Origin of the silent period in somato- and viscero-sympathetic responses

The origin of the period of postactivation depression of spike activity in the renal nerve during stimulation of afferent fibers of cutaneous (cutaneous branch of the peroneal nerve) and visceral (greater splanchnic nerve) nerves was studied in unanesthetized, anesthetized, decerebrate, and spinal cats. This silent period was shown to be considerably prolonged after administration of general anesthetics to unanesthetized animals or after decerebration. Analeptics (strychnine, leptazol, picrotoxin) or division of the spinal cord at the level of the lower cervical segments weaken postactivation depression. The causes of origin of the silent period are discussed and the localization of the structures responsible for its appearance is examined.I. P. Pavlov First Leningrad Medical Institute. Translated from Neirofiziologiya, Vol. 4, No. 5, pp. 501–509, September–October, 1972.     

Action of ammonium acetate on central primary afferent depolarization

Experiments on anesthetized spinal cats showed that ammonium acetate, injected intravenously (2–4 mmoles/kg) inhibits the depolarization of the central endings of primary afferent fibers activated by stimulation of afferent nerves. Inhibition of primary afferent depolarization is transient in character and develops parallel with depression of postsynaptic inhibition of monosynaptic reflexes. The depression produced by the action of ammonium was not due to blocking of negative postsynaptic potentials of the dorsal surface of the spinal cord or blocking of reflex electrical discharges in the ventral spinal roots. It is suggested that depression of primary afferent depolarization is due to a decrease in the emf for synaptic ion currents producing depolarization.Allergologic Research Laboratory, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 9, No. 1, pp. 52–60, January–February, 1977.     

On mechanisms of depression in pathways of presynaptic inhibition

We investigated inhibition of the N₁-component of the spinal cord dorsal potential (CDP) evoked by experimental stimulation of the n. peroneus in spinal cats. Stimulation was carried out following two conditioning stimuli administered at different time intervals to the same or different cutaneous nerves. The interval between the last conditioning stimulus and the experimental one remained constant (20 msec). It is demonstrated that there is no dependence between weakening of inhibitory action of the second conditioning stimulus and inhibition of the dorsal horn interneurons excited by it that generate the N₁-component of the CDP. It is hypothesized that mechanisms which act on the principle of negative feedback are present in the vincinity of the synaptic junctions of cutaneous afferent fibers with neurons of the substantia gelationsa, and that these mechanisms restrict the development of presynaptic inhibition during inflow of a series of afferent impulses into the cord.Dnepropetrovsk State University. Translated from Neirofiziologia, Vol. 1, No. 3, pp. 253–261, November–December, 1969.     

Mechanism of inhibition following reflex discharge in spinal cord interneurons

Using the method of microelectrode (intracellular and extracellular) recording, the mechanism of inhibition following reflex discharge in interneurons of the lumbosacral section of the spinal cord of cats on activation of cutaneous and high-threshold muscle afferents was studied. It was shown that the postdischarge depression of the reflex responses 10–20 msec after the moment of activation of the neuron is due to afterprocesses in the same neuron and presynaptic pathways. The depression of spike potentials from the 20th to the 100th msec is produced by inhibitory postsynaptic potentials (IPSP). During the development of IPSP the inhibition of spike potentials can be due to both a decrease of the depolarization of the postsynaptic membrane below the critical threshold and a decrease of sensitivity of the cell membrane to the depolarizing action of the excitatory postsynaptic potential (EPSP). At intervals between the stimuli of 30–100 msec the duration of EPSP after the first stimulus does not differ from that after the second stimulus. Hence, it is suggested that the presynaptic mechanisms do not play an essential part in this type of inhibition of interneurons. The inhibition following the excitation favors the formation of a discrete message to the neurons of higher orders.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 2, No. 1, pp. 3–9, January–February, 1970.     

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.     

