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D Kernell

Publications and source records attributed to D Kernell.

At least 73 records · Page 4Linked to original sources
PubMed1984

The meaning of discharge rate: excitation-to-frequency transduction as studied in spinal motoneurones.

In many neurobiological studies, the excitation and inhibition of nerve cells is studied by means of extracellular recordings of impulse discharges. For the interpretation of such data in terms of synaptic effects, it is necessary to know how activating currents give rise to repetitive impulse firing in neurones. In the present contribution, a review will be given of earlier as well as recent findings concerning the excitation-to-frequency transduction in alpha motoneurones of the cat's spinal cord. The transduction properties were investigated by recording the neuronal responses to long-lasting depolarizing currents, injected directly into the cell via an intracellular microelectrode. Such studies have revealed that there is no simple proportionality between the intensity of excitation and the resulting discharge rate. Different motoneurones may show considerable differences with respect to, for instance, (i) the frequency range of maintained firing, (ii) the amount of drop in discharge frequency during a period of constant activation (adaptation). Among spinal motoneurones, there are functionally relevant differences in transduction properties between cells with different kinds of muscle units and motor tasks. To an important extent the transduction properties are determined by the prolonged post-spike permeability changes that give rise to the after-hyperpolarization.

Action Potentials↗
PubMed1983

Motor unit categorization on basis of contractile properties: an experimental analysis of the composition of the cat's m. peroneus longus.

Recordings were made of isometric contractions of single motor units of the cat's m. peroneus longus (PerL). The units were activated by stimulation of dissected filaments of ventral roots. In accordance with the general principles introduced by Burke et al. (1973), the 80 isolated PerL units were classified into three or four type-categories according to their contractile speed and endurance. Three currently used varieties of a "fatigue index" were calculated and found to give equivalent results. Units with a high, intermediate and low resistance to fatigue were responsible for about 22.5, 25.4 and 52.1% respectively of the total muscle force. Two alternative methods for fast/slow categorization were compared: (i) classifying all units as slow that failed to show a "sag" in partly fused contractions ("sag-criterion", Burke et al. 1973) and (ii) classifying all units as slow that had a more prolonged twitch contraction time than that of fatigue-sensitive units ('FF vs. S-criterion'). The relative contribution of slow units to total muscle force was about 2.8 times as great (14 versus 5%) for a classification by the FF vs. S-criterion than for a subdivision according to sagging behaviour. When compared to equivalent data from previously published studies of feline hindlimb muscles, peroneus longus was found to resemble gastrocnemius medialis in relative motor unit composition. The maximum force of individual PerL units was, however, on average less than or equal to 50% of that reported for corresponding types of gastrocnemius units.

Animals↗
PubMed1983

Relation between isometric force and stimulus rate in cat's hindlimb motor units of different twitch contraction time.

The relation between isometric force and rate (or pulse interval) of repetitive stimulation was studied for 77 motor units from m. peroneus longus of the cat. The units were activated by constant-frequency bursts of 1 s, and the stimulus interval needed for producing half the maximum tension was strongly correlated to twitch contraction time (twitch CT, non-potentiated values 13-42 ms). This remained true for comparisons within groups of fast and slow units respectively (fast/slow classification according to criteria of Burke et al. 1973). A mean contractile force of half maximum amplitude (0.5 PO) was produced by repetitive stimuli with a pulse interval of about 1.5 CT in fast and 2 CT in slow units. Among both kinds of unit, however, these stimulus rates corresponded to pulse intervals of about 1.4 times the half-relaxation time of the twitch. At half-maximum force, the rise of tension per Hz rise of stimulus frequency was about 2.5% PO for fast and 5.8% PO for slow units. Fast-twitch fatigue-sensitive (FF) and twitch fatigue-resistant (FR) units showed similar tension-frequency relations. Comparisons to results from m. gastrocnemius medialis showed that, for corresponding types of fast units (FF units), the twitch CT tended to be about 25% longer for gastrocnemius than for peroneus. The stimulus rate needed for a half-maximum contraction was, however, not lower for FF units from gastrocnemius than for those from the peroneus muscle.

