Ia EPSPs Research Articles

Further observations on the depression of group Ia facilitation of motoneurons by vibration in man.

Stimulation of low threshold afferents in the peroneal nerve causes a short latency facilitation of individual tibialis anterior motor units considered to be due to the composite Ia EPSP. This facilitation is depressed by vibration 20 to 128 Hz applied over the tibialis anterior. The depression occurs without any change in the firing rate of the motor unit or in the facilitation from cutaneous afferents and so is unlikely to be due to postsynaptic inhibition. The depression can occur with vibration frequencies as low as 40 Hz and is therefore unlikely to be due to occlusion in Ia afferents. There is no evidence that vibration alters the electrical threshold of large afferents. A similar facilitation of soleus motor units resulting from stimulation of low threshold afferents in the tibial nerve is depressed for up to 75 ms following a 40 ms burst of 50 Hz vibration applied to the tendon of the tibialis anterior. The burst of vibration itself did not facilitate soleus motor units so there is no evidence to suggest that the vibration spread to soleus spindles. Homosynaptic depression is, therefore, unlikely. These findings provide further evidence that presynaptic inhibition of Ia afferents occurs in man.

Evidence that low threshold afferents both evoke and depress polysynaptic excitation of wrist flexor motoneurones in man.

The post-stimulus time histogram method was used to measure the time course of the firing probability of voluntarily activated wrist flexors motor units (MUs) after various stimulations. Stimulating the median and the ulnar nerve at an intensity equal to the motor threshold evoked an early increase in firing probability attributed to the monosynaptic Ia EPSP. Reducing the stimulation strength led to the disappearance of this peak and to the appearance of a second peak, whose latency was 3-5 ms longer than that of the monosynaptic peak. It is shown that both group I and cutaneous fibres contribute to the long-latency response but that group I fibres alone are responsible for its onset. Combined stimulation of the same afferents (group I and low threshold cutaneous fibres) reduced the amplitude of the second response. It is argued that this inhibition is exerted onto the interneurones mediating the non-monosynaptic excitation to motoneurones.

Reflex pathways from group II muscle afferents

The convergence of group II muscle afferents on interneurones in reflex pathways has been elucidated by investigating interaction in transmission to motoneurones. Recording was also made from interneurones activated from group II afferents. Maximal group II EPSPs evoked in motoneurones from different muscles (extensors or flexors and extensors) did not summate linearly but with a deficit of 35-40%. The corresponding deficit in summation with Ia EPSPs was 7%. It is suggested that the difference in deficit is caused largely by occlusion due to shared interneuronal discharge zones and that it gives an approximate minimal measure of the convergence of group II afferents from different muscles on the interneurones. Tests with weak group II volleys from different muscles gave no or little evidence for spatial facilitation in the disynaptic excitatory pathway to flexor motoneurones, and there was no or little temporal facilitation of transmission in this pathway. It is suggested that group II excitation of the interneurones in this pathway depends on few afferents giving large unitary EPSPs. Convergence of cutaneous afferents and joint afferents on the interneurones was evidenced by spatial facilitation from these afferents of group II transmission to motoneurones. Convergence on interneurones in the trisynaptic inhibitory pathway from group II afferents to extensor motoneurones was also investigated with the spatial facilitation technique. There was convergence on common interneurones of group II afferents from different muscles (extensors or flexors and extensors) and from cutaneous afferents as well as joint afferents. Trisynaptic group II IPSPs, including those depending on spatial facilitation from different muscles were resistant to recurrent depression from motor axon collaterals and are therefore not mediated by the reciprocal Ia inhibitory pathway. Interneurones with monosynaptic group II EPSPs were recorded from in the dorsal horn and intermediate region. Graded stimulation revealed large unitary EPSPs from few group II afferents. The EPSP evoked by a single group II afferent may produce firing (extracellular recording). Convergence of monosynaptic group II EPSPs from different muscles was rather limited but could be from flexors and extensors. Extensive multisensory convergence onto some of these interneurones was indicated by di- or polysynaptic EPSPs from group II and III muscle afferents, from joint afferents and from cutaneous afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Integration in descending motor pathways controlling the forelimb in the cat. 15. Comparison of the projection from excitatory C3-C4 propriospinal neurones to different species of forelimb motoneurones.

