Mean Axonal Conduction Velocity Research Articles

Segmental demyelination in Peripheral Nerves of Old Cats

This study of the fine structure of sciatic nerve branches in normal old cats provides evidence indicating that segmental demyelination may account, in part, for the significant decrease with age in the mean axonal conduction velocity in these hindlimb nerves. Fibers of different diameters exhibited focal abnormalities of their myelin sheath. Lipid-like droplets and granulo-vacuolar debris were present in distended portions of the inner adaxonal rim and in the outer cytoplasmic compartment of the Schwann cell. These inclusions extended into the cytoplasm of the paranodal myelin loops and clefts of Schmidt-Lantermann. There also occurred disruption of the axoglial junctions and separation of the myelin loops from the paranodal axolemma which widens the nodes of Ranvier. Complete disruption of one or more contiguous segments of the myelin sheath was produced by interlamellar splitting and ballooning along the major dense and intraperiod lines. Axonal degeneration occurred less frequently and was not present in all hindlimb nerves.

Electrophysiological properties of developing phrenic motoneurons in the cat

1. Intracellular recordings were made in 427 phrenic motoneurons from kittens (in four stages of postnatal development, ranging from 2 to 14 wk) and in 72 motoneurons from adult cats. These experiments were performed to determine how the pattern of spontaneous discharge changes in phrenic motoneurons during development and how these changes might be influenced by alterations in the electrophysiological properties of these neurons. 2. The mean axonal conduction velocity increased significantly (P less than 0.0001) throughout this period of development, with the most rapid increase occurring between weeks 2 and 5 (18.5 +/- 5.4 and 32.4 +/- 5.6 m/s, respectively, mean +/- SD). 3. There was no change in the magnitude of the membrane potential, antidromic action potential, or positive overshoot; whereas there was a decrease in the half-width of the action potential from 2 (652 +/- 184 ms) to 14 (525 +/- 116 ms) wk postnatal. 4. The mean duration of the afterhyperpolarization (AHPdur) decreased from 69 +/- 20 ms at 2 wk to 60 +/- 16 ms by 9 wk, then increased to 66 +/- 18 ms by 14 wk of age and to 75 +/- 21 ms in the adult. The mean amplitude of the afterhyperpolarization (AHPamp) in the 2-wk-old group (4.9 +/- 1.8 mV) was larger than that at weeks 5 (3.9 +/- 1.7 mV) and 9 (3.9 +/- 1.6 mV), whereas the mean AHPamp of the adult (3.1 +/- 1.2 mV) was significantly smaller than the mean of any younger group. A significant negative correlation was found between AHPdur and axonal conduction velocity in all age groups studied, including the adult.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal development of phrenic motoneurons in the cat

The postnatal growth of phrenic motoneurons in the cat was studied using retrograde transport of horseradish peroxidase (HRP). The mean somal surface area of these developing motoneurons increased 2.5 times from day 3 to adult while the mean somal volume increased four-fold. This change in mean somal surface area during postnatal development was found to be correlated with the change in mean axonal conduction velocity measured from phrenic motoneurons.

The activity pattern of phrenic motoneurons during the aspiration reflex: an intracellular study

Membrane potential trajectories and discharge characteristics were measured intracellularly in 29 phrenic motoneurons of anesthetized, paralyzed and artificially ventilated cats during hypercapnic respiration and the aspiration reflex. Fifteen ‘active’ cells discharged spontaneously during inspiration, and the remaining 14 ‘quiescent’ cells exhibited no discharge in spite of strong central respiratory drive. The mean membrane potential of the quiescent cells during inspiration (−62 ± 4 mV) was significantly lower than the threshold level determined for the active cells (−52 ± 4 mV). The mean axonal conduction velocity was slower for the active (60.4 ± 8.7 m/s) than quiescent cells (67.4 ± 6.9 m/s). All phrenic motoneurons discharged during the aspiration reflex with maximum instantaneous frequencies ranging from 6 to 357 Hz. No differences were found for the maximum discharge frequency during the reflex between the active and quiescent cells. Although there were differences in the slopes of the depolarization during inspiration between the groups of cells, no such difference existed in the slopes during the aspiration reflex. The threshold level for the first spike during the reflex was the same as that during inspiration but the level for successive spikes became progressively less negative while spike amplitude decreased and duration increased. Stimulation of the nasopharynx to elicit the aspiration reflex was found to alter the timing of the subsequent respiratory cycles.

