Depolarization of primary afferents evoked by cyclically modulated natural activity of proprioceptors of the hindlimb of the cat

The primary afferent depolarization (PAD) evoked during passive sinusoidal movements of a hindlimb in the ankle joint was investigated in decerebrated cats. The frequency of movements varied within 0.14-5.0 Hz, the amplitude of the joint angle with respect to the axis of the tibia changed from 90 degrees to 130 degrees. The dorsal root potential (DRP) negativity increased both during flexion and during extension of the joint. The amplitude of the evoked DRPs was about 50-100 mV. A strong negative correlation was observed between the latency and rise time of the DRP and the frequency of the joint angle changes. During flexion the latency changed from 650 ms at 0.14-0.16 Hz frequency to 100-110 ms at 2.0 Hz and higher frequencies; during extension at the same frequencies the latency changed from 300 ms to 80-85 ms. The latency and rise time became minimal at 2.0 Hz frequency and practically did not change during the further increase of the oscillation frequency. The cord dorsum potential (CDP) evoked by the cutaneous nerve stimulation was recorded in parallel with the DRP. Periodical changes of the N-component of the CDP were in the opposite phase to changes of the DRP. Mechanisms of the observed changes of the PAD and functional significance of these changes during rhythmical motor acts are discussed.     

Discharges of forelimb spindle primary afferents during locomotor activity in the decorticate cat

In the acute decorticate cat, the discharge patterns of single spindle primary afferents from various forelimb muscles during locomotion have been examined in relation to the extrafusal activity. In a given muscle under isometric conditions, the activation of spindle primary endings was closely related to that of extrafusal muscle fibers. Since this increase in spindle discharge could occur in the absence of detectable electromyographic activity in the parent muscle, it resulted, at least to some extent from an enhanced activity in gamma efferents. The data are discussed in relation to previous studies of forelimb efferent activity and hindlimb fusimotor control in the same preparation.     

Activity of rubrospinal neurons during locomotion and scratching in the cat

It is now well established that locomotion and scratching in vertebrates can result from the activation of a spinal central generator. The possibility of control of these rhythmic motor activities by the red nucleus has been analyzed in the thalamic cat, in which efferent nerve discharges representing fictive locomotion or fictive scratching can still be recorded following paralysis by curarization. It was found that the discharge of lumbar-projecting rubrospinal neurons is modulated in relation to the intensity and frequency of the rhythmic efferent activity in the contralateral hindlimb. The average firing frequency was minimal at the transition between the extensor and flexor efferent bursts and increased progressively to reach a maximum in the second part of the flexor burst. Comparison of the rubrospinal activities during real and fictive rhythmic motor activities revealed only minor influences of phasic afferent inputs. Analysis of the relations between the rhythmic discharges found in rubrospinal neurons, cerebellar neurons (interpositus nucleus and paravermal Purkinje cells of the cerebellar anterior lobe) and neurons of an ascending pathway (ventral spinocerebellar tract) leads to the conclusion that the rubrospinal tract belongs to an internal loop between spinal and supraspinal centres. However, until now, the results do not allow the evaluation of its contribution to the motor performance, even in situations which, like those studied here, do not involve the complex motor control present in the intact cat.     

Changes in the segmental reactions in the decerebrate cat during scratching and locomotion

Experiments on decerebrate cats have shown that N1 component potential of dorsal surface and dorsal root potential elicited by electrical stimulation of paw plantar surface tonically decrease during locomotion and scratching. Phase changes occur against a background of tonic changes in the rhythm of limb movements. The dynamics of the observed changes correlates well with that of primary afferent depolarization during these movements. It is concluded that presynaptic inhibition underlies the changes in segmental reactions.     

Phase-dependent modulation of primary afferent depolarization in single cutaneous primary afferents evoked by peripheral stimulation during fictive locomotion in the cat

Previous results from our laboratory have shown with intra-axonal recordings that hindfoot cutaneous primary afferents are subjected to rhythmic depolarizations during fictive locomotion (L-PAD) suggesting that cutaneous presynaptic mechanisms are activated by the central locomotor program. In this study, we examined the transmission in pathways responsible for primary afferent depolarizations (PAD) of cutaneous fibres during spontaneous fictive locomotion in decorticate cats and in spinal cats injected with nialamide and L-DOPA. PADs were evoked (E-PADs) by electrical stimulation of peripheral nerves and recorded intra-axonally with micropipettes in identified superficialis peroneal (SP; n = 7) and tibialis posterior (TP; n = 17) cutaneous primary afferents. Results showed that the amplitude of E-PADs, which were superimposed on the L-PAD, was deeply modulated throughout the locomotor cycle; decreasing to reach a minimum during the flexor phase and increasing to a maximum during the extensor phase. The results were not statistically different in fibres of the two nerves and in both types of preparation. The amplitude of E-PADs was always maximum during the extensor phase whether there was a large L-PAD or not during that phase. This suggests that the presynaptic mechanisms activated by central locomotor networks (L-PAD) and those activated by peripheral inputs (E-PAD) may in part be controlled differently. The results thus show that the transmission in PAD pathways activated by cutaneous inputs is phasically modulated by the central pattern generator for locomotion. This strongly suggests that the presynaptic inhibition in cutaneous fibres evoked by the movement-related feedback during real locomotion could be similarly modulated.     