In the preceding report (Alstermark and Sasaki 1986) it was shown that a stimulus of 500 microA applied in the lateral reticular nucleus (LRN) evokes a maximal or near monosynaptic EPSP (LRN EPSP) in forelimb motoneurones. This EPSP which is assumed to be selectively mediated by C3-C4 propriospinal neurones (PNs), was used to estimate the strength of the excitatory projection from C3-C4 PNs. A systematic comparison was made of the size and time course of the maximal LRN EPSP in various species of forelimb alpha-motoneurones innervating shoulder, elbow, wrist and digit muscles. The LRN EPSP was evoked in all investigated species of forelimb motoneurones. When either the peak amplitude or the underlying area of the LRN EPSP was compared, a three-fold range was found with some tendency for the size to vary in the order of wrist greater than shoulder approximately equal to elbow greater than digit greater than intrinsic paw motor nuclei. Generally, a positive correlation was found in each motor nucleus between the peak amplitude of the LRN EPSP versus the monosynaptic homonymous group Ia EPSP, input resistance and afterhyperpolarization duration respectively (cf. Alstermark and Sasaki 1986). It is therefore postulated, that the LRN EPSP peak amplitude is correlated with motor unit type. Comparison of the time course of the LRN EPSPs was made by measuring the time-to-peak (T-t-p) and half-width (H-w). The finding in the preceding report that the T-t-p and H-w is longer in slow than in fast motoneurones was confirmed and extended to all the investigated motor nuclei. The hypothesis that both fast slow motoneurones receive projection from a group of fast C3-C4 PNs, while slow motoneurones receive an additional projection from a group with lower conduction velocity, can therefore be applied to all forelimb motor nuclei. In addition, it is proposed that some slow shoulder, wrist and digit motoneurones receive projection from a special subpopulation of C3-C4 PNs with very slow conduction velocity.

Integration in descending motor pathways controlling the forelimb in the cat. 14. Differential projection to fast and slow motoneurones from excitatory C3-C4 propriospinal neurones.

The projection of C3-C4 propriospinal neurones (PNs) to alpha-motoneurones of forelimb muscles has been analysed with the aid of antidromic stimulation of the ascending branch of the PNs to the lateral reticular nucleus (LRN). A single stimulus of 500 microA applied in the caudo-dorsal part of the LRN evoked a maximal or greater than 90% maximal monosynaptic EPSP in the motoneurones. Systematic mapping of EPSPs evoked by stimulation of 500 microA in and around the LRN revealed that at this strength there was hardly any co-activation of a medial system (Peterson et al. 1979) which evoked small monosynaptic EPSPs with shorter latency and faster time course. The LRN EPSP amplitude was positively correlated with the homonymous group Ia EPSP amplitude, the input resistance and the afterhyperpolarization (AHP) duration. It is therefore postulated that the LRN EPSP amplitude is correlated with motor unit type (Burke 1967, 1968; Burke et al. 1973) with the largest EPSPs in slow (S), the smallest in fast, fatiguable (FF) and possibly intermediate sized in fast, fatigue resistant (FR) units. There was only a small difference in latency of the LRN EPSP in fast and slow motoneurones, while the time course was considerably slower in the latter. It is suggested that slow motoneurones receive projection both from fast and slowly conducting PNs but fast motoneurones mainly from fast PNs. Comparison of the disynaptic pyramidal EPSPs and the LRN EPSPs revealed a positive correlation, but the amplitude ratio pyramidal EPSP: LRN EPSP was smaller in slow than in fast motoneurones. A negative correlation was found between this amplitude ratio and the latency of the disynaptic pyramidal EPSP. It is suggested that this correlation reflects the excitability level in the PNs and that low excitability is due to inhibition of the PNs.

