Cat Motoneurons Research Articles

Ascending long spinal actions on forelimb motoneurons in the acute spinal cat.

Ascending long spinal reflex system were investigated by means of monosynaptic reflex testing in the acutely spinalized unanesthetized cat. 1. Hindlimb nerve stimulation gave bilateral facilitatory effects on the motoneuron pools of pectoralis major and physiological flexors of the forelimb such as biceps brachii, extensor carpi radialis, extensor digitorum communis, but elicited depressive effects on the physiological extensors such as triceps brachii, flexor carpi radialis, flexor digitorum profundus. 2. In the latissimus dorsi, which is the antagonist of pectoralis major, a depressive effect was elicited by the stimulation of ipsilateral hindlimb nerves, and a facilitatory effect by contralateral stimulation. 3. These effects were evoked mainly from group II afferent fibers in muscle as well as cutaneous nerves. 4. Intracellular recordings from motoneurons of extensor carpi radialis revealed EPSPs following stimulation of hindlimb nerves with time courses corresponding to those of the facilitatory effects. We failed to detect any potential changes in the motoneurons of flexor carpi radialis following stimulation of hindlimb nerves.

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.

Intracellular Mg2+ increases neuronal excitability.

Injection of Mg2+ into spinal motoneurons of cats leads to a depolarization, associated with a fall in membrane conductance, diminution in post-spike hyperpolarization, and increased excitability. This action has an apparent reversal level substantially more negative than the resting potential, and can be ascribed to a fall in K+ membrane conductance. Since these effects are opposite to those produced by intracellular Ca2+, it is suggested that Mg2+ probably competes with Ca2+ at the Ca2+-activated K+ ionophoreal free ionophores. Neuronal excitability can be regulated by the ratio of internal free Ca2+/Mg2+.

Differentiation of motoneurones and skeletal muscles in kittens.

1. Isometric contractions of the medial gastrocnemius (fast switch) and soleus (slow twitch) muscles were recorded in kittens ranging in age from 3 to 112 days, as well as in adult cats. 2. It was confirmed that the speed of contraction of the gastrocnemius muscle becomes progressively faster during the first few weeks after birth, whereas contraction times of the soleus muscle show little changes or a slight prolongation during the period of post-natal development. 3. The properties of gastrocnemius (fast alpha) and soleus (slow alpha) motoneurons were examined with intracellular electrodes in kittens at three different stages; 16-20, 61-71 and 100-112 days in age. 4. The axonal conduction velocities of both gastrocnemius and soleus motoneurones increased monotonically throughout the period of development and showed no correlation with post-natal changes in contraction times of the innervated muscles. 5. The duration of after-hyperpolarization in soleus motoneurones became progressively longer with age, while that in gastrocnemius motoneurones remained virtually unchanged during development. 6. The relation between the duration of after-hyperpolarization and the axonal conduction velocity in kitten motoneurones was similar to that observed in axotomized motoneurones of adult cats. 7. It is suggested that fast and slow alpha motoneurones show postnatal differentiation in terms of the duration of after-hyperpolarization and that axotomy leads to 'dedifferentiation' of the motoneurone properties. 8. Post-natal changes in the contractile properties of skeletal muscles were independent of the changes in the duration of after-hyperpolarization of the innervating motoneurones. However, it remains uncertain whether muscle differentiation is independent of the discharge pattern of the innervating motoneurones.

Stochastic properties of motoneuron activity and the effect of muscular length

The activity of single motor units contributing to small tonic isometric contractions in human muscle at different muscular lengths was analyzed. The form of motor unit firing patterns shows that the interspike intervals compose independent sequences with about a 10% coefficient of variation and have a gamma distribution. The variability and the distribution shape curves show that as the mean interval decreases the variance also decreases and the interval density function becomes more symmetric. More significant is the fact that the form of the firing pattern remains unchanged when a motor unit has the same mean interval but with the muscle at different lengths. Comparison of these facts with experimental data from neuron models and cat motoneurons indicates that in the human the only relevant input-output relationship in motoneurons is that the net excitation adjusts the firing rate.

