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Biomedical subjects

P Bawa

Publications and source records attributed to P Bawa.

51 records · Page 3Linked to original sources
PubMed1983

Repetitive doublets in human flexor carpi radialis muscle.

Single-motor-unit activity was recorded from flexor carpi radialis of two human subjects. 2. A large number of units showed repetitive doublets at the onset of slow recruitment. A unit starting with doublets would transfer to a normal firing pattern as the force increased. 3. At different speeds of ramp contractions, the number of doublets discharging at the onset of contraction decreased as the speed of contraction increased. 4. Both low- and high-threshold units discharged repetitive doublets. Motor units which could discharge doublets showed higher maximal firing rates than those units which did not fire doublets. 5. Short interspike intervals were also observed at the onset of ballistic movements. From the comparison of these short interspike intervals and the short intradoublet intervals we suggest that the two arise from two distinct phenomena in the spinal cord. 6. Linked potentials were observed both with single spikes and doublets. Their origin may lie in the spinal cord or the muscle unit itself or both.

Action Potentials↗
PubMed1982

Synchronisation of motor firing by vibration during stretch evoked responses of the human wrist flexors.

The wrist was rapidly dorsiflexed while the flexor muscles were being vibrated at 120 Hz. During both the M1 and the M2 responses the EMG showed a series of waves at the vibration frequency. When the vibration was turned off after overlapping with the stretch for only three to four cycles these EMG waves ceased about 30 ms later. Spinal reflexes are thus shown to continue to influence the discharging motoneurones throughout the human M2 "long-latency" response.

Electromyography↗
PubMed1982

Interaction of excitatory and inhibitory inputs to motoneurons in man.

Two separate paradigms were used to study the interaction of excitatory and inhibitory inputs to the motoneuron pools of lower leg muscles in human subjects. During inhibition of tibialis anterior muscle from Ia soleus afferents, the probability of firing of the higher threshold unit decreased more than that of the lower theshold unit in 14 out of 18 pairs. When soleus motor units were recruited by tendon vibration reflex and the subject voluntarily inhibited one unit, the higher threshold unit was silenced first in 55 pairs, whereas the lower threshold unit ceased firing first in 29 pairs, and 16 showed inconsistencies. It is concluded that although under standard conditions there is orderly derecruitment of units which is the reverse of recruitment, vibration of the intact human tendon brings about synaptic effects in the spinal cord which affect the normal patterns of recruitment and derecruitment of motor units.

Adult↗
PubMed1981

Contribution of joint and cutaneous afferents to longer-latency reflexes in man.

Electromyographic records from wrist extensors and flexors show a short latency reflex response, M1 and a longer latency response comprising of M2 and M3 peaks. M1 corresponds to the spinal stretch reflex and hence mediated by spindle afferents. In order to determine the contribution of various afferent types to M2-M3 components and simple reaction times, reflexes were elicited before and after anaesthetic blocks of palm cutaneous and wrist joint afferents in human subjects. The results show that joint and cutaneous afferents have no significant contribution to the longer latency reflexes or simple reaction times.

Adult↗
PubMed1981

Neural development in children: a neurophysiological study.

In adult human subjects torque motor imposed angular displacements of the upper limb joints result in two major reflex EMG components as identified by their latency. The shorter latency (SL) component is probably the spinal stretch reflex and the longer latency (LL) reflex component may involve supraspinal structures. Tendon jerk, arising via the spinal reflex pathway, is present in children at birth. In this study the presence and the development of the LL component in wrist flexors was investigated in children between the ages of 2 and 13 years. Kinaesthetic reaction times were tested simultaneously. In young children, the duration of the LL component was much longer than that in adults. The duration decreased slowly from 2 to 6 years, following which a relatively abrupt decrease between the ages of 6 and 8 years took place. After 8 years of age responses looked more adult-like. Kinaesthetic reaction times attained adult-like values after 10 years of age. Various possibilities underlying these observations are discussed.

Adolescent↗
PubMed1979

Motor unit responses in muscles stretched by imposed displacements of the monkey wrist.

Imposed angular displacements of monkey's wrist produce three major peaks of activity (terms M1, M2, and M3 peaks) in the averaged gross EMG activity ot the stretched muscles. The displacements were imposed on monkey's wrist by computer controlled step loads (range 60--540 g). Gross EMG was recorded simultaneously with the activity of single motor units (SMUs) in the stretched muscles. SMUs were identified and separated using a computer "shape-fitting" program. Average response histograms (ARHs) were constructed for each SMU's responses to randomly presented repetitions of up to five different step loads. Eighty-three percent of the SMUs showed a single excitatory response peak that was limited to a time interval corresponding to that of only one of the gross EMG peaks. Despite marked differences in the time courses of the imposed displacements, the time courses of the SMU excitatory responses were maintained. By increasing the background load and hence the tonic firing of the individual SMUs, the histograms show that the long latency of the M2 peak is not due to an inhibitory-excitatory sequence. The SMUs responding at longer latencies (M2 and M3 SMUs) show firing characteristics in keeping with those reported for fast twitch motor units while those of M1 SMUs correlate with those found for slow twitch units. These results establish that the M1, M2, and M3 peaks do not merely represent synchronized oscillatory activity of the motoneurons but largely result from the reflex excitation of separately responding motoneuron "subpopulations", each of which contributes to the generation of only one of the gross EMG peaks.

Afferent Pathways↗
PubMed1979

Input-output properties of motor unit responses in muscles stretched by imposed displacements of the monkey wrist.

