High-threshold Group Research Articles

Excitatory and inhibitory effects of repetitive stimulation of group I and II extensor afferents on homonymous motoneurons.

The repetitive firing of functionally isolated 143 extensor alpha motoneurons in response to homonymous group I and II tetanizations was studied in decerebrate cats. Of a total sample of 92 group I activated motoneuronal units, 76 motoneurons (82.6%) discharged phasically and 16 motoneurons (17.4%) discharged tonically at threshold or just suprathreshold tetanizations. Only 6 of the phasic motoneurons fired tonically when the stimulus strength was increased within the group I range. The firing rate of 8 phasic motoneurons increased at group I and II stimulations, but they retained their phasic discharge characteristics. The remaining 46 group I activated phasic motoneurons (60.1%) delivered tonic discharges when group II afferents were stimulated. Another group of 16 phasic motoneurons (21.1%) became tonic at group II tetanus, but their tonic discharge was usually interrupted by an inhibitory period, which coincided with an escape reaction of the animal. Out of 38 group II activated motoneuronal units, only 8 (21.1%) motoneurons discharged tonically at threshold or just suprathreshold tetanizations. The repetitive activity of these motoneurons was usually interrupted by an inhibitory period, which coincided with an escape reaction of the animal. This inhibition disappeared after procain application to the GS nerve, and was attributed to dominant inhibitory action of group II afferents mediating the sensation of pain. The remaining 30 (78,9%) motoneurons discharged phasically. Six of them remained phasic even at high threshold group II tetanus. The others gave tonic discharges when the stimulus strength was increased within the group II range. A great percentage of motoneurons discharged regularly with a fixed decoding ratio reflecting monosynaptic excitation. Some motoneurons discharged in multiple bands of impulse intervals due to a variable decoding ratio, which can be attributed to both monosynaptic and polysynaptic reflex discharge elicited by group I and/or II tetanizations. There were 6 motoneurons (4.2%), which discharged repetitively at stimulations of the GS, DP, and SUR nervers. These motoneurons were referred to "common motoneurons", which may participate in multiple motor actions such as fixation of a limb or as a part of the flexor reflex. SOL motoneurons usually discharged tonically at tetanizations within the group I range. The firing rate of these motoneurons usually decreased at tetanizations of the group II afferents of MG nerve. Conversely, tetanization of the group II afferents of SOL nerve usually exerted facilitatory effects on group II activated MG motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Summating potentials in Meni�re's disease

The summating potentials in response to tonebursts of 2, 4, and 8 kHz were studied in a group of 112 patients diagnosed as having Menière's disease, and the results were compared with recordings from 27 normal human ears. Subdivision of the Menière ears into a low-threshold (threshold less than 50 dB HL) and a high-threshold group (greater than 50 dB HL) proved to be functional in relation to the behaviour of the SP. The data of the low-threshold Menière group did not differ significantly from the normal data, whereas the high-threshold Menière group showed significantly smaller summating potentials. Up to hearing thresholds of 50 dB, the detection threshold for the SP was independent of the hearing threshold; at higher hearing losses, the increase in SP threshold equalled the increase in hearing loss. Analysis showed that the correlation found between the changes in SP and hearing threshold in individual patients did not differ significantly from zero for threshold values below 50 dB. The overall conclusion is that in Menière ears hearing losses up to about 50 dB are not related to changes in the hair cells, since the SP does not change, whereas the increase in the amount of hearing loss above 50 dB HL is paralleled by a loss in sensitivity of the summating potential and is therefore related to a functional loss of hair cells.

A seven-year review of measured hearing levels in male manual steelworkers with high initial thresholds.

ABSTRACT Audiometric records of 449 male manual steelworkers were reviewed to see whether the hearing of men with high initial threshold levels deteriorated more rapidly than that of those men with more normal thresholds when exposed to similar noise levels. Subjects were between 15 and 54 years old at the time of initial examination, and had a repeat audiogram 6-8 years later. They were classified into three occupational noise exposure groups: below 90 dB(A), 90-99 dB(A), and 100 dB(A) or over, without knowledge of their audiometric threshold levels, age, or aural history. Measurements at 0·5, 1, 2, 3, 4 and 6 kHz for the right ear were considered, first as the mean for all six frequencies, then for 0·5, 1 and 2 kHz only, and finally for 6 kHz only. After standardisation for age, it was found that those in the high initial threshold groups deteriorated no faster than the remainder of the series. At the 90-99 dB(A) noise exposure level, χ² tests showed that the differences between the groups, in terms of mean measured hearing loss, were not significant at the 0·05 probability level. The age-standardised mean threshold shifts for the 0·5-6 kHz range of frequencies over the seven-year review period were 7·5, 8·7 and 7·1 dB at a noise exposure level of 100 dB(A) or more, for workers with an initial threshold level of <12 dB, 12-26 dB and > 26 dB respectively; for those exposed to noise of 90-99 dB(A) the corresponding mean shifts were 7·8, 6·8 and 7·3 dB respectively; while for those exposed to noise of less than 90 dB(A) the mean shifts were 6·2, 5·0 and 5·2 dB respectively.

