Common interneurones mediating cortical and tectal excitation of abducens motoneurones in the cat

Summary Tectal and cortical effects on abducens motoneurones were examined with intracellular recording techniques in cats under chloralose anaesthesia. Abducens motoneurones exhibited disynaptic EPSPs after stimulation of the contralateral superior colliculus and cerebral peduncle. The tectal disynaptic EPSPs were observed invariably in all motoneurones tested, while the peduncular EPSPs were observed only in 40% of motoneurones after stimulation of the contralateral cerebral peduncle. However, the tectal disynaptic EPSPs were consistently facilitated by conditioning pedunclar stimulation in all motoneurones tested. These results indicated that the disynaptic excitatory tecto-abducens and cortico-abducens pathways shared common premotor interneurones. The common interneurones which mediated the tectal and cortical disynaptic excitation of abducens motoneurones were explored in the pons. These interneurones were identified by the criteria that they were fired monosynaptically from both the tectum and the cerebral peduncle and were activated antidromically from the abducens nucleus. Systematic threshold mapping for the antidromic activation in and around the abducens nucleus indicated that they gave off many collateral branches in the nucleus. Such neurones were found in the nucleus reticularis pontis caudalis, being distributed in the area extending 0.8–3 mm rostral to the rostral pole of the abducens nucleus, 1.3–2.7 mm deep from the dorsal surface of the brain stem, and 0.8–1.8 mm lateral from the midline. The present experiments strongly suggest that a group of neurones in the paramedian pontine reticular formation make direct excitatory connexions with abducens motoneurones and play a role of common interneurones that transmit both tectal and cortical commands.     

L-glutamate and N-methyl-D-asparatate actions on membrane potential and conductance of cat abducens motoneurones

The actions of L-glutamate and N-methyl-D-aspartate (NMDA) were studied with intracellular recordings from cat abducens motoneurones. Amino acids were electrophoresed extracellularly from the same 7-barreled electrode types as those used in the spinal cord. Depolarization development, conductance changes and firing pattern induced by amino acids were very similar to those described for spinal motoneurones. The shape of NMDA depolarization suggests a uniform distribution of the involved receptors on the membrane of the motoneurone.     

Alterations of synaptic action in chromatolysed motoneurones of the cat

1. Monosynaptic EPSPs in lumbosacral motoneurones undergoing chromatolysis were studied by intracellular recording from 7 to 20 days after section of the appropriate ventral roots of the cat.2. The maximum monosynaptic EPSPs evoked in chromatolysed motoneurones by afferent volleys from the biceps-semitendinosus or the triceps surae muscles ranged from 1.0 to 9.5 mV in amplitude. The time-to-peak of these EPSPs was 1.7 msec on the average. These values were significantly smaller and longer, respectively, than the amplitude and the time-to-peak of monosynaptic EPSPs observed in normal motoneurones. The long time-to-peak of EPSPs in chromatolysed motoneurones could not be accounted for by asynchronous transmitter release.3. The mean number of unit EPSPs responding to a single afferent impulse (m) in chromatolysed motoneurones was comparable to that found in normal motoneurones.4. The amplitude of unit EPSPs estimated from the mean EPSP amplitude and the m value following stimulation of a single afferent fibre was significantly smaller in chromatolysed motoneurones than in normal motoneurones. This difference was attributed to a difference in synaptic location.5. The shape of monosynaptic EPSPs evoked in chromatolysed motoneurones by stimulation of single afferent fibres was analysed on the basis of Rall's compartment model. The analysis suggested that there is a lack of the excitatory synaptic input to the cell body in chromatolysed motoneurones.6. Similar alterations were also found in IPSPs. The degree of change in synaptic responses evoked by stimulation of various pathways appears to depend on the synaptic location.7. Following the study of the interaction of several inputs on the motoneurone and of their dependence on membrane potential, a tentative model of the synaptic distribution of different pathways is proposed.     

