Spinal control of afferent temperature information in the cat

The effect of picrotoxin (at sub-convulsant doses, i.v.) on the background and invoked spike activity of cat dorsal horn internerons was investigated while thermal and mechanical influences were at work on the skin receptor areas. It was found that information on skin temperature is modulated at the presynaptic level and largely mediated by competitive interaction between large and small diameter fibers.Institute of Physiology, Academy of Sciences of the Kazakh SSR, Alma Ata. Translated from Neirofiziologiya, Vol. 18, No. 6, pp. 773–779, November–December, 1986.     

Neural encoding schemes of tactile information in afferent activity of the vibrissal system

When rats acquire sensory information by actively moving their vibrissae, a neural code is manifested at different levels of the sensory system. Behavioral studies in tactile discrimination agree that rats can distinguish different roughness surfaces by whisking their vibrissae. The present study explores the existence of neural encoding in the afferent activity of one vibrissal nerve. Two neural encoding schemes based on “events” were proposed (cumulative event count and median inter-event time). The events were detected by using an event detection algorithm based on multiscale decomposition of the signal (Continuous Wavelet Transform). The encoding schemes were quantitatively evaluated through the maximum amount of information which was obtained by the Shannon’s mutual information formula. Moreover, the effect of difference distances between rat snout and swept surfaces on the information values was also studied. We found that roughness information was encoded by events of 0.8 ms duration in the cumulative event count and event of 1.0 to 1.6 ms duration in the median inter-event count. It was also observed that an extreme decrease of the distance between rat snout and swept surfaces significantly reduces the information values and the capacity to discriminate among the sweep situations.     

Extra spike formation in sensory neurons and the disruption of afferent spike patterning

The peculiar pseudounipolar geometry of primary sensory neurons can lead to ectopic generation of "extra spikes" in the region of the dorsal root ganglion potentially disrupting the fidelity of afferent signaling. We have used an explicit model of myelinated vertebrate sensory neurons to investigate the location and mechanism of extra spike formation, and its consequences for distortion of afferent impulse patterning. Extra spikes originate in the initial segment axon under conditions in which the soma spike becomes delayed and broadened. The broadened soma spike then re-excites membrane it has just passed over, initiating an extra spike which propagates outwards into the main conducting axon. Extra spike formation depends on cell geometry, electrical excitability, and the recent history of impulse activity. Extra spikes add to the impulse barrage traveling toward the spinal cord, but they also travel antidromically in the peripheral nerve colliding with and occluding normal orthodromic spikes. As a result there is no net increase in afferent spike number. However, extra spikes render firing more staccato by increasing the number of short and long interspike intervals in the train at the expense of intermediate intervals. There may also be more complex changes in the pattern of afferent spike trains, and hence in afferent signaling.     

Convergence of afferent impulses from individual semicircular canals of pontine reticular units.

The effect of afferent impulses on single pontine RF units was recorded by means of tungsten microelectrodes in 22 curarized guinea pigs followed thermic stimulation of individual semicircular canals. The labyrinthine stimulation consisted of short-lasting circumscribed heating (45 degrees C) of the canals through a silver thermode. Unitary response to labyrinthine stimulation was characterized by excitation and inhibition of rest discharge rate. Out of 86 units, 66 received bilateral labyrinthine influences responding in greatest number (20 units) to four semicircular canals. Only two units were unaffected. No significant difference in RF responses was noticed between concordant and antagonistic canals. Reticular formation units showed great degree of facilitation of response and convergence of impulses from the single six semicircular canals.     

Characteristics of capsaicin-sensitive afferent neurons of spinal nerve ganglia

Morphological peculiarities of neurons that had TRPV1, SP, CGRP, and NF200 markers in thoracic spinal ganglia nerves were studied in 3-month-old rats subjected to chemical deafferentation produced by capsaicin. The obtained results have shown that from 6.5 to 41.3% of ganglion neurons in the control group of animals contain, as a rule, one of the above-mentioned markers. The heterogeneity of nociceptive neurons observed in a control group of rats was also preserved in the capsaicin-treated animals. In both groups, the spinal ganglion TRPV1 neurons were predominant, whereas populations of SP, CGRP, and NF200 neurons formed smaller groups. In each population, sensitivity to capsaicin was shown in the largest neurons, regardless of marker; this sensitivity was pronounced to the greatest degree in the group of TRPV1 neurons.     