Intracellular study of nucleus parabrachialis and nucleus tractus solitarii interconnections

Responses of the nucleus parabrachialis (PBN) neurons to electrical stimulation of the nucleus tractus solitarii (NTS) were investigated by intracellular recording technique in anesthetized rats. Excitatory postsynaptic potentials (EPSPs) were evoked by ipsilateral NTS stimulation in 38 PBN neurons. They were considered monosynaptic because their latencies did not change with either variations in stimulus intensity or high-frequency repetitive stimulation. The latencies of EPSPs ranged from 1.2 to 6.9 ms. PBN neurons were also antidromically activated by NTS stimulation, giving a mean axonal conduction velocity of 4.6 m/s. Some of these neurons also responded with monosynaptic EPSPs to NTS stimulation. Direct stimulation of these neurons by depolarizing current pulses elicited repetitive firing with frequencies up to 350 Hz. The morphological analysis of 5 PBN neurons labeled with horseradish peroxidase (HRP) indicates that the soma were fusiform in shape, and the size varied from 163 to 783 μm 2. All neurons had 3–5 spiny primary dendrites which extended in a predominantly mediolateral direction. Axons arose from a proximal dendritic trunk, close to the soma. The results indicated that PBN is reciprocally connected with the NTS which elicits an excitatory effect on PBN neurons.

Response properties of spinal neurons projecting to midbrain or midbrain-thalamus in the monkey

Response and receptive field properties were determined for 24 spinal neurons backfired from midbrain or midbrain-thalamus in the anesthetized monkey. Recording sites were located in laminae I (11 cells), IV (6). V (6) and VI (1) in the lumbosacral spinal cord. The mean axonal conduction velocity of cells projecting to midbrain was significantly less than that of cells projecting to midbrain-thalamus. Cells backfired only from midbrain had complex excitatory receptive fields whereas those projecting to midbrain-thalamus had excitatory receptive fields confined to a single limb. Cells in both populations had complex inhibitory receptive fields.

Axonal projection patterns of ventrolateral medullospinal sympathoexcitatory neurons.

We studied the following properties of cat ventrolateral medullary (VLM) neurons that projected to the thoracic spinal cord: the relationship between their spontaneous activity and that in the inferior cardiac postganglionic sympathetic nerve, their responses to baroreceptor-reflex activation, their axonal conduction velocities, the funicular trajectories of their axons, the likely sites of termination of their axons, and their axonal branching patterns. Microstimulation in the second thoracic spinal segment (T2) antidromically activated 67 VLM neurons (as determined with time-controlled collision of spontaneous and evoked action potentials), whose activity was correlated to inferior cardiac sympathetic nerve discharge (as determined with spike-triggered averaging). We tested the effect of baroreceptor-reflex activation on the firing rate of 20 of these VLM-spinal neurons. Because the firing rate decreased in each instance, these neurons apparently subserved a sympathoexcitatory function. The axonal branching patterns of 51 VLM-spinal sympathoexcitatory neurons were studied. Thirty-four neurons were antidromically activated by stimulation in the T2 gray matter and in more caudal thoracic spinal segments (T11 and/or T6). In each case, the antidromic response evoked by stimulation in the T2 gray matter was due to activation of an axonal branch rather than the main axon (via current spread to the white matter). This was demonstrated with tests that included time-controlled collision of the action potentials initiated by stimulation in T2 and a more caudal thoracic spinal segment. Some VLM-spinal axons that projected to T11 branched in T6 as well as in T2. These data indicate that some VLM-spinal neurons exerted widespread excitatory influences on sympathetic outflow. Seventeen VLM sympathoexcitatory neurons that innervated the T2 gray matter could not be antidromically activated by stimulation in T5, T6, and T11 despite an extensive search at each level. Thus the axonal projections of some VLM-spinal neurons were restricted to upper thoracic segments. Antidromic mapping in T2 revealed that the axons of VLM sympathoexcitatory neurons coursed through the dorsolateral or ventrolateral funiculus to innervate the region of the intermediolateral nucleus. Mean axonal conduction velocity was 3.5 +/- 0.3 m/s. Those VLM-spinal axons restricted to upper thoracic segments generally were located dorsally and/or medially to those that innervated widely separated thoracic segments. The discharges of 35 other VLM neurons that were antidromically activated by T2 stimulation were not related to sympathetic nerve activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Axonal projections of single bulbospinal inspiratory neurons revealed by spike-triggered averaging and antidromic activation

