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Biomedical subjects

G R Hammond

Publications and source records attributed to G R Hammond.

At least 19 recordsLinked to original sources
PubMed1992

Short latency cutaneous reflex responses of gamma-efferents in the decerebrate cat.

The effect of single shock stimulation, up to 20 x threshold (T), of the sural nerve on the discharges of triceps surae gamma-efferents was investigated in decerebrate cats. Units were classified as static (12) or dynamic (7) on the basis of their resting discharge rates (Murphy et al. 1984). All neurones were excited at short latency by sural nerve stimulation and response size was graded with stimulus intensity. Short latency mixed or inhibitory responses were not evident. Although reflex effects first occurred at low stimulus strengths (less than or equal to 1.5T) in both types of efferent, most responses appeared at higher intensities (greater than 1.5T). The estimated central delays of the responses of static (3.0 +/- 1.1 ms, mean +/- SD) and dynamic (3.4 +/- 1.0 ms) gamma-motoneurones were not significantly different and are consistent with spinal oligosynaptic pathways. The present results differ from those of the only previous study (Johansson and Sojka 1985) of the short latency responses of triceps surae static and dynamic gamma-motoneurones to sural nerve stimulation, in which mixed and inhibitory effects were common in anaesthetised cats. Although differences in recording techniques and gamma sampling may account for the apparent disparity between these studies, it is also feasible that a difference in the setting of interneuronal pathways in the two types of preparation is responsible. The results are discussed in relation to the control of gamma-motoneurones with particular reference to the "final common input" hypothesis (Johansson 1981; Appelberg et al. 1983).

Animals↗
PubMed1991

The role of cutaneous afferents in the control of gamma-motoneurones during locomotion in the decerebrate cat.

1. The effect of electrical stimulation, up to 20x threshold (T), of the sural nerve on the discharges of single medial gastrocnemius static and dynamic gamma-motoneurones has been investigated at rest and during locomotion in the decerebrate cat. 2. A total of twenty-three gamma-motoneurones were recorded. The neurones were identified as static (15) or dynamic (8) on the basis of their discharge characteristics (Murphy, Stein & Taylor, 1984). 3. Low intensity stimulation (less than or equal to 1.5T) had no effect on the discharges of most (22 of 23) gamma-efferents at rest or during locomotion. Hence the largest afferents in the sural nerve had little influence on the discharges of static or dynamic gamma-motoneurones in either condition. 4. Higher intensity stimulation (greater than 1.5T) excited both types of gamma-efferent in the resting state and response size was graded with stimulus intensity. For most neurones (20 of 23) excitatory effects appeared in the range 1.5-2T. 5. Stimulation at intensities greater than 1.5T also excited dynamic and some static gamma-motoneurones during locomotion. The responses of dynamic gamma-motoneurones were unchanged during locomotion compared to the resting state. In contrast, the responses of static neurones were significantly reduced, or even abolished, during locomotion and stimuli less than or equal to 3T generally (12 of 13) had no effect. Thus the responses of static, but not dynamic, gamma-efferents were task dependent. Further, the thresholds of responses indicate that activation of low threshold mechanoreceptors in the sural receptive field excites both types of gamma-efferent at rest, and dynamic neurones during locomotion. In contrast, it is proposed that the same peripheral input does not affect static gamma-efferents during locomotion. 6. The responses of static and dynamic gamma-motoneurones during locomotion were not obviously related to step cycle phase, or gamma rate, and responses occurring during or between homonymous electromyogram (EMG) bursts were not significantly different. Thus gamma responses during locomotion were not phase dependent. 7. Stimulation at intensities greater than 3T excited dynamic and some static gamma-motoneurones during locomotion but simultaneously inhibited on-going EMG activity. Peripheral inputs are therefore capable of influencing alpha- and gamma-motoneurones independently during locomotion. 8. The significance of the results is discussed in relation to the control and function of gamma-motoneurones.

Animals↗
PubMed1990

Augmentation of the rat's acoustic startle reflex by nonreflexogenic stimuli.

The size of the rat's acoustic startle reflex was augmented by brief acoustic clicks (which did not themselves elicit startle) presented several milliseconds before the reflex-eliciting stimulus (RS). The same clicks presented after the RS gave relatively weak augmentation that was present in the 1st, but not the 2nd, testing session. Brief footshocks set to 75% of each animal's flinch threshold augmented startle when presented both before and after the RS in both testing sessions. Augmentation by a leading footshock increased with shock intensity and was unaffected by the intensity of the RS. Augmentation by a trailing footshock increased with shock intensity and also with the intensity of the RS. Reflex size is not fixed at the time of reflex elicitation but can be augmented by a later nonreflexogenic stimulus. Reflex augmentation may be caused by the 2nd member of a stimulus pair discharging elements of the reflex pathway that were partially activated by the 1st.