Role of afferent pathways of heat and cold in body temperature regulation

The detection of surface and internal temperatures is achieved by axons terminating at lamina I of the spinal dorsal horn, otherwise approached only by nociceptive afferents. Recent advances in thermal physiology research have disclosed that temperature-sensitive ion channels belonging to the transient receptor potential family exist in the peripheral sensory neurons and in the brain. Thermosensory, nociceptive and polymodal afferents project to different thalamic nuclei, and specific pathways to the insular cortex evoke the conscious experience of thermal sensation. The posterior insular region represents discriminative thermal sensation, while the largest correlation with subjective ratings of temperature is located in the orbitofrontal and anterior insular cortex. The insular cortex forms an integrative part of the limbic system and is closely tied with the hypothalamus, the amygdala, the anterior cingulate cortex and the orbitofrontal cortex and emerges as the main coordinator of behavioral, autonomic and endocrine responses to both non-noxious and noxious thermal stimuli. The firing rate of warm and cold receptors is not altered by pyrogens. A strong correlation between the onset of fever and production of superoxide by macrophages following the injection of pyrogens implicates reactive oxygen species as elicitors of fever, a hypothesis strengthened by the observation that oxygen radical scavengers or thiol reductants act as antipyretics. Oxidative stress appears to be sensed by the brain and a likely structure for its detection may be the redox-sensitive site of the N-methyl-d-aspartate (NMDA) receptor for glutamate, in that oxidation of this site causes fever while its reduction lowers body temperature, effects which are abrogated by specific NMDA receptor blockers.     

Characteristics of unitary afferent discharges of neuromuscular spindles in man]

The primary ending of muscle spindle in man shows a dynamic and static sensitivity to stretch, but the dynamic and vibratory sensitivities as well as conduction velocity of the afferent fibres seem to be relatively low in comparison to those described in the cat.     

Response of motor units of the abdominal musculus rectus to afferent input in the course of the long-lasting activity in rats

In rats, in the course of a long-lasting m. rectus abdominis motor units activity combined with repetitive afferent stimulation of the ischiadic nerve, the units' firing rate decreased. The dependence of the motor units response to afferent stimulation on the background firing rate was preserved during the long-lasting activity in spite of changes in the background firing rate.     

Rigorous and extended application of information theory to the afferent visual system of the cat

1. Intracellular recording were obtained from P-cells of the LGN of the cat. The impulse trains of a single presynaptic retinal ganglion cell and the postsynaptic P-cell were separated by band-pass-filtering and subsequent amplitude discrimination.
2. The rates of information and transinformation for the visual channel from the eye to a ganglion cell and to the connected P-cell were calculated. Input signals to the channel were trains of light flashes of different rate, luminance and spatial distribution.
3. Transinformation was calculated without restrictive assumptions for the code.
4. The transient behaviour of the system in response to a flash was fully considered for information calculations. Additionally, it was ensured that the state of the (adaptive) channel was considered correctly.
5. Information theory was applied in an extended way. The time courses of information transfer were calculated for various flash stimuli and compared with each other.

     

Prostaglandin inhibitor indomethacin inhibits afferent activities of Adelta and C units in the saphenous nerve of diabetic hyperalgesic rats

The effects of a non-selective inhibitor of cyclo-oxygenase (COX) indomethacin, and exogenous prostaglandin E(2) (PGE(2)) on A(delta) units and C units in the saphenous nerve of diabetic hyperagesic rats were studied. The results showed that the conduction velocity of A(delta) units and C units and their mechanical threshold in diabetic hyperagesic rats were obviously decreased, and a small number of A(delta) units (4/24) and C units (2/18) produced increased spontaneous activities. Intraperitoneal injection of indomethacin in diabetic hyperagesic rats significantly relieved mechanical hyperalgesia, and resulted in a decrease in spontaneous afferent activities of the A(delta) units and C units. Subcutaneous injection of exogenous PGE(2) into the diabetic hyperagesic and control rats produced a significant decrease in mechanical threshold of the A(delta) units and C units, and elicited discharge from 3 A(delta) units (3/24) and 1 C unit (1/18) in diabetic hyperagesic rats and from 2 A(delta) units (2/13) in control rats. The present data suggest that the synthesis and release of PGs are increased in diabetic neuropathy, PGs can sensitize and /or activate A(delta) units and C units and elicit hyperagesia and allodynia in diabetic rats.     

The relation of serine-glycine metabolism to the formation of single-carbon units

Cultivation of serine-glycine auxotrophs of Escherichia coli in a medium containing serine and a purine base results in a temporary inability of the cells to grow upon transfer to a medium containing glycine as the nutrilite. It could be shown that this failure to grow is due to the inability of the cells to obtain single-carbon units from glycine. The formation of the enzyme system required for the synthesis of single-carbon units from glycine seems to be controlled through repression exerted by the intracellular single carbons. The intracellular concentration of single-carbon units derived from the β-carbon of serine is apparently increased by the addition of a purine base to the growth medium.