Activity was recorded extracellularly from 26 inspiratory bulbospinal neurons in anesthetized, paralyzed, artificially ventilated cats. All but one were located in the ventral respiratory group. A neuron was classified as either I alpha or I beta by comparing its firing pattern during inspiratory cycles with lung inflation to its pattern when lung inflation was withheld during the central inspiratory phase (2, 14, 15). In this study, the projection and conduction velocity of these axons were determined using two methods: antidromic activation (AA) of the bulbospinal neurons and spike-triggered averaging (STA) of the extracellular field potentials. These methods have been compared directly because the same electrode was used both for stimulating the axon of the bulbospinal neuron and recording its axonal potential in the same location. Axonal projections from these neurons were mapped in the contralateral spinal cord with a mobile electrode by determining where the lowest stimulus threshold occurs for AA and greatest axonal potential can be recorded with STA. The locations of these axons were in the ventral and lateral funiculi. Each method determined a similar location for an axon. Positions of 10 axons were determined at both the third (C3) and fourth (C4) cervical segments. Single axons maintained their positions in either descending tract from rostral C3 to mid-C4. In five of six cases where two "neighboring" medullary units were characterized, the axons of each pair projected together within 350 micron of each other in the cervical spinal cord. Estimates of mean axonal conduction velocity (CV) from antidromic activation from a single stimulus site, "single-point AA," were as much as 42% less than corresponding estimates from STA extracellular field potentials at that point (P less than 0.001). Such single-point estimates were less than determinations that were calculated from the difference in conduction time and the difference in conduction distance from two points in the spinal cord. These two-point determinations averaged 55.4 +/- 13.1 m/s (using AA) and 53.3 +/- 13.1 (using STA) for 10 neurons. These values were not significantly (P greater than 0.2) different from each other and are greater than most earlier reports, which used the single-point AA method. Either method, AA or STA, can be used to determine axonal position and CV. The advantages and disadvantages of each method are discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane electrical properties and prediction of motor-unit type of medial gastrocnemius motoneurons in the cat.

