Crosses and uncrossed synaptic actions on motoneurones of back muscles in the cat

A method to estimate the effects of parallel inputs on neuronal discharge probability

We here present a method to study the interaction of parallel neural input channels regarding their effects on a neurone. In particular, the method allows to disclose the effects of oligosynaptic pathways that may exist in parallel to direct monosynaptic connections to the cell. Two (or more) inputs (nerves) are stimulated with random patterns of stimuli. The response of the cell to these patterns is evaluated by the computation of peristimulus-time histograms (PSTHs). One of the two stimulus trains is selected as the one to yield reference events for the PSTH computation. From this stimulus train are selected those stimuli as reference events which are preceded, at defined mean intervals, by stimuli in the same or a parallel channel. These "conditioning" stimuli are determined (1) separately from each single stimulus train and (2) concomitantly from the two trains as events occurring simultaneously in both. The effects exerted by these various conditioning events on the effects of the "test" pulses on the cell response yield insights into the interactions between the two (or more) inputs. These methods are demonstrated on spinal Renshaw cells activated by independent random stimulation of two muscle nerves and on dorsal horn neurones responding to cutaneous nerve stimulation.

A short-latency crossed pathway from cutaneous afferents to rat hindlimb motoneurones

1. A novel pathway is described which mediates excitation of hindlimb moto-neurones from contralateral afferents. Stimulation of contralateral limb nerves evoked short-latency (less than 5 ms) EPSPs in 55% of motoneurones tested. 2. The EPSPs were evoked by fast-conducting contralateral afferents activated by electrical stimuli of 1.04-2.5 times nerve threshold. Stimulation of contralateral muscle nerves did not evoke short-latency EPSPs, whereas nerves which contain afferents from distal skin territories (sural, superficial peroneal, tibial and saphenous nerves) did. 3. Central latencies were determined for the EPSPs from the arrival of the earliest components of the afferent volleys at the spinal cord. The earliest latencies were very brief (1.5 ms). These are comparable to the latencies of disynaptic inhibition in rat motoneurones from ipsilateral group I muscle afferents (1.5-1.7 ms). We conclude that a disynaptic relay is likely to be responsible for the earliest of the crossed EPSPs. 4. Short-latency crossed EPSPs were found in all types of motoneurone tested (including both flexor and extensor motoneurones) except in six cells innervating intrinsic foot muscles. In view of the origin and distribution of the crossed EPSPs, their possible functional role is discussed.

Early and late components in cat Renshaw cell responses to random stimulation of motor axons: their differential sensitivity to preceding activation

Lumbosacral Renshaw cells were activated by random stimulation of motor axons in muscle nerves or ventral roots. The stimulus patterns had mean rates of 9.5-13 or 20-23 pulses per second. The Renshaw cell responses were evaluated by two kinds of peristimulus-time histograms. "Conventional" peristimulus-time histograms were calculated by averaging the cell discharge with respect to all the stimuli in a train. "Conditional" peristimulus-time histograms were determined by averaging the cell discharge with respect to the second ("test") stimulus in pairs of stimuli which were separated by varied intervals. The effects of the conditioning stimuli were evaluated after correcting for the effect of linear superposition of the conditioning and test stimuli. The conventional peristimulus-time histograms showed an excitatory response which often consisted of two distinct components: a narrow and high "early" peak and a broad and low "late" elevation of firing probability. The early and late excitatory components were conditioned in different ways. Whereas the late component was virtually always depressed, the early component showed three patterns: (1) uniform depression; (2) uniform facilitation; (3) a mixture of depression and facilitation. Frequency responses (coherence and gain estimates) were also calculated separately for the cell discharges underlying either the early or the late components. The estimates for the "late spikes" showed a stronger decline with increasing frequency than those for the "early spikes". The origin of the different conditioning effects probably lies in a combination of pre and postsynaptic factors. They may play a role in tremor mechanisms.

Reflex responses evoked from cats' coccygeal ventral roots by electrical stimulation of the tail nerves

In the terminal cone of the cat spinal cord, the coccygeal segments give rise to the ventrolateral and dorsomedial nerves of the tail. Extracellular recording was made from the ventral root of the coccygeal segments after stimulation of each or both ventrolateral and dorsomedial nerves. Stimulation of either nerve caused a polysynaptic response of 5- or 10-ms latency. A monosynaptic response was not seen unless posttetanic potentiation was carried out, or both nerves were stimulated simultaneously. It is suggested that the ventrolateral and dorsomedial nerves are linked by a common pathway, allowing spatial summation to take place, thus facilitating the elicitation of a monosynaptic response. The neural organization underlying the muscle movements of the cat's tail is discussed.

