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PubMed1981

The responses of I beta cells to increases in the rate of lung inflation.

The activity of inspiratory cells in the region of the nucleus of the tractus solitarius (NTS) was recorded extracellularly in paralyzed, artificially ventilated cats either during chloralose-urethane anesthesia or following midcollicular decerebration. Twenty-three of the 68 inspiratory cells recorded in the region of the NTS were classified as I beta cells on the basis of their response to withholding lung inflation. The dynamic sensitivity of I beta cells was determined by studying their response to increases in the rate of lung inflation at constant peak volume. The I beta cells in this study showed 3 distinct patterns of response to increases in the rate of inflation. Five cells showed no change in firing pattern (fixed firing pattern). Ten cells showed an increase in the rate of rise of cell activity but no change in peak frequency (low dynamic sensitivity). Eight cells showed increases in both the rate of rise of cell activity and peak frequency (high dynamic sensitivity). It was concluded that I beta cells are not a functionally homogeneous population, at least in terms of their dynamic sensitivity. Cells showing fixed firing patterns have the characteristics of off-switch neurons. Cells with low levels of dynamic sensitivity may receive afferents from pulmonary stretch receptors. Cells showing a high degree of dynamic sensitivity may receive afferents from rapidly adapting receptors. The fact that I beta cells are not a functionally homogeneous population may explain the many divergent observations reported from studies of these cells.

Afferent Pathways↗
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↗
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↗
PubMed1982

The effect of digital nerve stimulation on recruitment order of motor units in the first deep lumbrical muscle of the cat.

The normal recruitment order of EMG spikes of the first deep lumbrical muscle of the cat's hindpaw, usually seen during cortical stimulation, pad pinch and weak plantar nerve stimulation, was temporarily reversed after stimulation of the medial digital nerve of the foot at 50 Hz for 2 min, and normal order was recovered in 2 to 10 min. The longer the period of stimulation of the medial digital nerve was, the longer the time for recovery. Alteration of order is repeatable and reversible after an interval of more than 15 min. After prolonged medial digital nerve stimulation, EMG responses to pad pinch and plantar nerve stimulation were facilitated. Combination of some stimuli (e.g. cortical stimulation and plantar nerve stimulation, pad pinch and plantar nerve stimulation, plantar nerve and medial digital nerve stimulation) also produced reversal of recruitment order during the period of stimulation. A functionally single motor nerve fiber to the first deep lumbrical muscle was isolated from the motor nerve axons in L7 and S1 of the spinal cord of cats, and physiological properties of pairs of motor units whose recruitment order was temporarily altered by plantar nerve stimulation after prolonged stimulation of the medial digital nerve of a foot were examined. Motor units with large action potentials were more facilitated than motor units with small action potentials after prolonged stimulation of the medial digital nerve. The former motor units showed fast contraction time, large twitch tension, low resistance to fatigue and presence of sag-behavior. The latter motor units showed slow contraction time, small twitch tension and high resistance to fatigue. High threshold motor units with middle-sized surface EMG records were recruited after motor units with large potentials had been recruited; motor units with middle-size action potentials were beta motor axons branching to both intrafusal and extrafusal muscle fibers. The beta motor axons also showed fast conduction velocity and the presence of sag-behavior.

Animals↗
PubMed1983

Peripheral projections of nerve fibres in the human median nerve.

Elementary sensations of definable quality were evoked by intraneural electrical stimulation of sensory units through tungsten microelectrodes inserted in the median nerve of awake human subjects. The most commonly reported sensations were tapping, pressure, dull pain and sharp pain, which correlated with recording from RA units, SA I units, C nociceptors and A delta nociceptors, respectively. All sensations were projected more often to the fingertips than to the palm. The multifocal projections of sensations recruited by excitation of small groups of neighbouring nerve fibres were significantly more scattered in space than anticipated if these fibres had preserved an orderly intraneural map, implying lack of somatotopy in the nerve. Mean projected field sizes were 3.4 mm2 for sharp pain, 9.2 mm2 for tapping/vibration, 29.3 mm2 for pressure and 35.4 mm2 for dull pain, and in addition the latter two showed a rising size gradient from fingertip to palm. Consideration of projected field size as a variable in discriminative touch suggests that input from RA units and A delta nociceptors may be of importance in stimulus localization, while SA I units may play an additional role in two point discrimination.