Adenosine Triphosphatases↗
PubMed1982

Time course and properties of late adaptation in spinal motoneurones of the cat.

In the spinal cord of anaesthetized cats, motoneurones of m-gastrocnemius medialis were stimulated to repetitive firing by very long-lasting steady currents injected through an intracellular microelectrode (maximum duration 4 min). In such discharges, a gradual decline in impulse frequency was found to occur during several tens of seconds. Most of this "late adaptation" occurred during the first 30 s of firing. Comparisons between the responses of different cells showed that the frequency-drop during late adaptation was strongly correlated to the impulse rate at the beginning of the discharge. For one and the same cell, late adaptation was more prominent at strong than at weaker intensities of stimulation (i.e.., at high than at lower initial firing rates). In cells capable of discharging continuously for several minutes, a semi-stationary discharge rate tended to be reached after about 1 min or less.

Animals↗
PubMed1982

Limits of usefulness of electrophysiological methods for estimating dendritic length in neurones.

The present report deals with methodological problems concerning the electrophysiological analysis of multipolar neurones. In a nerve cell in which the dendrites may be considered as being equivalent to a single cable, the electrotonic length of the equivalent cable may be determined by analyzing the potential response to a step of current injected into the soma (Rall, 1969). In the present model study it is shown that this kind of analysis is unsuitable for discriminating between neurones with different dendritic lengths in excess of about 1.5-2.0 length constants.

Animals↗
PubMed1981

Threshold current for repetitive impulse firing in motoneurones innervating muscle fibres of different fatigue sensitivity in the cat.

In anaesthetized cats, alpha motoneurones of m. gastrocnemius medialis were activated by maintained currents that were injected via an intracellular microelectrode. There was a statistically significant correlation between the threshold current for maintained repetitive firing and axonal conduction velocity. These findings confirmed that slow-axoned motoneurones tend to be more excitable than those with faster axons. Among fast-twitch motoneurones of about the same size as judged by their axonal conduction velocity, the average threshold current was about twice as high for cells innervating fatigue-sensitive muscle fibres (FF units) than for those supplying more fatigue-resistant ones (FR units).

Animals↗
PubMed1981

Sizes of soma and stem dendrites in intracellularly labelled alpha-motoneurones of the cat.

The morphology of identified hindlimb motoneurones was studied after intracellular labelling with Procion yellow (59 cells), Procion red (19 cells) or horseradish peroxidase (9 cells). With respect to the measurements performed, all three intracellular labels gave similar results. As judged by their axonal conduction velocity (62-117 m/sec) all included cells were alpha-motoneurones. The motoneuronal cell bodies had cross-sectional areas of 816-3732 sq. microns, corresponding to diameters of about 32-69 microns. On average each neurone had 12 (5-20) dendritic stems. For all cells together, the number of dendritic stems per neurone was not strongly correlated to soma diameter. In the whole material, the dendritic stem diameters varied between about 0.5 and 19 microns. Stem dendrites of 4-5 microns were common in all kinds of cells, whereas thicker dendritic stems were preferentially distributed to cells with larger somas. The maximum as well as the mean stem-dendrite diameter (d) per cell was clearly correlated to, and roughly proportional to, the diameter of the cell body. The sum of the cross-sectional areas of all the dendritic stems emanating from a cell ('sum of dendritic holes' = sigma pi d2/4) was roughly proportional to the volume of the soma. Quadriceps motoneurones had a markedly greater number of dendritic stems per cell (mean 16.9) than other kinds of hindlimb motoneurones studied (mean 11.5; includes motoneurones of the hamstring muscles, triceps surae and intrinsic foot muscles). The many quadruceps dendrites were, however, also relatively thin, and the average ratio between 'sum of dendritic holes' and soma volume was the same for quadriceps motoneurones as for the other cells.

Animals↗
PubMed1981

Input conductance axonal conduction velocity and cell size among hindlimb motoneurones of the cat.