Partitioning of monosynaptic Ia excitatory postsynaptic potentials in the motor nucleus of the cat lateral gastrocnemius muscle.

Experiments were conducted to test the hypothesis that a partitioning of Ia monosynaptic excitatory postsynaptic potentials (Ia EPSPs) is present in motor nuclei supplying muscles with regions capable of different mechanical actions. Intracellular recordings of synaptic potentials were made in lateral gastrocnemius (LG) motoneurons in anesthetized low-spinal cats. The effects were tested of stimuli (group I range) to the four primary nerve branches of the LG nerve supplying muscle compartments LGm, LG1, LG2, and LG3 (terminology of English, Ref. 26) and the nerve to a heteronymous muscle, soleus. Stimulation of a given LG nerve branch produced monosynaptic Ia EPSPs of greater amplitude in "own-branch" motoneurons than "other-branch" cells. A significant partitioning of mean Ia EPSPs was found in three (LG1, LG2, LG3) out of the four homonymous pathways studied. An EPSP normalization (7) was performed to eliminate potential differences in cell type that might affect the amplitudes of the EPSPs between these four cell groups (e.g., differences in the number of cells supplying FF, FR, and S muscle units). This normalization confirmed that the partitioning of monosynaptic Ia inputs upon stimulation of LG1, LG2, and LG3 could not be attributed to differences in cell type. In addition, the effects of LGm stimulation were found to be significantly greater in the LGm motoneurons compared with the other cell groups. Heteronymous input (from soleus) to the LG motor nucleus showed some partitioned effects. Motoneurons innervating compartment LG2 received larger EPSPs from soleus than did the cells supplying compartments LG1, LG3, and LGm. The contributions of location specificity and species specificity (terminology of Scott and Mendell, Ref. 55) in the establishment of these Ia-afferent-motoneuronal connections were examined. Cell location sites within the spinal cord were consistent with location specificity making some contribution to the observed pattern of homonymous Ia connections. A more prominent role for species specificity was indicated by species-dependent differences in EPSP amplitude in pairs of LG motoneurons (e.g., LGm vs. LG2) at similar rostrocaudal locations upon stimulation of a given homonymous or heteronymous nerve/branch.

Long-lasting excitability changes of soleus alpha-motoneuron induced by midpontine stimulation in decerebrate, standing cat.

Stimulation of the dorsal portion of the caudal tegmental field (DTF) in the pons resulted in hyperpolarization of extensor alpha-motoneurons (alpha-MNs) that persisted for several minutes after cessation of the stimulation. The resulting inhibition of alpha-MN discharge led to a progressive reduction in the number of active motor units. Renshaw cells, persistently active at high levels of extensor muscle tone, were abruptly silenced by DTF stimulation. Active discharge was renewed at the time of cessation of the stimulation but at a frequency reduced in proportion to the persistently lowered level of extensor muscle tone. Ia primary afferents were tonically active during the high extensor tonus of reflex standing. DTF stimulation was accompanied by a brief, slight increase in Ia discharge frequency followed by a reduction in frequency variably correlated to the magnitude of extensor force reduction. Orthodromically elicited Ia EPSPs in the soleus alpha-MNs were reduced in peak voltage, time to peak, and half width during the hyperpolarization accompanying DTF stimulation. All of these parameters recovered beyond their prestimulus values with the cessation of DTF stimulation in spite of the persisting hyperpolarization. Antidromically initiated invasion of the somatodendritic (SD) segment of the motoneuron membrane was delayed and sometimes blocked during DTF stimulation. At the same time, the peak voltage of the SD action potential was reduced. There was an immediate recovery of these changes on termination of DTF stimulation, although the poststimulus hyperpolarization persisted. Intracellular injection of depolarizing current steps during DTF stimulation revealed a depression of membrane excitability that persisted during the hyperpolarization that followed the termination of the DTF stimulation. Depolarizing and hyperpolarizing steps of intracellular current were used to demonstrate a reduction of cellular input resistance during DTF stimulation. The resistance values rapidly returned to prestimulus levels following the cessation of DTF stimulation. It was demonstrated that the degree of resistance change is greater and that the magnitude of DTF-induced hyperpolarization is smaller for low-resistance cells than for high-resistance cells. Iontophoretically induced increase in intracellular Cl- resulted in a reversal of both Ia IPSPs and the hyperpolarization induced by DTF stimulation. The hyperpolarization enduring after DTF stimulation was not affected by the Cl- injection.