Adult↗
PubMed1984

Electrophysiological and morphological measurements in cat gastrocnemius and soleus alpha-motoneurones.

Intracellular recording and staining with HRP was used to study the electrical properties and anatomical size of medial gastrocnemius (MG) and soleus (SOL) alpha-motoneurones in curarized cats. The MG motoneurones were divided into two groups on the basis of their input resistance (RN), namely low-resistance MG-LR cells (RN less than 1.0 M omega) and high-resistance MG-HR cells (RN greater than 1.0 M omega). Analysis of the voltage transients following applied current pulses indicated that the SOL neurones had longer membrane time constants (tau o) than the MG-LR cells, while the MG-HR group exhibited intermediate values. Using Rall's equivalent cylinder model, a difference in specific membrane resistivity (Rm) between the MG-LR (low Rm) and SOL (high Rm) cells was obtained. This difference was observed also in neurones of similar anatomical size, and was consistent with the observed difference in tau o. In two neurones Rm was in addition calculated directly from anatomy and input resistance according to the general solution for a continuous neurone model with arbitrary geometry given by Rall. The latter method was found to yield significantly lower values for Rm, although the observed difference between the neurone types remained similar. Also the values for electrotonic length (L) were found to differ considerably between the calculations based on voltage transient analysis and those obtained from combined physiological and anatomical measurements. The observed variations in results are discussed in relation to possible sources of error in the experimental techniques and/or in the theoretical assumptions, particularly that of Rm being uniform over the entire soma-dendritic membrane. It is suggested that Rm might be larger in the dendritic regions than in the soma. A crude approximation of the dendrite to soma conductance ratio (Q) indicated that most cells (80%) had Q greater than 5.

Action Potentials↗
PubMed1984

Recruitment order of motor units during the stretch reflex in man.

The recruitment pattern of single motor units contributing to the short- and long-latency periods of the stretch reflex was investigated in human adults. When non-tonic, a given unit responded to stretch during the long-latency reflex period. When made tonic, the same unit showed a reflex response during the short-latency period, while a higher threshold, non-tonic unit would respond during the long-latency period. The data show an orderly recruitment of motor units during both phases of the stretch reflex in man.

Adult↗
PubMed1985

Motor unit recruitment and firing rates interaction in the control of human muscles.

Muscle contractions are modulated by the number of motor units recruited and their respective firing rates. The work described in this report documents an interplay between recruitment and firing rates of motor units. The recruitment of a new motor unit appears to have a disfacilitatory influence on the firing rates of previously activated motor units. It is speculated that this effect is likely to be mediated, at least partially, via the stretch reflex loop and possibly by the recurrent inhibition of the Renshaw circuit. Such a mechanism would be functionally useful in providing smooth control of muscle output via peripheral circuitry (consisting of proprioceptive reflexes and recurrent inhibition), thus lessening the amount of detailed supervision of the alpha-motoneuron pool required by the central nervous system.

Action Potentials↗
PubMed1985

Studies on pain. Effects of morphine on a spinal nociceptive flexion reflex and related pain sensation in man.