The effect of stimulation of Golgi tendon organs and spindle receptors from hindlimb extensor muscles on supraspinal descending inhibitory mechanisms.

Experiments were performed in precollicular decerebrate cats to investigate whether proprioceptive volleys originating from Golgi tendon organs and muscle spindles may activate supraspinal descending inhibitory mechanisms. Conditioning stimulation of the distal stump of ventral root filaments of L7 or S1 leading to isometric contraction of the gastrocnemius-soleus (GS) muscle inhibited the monosynaptic reflex elicited by stimulation of the ipsilateral plantaris-flexor digitorum and hallucis longus (Pl-FDHL) nerve. The amount and the time course of this Golgi inhibition were greatly increased by direct cross-excitation of the intramuscular branches of the group Ia afferents due to ephaptic stimulation of the sensory fibers, which occurred when a large number of a fibers had been synchronously activated. The postsynaptic and the presynaptic nature of these inhibitory effects, as well as their segmental origin, have been discussed. In no instance, however, did the stimulation of Golgi tendon organs elicit any late inhibition of the test monosynaptic reflex, which could be attributed to a spino-bulbo-spinal (SBS) reflex. Conditioning stimulation of both primary and secondary endings of muscle spindles, induced by dynamic stretch of the lateral gastrocnemius-soleus (LGS) muscle, was unable to elicit any late inhibition of the medial gastrocnemius (MG) monosynaptic reflex. The only changes observed in this experimental condition were a facilitation of the test reflex during the dynamic stretch of the LGS, followed at the end of the stimulus by a prolonged depression. These effects however were due to segmental interactions, since they persisted after postbrachial section of the spinal cord. Intravenous injection of an anticholinesterase, at a dose which greatly potentiated the SBS reflex inhibition produced by conditioning stimulation of the dorsal root L6, did not alter the changes in time course of the test reflex induced either by muscle contraction or by dynamic muscle stretch. Conditioning stimulation of a muscle nerve activated the supraspinal descending mechanism responsible for the inhibitory phase of the SBS reflex only when the high threshold group III muscle afferents (innervating pressure-pain receptors) had been recruited by the electric stimulus. This finding contrasts with the great availability of the system to the low threshold cutaneous afferents. The proprioceptive afferent volleys originating from Golgi tendon organs as well as from both primary and secondary endings of muscle spindles, contrary to the cutaneous and the high threshold muscle afferent volleys, were apparently unable to elicit not only a SBS reflex inhibition, but also any delayed facilitation of monosynaptic extensor reflexes attributable to inhibition of the cerebellar Purkinje cells.

The response of some sympathetic neurones to volleys in various afferent nerves.

1. Responses of sympathetic neurones to various afferent inputs have been analysed both in anaesthetized cats with intact neuraxis and in spinal cats.2. In anaesthetized cats electrical stimulation of low threshold group III afferent fibres in skin and muscle nerves inhibited sympathetic neurones and gave depressor responses. The silent period following a sympathetic reflex discharge is most likely due to inhibition brought on by a particular subgroup within this afferent group.3. High threshold group III and group IV afferent fibres excited sympathetic neurones and elicited pressor responses.4. Sympathetic reflex arcs could be temporally facilitated during the somatic afferent induced inhibition, by group IV, and to a lesser extent by group III volleys.5. Section of the spinal cord shortened the time course and lessened the degree of group III inhibition suggesting that suprasegmental pathways are involved in the long-lasting depression following a reflex.6. Baroreceptor afferent stimulation inhibited group III- and group IV- evoked reflexes even with some temporal facilitation in the pathway.7. The special properties of the sympathetic reflex arcs and the relation of the results to other work on B.P. reflexes are discussed. It is suggested that the group III and IV muscle afferents have a chemoreceptor function and are responsible for mediation of the pressor reflex during muscle exercise.

Myelinated afferent nerve fibers from the skin of the rabbit ear

Electrophysiological recordings were made by dissection and microelectrode methods from single myelinated fibers of the great auricular nerve responding to mechanical stimulation of hairs and skin. Two types of hair follicle afferent unit, both rapidly adapting, were observed: type G, conducting at 18 to 54 m/sec, excited by movement of up to six guard hairs, with amplitude thresholds of 8 to 36 μ; and type D, conducting below 18 m/sec, excited by many down hairs and guard hairs with thresholds of 2 to 30 μ. Units responding only to stimulation of the dorsal skin surface comprised a low-threshold group, excited also by movement of ventral hairs, and a high-threshold group, also excited by stimulation of the ventral skin surface. Slowly adapting mechanoreceptive units, touch corpuscles, and type T hair follicle units were not found. The innervation of rabbit car skin thus differs considerably from that of the trunk and limbs.