O-11 - Antagonism of group Ia epsp in the in vitro newborn rat spinal cord by glutamate antagonists

O-11 - Antagonism of group Ia epsp in the in vitro newborn rat spinal cord by glutamate antagonists

Primary afferent depolarization and presynaptic inhibition of monosynaptic group Ia EPSPs during posttetanic potentiation.

Primary afferent depolarization and presynaptic inhibition of monosynaptic group Ia EPSPs during posttetanic potentiation.

Characteristics of M spikes in cat motoneurons and their significance for the measurement of small composite Ia EPSPs.

Some characteristics of intracellularly recorded M spikes in cat motoneurons have been determined in order to evaluate techniques for subtracting this spike from recordings of homonymous monosynaptic composite Ia EPSPs.

Synaptic organization of defined motor-unit types in cat tibialis anterior.

1. Synaptic potentials were recorded intracellularly in tibialis anterior (TA) motoneurons following stimulation of a descending brain stem pathway, the medial longitudinal fasciculus (MLF), and three segmental inputs, the homonymous and heteronymous group Ia afferents, the group I afferents from the antagonist, and the cutaneous and muscle afferents. Intracellular stimulation of the motoneurons was used to classify them, based on the properties of the innervated muscle units, into types FF, F(int), FR, and S (6, 16). 2. The sum of the monosynaptic EPSP amplitudes resulting from stimulation of homonymous and heteronymous group Ia afferents (summed group Ia EPSP) was inversely related to motoneuron size, as assessed by motoneuron input resistance, and was inversely related to motor-unit tetanic tension. Type-FF, -FR, and -S motoneurons showed significant differences in the mean amplitude of their summed group Ia EPSPs. 3. The amplitudes of disynaptic IPSPs resulting from stimulation of group I afferents in the antagonist muscle also showed an inverse relationship to motoneuron size. The observed relationships between motoneuron size and the monosynaptic group Ia EPSP amplitude or the disynaptic group I IPSP amplitude are compatible with the "size principle" of motor-unit recruitment (26). 4. The amplitudes of the monosynaptic EPSPs evoked in TA motoneurons by stimulation of the MLF were distributed rather randomly among all types of TA motoneurons. A slight tendency of larger monosynaptic EPSPs to occur in motoneurons with larger tetanic tensions was observed. 5. The polysynaptic effects from cutaneous and muscle afferents in sural and gastrocnemius-soleus nerves were frequently excitatory on type-FF motoneurons, but were primarily inhibitory on type-FR and -S motoneurons. Clearly, the polysynaptic cutaneous and muscle inputs and the monosynaptic MLF input onto TA motoneurons show a different pattern of synaptic organization than the group I inputs. 6. In general, the synaptic organization of the TA motor nucleus is similar to that of its extensor antagonist, medial gastrocnemius (MG) (2--5, 7, 8), when analogous neural circuits are compared. This parallel organization suggests a commonality of motor-control systems for both flexor and extensor muscles.

Effect of compensatory hypertrophy studied in individual motor units in medial gastrocnemius muscle of the cat.