Determination of the inter-spike times of neurons receiving randomly arriving post-synaptik potentials

Stein's model for a neuron receiving randomly arriving post-synaptic potentials is studied from an analytic viewpoint, using some recent results in the theory of first passage times for temporally homogeneous Markov processes. The case when the only input is excitatory can be treated exactly. It is shown that the moments of the firing time are guaranteed to be finite so that the differential-difference equation for the expectation (and higher moments) of the time for the membrane potential to first reach threshold from resting level can be written down. Analytic solutions are obtained in a number of cases with main emphasis on the case when the threshold is twice the epsp magnitude. An invariance principie is formulated wherein at a given mean input frequency and for a given decay parameter, the distribution of firing times depends only on the ratio of threshld to epsp magnitude. For the case where this ratio is two, the variation in the mean discharge rate is obtained as a function of mean input frequency. The results are compared with the experimental data for the Poisson monosynaptic excitation of cat motoneurons by Redmanet al. Agreement between theoretical and experimental values is excellent at input frequencies near 102 sec-1, and theory underestimates the firing rate below that input frequency. Reasons for the discrepancy are discussed at length including the uncertainties in the neuronal parameters and the dependence of epsp magnitude on mean input frequency. The problem of including an inhibitory input process together with excitation is treated by an approximation procedure when the inhibition is considerably weaker than the excitation. At the input frequency investigated it is shown that when inhibition "half as weak" as the excitation occurs, the mean discharge frequency is approximately halved. In the final section a method of estimating neuronal parameters from the moments of the experimental inter-spike time distribution is outlined.

Influence of dendritic location and membrane properties on the effectiveness of synapses on cat motoneurones.

1. Measurements of the specific membrane properties and neuronal geometry of cat motoneurones were used to calculate the excitatory postsynaptic potentials (e.p.s.p.s) produced by unit (quantal) conductance changes occurring at various locations on the dendritic tree.2. Calculations demonstrate that conductance changes of 80-190 x 10(-10) mho are required to produce e.p.s.p.s having the same rise time and peak amplitude as the quantal e.p.s.p.s recorded in motoneurones by Kuno & Miyahara (1969b). Because quantal conductance changes are so large, synaptic activity can significantly reduce the effective specific resistance of the motoneuronal membrane.3. A quantal conductance change occurring at a high-impedance distal dendritic site is calculated to produce an e.p.s.p. of 15-20 mV peak amplitude at that site. Significant non-linear summation will occur between the e.p.s.p.s produced by conductance changes occurring simultaneously on the same dendritic branch.4. Calculations which take into account both non-linear summation and the loss of synaptic charge through dendritic membranes predict that for these motoneurones the time integral of soma-recorded quantal e.p.s.p.s originating on distal dendrites should be at least 20% as great as the time integral of a quantal e.p.s.p. originating directly on the soma. Quantal conductance changes occurring on 76% of the dendritic tree should produce soma e.p.s.p. time integrals at least 50% as great as those produced by somatic synapses.

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).

Lack of fixed order of recruitment in cat motoneuron pools.

Henneman et al. have shown that a wide variety of stimuli activate motoneurons in the order of their sizes, though some reversals occur. It is not clear which motoneurons should be included in the “pool” from which recruitment by size takes place. The “pool” may consist of 1. all the motoneurons to a given muscle 2. all the motoneurons in a given ventral root or 3. all the motoneurons activated by a given stimulus. We have examined recruitment in the pool of fibers to a given muscle. A variety of mechanical stimuli to cat's legs were used to elicit bursts of activity in gastrocnemius motor units. In 30% of pairs of units, one unit had a consistently lower threshold, i.e. it always began and ended a burst. In the other 70%, either unit could start or end a burst. For 48% of motor unit pairs recorded in either the medial or lateral gastrocnemius, each unit of the pair had runs of firing of at least 10 spikes while the other was totally inactive. By the same criterion, 23% of pairs of temporalis units showed differential activity. During spontaneous breathing there was no consistent recruitment order in 24% of pairs of units in throat respiratory muscles. These data indicate that reversals of recruitment order are relatively common for motoneurons innervating the same muscle. We discuss the possibility that size principle may hold for motoneurons in a single ventral rootlet whose somas are anatomical neighbors (even though they innervate different muscles), but not for motoneurons that are anatomically distant in the cord (even though they innervate the same muscle).

Data processing in the cat motoneuron system: averaging functions and rate limitation.

The variability in instantaneous frequency of cat motoneurons in response to various degrees of regularity and synchronization of input is used to examine several forms of data processing occurring in the cat motoneuron system. A theoretical basis is presented which distinguishes between spatial averaging and time averaging of information from muscle afferents and which suggests realizable experiments for assessing their relative strengths. The results of the experiments demonstrate spatial averaging to be a strong form of processing in the motoneuron system; time averaging is found to be much less so. Thus, temporal information is preserved to a greater extent by the motoneuron than is spatial information. Also, an inherent source of motoneuron variability is identified and linked with the process of rate limitation.