Reflex responses are "servo-like" where the output is graded with the input or are "triggered" where the output is independent of input once an input threshold is exceeded. Imposed displacements of monkey or human upper limb joints result in prolonged EMG output in the muscles stretched by the displacements. The longer-latency portions of the prolonged output have been variously reported to be servo-like or to be triggered in nature. In monkeys and humans, angular wrist displacements imposed by step loads result in three peaks (M1, M2 and M3) in the gross EMG recorded from the stretched muscles. Each gross EMG peak largely results from the firing of a separately-responding subpopulation of single motor units (SMUs). We studied the responses of SMUs to loads that were presented to the monkeys in a random order as to magnitude, duration and onset time. Average response histograms were constructed for the SMU responses for individual step load magnitudes. Averages were also constructed for the simultaneously-recorded gross EMG responses for each step load magnitude. The input parameters used were the initial velocity of displacement or the magnitude of step load, while the output was taken as the probability of firing/millisecond/presentation above baseline for the SMUs or the area under the response peaks above baseline for gross EMG. The results establish: 1) That it is not possible to unambiguously determine the input-output properties of the responses to imposed displacements utilizing the analysis of gress EMG activity due to the response characteristics of the various subpopulations of motor units contributing to the gross response. 2) That the SMU activity during all of the peak intervals is monotonically graded with increases in magnitude of the step load or the initial velocity of displacement. Hence, the long-latency portions of the EMG responses are servo-like in nature and are not preprogrammed or triggered responses. 3) That the gain (output/input) of the gross EMG responses almost entirely reflects the variation in the number of motoneurons recruited by changes in magnitude of the step loads rather than variation in the firing rates of motoneurons during the reflex responses.

Animals↗
PubMed1979

Dynamics of a long-latency reflex pathway in the monkey.

Angular wrist displacements in the monkey result in a short-latency (20--25 ms) "reflex" response of motor cortical cells in area 4 and separate peaks of activity in the gross EMG of the stretched muscles. Frequency domain analysis was carried out between (1) wrist position as input and motor cortical neuron response as output, (2) position as input and EMG response as output, and (3) motor cortical neuron response as input and EMG response as output. The results show that the dynamics of primary spindle afferents characterize the dynamics of the pathway to the motor cortex and that of the reflex loop. Results are discussed in terms of "long-loop" reflexes.

Animals↗
PubMed1978

Spinal reflexes as a basis for tremor in the premammillary cat.

A maintained tremor observed in premammillary cats has been studied by varying the load attached to the soleus muscle, by electrical recording, and stimulation. The frequency and amplitude of the tremor varies widely with changes in elastic or inertial loading (springs or flywheels). These variations agree well with predictions based on the idea that the tremor is maintained by relfexes reinforcing a tendency for oscillation in a mass-spring system (which includes the elasticity of the muscle). Spinal stretch reflexes are probably responsible since a relatively fixed delay of 8.5 +/- 2.9 ms is observed between the peaks of recorded afferent and electromyographic activity. Stimulation of the lateral gastrocnemius--soleus nerve completely resets the tremor, again suggesting a reflex origin.

Animals↗
PubMed1976

Frequency response of human soleus muscle.

1. The properties of human soleus muscle were studied by systems analysis. Single stimulus pulses and random stimulus pulse trains were applied to a branch of the nerve to soleus muscle and the resultant tension fluctuations were recorded. 2. The frequency-response function between stimulus pulses and tension conforms to that of a second-order, low-pass filter. The parameters of the second-order system, low frequency gain, natural frequency, and damping ratio, varied systematically with the angle of the ankle. As the ankle was flexed (the length of the muscle was increased), the low frequency gain increased, the natural frequency decreased, and the damping ratio was unaffected or increased slightly. 3. These results are discussed in relation to the twitch responses of human soleus muscles and the responses previously observed in cat muscles.

Adult↗
PubMed1976

Reflex responses of human soleus muscle to small perturbations.

1. A small branch of the nerve to soleus muscle in normal human subjects was stimulated intramuscularly with a needle electrode while the subjects were maintaining a steady voluntary contraction. The EMG and force fluctuations produced by these stimuli were recorded and averaged. 2. In addition to the M-wave produced directly by stimulating motoneurons and the resultant twitch contraction, one or more EMG waves were seen with a latency greater than 100 ms. These later waves produced further contractions, and when there were several later waves, the EMG and force fluctuations appeared as a damped oscillation with a frequency between 5.5 and 8 Hz. 3. By varying the angle of the ankle and hence the time course of the twitch contraction, the timing of the latter waves was shown to be closely related to the contraction time. Thus, the later waves appeared to be produced reflexly by the tension fluctuations, rather than directly by the stimulus. 4. The frequency response function between the tension fluctuations and the reflux EMG responses was computed. The gain agreed with that of primary muscle spindle afferents, but the phase data showed extra lags consistent with a time delay which was too long to be spinal in origin. This reflex probably involves supraspinal centers.

Action Potentials↗
PubMedDate not supplied

A comparison of human motoneuron data to simulated data using cat motoneuron models.

The response of repetitively firing human motoneurons to a composite excitatory input was evaluated. It was clearly shown that the response of the motoneurons to the transient input decreased with an increase in the background firing rate of the cell. The current model of repetitively firing human motoneurons could not account for this experimental result. Therefore, a compartmental modelling approach was used to simulate the repetitive firing properties of anaesthetised cat motoneurons under current clamp conditions. The modelled motoneurons were used in simulations similar to the experimental paradigms where the response to a composite excitatory input was evaluated at different background firing rates. The motoneuron models also showed a decrease in response to the excitatory input at faster background firing rates. The results suggest that human motoneurons are more comparable to motoneurons in the anaesthetised cat preparation than formerly thought. The results also demonstrate that the apparent efficacy of a synaptic input may be modulated by changes in background firing rate of the postsynaptic neuron.

Animals↗