Membrane electrical properties [time constant, action potential afterhyperpolarization (AHP), rheobase, input resistance, and axonal conduction velocity] were measured in motoneurons of cat medial gastrocnemius (MG) motor units. Motor units were classified on the basis of their mechanical responses as fast twitch, fast fatiguing (FF); fast twitch with intermediate fatigue resistance (FI); fast twitch, fatigue resistant (FR); or slow twitch, fatigue resistant (S; 11, 22). Motoneuron membrane time constant, estimated from the voltage response at the onset or termination of long (50-100 ms) current pulses and corrected for voltage-response nonlinearities (32), was found to differ significantly among the major motor-unit types, increasing in the order FF less than FR less than S. Afterhyperpolarization magnitude, half-decay time, and duration were all significantly greater for the fast (FF + FI + FR) versus the slow (S) motor units. The AHP half-decay time was correlated with muscle unit twitch time over the entire motoneuron population and within the type S motor-unit population. There was no significant correlation between twitch time and AHP half-decay time among the types FF and FR motor-unit populations. In agreement with previous studies, we found a significant difference in both rheobase and input resistance among the major motor-unit types, with rheobase increasing in the order S less than FR less than FF and input resistance decreasing in that order (S greater than FR greater than FF). The differences in input resistance were present both before and after correcting for voltage-response nonlinearities (32). Also in agreement with previous studies, the mean axonal conduction velocity was significantly faster among the fast (FF + FI + and FR) compared with the slow (S) motor units. These data were used to examine the properties alone to determine motor-unit type, which has traditionally been defined on the basis of the muscle unit's mechanical properties (11, 22). We used a discriminant analysis program to classify 73 mechanically typed motor units for which we had measures of rheobase, input resistance, membrane time constant, and AHP half-decay time. This model was able to properly classify 71 of the 73 motor units of this data set, indicating that the motor units of this data set could be grouped into three categories representing the three major motor-unit types (FF, FR, and S) on the basis of their rheobase, input resistance, membrane time constant, and AHP half-decay time.(ABSTRACT TRUNCATED AT 400 WORDS)

5-Hydroxytryptamine receptors of visceral primary afferent neurones on rabbit nodose ganglia.

1. The electrophysiological characteristics of 5-hydroxytryptamine (5-HT) receptors distributed on visceral primary afferent neurones (the nodose ganglion cells of the vagus) in rabbits were investigated with intracellular recording and voltage-clamp techniques.2. In response to 5-HT applied by superfusion (>/= 10 mum) or by ionophoresis (>/= 5 nA, 50 msec), the majority of type C neurones (mean axonal conduction velocity: 0.83+/-0.25 m/sec) showed a rapid depolarization of 20-30 mV in amplitude which was followed by a hyperpolarization of a few millivolts. Both the initial depolarization and afterhyperpolarization were associated with a reduction in membrane resistance.3. Type A neurones (mean axonal conduction velocity: 7.7+/-0.4 m/sec) did not show any significant alterations in membrane potential and resistance during or after application of 5-HT.4. The initial depolarization induced by 5-HT was abolished by Na(+)-free Krebs solution and showed a reduction of a few millivolts in K(+)-free or Ca(2+)-free Krebs solution. The response in normal Krebs solution was reversed at a membrane potential level of +7.3+/-1.1 mV.5. The afterhyperpolarization disappeared in Na(+)-free or Ca(2+)-free Krebs solution, while it was markedly enhanced in K(+)-free Krebs solution. The response in normal Krebs solution reversed at a membrane potential of -88.7+/-0.8 mV, and was abolished at membrane potentials more positive than -20 mV.6. Unlike 5-HT voltage responses, which were biphasic in the majority of neurones examined, 5-HT induced currents were usually monophasic when recorded at holding membrane levels ranging from -80 to +50 mV. The reversal potential of the inward current was +7.5+/-0.8 mV which was in good agreement with the reversal level for 5-HT-induced depolarizations. The reversal potentials for inward currents which were obtained at various concentrations of Na(+) or K(+) corresponded to the theoretical values calculated by the equivalent circuit equation.7. These results suggest that the initial depolarization induced by 5-HT is due mainly to simultaneous increases in Na(+) and K(+) conductances, while the afterhyperpolarization is brought about by an increase of K(+) conductance which is triggered by a voltage-dependent influx of Na(+) and Ca(2+).8. The mean value for the ;limiting slope' of conductance change vs. 5-HT concentration and the slope of 5-HT current vs. 5-HT concentration obtained by superfusion of 5-HT, were in good agreement, 1.84+/-0.26 and 1.88+/-0.31, respectively. On the other hand, the mean Hill coefficient obtained from the dose-response curves for the inward current induced by ionophoresis was 2.51+/-0.14.9. Tetrodotoxin (0.2 mum) blocked the soma action potential completely, but did not show any effect on 5-HT-induced responses.10. (+)-Lysergic acid diethylamide and methysergide (1-100 mum) had no depressant effect on the 5-HT-induced depolarization.11. (+)-Tubocurarine at low concentrations (1-5 mum) inhibited the 5-HT induced inward current competitively. The mode of its inhibitory action became noncompetitive at higher concentrations (10-20 mum).