1. Compensatory hypertrophy of the medial gastrocnemius (MG) muscle was produced by denervating or removing its synergists (i.e., the lateral gastrocnemius, soleus, and plantaris muscles) in adult cats. Following survival times of 14-32 wk, intracellular recording and stimulation techniques were used to study the motor-unit population in MG. The data obtained were compared with results from MG motor units in normal unoperated cats of the same body size and weight. 2. Using criteria employed for normal motor units, the units in hypertrophic MG muscles were readily classified into the same groups (types FF, F(int), FR, and S) as in normal MG. There was no detectable difference in the distribution of motor-unit types after hypertrophy. 3. When compared with a normal motor-unit sample, there was a large increase in mean tetanic tension, but no significant change in twitch tension, for each motor-unit type in the hypertrophied muscles. The most marked increase was found among the fatigue-resistant type S and type FR motor units. There was no alteration of twitch contraction times or fatigue resistance in any unit type after hypertrophy. 4. For each motor-unit type, the mean homonymous (MG) group Ia EPSP amplitude was the same in normal and hypertrophic MG populations. There was, however, a significant increase in the average conduction velocity of MG motor axons in the animals with uncomplicated MG synergist removal and maximal MG hypertrophy. 5. On the basis of histochemical staining, muscle fibers from comparable sections of hypertrophic and contralateral (unoperated) MG muscles were presumptively identified as belonging to FF, FR, or S units. There was no significant difference between hypertrophic and contralateral MG muscles in the percentage of each fiber type, although there was some variability in muscle composition from one cat to another. One muscle pair was studied in detail for fiber cross-sectional area. In this cat, with marked hypertrophy by muscle weight, there was a modest increase in the mean fiber areas of histochemical S and FR muscle fibers, but no evident change in FF fibers, on the hypertrophic side. 6. MG motor units were examined in several cats in which synergist removal resulted in scarring and marked limitation of passive ankle mobility, and no evident weight gain in MG. Motor units of all types in these animals showed a decrease in twitch tension and in mean twitch/tetanus ratios, with little alteration in mean tetanic tensions. 7. The main effect of compensatory hypertrophy under the present conditions was a large increase in tetanic tension output from individual motor units due, at least in part, to an increase in fiber cross-sectional area. There was no evidence indicating any "conversion" of motor units or of their muscle fibers from one type to another.

Effects of extensor and flexor group I afferent volleys on the excitability of individual soleus motoneurones in man.

The contour of the postsynaptic potential (PSP) produced in a neurone by an afferent volley can be derived from the contour of the post-stimulus time histogram (PSTH) of that neurone when it is discharging rhythmically. In the present study the PSTH of the firing of individual soleus motor units after stimulation of the popliteal or peroneal nerve was used to explore the effects of extensor and flexor group I afferent volleys on the excitability of single soleus motoneurones in man. Extensor group I volleys resulted in an early peak of increased impulse density in the PSTH of 75% of soleus motoneurones. The latency suggests an analogy with the Ia EPSP. The mean duration of the peak of increased impulse density, equivalent to the rise time of the EPSP, was 3.6 ms. Flexor group I volleys result in a period of reduced impulse density in the PSTH of five out of nine soleus motoneurones. The latency suggests an analogy with the Ia IPSP. We conclude that this method could be used to explore the afferent connections to single motoneurones in man and to derive some of the characteristics of the postsynaptic potentials from a variety of afferent nerve fibres in single human motoneurones.

Analysis of muscle receptor connections by spike-triggered averaging. 1. Spindle primary and tendon organ afferents.

