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.     

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.     

Influence of afferent synaptic innervation on the discharge variability of cat abducens motoneurones

The discharge variability of abducens motoneurones was studied after blocking inhibitory synaptic inputs or both excitatory and inhibitory inputs by means of an intramuscular (lateral rectus) injection of either a low (0.5 ng kg⁻¹) or a high dose (5 ng kg⁻¹) of tetanus neurotoxin (TeNT), respectively. Motoneuronal firing increased after low-dose TeNT. High-dose treatment, however, produced a firing depression, and in some cells, a total lack of modulation in relation to eye movements. Firing became increasingly more regular with larger TeNT doses as shown by significant reductions in the coefficient of variation after low- and high-dose treatments. Similarly, autocorrelation histograms of interspike intervals increased the number of resolvable peaks twofold in low-dose-treated motoneurones and sevenfold in high-dose-treated motoneurones. The plots of standard deviation versus the mean instantaneous firing frequency showed an upward deflexion with low firing frequencies. The upward deflexion occurred in controls at 39.9 ± 4.9 ms, an interval similar to the mean afterhyperpolarisation (AHP) duration (48.4 ± 8.8 ms). Low-dose TeNT treatment shifted the deflexion point to 20.9 ± 3.9 ms, whereas the high dose increased it to 60.7 ± 6.1 ms, in spite of the fact that no differences in AHP parameters between groups were found. The density of synaptophysin-immunoreactive boutons decreased by 14 % after the low-dose treatment and 40.5 % after the high-dose treatment, indicating that protracted synaptic blockade produces elimination of synaptic boutons. It is concluded that abducens motoneurone spike variability during spontaneous ocular fixations depends largely on the balance between inhibitory and excitatory synaptic innervation.     

Spatial synaptic distribution of recurrent and group Ia inhibitory systems in cat spinal motoneurones

1. The reversal potentials of several types of inhibitory post-synaptic potentials (IPSPs) have been studied in cat spinal motoneurones with and without modification of intracellular chloride ion (Cl(-)) concentration. Single barrel intracellular micropipette electrodes have been used.2. When studied with potassium citrate filled micropipettes, the reversal potential for IPSPs evoked by stimulation of antagonist group Ia afferents is the same as that for recurrent IPSPs evoked by antidromic stimulation of motoneurone axon collaterals, confirming earlier observations (Araki, Ito & Oscarsson, 1961; Coombs, Eccles & Fatt, 1955).3. Studied with potassium chloride filled micropipettes. the reversal potential for the group Ia IPSP was found to be different from that for the recurrent IPSP when the amount of Cl(-) diffusing or iontophoretically injected into the motoneurone was small. The amount of difference in reversal potential varied from cell to cell but when present the group Ia IPSP reversed to a depolarizing potential more readily than the recurrent IPSP in all cases.4. Interaction between recurrent IPSPs and monosynaptic excitatory post-synaptic potentials (EPSPs) was also studied and the amount of non-linearity of potential summation was measured.5. The results are consistent with the hypothesis that the terminations of Renshaw cells responsible for the recurrent IPSP are located largely on the proximal dendrites of motoneurones, while the terminations of the interneurones generating the group Ia IPSP appear to be closer to or on the cell somata.     

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.     

An analysis of synaptic transmission to motoneurones in the cat spinal cord using a new selective receptor blocker

We have investigated the role of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors on synaptic transmission in vivo between Ia primary afferents and cat spinal motoneurones using a selective non-N-methyl-d -aspartate (non-NMDA) receptor antagonist, GYKI 52466. Both microionophoretic and intravenous application of GYKI 52466 depressed the Ia excitatory post-synaptic potential (Ia EPSP) in a dose-dependent manner, without any apparent effect on membrane conductance or resting potential of the motoneurone. GYKI 52466 reduced selectively α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)- but not N-methyl-d -aspartate (NMDA)-induced depolarizations. Our results suggest that a large part of the Ia EPSP is mediated by AMPA receptors. The participation of other excitatory amino-acid receptors in the Ia EPSP is also discussed.     

