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Biomedical subjects

J M Shefner

Publications and source records attributed to J M Shefner.

68 records · Page 4Linked to original sources
PubMed1990

The use of sensory action potentials in the diagnosis of peripheral nerve disease.

We review the different measurements that can be derived from recordings of compound sensory action potentials, and describe how less commonly measured aspects may be of use in clinical situations. In addition to the frequently obtained maximum conduction velocity and amplitude, minimum conduction velocity may be measured if near nerve electrodes and response averaging are used. Minimum conduction velocity is a sensitive measure of both axonal and demyelinating peripheral nerve pathology. Activity-dependent conduction is another aspect of sensory conduction that may provide additional information for the clinician. Refractory period of transmission has been used to diagnose subtle demyelinating lesions, as well as some axonal disorders. Supernormal period and the conduction of trains of stimuli may also be useful, although, at present, they remain predominantly research tools.

Action Potentials↗
PubMed1988

Cold-induced peripheral nerve damage: involvement of touch receptors of the foot.

A 31-year-old male developed paresthesia and numbness of mainly the right foot following exposure to nonfreezing temperatures under moist conditions over a period of 1 week. The symptoms gradually improved over several months. When seen for electrophysiological studies 6 months after the injury, there was no sensory loss on clinical examination, although he continued to complain of distal numbness of the right foot. The right extensor digitorum brevis muscle was atrophic, and the distal motor latency in the peroneal nerve was prolonged. Conduction studies of the right sural nerve showed a predominantly distal diminution of the SAP evoked by electrical stimulation at the dorsum pedis. Action potentials evoked by tactile stimulation of Pacinian corpuscles showed a prolonged latency on the symptomatic side, suggesting that the most pronounced pathological changes in immersion injury may be localized to the very distal portion of the nerve at the nerve fiber-receptor junction.

Action Potentials↗
PubMed1987

Quantitative assessment of position sense at the ankle: a functional approach.

We studied position sense at the ankle in 22 normal subjects to develop a functionally meaningful method of identifying mild position sense loss in individuals with suspected posterior column disease. Subjects were asked to match the angular displacement of a passively positioned reference foot. The subjects tried to improve performance by training sessions and visual feedback during the task. The subjects made accurate matches over a wide range of positions, with an average error of about 3 degrees. These methods can be used to provide a sensitive assessment of proprioception.

Adult↗
PubMed1981

Distance-dependent interactions between the rod and the cone systems in goldfish retina.

We recorded from ganglion cell axons in the optic tracts of self-respiring goldfish, and examined the interaction of rod-effective and cone-effective stimuli within their receptive fields. The summation of influences due to the rod system and the long wavelength sensitive cone system was analyzed by the methods of response-summation and sensitivity-summation. Different spatial relationships between the rod- and cone-effective stimuli allowed examination of distance-dependent effects. Both the response-summation and sensitivity-summation analyses showed a difference in non-linearity between a configuration in which the rod- and cone-effective stimuli were spatially overlapped and a configuration in which they were not. This difference in both analyses demonstrates a distance dependent interaction between the rod and cone systems. Both analyses also showed a difference in non-linearity between a configuration in which the rod-and cone-effective stimuli were nearby (but not overlapped) and one in which they were more distant. This demonstrates that the interaction is not limited to receptors that are immediate neighbors. An estimate of the strength of interaction in each case showed that the differences among the three configurations were relatively slight, indicating a broad spread of the effect. The interaction was found to be relatively powerful; assuming a specific simple model for the interaction mechanism, we found that each system exerts an effect upon the other which accounts for about 1/3 of its signal.

Animals↗
PubMed1977

Interactions between rod and cone systems in the goldfish retina.

Signals from both the rod and the cone receptor systems converge upon the same retinal ganglion cell, but only one or the other of these systems appears to be effective at any particular level of adaptation. In this report we provide evidence that the change from one receptor system to the other is not simply due to the two systems having nonoverlapping dynamic ranges; rather, there is a distance-dependent interaction between the two systems.

Adaptation, Ocular↗
PubMed1977

A model for the variability of interspike intervals during sustained firing of a retinal neuron.

The statistics of the variability of interspike intervals of ganglion cells in the retina of goldfish are modeled by assuming the noise in an integrate-and-fire mechanism is proportional to the reciprocal of a normally distributed variable. This model meets the constraint that the coefficient of variation of the interspike. This does not change when the mean firing rate of the neuron changes. Alternative sources of variability of interspike intervals are discussed.

Action Potentials↗
PubMed1975

Independence of "on" and "off" responses of retinal ganglion cells.

Recordings of action potentials from retinal ganglion cells that are stimulated repetitively demonstrate two properties (i) variability introduced during the stimulus is not evident in the response that occurs at stimulus offset and (ii) variability in the ON response shows a different temporal structure than variability in the dark. Our findings demonstrate that these responses are generated independently.

Action Potentials↗
PubMedDate not supplied

Dietary intake of calcium, magnesium and antioxidants in relation to risk of amyotrophic lateral sclerosis.

Dietary factors have long been suspected of being risk factors for amyotrophic lateral sclerosis (ALS), but few human studies have been reported. To address several of the dietary hypotheses, a case-control study of risk factors for ALS conducted in New England in 1993-1996 included an abbreviated food frequency questionnaire. We examined the dietary intake of calcium, magnesium and antioxidants among 107 ALS cases and 262 community controls. Overall, these dietary factors were not related to risk of ALS, though modestly protective associations were suggested for magnesium and lycopene.

Adult↗