1. The synaptic connections of 44 single identified muscle spindle Ia afferents and of 21 Golgi tendon organ (Ib) afferents from medial gastrocnemius (MG) were studied in 46 cats by the spike-triggered averaging of synaptic noise in 803 motoneurons of various types. 2. The well-known monosynaptic Ia excitatory connections were confirmed and their characteristics examined in 113 cells. The method was used at greater sensitivity than before and revealed that, in addition to the larger EPSPs of the order of 300 muV, there were many below the previously reported lower 17-muV limit. 3. By studying the Ia disynaptic inhibitory pathway with quick stretch-evoked Ia volleys and by spike-triggered averaging (STA), it was shown that the latter method can reveal disynaptic and possibly trisynaptic excitatory connections. This is believed to depend on having continuous activity in the relevant interneurons. 4. Latencies of individual connections showed broad distributions and arguments are advanced for setting working limits to mono- and disynaptic paths for Ia excitation and inhibition. Monosynaptic EPSP latency from cord entry was 0.4-1.1 ms and disynaptic inhibition was 1.2-2.4 ms. It was recognized that the boundaries are not rigid and monosynaptic Ia EPSPs may have latencies up to 1.5 ms. 5. Rise times of disynaptic PSPs were, on average, significantly longer than monosynaptic, but individual disynaptic responses could have values within the monosynaptic range. 6. A small diphasic wave shortly preceding the monosynaptic EPSPs was interpreted as a presynaptic spike. Its timing was consistent with this and, as such, permitted estimates to be made of central conduction time. 7. An early negative wave (latency less than or equal 1.1 ms) of small emplitude was sometimes detected in antagonist motoneurons when triggering from Ia afferents. It was found tha extracellular fields could be detected due to single Ia afferent excitations and efforts were made to see if the early negative wave could be explained by this. In a few cases there was evidence that a very short-latency IPSP might be occuring. This evidence and its implications are discussed with attention to the new factors which have to be considered in using the spike-triggered averaging method at very high sensitivity. 8. Ib effect were di- or trisynaptic. They were excitatory to 18% of synergists and to 28% of antagonists. They were inhibitory to 41% of synergists and to 19% of antagonists. The Ib IPSPs were larger than the EPSPs.

Analysis of muscle receptor connections by spike-triggered averaging. 2. Spindle group II afferents.

1. The spike-triggered averaging (STA) method has been used to study synaptic connections of nine spindle group II afferents from medial gastrocnemius to 151 motoneurons of leg muscles in the cat. 2. EPSPs were found in 40 cells, predominantly of triceps surae with latency from cord entry ranging from 0.3 to 4.2 ms. Those with latency less than or equal to 1.4 ms were deduced to be monosynaptic in confirmation of Kirkwood and Sears (13). Mean amplitude for MG-LGS cells was 30.1 muV and mean rise time 1.0 ms (compared with 65.4 muV and 1.0 ms for monosynaptic Ia EPSPs from the preceding report (26)). It is argued that monosynaptic latency for spindle group II afferents could be as large as 1.65 ms. 3. The occurrence of a presynaptic spike permitted the division of EPSP latencies into central conduction time and synaptic delay components. Sindle group II central conduction times were significantly longer than those of Ia afferents, while there were no differences in the synaptic delays associated with the two afferent types. 4. EPSPs of longer latency were judged to be di- or trisynaptic. They were smaller and had longer rise times than the monosynaptic effects. Evidence is presented to show that short rise time of an individual PSP does not guarantee that it is monosynaptic. Rise times are different only on a population basis. 5. Inhibitory responses were found with latencies and mean rise times appropriate for di- and trisynaptic connections. Their mean amplitude was 4.6 muV. 6. The distribution of EPSPs and IPSPs was generally consistent with their exerting stretch reflex effects similar to that of Ia afferents and inconsistent with the inclusion of them in the grouping known as "flexor reflex afferents."

