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PubMed1984

Reflex effects from high threshold neck muscle afferents on hind limb extensor gamma motoneurones in the cat.

The supra-segmental control of hind limb gamma motoneurones from neck muscle receptors has been studied in decerebrated and in spinal cats. Stretch of individual dorsal neck muscles was not an adequate stimulus for evoking long spinal reflexes to gamma motoneurones of gastrocnemius-soleus (GS) muscles unless the stretch was maximal or excessive. Pressure applied to the neck muscles, or intramuscular injections of KCl solution (0.1 ml, 5%), did affect the discharge of GS gamma motoneurones. Excitation was more evident than inhibition. We conclude that the long spinal reflex effects originate from high threshold mechanoreceptors, or nociceptors, rather than muscle spindles.

Animals↗
PubMed1977

Response of rapidly and slowly adapting mechanoreceptors and vibratory sensitivity in human hairy skin.

Single unit activity was recorded percutaneously with microelectrodes from 38 rapidly adapting (RA) mechanoreceptors of the dorsal surface of the hand in 49 awake human subjects. Tuning characteristics were determined for 28 RA-fibers at various frequencies between 5 and 100 cps of sinusoidal mechanical stimulation. In separate experiments human thresholds of vibration perception were studied under comparable conditions. Several RA-mechanoreceptive fibers were broadly tuned, showing no clearly defined best frequency in the range between 5 and 20 cps. Other RA-fibers had a minimum of sensitivity between 20 and 40 cps. For either lower or higher frequencies, stronger stimuli were required to elicit one nerve impulse per stimulus cycle. These RA-receptors may be related to the perception of low frequency oscillation (flutter). They cannot account for human vibration sensitivity in the higher frequency range, since tuning of the receptors required considerably higher amplitudes than perception. Sixteen slowly adapting (SA) mechanoreceptors of Type I and II showed a frequency modulation in phase with low frequency mechanical oscillations of stimulus amplitudes far below human thresholds for perception of movement.

Action Potentials↗
PubMed1980

Brain stem auditory evoked potentials are related to interaural time discrimination in patients with multiple sclerosis.

In a combined psychophysical-electrophysiological study on 29 patients with multiple sclerosis (MS), a compromised ability to make interaural time discriminations was nearly always found to be associated with 'abnormal' brain stem potentials evoked by clicks to at least one ear. However, no obvious relationships were found between evoked brain stem potentials and several other auditory behavioral measures such as interaural intensity discrimination, pure-tone thresholds and speech discrimination.

Adolescent↗
PubMed1975

Influence on touch, vibration and cutaneous pain of dorsal column stimulation in man.

Five patients out of a group of ten who had dorsal column electrodes implanted for the relief of chronic pain were examined for the influence of the stimulation on the spontaneous pain and on the thresholds for touch, vibration and cutaneous pain induced by pinching. Stimulation producing paraesthesias resulted in an almost immediate abolishment of spontaneous pain and was accompanied by significant elevations of both tactile and vibratory thresholds. Elevation of thresholds was confined to segments below the site of implantation and occurred bilaterally also when the paraesthesias were restricted to one side. The changes of thresholds generally persisted for some time after the stimulation but these effects were short lasting in comparison with the effect on spontaneous pain. Elevation of sensory thresholds is presumably not due to blocking of the primary neurones but to central inhibitory mechanisms. The thresholds for induced cutaneous pain were not influenced by dorsal column stimulation except for one case in whom an abnormally low threshold within an hyperaestethic area became normalized.

Adult↗
PubMed1990

Effect of pinealectomy on cortically kindled rats.

Cortical kindling in pinealectomized rats was studied to observe the effect of pinealectomy on local afterdischarge thresholds and seizure development. Although the local afterdischarge thresholds were not affected by pinealectomy, the kindling process itself was hastened. These results suggest that the anticonvulsive function of the pineal body is due not to a decrease in local afterdischarge thresholds, but to retardation of secondary generalization.

Animals↗
PubMed1978

Movement discrimination capacities in the cat.

Cats were trained to discriminate moving from nonmoving targets or one direction of movement from another. Various stimulus changes, e.g., size, direction, and rate, were then introduced as a test for generalization of the dimension of movement. Thresholds for detection for minimal movement were also determined. The results showed that (a) for cats, discrimination of movement is more difficult than discriminations based on brightness; (b) the dimension of movement is completely generalized across stimulus configuration but incompletely generalized for direction of movement; (c) the mean movement detection threshold was found to be 3.3 degrees/sec; (d) the thresholds for minimal movement and direction of movement were essentially identical; and (e) stimulus-viewing strategies were found to play an important role in the threshold determinations. The results of the generalization tests are consistent with the physiological properties of neurons found to be sensitive to movement in the cat visual system. The movement threshold values were found to lend support to the view that resolution and slow movement thresholds are correlated.

Animals↗
PubMed1992

[Documentation of glaucomatous visual field defects in threshold and suprathreshold perimetry].