Brainstem reticular stimulation facilitates back muscle motoneuronal responses to pudendal nerve input

In urethane or pentobarbital anesthetized female rats, stimulation with bipolar electrodes in the medullary reticular formation evoked activity in lateral longissimus muscle nerves, when trains of pulses were applied. Combined stimulation of the medullary reticular formation and the pudendal nerve revealed a marked mutual facilitation of their two separate effects on the excitability of the lateral longissimus muscle nerves. These results match previous conclusions based on the effects of spinal column transection on the pudendal nerve-evoked response, and are consistent with a role for this mutual facilitation in the control of lordosis behavior.

Recurrent inhibitory connexions among neck motoneurones in the cat

1. Intracellular recordings were made from motoneurones innervating neck muscles in the cat. Dorsal roots were cut and muscle nerves electrically stimulated to activate alpha motor axons. 2. Recurrent inhibitory post-synaptic potentials (i.p.s.p.s) evoked by antidromic volleys in homonymous or heteronymous nerves were found in the majority of motoneurones studied, including those to dorsal neck muscles (biventer cervicis, splenius and complexus) as well as to occipitoscapularis and levator scapulae ventralis. 3. Central latencies of the recurrent i.p.s.p.s indicate disynaptic transmission. Amplitudes ranged from 100 microV (criterion level) to 2.2 mV. Average amplitudes were less than 0.6 mV. 4. The recurrent i.p.s.p.s were distributed to non-synergistic as well as to synergistic motoneurones. Analysis of relative strength of recurrent inhibition indicates influence of proximity of motoneurone pools, functional relatedness of muscles, as well as other factors. Variation in intrinsic motoneuronal properties probably underlies positive correlations (independent of variation in resting potential) between recurrent i.p.s.p.s evoked from different sources in motoneurones of a single pool. 5. Recordings (mainly extracellular) were also made from interneurones (Renshaw cells), located in the C3 and C4 segments of the spinal cord, that were excited by antidromic volleys in muscle nerves. The response varied from a single action potential to a burst of up to nineteen action potentials. Central latencies to the first response indicate monosynaptic transmission. Many Renshaw cells were excited by antidromic volleys in several muscle nerves, though this was restricted to nerves of the same segmental level as the Renshaw cell. All the muscle nerves studied were effective in activating Renshaw cells. 6. The results indicate that in many ways the recurrent i.p.s.p.s and the responses of Renshaw cells recorded in the neck segments resemble those in the hind-limb segments. Thus, the basic organization of recurrent inhibition in the neck segments resembles that occurring elsewhere in the spinal cord. A difference is the tendency for recurrent i.p.s.p.s in neck motoneurones to be relatively small in amplitude and Renshaw cell responses to be less strong than those recorded in the hind-limb segments. It is suggested that this is related to the segmentation of neck muscles and their motoneurone pools. 7. It is concluded that recurrent inhibition is a prominent feature of spinal organization governing neck muscles. It can therefore be expected to participate in control of head movements.

Electromyographic study of lumbar back muscles during locomotion in acute high decerebrate and in low spinal cats

The electromyographic (EMG) activity of the lumbar back muscles (multifidus, longissimus and iliocostalis) was investigated during treadmill locomotion in acute high decerebrate and in low spinal cats. During alternate stepping (0.7-2.0 m X s-1) in high decerebrate cats, the back muscles have two bursts of activity per step cycle. On the average these EMG bursts last about 170 ms and start some 25 ms before the onset of each vastus lateralis (VL). The two bursts in any one back muscle may have a different duration, the shortest burst on one side coinciding with the longest burst of the homologous contralateral muscle. There is often an overall asymmetry in the discharge wherein the bursts of activity in both ipsi- and contralateral muscles are longer at the onset of one of the VLs. Correlation analyses of several timing parameters of the bursts as a function of walking speeds were made. Different patterns of correlation were identified and it was found that, in most cases, the end of the bursts (with respect to the onset of VL activity) was best correlated with the speed of walking. During gallop, the back muscles activity is a single burst of about 200 ms duration which starts some 75 ms before the onset of VL. In low spinal cats walking after an injection of clonidine, the double burst pattern of EMG activation may be present if there is adequate weight support. However, when the animal steps with the hindlimbs extended and with insufficient weight support, these muscles have a tonic activity uncorrelated with the rhythmic activity of hindlimb muscles.