Relative strength of synaptic input from short-latency pathways to motor units of defined type in cat medial gastrocnemius

1. Intracellular recording and stimulation techniques were used in anesthetized cats to study the interrelations between amplitudes of PSPs produced by electrical stimulation of several short-latency pathways to MG alpha motoneurons and the mechanical properties of muscle units innervated by the same cells. Motor-unit types were identified by muscle-unit properties.2. The maximum amplitudes of monosynaptic EPSPs produced in MG motoneurons by activation of homonymous (MG) and heteronymous (LGS) group Ia afferents were clearly related to motor-unit type, being, on the average, largest in type S units, somewhat smaller in type FR and F(int) units, and smallest in type FF units. Correspondingly, group Ia EPSP amplitudes were inversely correlated with muscle-unit tension production and directly correlated with resistance to fatigue. The same input distribution was true for disynaptic IPSPs produced by group Ia afferents from antagonist ankle flexors.3. The amplitudes of monosynaptic EPSPs produced by fibers descending in the ipsilateral ventral funiculi of the low thoracic cord were not clearly related to MG motor-unit type or (therefore) to muscle-unit properties.4. A quantitative input-output model of the MG motor-unit pool, based in part on the present results, suggests that overall characteristics of MG motor units, and their relative numbers in the MG pool, reflect functional specializations determined by specific mechanical demands placed on the MG muscle by the usual motor behavior of the animal.

Modulation of synaptic effectiveness of Ia and descending fibers in cat spinal cord.

1. In the unanesthetized spinal cord, conditioning stimulation of low-threshold afferents (below 1.3 times threshold strength) in the biceps semitendinosus (BST) nerve often reduced the peak amplitude of the monosynaptic Ia EPSPs evoked in gastrocnemius motoneurons without affecting the monosynaptic component of the EPSPs evoked by stimulation of the ipsilateral ventral funiculus (VF) in the thoracic cord. 2. Volleys to the BST nerve comprising higher threshold afferents (usually above 1.4 times threshold strength) reduced the peak amplitude of the monosynaptic Ia and VF EPSPs and shortened their falling phase. 3. Conditioning volleys to low-threshold cutaneous afferents often increased the Ia-EPSP peak amplitude, sometimes without affecting the monosynaptic component of the VF EPSP. 4. In most cases the Ia nd VF monosynaptic EPSPs elicited in a given motoneuron summated nonlinearly. The amount of nonlinear summation between Ia and VF monosynaptic EPSPs was often reduced by low-threshold BST conditioning volleys. These observations suggest that in many instances, both species of fibers end in "electrotonically close" synaptic loci over the motoneuron surface. Therefore, amplitude changes of monosynaptic Ia EPSPs produced by conditioning afferent volleys without concomitant changes of monosynaptic VF EPSPs do not appear to result from postsynaptic remote conductance changes and may be attributed to a presynaptic mechanism. 5. At the time of occurrence of the Ia and VF monosynaptic EPSP the variance of the motoneuron membrane potential may be increased above base-line levels with a time course approximately matching the EPSP itself. Conditioning stimulation of BST afferents usually reduced Ia EPSP variance, often without affecting or even increasing the variance of the monosynaptic VF EPSPs. These observations provide additional evidence that Ia EPSP variability is introduced, at least in part, through the segmental pathways mediating primary afferent depolarization. 6. The possibility of a differential control of the information flow transmitted through two independent channels converging on a given cell ensemble is discussed.

Control by Preynaptic Correlation: a mechanism affecting information transmission from Ia fibers to motoneurons