Morphology of single primary spindle afferents of the intercostal muscles in the cat

A reconstruction was made of the trajectory of primary spindle afferents from the intercostal muscles in the spinal cord of the cat. Intraaxonal recordings were performed from the primary spindle afferents that were identified by their response to lung inflation and stimulus threshold to activate the action potentials. The afferents were stained by using intraaxonal injection of horseradish peroxidase (HRP). Results were obtained mainly from internal intercostal Ia fibers, which entered the spinal cord and bifurcated into ascending and descending branches. The ascending branches could be traced up to 10.7 mm, and the descending branches could be traced up to 7.3 mm. The ascending branches extended to the next segment. Collaterals ranging from one to six were given off from these branches. The distances between adjacent collaterals ranged from 0.9 mm to 4.7 mm. Each collateral had similar morphological characteristics. The collaterals entered the dorsal horn and ran toward lamina IX through the medial half of the gray matter. Fine branches and boutons were given off in laminae V, VII, VIII, and IX. The aggregations of these branches were found in lamina VII, mainly in the region of Clarke's column and in the ventral and ventrolateral regions thereof and in lamina IX, mainly in the nucleus lateromedialis. Most terminals did not contact the somata of target neurons in all laminae in which terminals were found. However, a few terminals were found to contact large neurons in lamina IX. In addition to these aggregates, there were some terminals scattered throughout the ventral horn. Thus, it was concluded that single intercostal Ia afferents project to the region of Clarke's column, to the intercostal motor nucleus, and to the intermediate regions. J. Comp. Neurol. 398:459–472, 1998. © 1998 Wiley-Liss, Inc.     

Rhythmic antidromic discharges of single primary afferents recorded in cut dorsal root filaments during locomotion in the cat

Extracellular recordings were made from 52 units in perifused 300 microns thick slices of rat hypothalamus taken parallel to the ventricular surface of the arcuate nucleus and kept at 37 degrees C in an organ bath. Addition of 8 nM adrenocorticotrophic hormone (ACTH) to the perfusate reversibly and rapidly inhibited 15 of the 38 units found within 300 microns of the ventricular surface (in the arcuate nucleus), and there were no comparable excitations. A prompt and reversible excitation of 6/14 units by ACTH was found when records were made from units found 300-600 microns lateral to the ventricle in the lateral arcuate region. It is suggested that ACTH may be an inhibitory transmitter in the arcuate nucleus and that this action may be an inhibition of ACTH-containing neurons.     

Phase-frequency characteristics of afferent activity and depolarization of primary afferents during passive displacement of the hindlimb of the cat

Frequency response characteristics of afferent activity and primary afferent depolarization (PAD) evoked by this activity were investigated in decerebrated cats during passive movement of hindlimb in ankle joint. At the frequencies of 0.2-1.7 Hz the lead of the level of depolarization was determined by the value of phase changes of afferent activity. Above 1.7 Hz a phase lag of the level of depolarization was observed. This lag was caused by the dynamic properties of PAD generation system.     

The effects of antidromic discharges on orthodromic firing of primary afferents in the cat

This study investigated the effects of antidromically conducted nerve impulses on the transmission of orthodromic volleys in primary afferents of the hindlimb in decerebrated paralyzed cats. Two protocols were used: (A) Single skin and muscle afferents (N=20) isolated from the distal part of cut dorsal rootlets (L7-S1) were recorded while stimulation was applied more caudally. The results showed that during the trains of three to 20 stimuli, the orthodromic firing frequency decreased or ceased, depending on the frequency of stimulation. Remarkably, subsequent to these trains, the occurrence of orthodromic spikes could be delayed for hundreds of ms (15/20 afferents) and sometimes stopped for several seconds (10/20 afferents). Longer stimulation trains, simulating antidromic bursts reported during locomotion, caused a progressive decrease, and a slow recovery of, orthodromic firing frequency (7/20 afferents), indicating a cumulative long-lasting depressing effect from successive bursts. (B) Identified stretch-sensitive muscle afferents were recorded intra-axonally and antidromic spikes were evoked by the injection of square pulses of current through the micropipette. In this case, one to three antidromic spikes were sufficient to delay the occurrence of the next orthodromic spike by more than one control inter-spike interval. If the control inter-spike interval was decreased by stretching the muscle, the delay evoked by antidromic spikes decreased proportionally. Overall, these findings suggest that antidromic activity could alter the mechanisms underlying spike generation in peripheral sensory receptors and modify the orthodromic discharges of afferents during locomotion.     