The effect of long-term immobilization on the motor unit population of the cat medial gastrocnemius muscle

The properties of medial gastrocnemius and soleus muscles and of individual medial gastrocnemius motor units were evaluated after chronic (3–29 weeks) unilateral immobilization of ankle and knee joints in 10 adult female cats. Joint fixation markedly impaired the mobility of the pinned limb and, when prolonged (> 17 weeks; 8 animals), produced marked muscle atrophy and some shortening of whole muscle isometric twitch contraction times (mean contraction time of pinned medial gastrocnemius was 83.3 percent of the contralateral muscle). However, the immobilized muscles were not truly disused, since the patterns of medial gastrocnemius and soleus electromyographic activity during posture and locomotion were qualitatively normal in the 4 animals examined. Despite severe whole muscle atrophy, all but 3 of the 239 individual medial gastrocnemius motor units studied were readily classified into one of the 4 motor unit types (types FF, F(int), FR and S) found in the normal muscle. The histochemical mosaic of immobilized muscles was also the same as in contralateral unoperated muscles. When compared to data from normal cats, there was a highly significant loss of maximum twitch and tetanic force output in all unit types after long-term immobilization (17–29 weeks; N = 187), most marked among the fatigue-resistant unit groups (types FR and S), and mean isometric twitch contraction times were shorter than expected in all unit type groups. The most surprising finding in the present work was that the mean homonymous (medial gastrocnemius) and heteronymous (lateral gastrocnemius-soleus) group Ia excitatory postsynaptic potentials were significantly smaller than expected in all motor unit groups in both short-term and long-term immobilization. The mean axonal conduction velocity of medial gastrocnemius motor axons was unchanged from the normal sample. These results indicate that the essential features that define the motor unit types in normal medial gastrocnemius muscles are resistant to change despite long-term immobilization. We conclude that the motor units types found in normal muscles are robust and essentially unaltered despite severe muscle atrophy. The effects on group Ia excitatory postsynaptic potentials suggest that limb immobilization produces altered patterns of impulse traffic in group Ia afferents that induce significant decreases in synaptic efficacy, although it is unclear whether or not Ia traffic is increased or decreased by immobilization.

Axonal projection of anterior olfactory nuclear neurons to the olfactory bulb bilaterally

The axonal projection of anterior olfactory nuclear (AON) neurons to the ipsilateral and contralateral olfactory bulbs and to the prepiriform cortex was analyzed electrophysiologically in the rabbit. Of 117 AON neurons which sent their axons to the anterior commissure, 46 cells (39%) and 55 cells (47%) were activated antidromically by ipsilateral and contralateral olfactory bulb stimulation, respectively, and 22 AON neurons (19%) were activated antidromically from both. The mean axonal conduction velocity of the AON neurons was 2.8 m/s in the AON—anterior commissure axonal segment, 1.6 m/s in the AON—contralateral offactory bulb segment, and 1.0 m/s in the AON—ipsilateral bulb segment. These results and the collision tests between the antidromically evoked spikes indicate that a number of AON neurons send their axons to the contralateral olfactory bulb via the anterior commissure and that the same neurons send thin axon collaterals to the ipsilateral bulb. These axonal projections are significant in relation to the synaptic influences of these axons upon olfactory bulb neurons.