The afferent innervation of two infrahyoid muscles of the cat

1. The afferent innervation of the straplike muscles of the infrahyoid region were investigated in two ways. The morphology of spindles and counts of tendon organs were investigated by the gold chloride technique in ten muscles. Spindle counts were made in forty pairs of thyrohyoid and infrahyoid muscles. De-efferenting of the nerves to these muscles was done in three cats and the calibre spectra of the afferent innervation investigated. These were compared with the total counts of fibres in intact nerves.2. In the thyrohyoid, spindles are frequently absent. No tendon organs were seen. In the large infrahyoid (combined sternohyoid and sternothyroid), spindle counts varied from 0 to 20 and the mean spindle count per gram of muscle was 3.5. A maximum of five tendon organs were seen in the muscle. Both spindle and tendon organ counts are low when compared with a limb muscle of similar weight and size.3. In the infrahyoid muscle complex spindles were about equal in number to simple spindles.4. Counts of spindles in the infrahyoid muscle in families of three or more siblings suggest that some families of kittens tend to have higher spindle counts than other families.5. The afferent innervation of the two muscles varied between 21 and 42% of the total fibre population and the fibre diameter spectrum is in keeping with the low counts of encapsulated endings.     

Non-linear summation of unit synaptic potentials in spinal motoneurones of the cat

1. Monosynaptic excitatory post-synaptic potentials (EPSPs) produced in spinal motoneurones of the cat by stimulation of a single afferent fibre were recorded with intracellular electrodes.

2. In total, seventy-three triceps surae motoneurones were studied with stimulation of thirty-six different afferent fibres.

3. The mean amplitude of the EPSPs evoked by single afferent impulses ranged from 0·06 to 2·0 mV with an average of 0·27 mV.

4. The mean number of unit EPSPs responding to a single afferent impulse (m) was calculated from the number of failures. The values ranged from 0·7 to more than 5. About 10% of the sample showed no failure of synaptic response in about 200 consecutive trials. The m values for these synaptic responses were estimated to range from 5 to 15.

5. In the majority of tests, the observed amplitude fluctuations of monosynaptic EPSPs evoked by stimulation of a single fibre were less than those expected from Poisson's law. This discrepancy may be accounted for by non-linear summation of the unit EPSPs at dendritic synaptic sites.

6. It is suggested that the synaptic responses initiated at different sites of a motoneurone may summate linearly at the soma, although summation of unit EPSPs is non-linear at individual synaptic sites.

     

Quantal components of the inhibitory synaptic potential in spinal motoneurones of the cat

1. Monosynaptic i.p.s.p.s were produced in spinal motoneurones of the cat by stimulation of a pool of interneurones following chronic degeneration of descending tracts and primary afferent fibres in the lumbosacral cord.2. Monosynaptic i.p.s.p.s so evoked by supramaximal stimuli often showed a fluctuation in amplitude with occasional failures of response.3. When two successive stimuli were applied at a short interval, the mean amplitude of the second i.p.s.p.s was greater than that of the first. This facilitation was associated with a decrease in the number of failures, a decrease in the coefficient of variation of the amplitude distribution and an increase in the probability of occurrences of large i.p.s.p.s.4. A statistical analysis of the i.p.s.p. amplitude fluctuation showed that the monosynaptic i.p.s.p. is composed of discrete unit potentials evoked with a certain probability in a manner described by a binomial law.5. The application of strychnine decreased the mean amplitude of i.p.s.p.s with little change in the coefficient of variation of the i.p.s.p. amplitude distribution.6. It is concluded that the release of inhibitory transmitter occurs in quantal steps and that strychnine blocks primarily the post-synaptic receptors for the inhibitory transmitter.     