Modification of primary afferent depolarization in cat group Ia afferents following high frequency intra-axonal tetanization of individual afferents

Intra-axonal tetanization of a single, functionally-identified group Ia afferent from the triceps surae muscle in the anesthetized cat produces marked enhancement and slowing of the primary afferent depolarization (PAD) generated in the Ia afferent by volleys in flexor muscle group Ia afferents, plus a pronounced transmembrane hyperpolarizing undershoot (HPU) which disappears more rapidly than the enhanced PAD. These alterations are qualitatively and quantitatively similar to those found after conditioning tetani are applied to the whole muscle nerve. The occurrence of these PAD changes after intra-axonal tetanization of a single group Ia afferent appears to rule out the participation of non-specific alterations in extracellular ionic concentrations or activation of polysynaptic pathways in their genesis.     

Readjustment of the efferent activity of the scratching generator in response to stimulation of cutaneous afferents of the hindlimb of the decerebrate immobilized cat

Rebuildings of the scratching generator efferent activity caused by the phasic electrical stimulation of ipsilateral hindlimb skin nerves during different hindlimb positions were studied in decerebrated immobilized cats. Stimulation was followed by short latency inhibition of the efferent activity. Stimulation did not cause correlation shifts in the common "aiming" and "scratching" activity. Changes in the efferent activity cycle duration and intensity depended on the stimulation phase. Inversion of intensity changes occurred with transition from the middle-force to strong stimulation. A functional role of the dependence of the efferent activity rebuilding on the stimulation phase is considered. The scratching generator is supposed to contain a model of the afferent inflow which enters the spinal cord during real scratching.     

Interrelation of the kinematics of hindlimb movement and efferent activity in the decerebrate cat during scratching

Statistical dependence between parameters of hindlimb scratching movements and muscle electrical activity was studied in decerebrated cat during real scratching. Power of electrical signal to scratching and aiming muscles and duration of aiming muscle activity are determining factors of movement parameters (amplitude, velocity and phase duration). Experiments with limb deafferentation had shown that statistical dependences between movement parameters and muscle electrical activity reflect the peculiarities of the scratch generator functioning. Some investigated statistical dependences change under the afferent signal influence.     

Somatic and visceral primary afferents in the lower thoracic dorsal root ganglia of the cat

Anterograde transport of horseradish peroxidase (HRP) through somatic and visceral nerves was used to estimate the proportions of somatic and visceral dorsal root ganglion (DRG) cells of the lower thoracic ganglia of the cat. A concentrated solution of HRP was applied for at least 5 hours to the central end of the right greater splanchnic nerve and of the left T9-intercostal nerve of adult cats. Some animals remained under chloralose anaesthesia for the duration of the HRP transport time (up to 53 hours) whereas longer HRP application and transport times (4-5 days) were allowed in animals that recovered from barbiturate anaesthesia. Visceral DRG cells were found in approximately equal numbers in all ganglia examined (T7-T11). Population estimates were obtained for the T8 and T9 ganglia where visceral DRG cells were found to be 6.2% (T8) and 5.2% (T9) of the total cell population. In contrast, somatic DRG cells were found in large numbers in the ganglia examined (T8 and T9) where they amounted to over 90% of the cell population. Measurement of cross-sectional areas and estimates of cell diameters of the DRG cells showed greater proportions of large somatic cells (diameter greater than 40 micron) than of large visceral cells. Similar distributions of cell size were found for both somatic and visceral DRG cells with diameters less than 40 micron. These results show that the proportion of visceral afferent fibres in the dorsal roots that mediate the spinal cord projection of the splanchnic nerve is very small. Since viscerosomatic convergence in the thoracic spinal cord is very extensive, the present results suggest considerable divergence of the visceral afferent input to the central nervous system.     