Specific membrane properties of cat motoneurones

1. Electrophysiological properties of cat motoneurones were measured using intracellular electrodes, after which Procion dye was injected iontophoretically into the neurone through the recording pipette.2. Histological procedures were chosen to minimize changes in neuronal morphology. Reconstructed motoneurones had more dendritic branches and larger surface areas than the Golgi-stained motoneurones of earlier reports.3. The sum of the 3/2 power of the dendritic diameters (the dendritic trunk parameter; Rall, 1959) of the reconstructed motoneurones was found to decrease with distance from the soma. Thus, the dendritic tree is not satisfactorily approximated by a non-tapering membrane cylinder.4. A computational technique was developed to allow calculation of the specific resistance (R(m)) of the membrane using the measured value of the input resistance of the motoneurone and a more detailed approximation of the dendritic tree. These calculations indicate that the average resting value of dendritic R(m) is at least 1800 Omega cm(2). The specific membrane capacity, calculated assuming uniform R(m), ranged between 2-3 muF/cm(2).     

Trigeminal excitation of dorsal neck motoneurones in the cat

Summary Excitation of dorsal neck motoneurones evoked by electrical stimulation of primary trigeminal afferents in the Gasserian ganglion has been investigated with intracellular recording from -motoneurones in the cat. Single stimulation in the Gasserian ganglion ipsi-and contralateral to the recording side evoked excitatory postsynaptic potentials (EPSPs) in motoneurones innervating the lateral head flexor muscle splenius (SPL) and the head elevator muscles biventer cervicis and complexus (BCC). The gasserian EPSPs were composed of early and late components which gave the EPSPs a hump-like shape. A short train of stimuli, consisting of two to three volleys, evoked temporal facilitation of both the early and late EPSP components. The latencies of the gasserian EPSPs ranged from 1.6 to 3.6 ms in SPL motoneurones and from 1.6 to 5.8 ms among BCC motoneurones. A rather similar latency distribution between 1.6 and 2.4 ms was found for ipsi- and contralateral EPSPs in SPL and BCC motoneurones, which is compatible with a minimal disynaptic linkage between primary trigeminal afferents and neck motoneurones. Systematic transections of the ipsi- and contralateral trigeminal tracts were performed in the brain stem between 3 and 12 mm rostral to the level of obex. The results demonstrate that both the ipsi- and contralateral disynaptic and late gasserian EPSPs can be mediated via trigeminospinal neurones which take their origin in the nucleus trigeminalis spinalis oralis. Transection of the midline showed that the contralateral trigeminospinal neurones cross in the brain stem. Systematic tracking in and around the ipsilateral trigeminal nuclei demonstrated that the axons of ipsilateral trigeminospinal neurones descend just medial to and/or in the medial part of the nucleus. Spinal cord lesions revealed a location of the axons of the ipsilateral trigeminospinal neurones in the lateral and ventral funiculi. Interaction between the ipsi- and contralateral gasserian EPSPs showed complete summation of the disynaptic EPSP component, while the late components were occluded by about 45%. These results show that the disynaptic EPSPs are mediated by separate trigeminospinal neurones from the ipsi- and contralateral side, while about half of the late EPSPs are mediated by common neurones which receive strong bilateral excitation from commissural neurones in the trigeminal nuclei. Spatial facilitation was found in the late gasserian EPSP but not in the disynaptic gasserian EPSP by conditioning stimulation of cortico- and tectofugal fibres. Disynaptic pyramidal and tectal EPSPs, which are mediated by reticulospinal neurones, were facilitated by a single stimulation in the gasserian ganglion at an optimal interval of 2 ms. It is suggested that primary trigeminal afferents can excite the reticulospinal neurones via a disynaptic trigeminoreticular pathway.     

Alterations of synapses on axotomized motoneurones.

1. Properties of medial gastrocnemius motoneurones in cats were examined with intracellular electrodes 1-77 days after partial transection of the medial gastrocnemius muscle nerve. 2. Three sequential stages of reaction were defined: (i) firstly, conduction velocity of axotomized motoneurones fell, but e.p.s.p. properties and Ia-motoneurone connectivity were normal; (ii) secondly, e.p.s.p.s became characterized by prolonged rise times and half-widths and by diminished amplitudes; (iii) finally, failures of connectivity were also seen, that is, Ia spindle afferent fibres connected with fewer axotomized than normal homonymous motoneurones. 3. The reason for the change of e.p.s.p. properties cannot be stated with certainty: among the factors may be (i) a change of motoneurone membrane properties and morphology; (ii) partial detachment of synaptic boutons before their total disconnexion, and (iii) detachment of Ia afferent branches terminating on the motoneurone soma before those on the dendrites. 4. The previous conclusion that alteration of e.p.s.p. profiles following motoneurone axotomy was due to total disconnexion of those Ia fibres making synapses on the soma and proximal dendrites ("somatic stripping") must be modified to account for the findings that e.p.s.p. changes are seen before connectivity changes.