Acoustic Stimulation↗
PubMed1990

Gamma-motoneurone discharge patterns during fictive locomotion in the decerebrate cat.

Triceps surae gamma-motoneurones were recorded during fictive locomotion in the paralysed high decerebrate cat. Two distinctive patterns of discharge were observed which were similar to those reported for static and dynamic gamma-motoneurones during locomotion in the same preparation, but without paralysis (Murphy, Stein & Taylor, 1984). These results suggest that movement-related afferent feedback is not essential for the generation of the basic patterns of static and dynamic gamma-motoneurone activity during locomotion. The results are discussed in relation to the generation of alpha and gamma locomotor rhythms.

Action Potentials↗
PubMed1989

Lateral asymmetry of the scalp distribution of somatosensory evoked potential amplitude.

Somatosensory potentials evoked by brief low-intensity electrical pulses delivered separately to the left and right index fingers were recorded from the scalp over the posterior half of the contralateral hemisphere in normal human subjects. In two experiments it was found that the scalp areas enclosed by 75 and 90% of maximum-amplitude isopotential contour lines of the early cortically generated complexes were more restricted over the left hemisphere. This asymmetrical evoked potential distribution supports the proposal by Semmes from her observations of brain-damaged patients that elementary somatosensory representation is focal in the left hemisphere and diffuse in the right.

Adult↗
PubMed1989

Temporal integration of acoustic and cutaneous stimuli shown in the blink reflex.

Temporal integration of pairs of brief blink-eliciting acoustic and cutaneous stimuli was investigated to determine if there was integration of stimuli from different modalities. Reflexes elicited by a tone burst or by a brief electrical shock to the supraorbital nerve followed by a second tone burst or shock at short stimulus onset asynchronies (SOAs) were larger and faster than control reflexes elicited by a single stimulus identical to the lead stimulus of the stimulus pairs. Reflex amplitude was augmented at longer SOAs where there was no effect on latency. Temporal integration was evident for all stimulus pairs, showing that it is due, at least in part, to processes that occur outside specific sensory pathways. Heterogeneous stimulus pairs produced greater reflex enhancement than did homogeneous stimulus pairs. This finding was examined further in Experiment 2, which showed that reflex enhancement with pairs of acoustic pulses was unaffected by the frequency of the second stimulus, suggesting that sensory masking was not acting to suppress reflex expression with acoustic pulse pairs. Integration of reflexogenic acoustic stimuli shown in the blink reflex is restricted to shorter intervals than is integration of acoustic stimuli shown by psychophysical procedures, suggesting that the two methods reflect different aspects of stimulus processing. Integration of reflexogenic stimuli may result from summation of activity associated more directly with reflex expression than with perceptual awareness.

Acoustic Stimulation↗
PubMed1982

Hemispheric differences in temporal resolution.

A review of the relevant clinical and experimental literature gives the conclusion that the cerebral hemispheres differ in temporal resolution of input, with the language-dominant hemisphere showing finer acuity. This conclusion is supported by evidence from performance of patients with unilateral brain damage on tests of temporal resolution, performance of developmental dyslexics on similar tasks, and left-right sensory field differences in temporal acuity in normal human subjects. While it is unlikely that a hemispheric difference in temporal resolution is sufficient to give a complete account of lateralized functions, such attempts to show more primitive physiological differences between the hemispheres are more likely to be fruitful than attempts which differentiate the hemispheres in terms of higher-order psychological functions.

Aphasia↗
PubMed1978

Properties of twitch motor units in snake costocutaneous muscle.

1. Single motor units of twitch muscle fibres were studied in isolated nerve-muscle preparations of m. costocutanei inferiores from grass and garter snakes. Preparations were superfused with Ringer solution at a controlled temperature of 22 degrees C.2. The peak and time-to-peak tension (contraction time) were measured for isometric twitches of forty-seven whole muscles and 83 motor units. The sample of motor units was drawn from an estimated total population of 213-355 twitch units. Peak tetanic tensions were also measured. The measurements were made at muscle lengths at which the twitch tension was maximal, and this length was not always the same for whole muscle and unit twitches. In fifty-nine cases 1 and in twelve cases 2 motor units were isolated from each muscle.3. Whole muscle contraction times ranged from 22-61 msec (mean +/- S.D. = 40.3 +/- 9.8 msec) and those for units from 18-92 msec (mean +/- S.D. = 46.9 +/- 15.9 msec). The wide range for whole muscles is discussed.4. The percentage of the whole muscle tetanic tension contributed by each unit (unit size) was calculated. Contraction time was inversely related to unit size.5. Twitch-tetanus ratios were calculated and found not to be related to unit contraction time.6. The conduction velocities of axons innervating 23 motor units were calculated from latency measurements at two points along the length of the nerve. They ranged from 1.9 to 10.4 m/sec. Axon conduction velocity was inversely related to unit contraction time, and directly related to unit size.