Morphology of single primary vestibular afferents originating from the horizontal semicircular canal in the cat

The central projections of physiologically characterized vestibular nerve fibers originating from the horizontal semicircular canal were studied in the vestibular nuclei of adult cats after intracellular staining with horseradish peroxidase (HRP). First, primary nerve fibers were physiologically classified as regular or irregular types on the basis of the regularity of the spontaneous discharge pattern. Then, these two types of fibers were morphologically analyzed and compared following HRP intraaxonal injection. The two types of axons showed a basically similar trajectory in the four major vestibular nuclei. They bifurcated into an ascending and a descending branch in the ventrolateral part of the lateral vestibular nucleus (LVN). The ascending branch extended rostrally and gave off one or two collaterals in the superior vestibular nucleus (SVN), although some of the ascending branches further ran rostrally into the cerebellum. The collaterals, while running medially, gave rise to fine terminal branches with en passant boutons in the SVN, and further coursing caudally, they entered the rostral part of the medial vestibular nucleus (MVN). The descending branch, while running caudally in the lateral part of the LVN and the inferior vestibular nucleus (IVN), gave off several thick collaterals to the MVN and extensive terminals were present in the IVN and MVN. In each primary axon, about one-third of the total terminal boutons were distributed in each of the SVN, the MVN, and the IVN. In contrast to this similarity of the overall axonal trajectory within the vestibular nuclei, both types of axons exhibited several marked differences in diameter and in the mode of terminal arborization. In almost every part of the ramification, the irregular-type fibers were thicker than the regular-type fibers. In the regular-type axons, many small terminal boutons (mean size, 2.4 x 1.4 microns, N = 2,739) were located in close proximity (100-150 microns) to the parent collateral. In the irregular-type axons, slightly larger terminal boutons (mean size, 3.0 x 1.7 microns, N = 1,287), were spread more widely (200-300 microns) around their collaterals. These clear morphological differences between the regular-type and the irregular-type terminal axons were consistently observed in any vestibular nucleus.     

Readjustment of the efferent activity of the scratching generator in response to stimulation of muscle afferents of the hindlimb of the decerebrate immobilized cat

Rebuildings of the scratching generator activity caused by phasic electrical stimulation of ipsilateral hindlimb muscle nerves during different hindlimb positions were studied in decerebrated immobilized cats. Strong dependence of these rebuildings on the stimulation phase was observed. The character of the "scratch" cycle duration rebuilding was formed by the scratching generator tendency to bring efferent activity into such correlation with the stimulus that the stimulation moment coincided with the moment of efferent activity phase triggering. Phasic altering of the efferent activity intensity rebuilding was observed against a background of "aiming" and "scratching" activity correlation shift in the direction of strengthening activation of muscles innervated by the stimulated nerve. This rebuilding was intensified when the hindlimb deflects from the aimed position in the direction of corresponding muscles stretching. Physiological sense of "rebuilding absence phases" is discussed. It is postulated that absence of the duration and intensity changes can be achieved simultaneously only with definite correlation between phase and intensity of the afferent impulsation burst.     

Changes in the intensity of integral afferent inflow from limb receptors and the level of polarization of primary afferent endings in the decerebrate cat during scratching

The experiments performed on decerebrated cats have shown that afferent activity during scratching consisted of two components--tonic and periodic phasic ones. The first component was determined by the limb position, the second was closely related to the amplitude and velocity of joint angle changes. Maximum of integral afferent activity in the cycle coincided with the scratching jerk phase. These two components of afferent activity evoked corresponding depolarization changes in primary afferent terminals and these changes added to those elicited by central generator. Statistical correlations between the aforementioned parameters were studied. The afferent control mechanisms of scratching generator are under discussion.     

Relation between parameters of efferent activity and limb position during fictive scratching in the decerebrate cat

The influence of tonic afferent inflow conditioned by ipsilateral hindlimb position on the spinal generator of scratching efferent activity parameters was investigated in immobilized decerebrated cats. When the afferent inflow was absent (after the hindlimb deafferentation) strong correlation between the motor activity parameters of successive "scratch" cycles was observed. In case of the intact afferentation this correlation was somewhat weaker and decreased with shifting the hindlimb from the aimed position to overaimed or deflecting backward. The discovered statistically significant correlations between the parameters of different motor activity phases and their dependence on the hindlimb position are organized in such a way that they can provide stability of the aimed position of the hindlimb carrying out oscillatory movements. According to the parameters of the efferent activity and their correlation type, the hindlimb deafferentation is most similar to the aimed position of the hindlimb.     

Depolarization of the terminals of different groups of afferent fibers of the lumbar region of the spinal cord during fictive scratching

Kinetics of lumbar primary afferent depolarization was investigated during fictitious scratching of decerebrated and decapitated cats. Fictitious scratching was accompanied by tonic and phasic primary afferent depolarization. Periodic depolarization of primary afferents was synphasic in different segments of the lumbosacral spinal cord. Tonic depolarization was observed in the terminals of all the investigated groups of fibres (low-threshold cutaneous and of groups 1a and 1b). Phasic changes took place in terminals of low-threshold cutaneous fibres and fibres of group 1b. Physiological significance of modulation of primary afferent depolarization by the spinal scratching generator is discussed on the basis of the data obtained.