Enhancement of synaptic transmission by dendritic potentials in chromatolysed motoneurones of the cat

1. Monosynaptic transmission in cat lumbosacral motoneurones undergoing chromatolysis was studied by intracellular recording from 7 to 20 days after section of the appropriate ventral roots.2. The average input resistance measured by passing polarizing currents across the cell membrane showed no significant difference between normal and chromatolysed motoneurones. Average rheobasic current for chromatolysed motoneurones was significantly lower (by about 30%) than that for normal motoneurones.3. Spike-like partial responses were commonly superimposed on monosynaptic EPSPs in chromatolysed motoneurones. These responses could be eliminated by stimulation of the bulbar inhibitory reticular formation, but could not be blocked by hyperpolarization applied to the motoneurone soma.4. The spike-like partial response in chromatolysed motoneurones showed a refractory period following (i) the antidromic invasion of the neurone generated by ventral root stimulation, and (ii) in response to two successive afferent stimuli. The refractory period ranged from 5 to 13 msec.5. Initiation of the partial response had no direct relation with the amplitude of the underlying EPSP. The partial response could be evoked by small EPSPs of about 0.5 mV.6. The action potential of a chromatolysed motoneurone arose from the partial response at different levels of depolarization, showing multiple trigger zones for spike initiation. Occasionally, chromatolysed motoneurones discharged in response to stimulation of a single afferent fibre.7. In neurones where more than one spike-like response was obtained, interaction between dendritic responses showed no refractoriness.8. It is concluded that the partial response is an all-or-none event originating at some discrete site on dendrites, and that its presence increases the efficacy of synaptic excitation in chromatolysed motoneurones.     

Ultrastructural and electrophysiological properties of accessory abducens nucleus motoneurones: An intracellular horseradish peroxidase study in the cat

The physiological and morphological (light and electron microscopy) properties of six retractor bulbi motoneurones were analysed using the technique of intracellular recording and intracellular labelling with horseradish peroxidase. The retractor bulbi motoneurones were identified by antidromic invasion and orthodromic responses following stimulation of trigeminal afferents were studied. Two of these motoneurones were examined ultrastructurally. Terminal boutons forming synapses with labelled soma, labelled proximal and distal dendrites were characterized. Serial sections allowed the axon hillock to be analyzed and the initial segment of a presumed motoneurone to be observed in the section where the injected motoneurone was described. The ultrastructure of unidentified elements observed in the accessory abducens nucleus is stressed.     

Control from the brainstem of synchrony of discharge between gamma motoneurones in the cat

Summary An assessment has been made of the effect of partial and complete section of the spinal cord on the discharges of gamma motoneurones to hind limb muscles in the decerebrated cat. The degree to which the discharges of pairs of individual gamma motoneurones exhibit short-term synchrony and the variability in interspike intervals of the discharge in individual neurones was measured. Variability of discharge was assessed as coefficient of variation of interspike intervals and degree of synchronization assessed from cross correlation analysis. The discharges of gamma motoneurones in the decerebrated cat with intact spinal cord are regular (low coefficient of variation) and show no tendency to synchrony for up to 24 h following decerebration. Section of the more medial part of the dorsolateral funiculus, on either side of the thoracic spinal cord, resulted in a less regular discharge and the development of short-term synchrony between gamma motoneurones. A dependence of synchrony on variability of discharge was established. The synchrony and increased variability in the spinal cat persisted for at least 24 h. Both were markedly reduced following administration of the precursors of monoamines (either L-Dopa or 5-HTP). We conclude that a bilateral, monoaminergic pathway descending in the dorsolateral funiculus from the brainstem controls synchrony of gamma motoneurone discharge in the decerebrated cat. The possibility is discussed that synchrony of discharge between alpha motoneurones may be controlled by a similar pathway.     