Properties of Afferent Connections to the Rostral Spinocerebellar Tract in the Cat

AbstractOscarsson, O. and N. Uddenberg. Properties of afferent connections to the rostral spinocerebellar tract in the cat. Acta physiol. scand. 1965. 64. 143–153. – Properties of the rostral spinocerebellar tract (RSCT) have been studied and compared with those of its functional hindlimb equivalent, the ventral spinocerebellar tract (VSCT). Both RSCT and VSCT are monosynaptically activated from high threshold Group I muscle afferents and polysynaptically influenced from the flexor reflex afferents. Excitation from Group I afferents is mono‐, di‐, and polysynaptic in the RSCT, but exclusively monosynaptic in the VSCT. The monosynaptic linkage has different properties in the two tracts. The effects from the flexor reflex afferents are predominantly excitatory in the RSCT and predominantly inhibitory in the VSCT. The significance of similarities and differences in the organization of the two tracts is discussed.

IDENTIFICATION OF A SPINOCEREBELLAR TRACT ACTIVATED FROM FORELIMB AFFERENTS IN THE CAT.

AbstractA spinocerebellar tract activated from ipsilateral forelimb nerves has been identified by antidromic activation from the cerebellar cortex. This tract has been denoted the rostral spinocerebellar tract (RSCT). It originates from cell bodies at, or slightly above, the level of the dorsal roots supplying excitation. The RSCT ascends in the middle third of the lateral funiculus where it has a more ventral position than the ventral spinocerebellar tract (VSCT) which it partly overlaps. About two thirds of the RSCT enter cerebellum together with the VSCT through the brachium conjunctivum. About one third of the RSCT shifts dorsally, becomes contiguous to the dorsal spinocerebellar tract (DSCT), and enters cerebellum through the restiform body. The RSCT terminates mainly in the anterior lobe but some fibres send branches also to the pyramis and the ipsilateral paramedian lobule. In the anterior lobe the RSCT terminates in bilateral zones consisting of a medial strip of the intermediate cortex and a lateral strip of the vermal cortex. The termination is more intense ipsilaterally and the fibres reach equally often the “hindlimb” as the “forelimb area”. — The RSCT receives monosynaptic excitation from high threshold group I muscle afferents and polysynaptic excitation from the flexor reflex afferents. The functional organization and the characteristic cerebellar termination suggest that the RSCT is a forelimb equivalent of the VSCT.

Effects of visceral and somatic afferent nerve stimulation on intra-abdominal pressure reflex of pelvic nerve origin

Effects of excitation of visceral and somatic nerve afferents on the intra-abdominal pressure reflex produced by pelvic nerve stimulation were studied in cats. Stimulation of vagus, splanchnic and somatic mixed and skin nerves mostly depressed this reflex. A similar inhibitory interaction was obtained by stimulating branches of the pelvic nerve itself. From hind-limb nerves, inhibition of the reflex was effected by excitation of Group III and probably high-threshold Group II fibers. Judging from the threshold voltage for inhibitory action, the same was considered to be true for the remaining nerves examined. The interaction appeared to occur extensively in the central pathways of the reflex, but the principal place of blocking varied with the kind of nerves stimulated. Depression of the reflex following cranial and brachial nerve stimulations appeared to be mostly dependent upon increased supraspinal inhibition or occulusion of the bulbar reflex center. On the other hand, the study of hind limb afferents presented evidence that the motoneuron pool for the abdominal muscles could also be suppressed directly having a short latency.

Three Ascending Tracts Activated from Group I Afferents in Forelimb Nerves of the Cat

Publisher Summary Group I muscle afferents in hindlimb nerves activate two ascending tracts—the dorsal and the ventral spino-cerebellar tract (DSCT and VSCT). Group I afferents in forelimb nerves activate three ascending pathways. The rostra1 spino-cerebellar tract (RSCT) originates from cell bodies at, or slightly above, the level of the dorsal root entrance, ascends ipsilaterally in the middle third of the lateral funiculus, and terminates in a characteristic manner in the anterior cerebellar lobe. It receives monosynaptic excitation from high threshold group I muscle afferents, identical with tendon organ afferents. The cuneo-cerebellar tract contains one component that is monosynaptically activated from very low threshold group I muscle afferents, identical with muscle spindle afferents. The third pathway is a projection to the cerebral cortex of large muscle spindle afferents. The group I afferents ascend in the dorsal funiculus and activate monosynaptically cells in the main cuneate nucleus, which give origin to a component of the medial lemniscus.