Action Potentials↗
PubMed1978

Post-tetanic potentiation of twitch motor units in snake costocutaneous muscle.

1. Isometric twitch responses of single motor units in snake costocutaneous muscle have been recorded before and after conditioning tetanic stimulation. 2. Most units showed post-tetanic potentiation of twitch tension, associated with increased maximal rate of rise of twitch tension, and in some cases also associated with prolongation of twitch contraction time. A few units showed a short phase of actual depression of the post-tetanic twich responses, followed by potentiation. 3. The time course and magnitude of post-tetanic changes of twitch tension and maximal rate of rise of tension could be described by the sum of three processes which are assumed to be maximal close to the end of the conditioning tetanus: (i) a rapidly declining potentiation (called here early potentiation, which lasted less than 2 sec) which may have a purely mechanical origin; (ii) a much slower-declining potentiation (called here prolonged potentiation, which lasted up to 15 min); and (iii) a process which lasted up to 60 sec during which twitch potentiation was reduced. The latter process (called here depressed potentiation) was usually apparent as a marked trough in the plots of twitch amplitude versus time after the conditioning tetanus, and occassionally was evident as an actual transient depression of twitch amplitude after the tetanus compared with that before. 4. The effects of the prolonged potentiation and the depressed potentiation on the maximal effect of each process close to the end of the tetanus were extracted by fitting single exponential equations to different portions of the data, using a computer program. 5. Twitch potentiation associated with increased maximal rate of rise of tension seemed to be a separate phenomenon to that associated with prolongation of contraction time, seen when conditioning tetani of higher frequency and numbers of stimuli were employed. The depressed potentiation of twitch tension tended to be partly masked in cases where contraction time was prolonged, but this did not affect the depressed potentiation of maximal rate of rise of tension. 6. The post-tetanic potentiation shown by a unit was related to the contraction time of the unit, in addition to the well known relationship to the initial twitch-tetanus ratio. The depressed potentiation may correlat more closely with the initial twitch-tetanus ratio than with the unit contraction time. 7. The magnitude of maximal depressed potentiation shown by a unit may be directly correlated to that of maximal prolonged potentiation. 8. The time constant of decay for prolonged potentiation of twitch tension tended to be related inversely to unit contraction time and directly to unit size and the maximal value of prolonged potentiation oftwitch tension. The time constant of decay for prolonged potentiation of maximal rate of rise of tension tended to be related to unit size and initial twitch-tetanus ratio, and the time constant of decay for depressed potentiation of rate of rise of tension tended to be related to unit size. 9...

Action Potentials↗
PubMed1977

Inhibition of the rapid movement of optically detectable axonal particles colchicine and vinblastine.

The rapid saltatory motion of intra-axonal particles detected by dark-field microscopy in myelinated axons isolated from sciatic nerves of adult Xenopus laevis was inhibited by colchicine or vinblastine at a concentration of larger than or equal to 0.1 mM. Both the predominant somatopetal transport and the somatofugal transport of these round particles were inhibited. The reduction in numbers of moving particles was apparent first in the juxtanodal portions of the isolated axons within about 1 h. No particles could be detected moving by 3-5 h after application of the colchicine or vinblastine. During the phase of partial inhibition, those particles that were still progressing along the axon did so at apparently normal velocities while they were in motion, but remained stationary increasingly frequently and for progressively longer periods. Colchicine or vinblastine at a concentration of less than or equal to 10 micronM caused no observable inhibition within 4 h of application. Colchicine at a concentration of larger than or equal to 10 mM caused local accumulation of round particles, and vinblastine at a concentration of larger than or equal to 2.5 mM caused fragmentation of rod-shaped organelles, believed to be mitochondria. Electron microscopy of nerve fibers treated with 5 mM colchicine showed a progressive loss of microtubules from the axoplasm, such that approximately 70% of the microtubules had disappeared after 4h.

Animals↗