Distribution of monosynaptic Ia excitatory post-synaptic potentials in the motor nucleus of the cat semitendinosus muscle

Evidence is presented for a lack of localization of monosynaptic Ia excitatory post-synaptic potentials (e.p.s.p.s) in the motor nucleus supplying the atypical cat hind limb muscle semitendinosus, which has anatomically distinct in-series compartments. Recordings were made from dorsal root filaments containing functionally isolated Ia, spindle group II and Ib axons from the proximal and distal compartments of semitendinosus. Twitch of either of these in-series compartments resulted in accelerated discharge of Ia and spindle group II fibres in the other compartment. Ib fibres of either compartment showed an in-series response to twitch of a single compartment which was weaker than twitch of the whole muscle, a finding which was consistent with the diminished force produced by twitch of either compartment alone. In addition, intracellular recordings were made from semitendinosus motoneurones in anaesthetized low-spinal cats during electrical stimulation of the nerve branches to proximal semitendinosus and distal semitendinosus. Comparison of proximal semitendinosus and distal semitendinosus motoneurones failed to reveal any difference between the two cell groups with respect to the average Ia e.p.s.p. amplitude produced by either the proximal or distal semitendinosus nerve branch. However, e.p.s.p.s due to stimulation of distal semitendinosus were approximately 65% larger, on average, than those due to stimulation of proximal semitendinosus in either motoneurone group. Analysis of cell location along the rostro-caudal axis of the spinal cord indicated that the proximal and distal semitendinosus cell groups are largely co-extensive. Recordings of volleys in the proximal and distal semitendinosus nerve branches in response to stimulation of the L6, L7 and S1 dorsal roots showed that group I afferents from the proximal semitendinosus compartment tend to have a more rostral entry point to the spinal cord than do distal semitendinosus afferents. E.p.s.p. amplitude in either cell group due to stimulation of either nerve branch showed little dependence on cell location in the spinal cord. The results are discussed with respect to the relation between muscle function and the distribution of monosynaptic Ia connexions.

Supraspinal and segmental input to lumbar epaxial motoneurons in the rat

Inputs to medial longissimus (ML) and lateral longissimus (LL) motoneurons were studied in urethane or urethane-chloralose anesthetized rats by recording from ML and LL nerves while stimulating ipsilateral lumbosacral dorsal roots, medial medullary reticular formation (RF), vestibular nuclei (VN), dorsal midbrain (MDBR), or ventromedial hypothalamus (VMH). Stimulation of appropriate dorsal roots produced short-latency responses (1.5-3.0 ms) in nerves to medial longissimus or lateral longissimus. The connections underlying these responses, which could be monosynaptic, are weak, since generally two or more stimuli were necessary for a response to occur. Short-latency LL nerve responses required more dorsal root stimuli than did ML nerve responses and stable LL responses sometimes could not be obtained, suggesting that segmental reflexes to a back muscle (LL) could be weaker than those to a proximal tail muscle (ML). Trains of conditioning stimuli delivered to the RF, VN, and MDBR facilitated segmental responses in ML nerves or LL nerves. Temporal profiles of facilitation of ML differed for the three regions. On one extreme, the facilitation produced by RF conditioning required few stimuli (median, 3 shocks) and peak facilitation occurred at short condition-test intervals (median, 1.5 ms). On the other extreme, facilitation produced by MDBR conditioning required long trains (median, 14 stimuli) and peak facilitation occurred at longer condition-test intervals (median, 10 ms). Stimulation within the VMH never facilitated ML or LL nerve activity. These results demonstrate excitatory connections from reticular formation, vestibular nuclei and the dorsal midbrain to medial longissimus and lateral longissimus. Such connections could be involved in behaviors mediated by midbrain, and in postural regulation through brain stem control of axial musculature. Motoneuron cell bodies for LL, ML and lumbar transversospinalis (TS) muscles were localized by ejecting dye at sites where unitary antidromic responses to muscle nerve stimulation were recorded extracellularly. ML cells were found ventrolaterally in the L6-S1 ventral horn. LL and TS cells were found medially in the ventral horn of the lumbar enlargement.