Role of propriospinal neurons in inhibition of the afferent flow

The effects of stimulation of the dorsal funiculus on dorsal surface potentials (DSPs) of the spinal cord evoked by stimulation of a peripheral nerve and on antidromic action potentials (AAPs) evoked by stimulation of terminal branches of primary afferent fibers and recorded from the afferent nerve or dorsal root, were investigated in acute experiments on spinal cats and on cats anesthetized with pentobarbital and chloralose. Stimulation of the dorsal funiculus led to biphasic inhibition of the N₁-component of the DSP with maxima at the 15th–30th and 60th–80th milliseconds between the conditioning and testing stimuli. Maximal reinforcement of the AAP was found with these intervals. Bilateral division of the dorsal funiculi between the point of application of the conditioning stimuli and the point of recording the DSP abolished the first wave of inhibition of the DSP and the reinforcement of the AAP. After total transection of the cord above the site of conditioning stimulation the picture was unchanged. It is concluded that the initial changes in DSP and AAP are due to activation of the presynaptic inhibition mechanism by antidromic impulses traveling along nerve fibers running in the dorsal funiculus. Repeated inhibition of the DSP, like reinforcement of the AAP, can possibly be attributed to activation of similar inhibitory mechanisms through the propriospinal neurons of the spinal cord.Dnepropetrovsk State University. Translated from Neirofiziologiya, Vol. 5, No. 4, pp. 401–405, July–August, 1973.     

Effects of partial bulbar section on inhibition of jaw-opening reflexes induced by central gray matter stimulation in the cat

The effects of severing the spinal trigeminal tract and its caudal nucleus on high-threshold jaw-opening reflex elicited by tooth pulp stimulation were investigated during experiments on cats under chloralose-Nembutal anesthesia. Low-threshold jaw-opening reflex produced by stimulating the A--infraorbital nerve at an intensity 2–3 thresholds in relation to the most excitable fibers on this nerve was also observed, as well as suppression of these reflexes induced by central gray matter stimulation. It was found that spinal trigeminal tract section produces a 8–52% increase in high-threshold reflex. The amplitude of low-threshold reflex either remained unchanged or showed a slight tendency to rise or fall. Brief stimulation of the central gray matter produced a 100% decrease in high-threshold reflex in intact animals compared with a 40–60% decrease after section of the trigeminal tract. Protracted stimulation of the central gray brought about an 80% decline in high-threshold reflex in intact animals as against 25–30% after section. The degree to which brief stimulation of the central gray produced depression of low-threshold stimulation remained unchanged by trigeminal tract section. Protracted stimulation of the central gray matter brought about a 25–50% reduction in low-threshold reflex in intact animals and a reduction of 75% in three animals and 15–20% in four animals. This implied that the caudal nucleus of the spinal trigeminal tract exerts a more substantial influence on the process of high- than low-threshold reflex inhibition when the central gray matter is stimulated.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 3, pp. 362–368, May–June, 1987.     

Reticular structures and reticulospinal pathways participating in the initiation and inhibition of spino-bulbo-spinal activity

Reflex discharges in intercostal nerves and activity of reticulospinal fibers of the ventral and lateral funiculi, evoked by stimulation of the reticular formation and of the splanchnic and intercostal nerves were investigated in cats anesthetized with chloralose (50 mg/kg). Brain-stem neuronal structures participating in the "relaying" of spino-bulbo-spinal activity were shown to lie both in the medial zones of the medullary and pontine reticular formation and in its more lateral regions; they include reticulospinal neurons and also neurons with no projection into the spinal cord. Structures whose stimulation led to prolonged (300–800 msec) inhibition of reflex spino-bulbo-spinal activity were widely represented in the brain stem, especially in the pons. Analogous inhibition of this activity was observed during conditioning stimulation of the nerves. Reticulospinal fibers of the ventral (conduction velocity 16–120 m/sec) and lateral (17–100 m/sec) funiculi were shown to be able to participate in the conduction of spino-bulbo-spinal activity to spinal neurons. In the first case fibers with conduction velocities of 40–120 m/sec were evidently most effective. Evidence was obtained that prolonged inhibition of this activity can take place at the supraspinal level.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Institute of Normal and Pathological Physiology, Slovak Academy of Sciences, Bratislava, Czechoslovakia. Translated from Neirofiziologiya, Vol. 8, No. 4, pp. 373–383, July–August, 1976.     