In contrast to normal subjects, the individual stimulus threshold for differential light sensitivity (DLS) in glaucoma patients fluctuates considerably. The sensitivity and the size of the defect are correlated: with a higher individual stimulus threshold, i.e. when sensitivity deteriorates, the number of defects decreases. Conversely, when the individual stimulus threshold drops, i.e. when sensitivity improves, the number of defects recorded increases. The trend is the same with extensive Grade 3 defects. This correlation between sensitivity and defect size means that the number and depth of individual defects change constantly, due to the fluctuation of the individual stimulus threshold. In follow-up examinations test positions with permanent defects (a defects) can be distinguished from those where defects have been detected several times, but not permanently (b defects) and, respectively, only once in 2 1/2 years (c defects). Defects must therefore be classified not only according to depth, but also according to behaviour at individual test positions. Such a classification can be accomplished by tabulating all newly numbered test positions and the defects identified there. After 5 visual field examinations the defects are then classified as a, b, or c defects. They are then identified by colour symbols and transferred to a test position chart representing a synopsis of 5 visual fields. The chart not only permits exact topographical analysis but also furnishes information on certain structural features of the glaucomatous visual field: stable nuclear areas of a defects surrounded by fluctuating peripheral defects (b defects) or concomitant c defects. After a further 1 1/2-2 years, 5 more follow-ups are summarized, classified and transferred to a follow-up test position chart. Comparison of these two test position charts furnishes information of major prognostic value.

Aged↗
PubMed1996

T-type Ca2+ channel lowers the threshold of spike generation in the newt olfactory receptor cell.

Mechanisms underlying action potential generation in the newt olfactory receptor cell were investigated by using the whole-cell version of the patch-clamp technique. Isolated olfactory cells had a resting membrane potential of -70 +/- 9 mV. Injection of a depolarizing current step triggered action potentials under current clamp condition. The amplitude of the action potential was reduced by lowering external Na+ concentration. After a complete removal of Na+, however, cells still showed action potentials which was abolished either by Ca2+ removal or by an application of Ca2+ channel blocker (Co2+ or Ni2+), indicating an involvement of Ca2+ current in spike generation of newt olfactory receptor cells. Under the voltage clamp condition, depolarization of the cell to -40 mV from the holding voltage of -100 mV induced a fast transient inward current, which consisted of Na+ (INa) and T-type Ca2+ (ICa.T) currents. The amplitude of ICa,T was about one fourth of that of INa. Depolarization to more positive voltages also induced L-type Ca2+ current (ICa,L). ICa,L was as small as a few pA in normal Ringer solution. The activating voltage of ICa,T was approximately 10 mV more negative than that of INa. Under current clamp, action potentials generated by a least effective depolarization was almost completely blocked by 0.1 mM Ni2+ (a specific T-type Ca2+ channel blocker) even in the presence of Na+. These results suggest that ICa,T contributes to action potential in the newt olfactory receptor cell and lowers the threshold of spike generation.

Action Potentials↗
PubMed2003

ECS seizure threshold: normal variations, and kindling effects of subconvulsive stimuli.

BACKGROUND: The efficacy and adverse effects of electroconvulsive therapy are generally believed to depend upon the extent to which an administered stimulus is suprathreshold. The seizure threshold is therefore an important biologic marker. We sought to examine the variability of the electroconvulsive shock (ECS) seizure threshold in rats, and to identify factors influencing the threshold, to guide future research using animal models. MATERIALS AND METHODS: We administered once-daily subconvulsive stimuli to Wistar rats beginning at a charge of either 1 mC (n = 25) or 5 mC (n = 25) and titrated the dose upward in 1-mC steps until the baseline seizure threshold was identified. Two weeks later, we divided each group into two subgroups and administered stimuli that were either at or 2 mC below the baseline threshold, and titrated the dose upward, again in 1-mC steps once daily, until the final threshold was identified. RESULTS: The mean baseline seizure threshold was 3.8 mC when upward titration was begun at 1 mC, and 6.7 mC when upward titration was begun at 5 mC (p < 0.001). Two weeks later, titration from baseline-subthreshold stimuli was associated with a lower final threshold in the 5-mC group, while titration from baseline-threshold stimuli was associated with a higher final threshold in the 1-mC group (p < 0.006). CONCLUSIONS: The ECS seizure threshold ranged from 3 to 7 mC in this sample of rats; since the twofold variation is very small relative to clinical contexts, it is unlikely that ECS research needs to be threshold-based. The administration of low-dose, once-daily subconvulsive stimuli significantly lowered the seizure threshold; while this kindling effect wore off within 2 weeks, thresholds otherwise identified remained stable at the 2-week time point.

Animals↗
PubMed1991

Identification of a low-threshold T-type calcium channel in bovine ciliary epithelial cells.