1. In the unanesthetized spinal cord of the cat, simultaneous intracellular recordings were made from two motoneurons belonging to the gastronemius motor nucleus. 2. Supramaximal iterative stimulation of small branches of the gastrocnemius nerve produced monosynaptic EPSPs (Ia EPSPs) of varying amplitude superimposed on a fluctuating base line. 3. In most cases the variance of the motoneuron membrane potential was increased above base-line levels with a time course approximately matching the Ia EPSP. This suggests that Ia EPSP fluctuations are greater than can be accounted for by the base-line fluctuations alone. 4. For a given series of Ia EPSPs, the smaller responses in the series had about the same decay phase as the larger EPSPs, suggesting that most of the Ia EPSP fluctuations were not due to systematic changes in postsynaptic conductances produced by ongoing activity, but rather to a presynaptic mechanism. 5. Simultaneous recording from two motoneurons showed that base-line fluctuations were positively correlated. In most cases, however, there was an additional increased correlation above base-line levels resembling the time course of the Ia EPSPs, indicating positive correlation between EPSP fluctuations which is attributed to a presynaptic mechanism. 6. Conditioning volleys to group I muscle afferents or to low-threshold cutaneous afferents reduced the variance of the Ia EPSPs and also their correlation in motoneuron pairs, often without changing the mean Ia EPSPs. 7. It is concluded that, in the unanesthetized spinal cord, in addition to the random process which governs transmitter release intrinsic to a given synaptic terminal, there is another stochastic process affecting, in a correlated manner, transmitter release in large sets of Ia synaptic terminals. Most likely, the correlation in transmitter release is achieved by membrane potential fluctuations imposed on the Ia terminal arborizations by ongoing activity of the segmental mechanism mediating primary afferent depolarization. 8. The effects of such a correlating influence on cell firing behavior have been analyzed. The results suggest that this mechanism, referred to as control by presynaptic correlation, is able to modulate the information transmitted from Ia fibers to motoneurons.

Effects of trans-membrane polarization and TEA injection on monosynaptic actions from motor cortex, red nucleus and group Ia afferents on lumbar motoneurons in the monkey.

Abstract The monosynaptic EPSPs evoked from supraspinal structures and muscle afferents in alpha-motoneurons were investigated in the rhesus monkeys with the aid of intracellular recording. There was a statistically significant difference in the time course of cortico-motoneuronal, rubro-motoneuronal and group Ia monosynaptic EPSPs but a relatively small difference between these inputs in the sensitivity to the artificial polarization. No correlation was found between the sensitivity of monosynaptic actions to injected currents and the rise time of EPSP. Tetraethylammonium (TEA) injection into the neuroplasm induced a large increase in the ability of the applied currents to affect the magnitude of the monosynaptic EPSPs, and shifted the extrapolated equilibrium potential in the negative direction. TEA accentuated the trend of cortico-motoneuronal EPSPs toward a more negative equilibrium potential as compared with the group Ia and rubro-motoneuronal EPSPs recorded in the same cell. The early phase of all monosynaptic EPSPs could not be reversed by depolarizing currents in normal and TEA-treated motoneurons and when superimposed on the plateau of depolarization of TEA-prolonged action potential. The results are discussed in relation to the ionic mechanisms of excitatory subsynaptic membrane and to the location of the synaptic sites.

The effect of dopa on the spinal cord. 8. Presynaptic and "remote" inhibition of transmission from Ia afferents to alpha motoneurones.

AbstractIn unanesthetized spinal cats, injected with l‐DOPA, intra‐axonal recording was used to investigate the primary afferent depolarization (PAD) evoked in different species of afferents by volleys in the flexor reflex afferents (FRA). It was confirmed that a late PAD is evoked in la afferents and usually not in Ib or cutaneous afferents. Some late PAD was observed in a few group II muscle afferents. Intracellular recording from motoneurones revealed a considerable depression of the Ia EPSP during the late PAD evoked from the FRA after DOPA, but it was diffkult to attribute this entirely to the PAD in Ia afferents since there was also a late postsynaptic conductance increase in the motoneurones. A further analysis was made on acute spinal cats not injected with DOPA, in which a late P.4D sometimes is evoked by FR24 volleys, without the late postsynaptic soma effects in motoneurones. The marked depression of the Ia EPSP evoked from the FRA under these conditions has a longer duration than the PAD in la afferent terminals. It is postulated that two mechanisms contribute to the Ia EPSP depression, presynaptic inhibition and “remote inhibition”; the latter action persisting after the repolarization of Ia afferents. The results are discussed in relation to reflex regulation of stepping.