Changes in antidromic discharges in peripheral nerves during polarization of the cat spinal cord by a steady current

The effect of a steady current passed through the spinal cord on antidromic discharges in primary afferent groups of Agb cutaneous nerves of the hind limb, evoked by single and paired stimulation of the terminals of these fibers, was investigated by Wall's technique in acute experiments on spinal and anesthetized cats. A current of up to 50–100 µA, flowing in the dorso-ventral direction, led to an increase in amplitude of antidromic dischanges evoked by single stimulation of afferent terminals; if the current flowed in the opposite direction, the opposite effect was observed. The relative degree of facilitation of antidromic discharges caused by conditioning stimulation of these same fibers was reduced by a polarizing current in either direction. It is suggested that the effects of the action of a steady current flowing through the spinal cord observed in these experiments are due mainly to shifts of membrane potential in primary afferent terminals.Dnepropetrovskii State University. Translated from Neirofiziologiya, Vol. 14, No. 4, pp. 386–391, July–August, 1982.     

Responses of motor cortex neurons of the cat to auditory stimulation and their role in the instrumental food reflex

The responses of motor cortex neurons in the cat to the presentation of a single auditory click and a series of 10 clicks presented with 1,000/sec frequency were studied under conditions of chronic experiments before and after the development of an instrumental food reflex. After reflex development a single presentation of a positive conditioned stimulus (single click) markedly influenced for 7 sec the appearance of instrumental movements. At the same time, the immediate responses of motor cortex neurons to presentation of the conditioned auditory stimulus had no impact on the appearance in the motor cortex of discharges leading to the realization of instrumental movements. Consequently, motor cortex neurons do not require activation from afferent sensory inputs for the generation of such discharges. The immediate neuronal responses to conditioned stimulation did not inhibit the realization of the instrumental reflex. It is proposed that they are associated with the realization of motor function in the unconditioned defensive response evoked by the presentation of an auditory stimulus. The presence or absence of responses to auditory conditioned stimulation was dependent upon the signal meaning of the stimulus, its physical parameters, and the degree of excitability of the animal.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 17, No. 4, pp. 539–550, July–August, 1985.     

Neuronal and synaptic mechanisms of cortical inhibition

The latent periods, amplitude, and duration of IPSPs arising in neurons in different parts of the cat cortex in response to afferent stimuli, stimulation of thalamocortical fibers, and intracortical microstimulation are described. The duration of IPSPs evoked in cortical neurons in response to single afferent stimuli varied from 20 to 250 msec (most common frequency 30–60 msec). During intracortical microstimulation of the auditory cortex, IPSPs with a duration of 5–10 msec also appeared. Barbiturates and chloralose increased the duration of the IPSPs to 300–500 msec. The latent period of 73% of IPSPs arising in auditory cortical neurons in response to stimulation of thalamocortical fibers was 1.2 msec longer than the latent period of monosynaptic EPSPs evoked in the same way. It is concluded from these data that inhibition arising in most neurons of cortical projection areas as a result of the arrival of corresponding afferent impulsation is direct afferent inhibition involving the participation of cortical inhibitory interneurons. A mechanism of recurrent inhibition takes part in the development of inhibition in a certain proportion of neurons. IPSPs arise monosynaptically in 2% of cells. A study of responses of cortical neurons to intracortical microstimulation showed that synaptic delay of IPSPs in these cells is 0.3–0.4 msec. The length of axons of inhibitory neurons in layer IV of the auditory cortex reaches 1.5 mm. The velocity of spread of excitation along these axons is 1.6–2.8 msec (mean 2.2 msec).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 394–403, May–June, 1984.     

Participation of nonsegmentary neurons of the spinal cord in execution of presynaptic control

Experiments were carried out on cats six days after complete transection of the spinal cord. Cord dorsum potentials (CDP) were recorded in the vicinity of the third lumbar segment during stimulation of the isolated dorsolateral funiculus (DLF). The CDP consist of a rapid monophasic potential (which apparently reflects antidromic excitation of the cells of Clarke's column) and two subsequent slow negative waves, which are replaced by a long positive oscillation. In form, time characteristics, and behavior during thythmic stimulation, this potential differs considerably from the CDP recorded during stimulation of the afferent nerves. The presence of a positive phase of the CDP indicates that stimulation of the DLF evokes primary afferent depolarization (PAD). Stimulation of the DLF causes inhibition of the CDP evoked by stimulation of the afferent nerve. The time course of this inhibition corresponds to the time course of presynaptic inhibition. It is demonstrated that stimulation of the afferent nerve (n. femoralis) inhibits slow components of the CDP evoked by stimulation of the DLF. This inhibition reaches a maximum at the 16th millisecond; its duration exceeds 300 msec. Stronger and more prolonged inhibition of the same components is observed when both the conditioning and the testing stimuli are administered to the DLF. Since primary afferents do not take part in CDP emergence during stimulation of the DLF, it may be hypothesized that the observed inhibition develops as a result of depolarization of interneuron axon terminals.Dnepropetrovsk State University. Translated from Neirofiziologiya, Vol. 2, No. 5, pp. 520–527, September–October, 1970.     