Temperature effects on resting potential and spike parameters of cat motoneurons.

During intracellular recording from cat spinal motoneurons, the lumbo-sacral cord was subjected to rapid local temperature changes. Cooling to 30° C causes a decrease in the resting membrane potential (RMP), a transient increase in spike overshoot, and increases in the input resistance and the durations of both the action potential and the hyperpolarizing after-potential (HAP). Warming causes these membrane parameters to change in the opposite direction and can result in a failure of antidromic activation. The RMP changes are dependent upon the RMP at normal temperature, being negligible in cells having an initial RMP greater than 70 mV. The Q10-values of the three peaks of the differentiated spike are relatively high and nearly identical to the Q10-values of the corresponding rate constants for sodium activation and inactivation and potassium activation measured by Frankenhaeuser and Moore in toad nerve fibers, i.e. 1.71, 2.35 and 3.12, respectively. Delayed sodium inactivation and potassium activation explain the increase in spike overshoot observed during cooling, and the high Q10 of potassium activation is nearly identical to the Q10 of the time to peak of the HAP (3.12 vs. 2.97). The decrease in RMP during cooling is attributed to a decrease in the PK∶PNa ratio rather than to a blockage of an electrogenic sodium pump; the differences in RMP temperature sensitivities between cells with high and low initial RMP's might be due to a higher resting PK (anomalous rectification) at higher RMP. We conclude that the electrogenic pump does not normally contribute significantly to the RMP of cat motoneurons.

Facilitatory interaction in transmission to motoneurones from vestibulospinal fibres and contralateral primary afferents.

Interaction between volleys in the vestibulospinal tract and contralateral primary afferents in transmission to motoneurones has been investigated with intracellular recording from motoneurones in cats.

Spontaneous and reflexly evoked activity in pharyngeal, laryngeal, and phrenic motoneurons of cat

Experiments were performed on spontaneously breathing cats that were anesthetized with chloralose-urethane in order to study changing excitability in pharyngeal, laryngeal, and phrenic motoneurons. Pharyngeal constrictor motoneurons chiefly were spontaneously active during expiration; the stylopharyngeus and certain motoneurons serving the superior pharyngeal constrictors were inactive during either phase of tidal respiration. While confirming work of previous investigators that feeble glossopharyngeal (epipharyngeal branch) and superior laryngeal nerve stimulation transiently attenuated on-going phrenic nerve activity, such evoked afferent volleys excited laryngeal motoneurons independent of phases of respiration. On-going pharyngeal constrictor nerve activity (expiration) was attenuated for 20–50 msec by superior laryngeal nerve stimulation and for 100–300 msec by glosso-pharyngeal nerve stimulation. Intercostal nerve stimulation evoked occasional pharyngeal constrictor nerve discharges but more commonly attenuated its spontaneous nerve activity. Stylopharyngeus and an unknown portion of the superior pharyngeal constrictor motoneurons were excited by afferent volleys evoked in glossopharyngeal and superior laryngeal nerves. These aforementioned changes in excitability were unaffected by total neuromuscular paralysis while maintaining artificial respiration. It is concluded that certain pharyngeal constrictor motoneurons of the cat are spontaneously active during tidal expiration, are under central respiratory control, and that their intermittence along with those serving the diaphragm and larynx by modest afferent volleys in the ninth and tenth cranial nerves stage the act of swallow or expulsive reflexes, or both.

Bilateral effects of the afferent impulses from the masseteric muscle on the trigeminal motoneuron of the cat

Summary (1) Effects of stimulation of the masseteric nerve were investigated on the ipsilateral and contralateral trigeminal motoneurons of cats. (2) No group I effects were found on the contralateral masseteric reflex, but the group II afferents exerted the crossed effect consisting of a brief facilitation followed by a prolonged suppression. The latter was consistently induced and could be divided into an early and a late phase, whereas the former was found only in a few cases tested. (3) Masseteric group II afferents induced a sequence of a depolarization and a hyperpolarization in the contralateral masseteric motoneurons. But only the hyperpolarization was observed in the majority of the motoneurons and this could be divided into an early and a late phase. The time course of this depolarization and hyperpolarization corresponded with that of the crossed effects on the masseteric reflex. The hyperpolarizing potential with an almost identical latency was evoked also in the ipsilateral masseteric motoneuron by stimulation of the group II fibers of the masseteric nerve. (4) Masseteric group II afferents evoked the reflex discharges in the digastric nerve and the EPSPs with spikes in the digastric motoneuron bilaterally. The onset latency of the contralateral EPSPs was longer than that of the ipsilateral. (5) It was concluded that the group II afferents from the masseteric muscle on the one side exerted bilaterally symmetrical effects on the trigeminal motoneurons, excitatory on the digastric and mainly inhibitory on the masseteric motoneuron on both sides.