The characteristics of an inward current in single pigmented cells from the bovine ciliary body were examined using the whole cell recording technique. The inward current appeared to activate at potentials 30 mV positive to a holding potential of -80 mV, although activation studies revealed that from more negative holding potentials the current activated at about -75 mV, peaked at around -10 mV, and reversed at a potential of +40 mV. The current was small, peak value -72 +/- 24 pA (n = 13), and exhibited rapid activation and slower inactivation kinetics, both processes being voltage sensitive. Inactivation occurred at holding potentials more positive than -120 mV and was complete at -50 mV with a half-maximal membrane potential of -88 mV. The overlap of the inactivation and activation curves between -75 and -50 mV means that, in the steady state, entry of calcium will be greatest in this range and calcium will thus enter the cell at the resting membrane potential. The current was resistant to tetrodotoxin and nifedipine. It was partially blocked by Cd2+ and almost completely blocked by Ni2+. It is concluded that this current is a low-threshold T-type calcium current.

Animals↗
PubMed1983

Ventilatory thresholds during short- and long-term exercise.

The ventilatory (anaerobic) threshold for short-term exercise was defined as the work rate or O2 uptake (VO2) immediately below the work rate at which ventilation increased disproportionately relative to work rate or VO2, and the ventilatory threshold for long-term exercise as the work rate or VO2 immediately below the work rate at which ventilation continued to increase with time rather than attain a steady state. The purpose of the present study was to investigate how both thresholds relate to each other and how they relate to other measures of physical performance capacity. The subjects were eight healthy males, 20-53 yr of age. Maximal performance capacity was estimated by measurements of maximal O2 uptake (VO2 max) and by endurance performance during a 12-min distance run. A high interrelationship was found between the two thresholds (r = 0.84), and each threshold expressed in VO2 (ml X min-1 X kg-1) correlated highly with VO2 max (r = 0.87 and r = 0.75, for short-term and long-term exercise, respectively). When the two thresholds were expressed as a percentage of VO2 max, neither threshold showed a significant relationship with VO2 max. Endurance performance was significantly correlated with both the ventilatory threshold for short-term and long-term exercise (r = 0.73 and 0.82, respectively). A stepwise multiple regression analysis indicated that the distance run in 12 min was best predicted by VO2 max (R2 = 0.66) or the ventilatory threshold for long-term exercise (R2 = 0.63). It is concluded that the ventilatory threshold for long-term exercise is a more specific measure to explain running performance than is the threshold during graded exercise.

Adult↗
PubMed1976

Mechanoreceptors for near-field water displacements in crayfish.

1. Mechanosensory hairs on the surface of the crayfish telson are dually innervated, one sensory cell responding to headward, the other to tailward deflection of the hair. The average conduction velocity of headward elements was 0.8 m/s (variance 0.08) and of tailward elements 1.2 m/s (variance 0.19). In a frequency range from 0.05 to 200 Hz, thresholds were lowest near 20 Hz: 0.08 mum (pp) for headward-sensitive and 0.1 mum (pp) for tailward-sensitive cells. 2. The receptors are displacement sensitive since thresholds are of the same order of magnitude over the frequency range 1-70 Hz when the hair is moved by a vibrating wire loop. With natural stimuli (surface waves), the velocity component of the particle movement (and consequently force) becomes influential. The coding of a broad range of stimulus intensities is aided by variations in mechanical properties of the hair. 3. Marked directionality (better than 4:1), in addition to the dual innervation, enhances vector detection. At least part of this characteristic stems from the hingelike articulation of the hair on the body surface: the hair can be moved easily 40 degrees tailward and 20 degrees headward, but must be forced in the orthogonal direction. Morphological studies indicate the presence of a double pivoted hinge, with rigid guides for movement of the hair shaft. Preliminary results of electron microscope examination show a clearly polarized arrangement of densely packed microtubules in the two dendrites; they appear interconnected in groups of two and three along a line parallel to the sensitivity plane of the receptor. 4. The 50-fold threshold difference between the results of behavioral experiments in lobsters (24) and the data for the individual receptors reported here may be due to improvement in signal-to-noise ratio by central nervous averaging of the input from an estimated 2 X 10(3) receptors (Procambarus), and/or to the kind of threshold criteria applied to individual receptor thresholds. As would be expected (35), the sensory cells of each directional class synapse with separate interneurons: in this way, the organism might employ differential microphones to reduce background noise. 5. The receptors are analogous to those of the lateral-line system in lower vertebrates in having receptors with sensitivities polarized by 180 degrees. These similarities suggest that in both cases monitoring of near field water displacements has proved in essential way of orienting in opaque waters.

Animals↗
PubMed2000

Pulse-duration threshold functions obtained at the MWS/LWS cone antagonistic locus.

Pulse-duration threshold functions were determined for spectral increments in the region of the Sloan notch (580 nm) as well as for 440 and 640 nm and white increments flashed on a spatially coincident white background. While increments of 440 and 640 nm elicited traditional pulse-duration functions with decreasing thresholds as the duration increased, functions obtained with 580 nm and white increments manifested a minima at about 50 ms. These data support the notion that stimuli in the region of Sloan's notch can be detected by an achromatic mechanism that may integrate sub-threshold ON and OFF responses, thereby maximizing detection sensitivity.

Color Perception↗