Functional organization of mechanisms of presynaptic inhibition induced by stimulation of cutaneous afferents

Changes in the N₁-component and P-phase of the dorsal surface potential (DSP) of the spinal cord evoked by test stimulation of the posterior tibial nerve after conditioning stimulation of the sural nerve were investigated in anesthetized cats. The test responses were inhibited if stimulation was applied at short intervals. They then recovered to some extent, but after 1.8–2.2 msec, a further prolonged period of inhibition began. The initial inhibition was connected with occlusion of synaptic action, and the subsequent prolonged inhibition with the development of presynaptic inhibition. The latent periods of prolonged inhibition of the N₁-component and P-phase of the DSP (2 msec) were almost exactly identical, and the curves showing the diminution of the initial occlusion of these components were very similar. The results demonstrate that presynaptic inhibition of the interneurons generating the N₁-component of the DSP and of cells of the substantia gelatinosa which participate in depolarization of the presynaptic terminals of the cutaneous afferents is due to the action of depolarizing systems with similar temporal characteristics.Dnepropetrovsk State University. Translated from Neirofiziologiya, Vol. 4, No. 5, pp. 510–515, September–October, 1972.     

Convergence of autonomic,somatic, and vestibular afferent impulses on single units of the medullary vasomotor center of the cat

Responses of vasomotor neurons of the cat medulla to electrical stimulation of the depressor nerve and of mixed nerves of the limbs and to adequate stimulation of the vestibular apparatus were investigated. Evoked unit activity was demonstrated as groups of action potentials followed by inhibition of spontaneous activity. Three types of unit responses to stimulation of the depressor nerve and somatic afferent fibers and changes in unit activity in response to vestibular stimulation are described. The features distinguishing the convergence of afferent impulses on vasomotor neurons are discussed.Institute of Medico-Biological Problems, Ministry of Health of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 5, No. 5, pp. 460–467, September–October, 1973.     

A biphasic excitability change in hindlimb motoneurons evoked by stimulation of the nucleus raphes magnus in the cat

1. The influence of electrical stimulation of the nucleus raphes magnus (RM) on spinal segmental systems were examined. 2. RM stimulation produced an initial increase and a subsequent suppression of the amplitude of both fiextor and extensor lumbar monosynaptic reflex potentials (MSRs). 3. Intracellular recordings were made from alpha-motoneurons of the common peroneal nerve (flexor) and the tibial nerve (extensor). RM stimulation evoked postsynaptic potentials with a time course similar to that of MSR facilitation. 4. RM stimulation inhibited the aggregate excitatory synaptic potential (EPSP) evoked by stimulation of group I afferent fibers without apparent changes in the motoneuronal membrane potential. 5. These data suggest that the RM-evoked biphasic effect on MSR consists of early facilitation due to EPSP, and late inhibition possibly due to presynaptic inhibition of group I afferent fibers.     

State of monosynaptic reflex (H reflex) arc during voluntary muscle contraction in humans

Effect of an isometric voluntary contraction of the soleus muscle (5–6 sec in duration) on the H reflex was studied in persons aged 25–45 years. A sharp enhancement of the H reflex at a dynamic phase of fast contraction (in the force increase period) and its less pronounced increase at a static phase of contraction were shown. When a paired stimulation of the nerve was used (interstimulus interval, 500 msec), a voluntary contraction of the muscle fully abolished the first stimulus-related suppression of the second H reflex. When the muscle contracted against the background of vibration, the suppressed H reflex recovered up the the level observed in the resting muscle, but did not reach the level characteristic for the contracting muscle. The findings show that the two pathways controlling the H reflex (descending and afferent) function independently, and apparently there exist at least two corresponding groups of interneurons causing a presynaptic inhibition of the H reflex.Neirofiziologiya/Neurophysiology, Vol. 25, No. 5, pp. 365–371, September–October, 1993.     