The nociceptive flexion reflex and the corresponding subjective pain score elicited by sural nerve stimulation were studied in 6 healthy volunteers. A significant correlation was found between the respective recruitment curves of the reflex and of the pain score as a function of stimulus intensity. Consequently, the reflex (Tr) and the pain (Tp) thresholds were found to be almost identical (mean: 10.6 and 10.3 mA, respectively). Similarly, the threshold of the maximal reflex response (Tmr) was very close to that of intolerable pain (Tip): 37.1 and 38.8 mA, respectively. These four parameters were studied before and after intravenous administration of morphine chlorhydrate (0.05, 0.1, 0.2 and 0.3 mg/kg) and subsequent administration of naloxone hydrochloride (0.02 mg/kg; i.v.). While 0.05 mg/kg morphine remained without any effect, higher doses produced an increase in the four thresholds (Tr, Tp, Tmr, Tip). Furthermore, a very significant linear relationship was found between the importance of the increase and the dose of morphine. Morphine also depressed in a dose-dependent fashion, the nociceptive reflexes elicited by a constant stimulation intensity (1.2-1.3 Tr). All these effects were immediately reversed by subsequent naloxone. During all the pharmacological situations, variations in Tr and Tp as well as in Tmr and Tip were found to be very significantly linearly related, indicating a close relationship between the effects of morphine on the nociceptive reflex and on the related pain sensation. These results suggest that, in our model involving a brief 'epicritic' nociceptive stimulus, the mechanisms of morphine-induced analgesia in man can be explained by a depressive effect on the nociceptive transmission directly at a spinal level.

Adult↗
PubMed1985

Recruitment patterns in the rat hindlimb muscle during swimming.

Cycle periods and electromyographic (EMG) burst durations of selected extensors and flexors are shorter during swimming than during running at 27 m/min, while the relative ON-OFF timing of these interrelationships are similar. The mean EMG activities of the tibialis anterior and adductor longus were higher and the soleus and medial gastrocnemius were lower during swimming than during running. Both fast and slow extensors are activated during both forms of locomotion, thus demonstrating that there is not a selective recruitment of fast or selective inhibition of slow ankle extensors during swimming as measures of muscle blood flow have suggested.

Animals↗
PubMed1986

Joint angle signaling by muscle spindle receptors.

Nerve impulses were recorded from sensory fibers supplying the tibialis anterior and soleus muscles of anesthetized cats as the ankle joint was moved from one end of the flexion-extension axis to the other and back again in steps of 6-7 degrees. The rate of movement from one position to the next was 40 deg/s and each position was held for 16-18 s. Plots were made of receptor discharge frequency as a function of ankle joint angle during joint movement (dynamic input-output (I-O) functions) as well as 2 and 15 s after movement terminated (2 and 15 s static I-O functions). Only receptors with a sustained (5s) static response within the physiological range were studied. A total of 229 tibialis anterior receptors met this criterion, of which 11 were identified as tendon organs. One hundred and five soleus receptors were studied, of which 6 were tendon organs. Thus tendon organ activity accounted for only a small part of the muscle afferent signal under passive conditions. The spindle receptors in soleus and tibialis anterior divided the ankle flexion-extension range about equally between them, those in soleus signaling over the flexion half of the range and those in tibialis anterior over the extension half. At angles where the receptors in a particular muscle did not signal joint angle, the tendon of the muscle was observed to be slack. Thus the total muscle afferent discharge in a relaxed animal is high at one end of the range, declines progressively as the ankle is displaced to an intermediate position, and then increases again as the joint moves toward the opposite end of the range. The spindle receptors within an individual muscle were recruited rather early as the muscle came under tension so that over most of a muscle's signaling range joint angle could have been coded by changes in receptor discharge frequency but not by which spindle receptors were active. To evaluate the information signaled by individual muscle spindle receptors, the following measurements were made from plots of impulse frequency vs joint angle: dynamic response, defined as the frequency difference between the dynamic and 2 s static I-O functions during muscle lengthening; adaptation, defined as the frequency difference between the 2 and 15 s static I-O functions during muscle lengthening.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗
PubMed1986

Increased EMG of rat plantaris during locomotion following surgical removal of its synergists.

A chronic EMG electrode implant system was used to determine recruitment patterns of rat plantaris, during treadmill locomotion, before and after surgical removal of its synergists, gastrocnemius and soleus. Bilateral synergist removal resulted in increased plantaris muscle weight and myofibrillar protein content of 59% and 44%, respectively, by 30 days following surgery. Evidence of increased plantaris EMG during treadmill walking (increases in amplitude and integral of EMG bursts to ca. 200% of pre-excision values) occurred at 15-30 days and decreased to non-significant levels at 35 days postsurgery. No corresponding alterations occurred in sham-operated controls. At 25 and 30 days, EMG of plantaris during locomotion was still submaximal, since burst amplitudes were 53% to 67% of those recorded during a dynamic grid-climbing task. The magnitude and time course of changes in EMG of overloaded plantaris during a standardized locomotor task, which reflect increased recruitment and rate of discharge of motor units, are consistent with the chronology of morphological and metabolic events previously described for this model.

