory".

A recent article by Crenna et al. (1987) has shown that fast, forward bending movements are accompanied by a backwards motion of the hips and lower limbs. The ongoing research presented in this brief note expands upon the experimental data described by Crenna and colleagues, concerning the postural activities associated with rapid forward bending in standing man. Our primary experimental tool is the computer simulation method, with the standing subject being represented by a double-joint system: the trunk is modeled as a rigid link mechanically coupled (via a "hip" joint) to the lower body link fixed to the ground (via an "ankle" joint). Each of the two joints in this system is independently controlled by a neurological control model for single joint movements, consisting of an idealized pair of antagonistic muscles (flexor and extensor), their common load, and proprioception from the muscle spindles. This model thereby integrates descending commands with proprioceptive feedback in controlling the joint movements. Our early simulation experiments determine a "reduced trajectory", that is, the physical perturbation to the postural system, due to the voluntary movement, in the absence of any stabilizing activities. These simulation experiments clearly show that an important component of the backward movements in the hips and lower limbs during forward bending is indeed due to the mechanical (physical) coupling between the upper and lower body segments and thus not solely a consequence of the anticipatory postural muscle activity. Simulations also predict that any postural activities in the hips and lower limbs should be a two-fold process: first, some preprogrammed, descending control to the lower body would be required to actively enhance the passive, backwards motion (this is consistent with, though not strictly identical to, the hypothesis of Crenna and colleagues); secondly, there must be a subsequent activation in the anterior muscles of the lower body in order to arrest this backwards motion, since otherwise the uncountered momentum would carry the body backward to the floor in less than half a second after the upper body movement has terminated.

Computer Simulation↗

PubMed1990

Behaviour space of a stretch reflex model and its implications for the neural control of voluntary movement.

A nonlinear model for the stretch reflex has recently been used to study the interactions between voluntary and reflex controls during fast, targeted movements. The present study explores the topography of a 'behaviour space' generated by computer simulations of this model under various combinations of values for the gain parameters and time constants in the model's feedback loops. In general, we define a behaviour space to be any set of behavioural characteristics of the simulated movement, such as movement time, peak acceleration or peak velocity. The mathematical model can therefore be viewed as an M x N dimensional map from its parameter space N to a behaviour space M. Here, a one-dimensional behaviour space is explored. This provides a method for quantitatively comparing the different control strategies that might be employed by the nervous system for integrating reflex and descending signals during fast, voluntary movements. The results indicate that an optimal strategy will employ proprioceptive feedback as a means of fine-tuning the braking and clamping activities of fast, goal-directed movements and that descending signals are primarily important for initiating the movement and for controlling reciprocal patterns of muscle activity during the end phase of the movement.

Feedback↗

PubMed1989

A technical improvement for the thyroid peroxidase iodination assay.

Thyroid peroxidase (TPO) iodination activity is generally evaluated in vitro by the iodination of poorly iodinated thyroglobulin or bovine serum albumin, followed by separation of protein-bound and inorganic iodide by paper chromatography. Precipitation of protein-bound iodine by trichloroacetic acid (TCA) was evaluated as an alternative to the time-consuming paper chromatographic separation (PC) in normal rat TPO preparations. The protein-bound iodine estimates as well as the iodination activities determined by these two procedures were significantly correlated (r = 0.95 and 0.98, respectively, P less than 0.001), and the iodination activities regression line slope (b = 0.97 +/- 0.11) was not different from 1. The protein-bound iodine separation by TCA is simpler and faster, without loss of precision. Thus, it can be a useful alternative step in the thyroid peroxidase iodination assay.

Animals↗

PubMed1987

Thyroid function in fasting rats: variations in 131I uptake and transient decrease in peroxidase activity.

Serum thyroxine and triiodothyronine, radioiodide thyroid uptake and thyroid peroxidase (TPO) activity were studied over a 2 to 5 day period in fasting rats treated (F+) or not (F-) with TSH. In F- rats, TPO activity was transiently decreased on the 3rd day, whereas in F+ it was always higher than in controls. On the 5th day, the 2 h thyroid uptake of 131I decreased in F-, while the 24 h uptake increased in both F- and F+. Serum T3 and T4 decreased in both fasting groups. Thus, not all effects of fasting on rat thyroid function are reverted by TSH administration, suggesting intrinsic impairment of glandular function.

Animals↗