Input conductance and axonal conduction velocity were measured for hindlimb motoneurones that were anatomically labelled by substances injected through the intracellular microelectrode (Procion dyes, horseradish peroxidase). We confirmed that there is a good correlation between the axonal conduction velocity of a hindlimb motoneurone and the size of its cell body. Furthermore, we confirmed that the power relation between neuronal input conductance and axonal conduction velocity has an exponent of about 3-4. If large motoneurones were simply scaled-up versions of the smaller ones, this exponent should have been between 1.5 and 2.0. We showed that the unexpectedly high input conductance of fast-axoned motoneurones, compared to that of the more slow-axoned ones, was not due to a corresponding disproportion between the axonal conduction velocity and the size of the cell body. Neither could it be explained by differences between large and small cells with respect to the relative sizes and numbers of dendritic stems. The unexpectedly high input conductance of large cells seems likely to be largely caused by a lower average value for the specific membrane resistance among these cells than among the smaller ones. Hitherto unknown differences in dendritic architecture between large and smaller cells might conceivably be of some importance as well. Our results are consistent with the view that, in muscle contractions evoked by the central nervous system, thin-axoned motoneurones might be recruited more easily than more thick-axoned ones even if all the cells were activated by the same density of equipotent synapses.

Animals↗
PubMed1980

The duration of after-hyperpolarization in hindlimb alpha motoneurones of different sizes in the cat.

In the present intracellular investigation, direct measurements were made of the size of the cell body and the duration of after-hyperpolarization (AHP) among 76 hindlimb motoneurones of the cat. Following the penetration and intracellular recording of action- and after-potentials, each one of the cells was individually labelled by an injection of Procion yellow, Procion red or horseradish peroxidase from a microelectrode. In accordance with expectations, a significant negative correlation was found between the size of a motoneuronal cell body and the duration of its AHP. Motoneurones with a mean soma diameter exceeding about 50 microns usually had an AHP shorter than 110 msec. Cells smaller than 50 microns could, however, have AHPs of widely varying durations (range from 55 to 265 msec).

Animals↗
PubMed1975

Properties of motor units in the first deep lumbrical muscle of the cat's foot.

Isometric contractions of single motor units were studied in the first deep lumbrical muscle of the cat's hind-foot. Motor units with short twitch contraction times (15-20 msec) generally differed from those with longer ones (23-50 msec; contraction time measured in unpotentiated twitches) in showing (1) a greater maximum tetanic tension, (2) a smaller resistance to fatigue, (3) more post-tetanic potentiation of twitch tension, and (4) no post-tetanic occurrence of repetitive activity in response to single nerve stimuli (such "post-tetanic repetitive activity" was seen in several of the slower units). The ratio between unpotentiated twitch tension and maximum tetanic tension was similar for units with brief and long contraction times. The peak-to-peak amplitude of a single motor unit spike, recorded with gross electrodes, tended to be directly proportional to the maximum tetanic tension of the same motor unit.

Animals↗
PubMed1975

Recruitment and firing rate modulation of motor unit tension in a small muscle of the cat's foot.

Maintained contractions were elicited in the first deep lumbrical muscle of the cat's foot by electrical stimulation of the contralateral motor cortex or, reflexly, by pinching of the foot pad. The discharges of all significant motor units of the muscle were monitored by electromyography, and contractions of the various motor units were observed in isometric recordings of muscle tension. Over a wide range, muscle tension could be enhanced by an increased intensity of pad pinching or cortical stimulation. This increase in muscle tension was caused by a recruitment of new motor units as well as by an increase in the firing rate of already active motor units. The latter mechanism was clearly of great importance. Pad pinching or cortical stimulation could sometimes cause the muscle to produce a tension close to that of a maximum tetanic contraction. This was several times greater than the mean tension that would have been caused by motor unit recruitment alone (i.e. by the motor units firing at their minimum steady rate). Cortical stimulation as well as pad pinching commonly recruited weak units more easily than stronger ones of the same muscle. The recruitment order obtained in response to pad pinching often differed, however, in various details from the recruitment caused by cortical stimulation.

Animals↗