Penicillin-induced segmental myoclonus. I. Motor responses and intracellular recording from motoneurons.

The lumbar spinal cord of rats and cats has been used as a model system to study the epileptogenic effects of penicillin. Within a few minutes after the application of penicillin, segmental myoclonus develops. Intracellular recording from cat motoneurons demonstrates that penicillin induces periodic large spontaneously occurring depolarizing synaptic potentials. Either these responses or directly applied intracellular current pulses evoked prolonged depolarization of the motoneuron soma, which represent an alteration in membrane properties of motoneurons. The prolonged depolarization has a threshold for termination by a hyperpolarizing current.

Ultrastructural identification of degenerating boutons of monosynaptic pathways to the lumbosacral segments in the cat after spinal hemisection

Electrophysiological results suggest that the supra-spinal monosynaptic pathways to the lumbosacral motoneurones in cats are located in the vestibulospinal and reticulospinal tracts. In the present study these pathways were interrupted in the upper thoracic region by hemisection of the spinal cord. The boutons undergoing degeneration were then studied both qualitatively and quantitatively by means of serial and single sections of selected areas of the ventral horn in spinal cord segments L 6, L 7 and S 1. The degenerating boutons were distributed over the cell soma and larger dendrites (but not the most proximal portions) of the motoneurones. Two days postoperatively boutons showing signs of degeneration were readily identified in the latero-ventral motoneurone nucleus and Rexed's lamina VIII. These boutons which were swollen had a greatly increased number of neurofilaments and a decreased number of synaptic vesicles. Five days postoperatively these boutons which were now shrunken, contained swollen mitochondria and a dense, granular material. Between 5 and 8 days post-operatively the boutons were invaded by glial cell processes and digested. A small percentage of the F- and S-type boutons (approximately 0.5 and 3.0% respectively) were identified as forming the terminals of the experimentally interrupted pathways. This study did not exclude the possibility that a proportion of these boutons were the terminals of the propriospinal, monosynaptic fibers derived from the cervical region of the cord.

Specific membrane resistivity of dye-injected cat motoneurons

Specific membrane resistivity of dye-injected cat motoneurons

The effect of hexobarbital on the duration of the recurrent IPSP in cat motoneurons

The effect of hexobarbital on the duration of the recurrent IPSP in cat motoneurons

Fluorescent staining of cat motoneurons in vivo with beveled micropipettes

Fluorescent staining of cat motoneurons in vivo with beveled micropipettes

Excitation and axonal flow: Autoradiographic study on motoneurons intracellularly injected with a 3H-amino acid

Tritiated glycine has been injected intracellularly into cat motoneurons by means of cross barrel iontophoresis through microelectrodes. The amino acid is incorporated into proteins in the cell body and the synthesized radioactive proteins have been localized by autoradiography. The single-cell-injection technique has several advantages over the usual intraperitoneal, intravenous or intrathecal injection, e.g. distribution of the radiochemical is strictly confined to the injected neuron; the amount of injected substance can be estimated; it is easy to investigate simultaneously electrophysiological and morphological characteristics of a defined neuron. Since the injected cell body releases radioactive proteins into the axon, the transport of these substances can be studied under various conditions. In this study the influence of antidromic stimulation has been studied. Stimulated motoneurons demonstrate higher radioactivity in their cell bodies than unstimulated ones, reflecting an increased protein synthesis. The differences are even more pronounced in the axons of stimulated cells when compared with unstimulated ones. A significant, higher amount of radioactive material — up to 100% increase as demonstrated by silver grain counting — can be seen in the axons of stimulated neurons. Although considerable differences seem to exist for the quantity of the exported proteins no significant differences have been detected for the velocity of transportation. In axons of stimulated and unstimulated neurons proteins advance toward the periphery at the same rate.