Effect of nociceptive stimulation of signal transmission from low-threshold cutaneous afferents in the somatosensory system

In cats anesthetized with chloralose nociceptive heating of the skin of the foot to 44–60°C led to a two- to fourfold increase in amplitude of primary cortical responses to direct stimulation of neurons of the spinocervical tract receiving information from the heated area of skin, but did not affect primary responses evoked by stimulation of axons of these neurons in the dorsolateral funiculus, and actually inhibited the response to stimulation of the nerve innervating the heated area of skin. Inhibition was accompanied by depolarization of central terminal of low-threshold fibers of this nerve: During heating the amplitude of the antidromic discharges evoked in the nerve by stimulation of its presynaptic endings in the spinal cord was increased two- to threefold. After abolition of presynaptic depolarization with picrotoxin (0.2–0.7 mg/kg, intravenously) or as a result of asphyxia, nociceptive heating acquired the ability to facilitate primary responses arising as a result of stimulation of the nerve also. The amplitude of the responses was increased under these circumstances by 3–20 times. It is concluded that acute nociceptive stimulation causes such powerful presynaptic inhibition of impulse transmission from low-threshold fibers of the cutaneous nerve that it virtually abolishes the facilitating effect of nociceptive impulses on sensory neurons of the spinal cord. It is suggested that it is this inhibitory mechanism which prevents the development of hyperalgesia during acute nociceptive stimulation.Institute of General Pathology and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 13, No. 6, pp. 621–627, November–December, 1981.     

Segmental inhibitory response in the frog spinal cord during orthodromic motoneuron activation

Different types of reflex discharges were produced in various preparations by stimulating the dorsal root of isolated frog spinal cord. These ranged from multiphasic low-amplitude waves to distinctly synchronized monosynaptic response. The discharges were followed by facilitation in the former and deep, protracted inhibition of response to test dorsal root stimulation in the latter. When interstimulus intervals measured 40–50 msec, inhibitory action was less pronounced than at shorter (15–30 msec) or longer (60–100 msec) intervals, thus indicating that at least two types of inhibition were at work, one at an earlier and the other at a later stage. Strychnine at a concentration of 10^–5 M effectively reinforced the former and blocked the latter, while 10^–4 M d-tubocurarine attenuated both types of inhibition substantially. It is concluded that inhibition of response occurs mainly as a result of recurrent activation of inhibitory systems via recurrent motoneuron axon collaterals when frog spinal cord afferents are excited. Intensity of the later (presynaptic) and earlier (postsynaptic) inhibition of reflex transmission is determined by the degree of synchrony in motoneuronal discharge in response to orthodromic stimulation.Institute of Medical Radiology, Academy of Medical Sciences of the USSR, Obninsk, Kaluga Oblast. Translated from Neirofiziologiya, Vol. 19, No. 3, pp. 343–350, May–June, 1987.     

Characteristics of the formation of temperature information in afferent units

The impulse activity of single afferent fibers of the dorsal roots of the cat spinal cord is studied for local mechanical, heat, and cold influences on the skin receptor fields. A probability analysis of the impulse flux suggests that a change occurs in the distributions of the intervals between impulses in accordance with the stimuli presented, regardless of the variations in the mean frequency of impulsation. It is hypothesized that the afferent fibers acquire polyfunctional properties on account of their multichannel information.Institute of Physiology, Kazakhstan, Academy of Sciences, Alma-Ata. Translated from Neirofiziologiya, Vol. 24, No. 5, pp. 582–591, September–October, 1992.     

Suppression of postsynaptic response in trigeminal motoneurons by stimulating the midbrain central gray matter

The effects of stimulating the midbrain central gray matter (CGM) on motoneuronal response in trigeminal nerves were investigated in anesthetized cat. It was found that stimulating the CGM did not induce postsynaptic response in these motoneurones. Conditioning stimulation of the CGM brought about suppression of motoneuronal postsynaptic response to stimulation of tooth pulp and high threshold infraorbital nerve afferents without affecting motoneuronal antidromic response and jaw-opening reflex as induced by stimulating the caudal nucleus of the spinal trigeminal tract. It was thus concluded that stimulating the CGM exerts no direct effect on motoneurons but does have an influence on postsynaptic response — a result of modulation of the afferent spike flow at interneuronal level.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 543–549, July–August, 1990.