Animals↗
PubMed1986

Recruitment threshold force and its changing type of motor units during voluntary contraction at various speeds in man.

The motor unit discharges in human hand muscles were recorded during voluntary isometric contraction. Bipolar wire electrodes were inserted in the right adductor pollicis muscle (AP) and first dorsal interosseous muscle (FDI) for the recordings. Motor unit activities from these muscles were investigated when subjects exerted voluntary force to each target force at various speeds. On 27 motor units of AP and 28 motor units of FDI, ramp RTF and ballistic RTF were compared. Furthermore, RTF changes of motor units with the different force speeds were investigated. It is clear that the RTF of the motor units during ballistic contractions were lower than those during ramp contraction. The processes of motor unit RTF changes were classified into 3 types: RTF of type I motor units were relatively low and decreased slightly even when force speed increased widely; RTF of type II motor units were relatively high and decreased irregularly; and RTF of type III motor units, which appeared rarely, decreased their relatively high RTF with increases of force speed increment. It is suggested that the volitional outflow related with the force speed change to the spinal motor pool might cause the different RTF change effects in the various motor units.

Action Potentials↗
PubMed1987

The recruitment order of stapedius motoneurons in the acoustic reflex varies with sound laterality.

In many muscles, motor units are recruited in a fixed order with increasing strength of muscular contraction. We show that for the stapedius muscle of the cat, vastly different recruitment orders can be obtained, depending on which ear is acoustically stimulated. The data support the idea that the distribution of inputs to a motoneuron pool can be inhomogeneous and a significant factor in determining recruitment order.

Animals↗
PubMed1990

Synaptic effects on recruitment gain: a mechanism of importance for the input-output relations of motoneurone pools?

The present theoretical analysis concerns quantitative aspects of the graduation of motoneuronal recruitment. It is pointed out that variations in the relative distribution of synaptic input to low- and high-threshold cells will serve to alter the ultimate magnitude of the threshold differences between them. These threshold differences determine the pool's 'recruitment gain', i.e. the ease with which a variation of excitatory drive to the pool may alter the number of active motoneurones. In this context, not only the command signal itself but also any steady synaptic 'background' activity is of importance, whereby the background effects on recruitment gain depend on the presence of differences in intra-pool distribution between these synapses and those of the command signal. Given the appropriate intra-pool distribution, even inhibitory background effects might serve to increase the recruitment gain. It is stressed that synaptic effects on recruitment gain should be taken into account when analyzing the input-output relations of motoneurone pools.

Models, Neurological↗
PubMed1991

Instability of motor unit firing rates during prolonged isometric contractions in human masseter.

The firing patterns of up to 4 concurrently active masseter motor units were studied with intramuscular electrodes during a continuous isometric contraction of 15 min duration, in which the subject maintained the mean firing rate of one selected unit at 10 Hz. With this paradigm the net excitation (i.e. mean firing rate) of one unit in the muscle was controlled. This served as the reference for the functional state of other active units during the prolonged contraction. With the mean firing rate of one unit in the muscle fixed, 58% of other active units showed a slow, statistically-significant change in mean firing rate over the 15 min. The initial firing rate of the units did not influence the change in rate. The original firing rate hierarchy, which in short-term contractions reflects the recruitment order, was altered during the prolonged contraction. The explanation for these differential changes in motoneuron net excitation is not clear; they could be intrinsic to the motoneurons or perhaps mediated by reflex pathways. The selective facilitation or suppression of some motor units with continuous activation means that the original size-structured combination of motor units can be modified during a prolonged contraction.

Adolescent↗