Electrophysiological and morphological properties of rat abducens motoneurones

Summary The electrical and morphological properties of abducens motoneurones were investigated in the rat with intracellular recordings and intracellular HRP-staining. Motoneurones were identified by their antidromic response to electrical stimulation of the lateral rectus muscle. The antidromic action potential was followed by a delayed depolarization and an after hyperpolarization lasting 20 ms to 45 ms. The whole neurone input resistance (R_N) calculated from I/V curves, was found to lie between 2 M and 15 M with a bimodal distribution (mean values 4.9 M and 12 M). In some cases, anomalous rectification was observed with low current intensities. Prolonged hyperpolarizing current pulses revealed the presence of a time dependant inward rectification and slow rebound depolarization. The intensity/frequency curves suggest the existence of three ranges of discharge. The average intensity frequency slope during the steady state was 43 imp/s/ nA. Eight abducens motoneurones were intracellularly labelled with HRP and fully reconstructed. The soma (23 m to 40 m in diameter) gave off 5 to 7 primary dendrites. The general organization and extension of the dendritic trees depended on the location of the soma within the abducens nucleus. The mean diameter of primary dendrites was 4.17 m with similar average values in all motoneurones. The soma size of abducens motoneurones was not correlated with either the size of the proximal tree or the whole neurone input resistance.     

Direct inhibitory synaptic linkage of pontomedullary reticular burst neurons with abducens motoneurons in the cat

1. Unit spikes of burst neurons were extracellularly recorded in the pontomedullary reticular formation of the cat. These neurons were identified by their burst activity coincident with the quick inhibitory phase of the contralateral abducens nerve during vestibular nystagmus and their antidromic activation from the contralateral abducens nucleus. 2. When the extracellular field potentials in and near the abducens nucleus were triggered by spikes of a contralateral burs neuron, the averaged potential consisted of an early di- or triphasic spike and a late slow positive wave. The early spike was an action current caused by impulses conducting along the axon of the burst neuron. 3. The action potentials of a contralateral burst neuron. 3. The action potentials of a contralateral burst neuron were employed to trigger a post-spike average of the membrane potential of abducens motoneurons. Then unitary IPSPs with monosynaptic latencies were revealed. This provided direct evidence that the burst neurons are inhibitory in nature. The amplitudes of unitary IPSPs ranged from 18 to 220 mu V. Each inhibitory burst neuron branched widely in the abducens nucleus and was estimated to make inhibitory connections with approximately 60% of the motoneuron pool. 4. The post-spike average of compound potentials of the abducens nerve triggered by action potentials of contralateral single inhibitory burst neurons revealed inhibition of spike activity with latencies and time courses compatible with those of unitary IPSPs in motoneurons. The inhibition was observed with all inhibitory burst neurons tested.     

Variable amplification of synaptic input to cat spinal motoneurones by dendritic persistent inward current

The circadian pacemaker is differentially sensitive to the resetting effects of retinal light exposure, depending upon the circadian phase at which the light exposure occurs. Previously reported human phase response curves (PRCs) to single bright light exposures have employed small sample sizes, and were often based on relatively imprecise estimates of circadian phase and phase resetting. In the present study, 21 healthy, entrained subjects underwent pre- and post-stimulus constant routines (CRs) in dim light (∼2–7 lx) with maintained wakefulness in a semi-recumbent posture. The 6.7 h bright light exposure stimulus consisted of alternating 6 min fixed gaze (∼10 000 lx) and free gaze (∼5000–9000 lx) exposures. Light exposures were scheduled across the circadian cycle in different subjects so as to derive a PRC. Plasma melatonin was used to determine the phase of the onset, offset, and midpoint of the melatonin profiles during the CRs. Phase shifts were calculated as the difference in phase between the pre- and post-stimulus CRs. The resultant PRC of the midpoint of the melatonin rhythm revealed a characteristic type 1 PRC with a significant peak-to-trough amplitude of 5.02 h. Phase delays occurred when the light stimulus was centred prior to the critical phase at the core body temperature minimum, phase advances occurred when the light stimulus was centred after the critical phase, and no phase shift occurred at the critical phase. During the subjective day, no prolonged 'dead zone' of photic insensitivity was apparent. Phase shifts derived using the melatonin onsets showed larger magnitudes than those derived from the melatonin offsets. These data provide a comprehensive characterization of the human PRC under highly controlled laboratory conditions.