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Biomedical subjects

G Westling

Publications and source records attributed to G Westling.

29 records · Page 2Linked to original sources
PubMed1990

Measurement of contractile and electrical properties of single human thenar motor units in response to intraneural motor-axon stimulation.

1. A method is described for measuring contractile properties of single human motor units. Conventional human microneurographic techniques were adapted to stimulate individual motor axons in the median nerve, with the use of negative current pulses and a tungsten microelectrode, while recording motor-unit electromyographic activity (EMG) and isometric force responses from the thenar muscles. 2. EMG signals were recorded from both proximal and distal thenar muscle surfaces. Force was recorded in two directions (thumb flexion and abduction). This allowed calculation of the direction and magnitude of resultant force exerted by each unit. 3. Data accepted as originating from a single unit satisfied all the traditional "all-or-none" criteria. Additional criteria also required the following: 1) a wide safety margin between the threshold for unit activation and the current intensity needed to elicit responses from other units; 2) that the characteristic direction in which each unit generated force did not change during the recording period; and 3) whenever F-responses were encountered, the second EMG waveform was identical to the first--a highly improbable event if more than one unit had been excited. 4. Respiration and blood pressure waves introduced baseline fluctuations that distorted the force measurements. These fluctuations were minimized by synchronizing stimuli to the pulse pressure cycle and resetting the baseline electronically just before stimulus onset. 5. Combining motor-axon stimulation at a site remote from the muscle with electronic resetting of the force baseline and delivery of stimuli at fixed intervals after the pulse pressure waves allowed the full time course of human motor-unit twitch and tetanic force and EMG signals to be recorded accurately without signal averaging.

Axons↗
PubMed1990

Twitch properties of human thenar motor units measured in response to intraneural motor-axon stimulation.

1. The twitch properties of human thenar motor units were examined in response to intraneural motor-axon stimulation. Force components of thumb abduction and flexion were measured before and after tetanic stimulation. The magnitude, direction, and time derivatives of resultant forces, together with axon conduction velocities, were calculated for each unit. 2. Various indexes of contraction and relaxation rate were measured including contraction time (time from force onset to peak), one-half relaxation time (time from peak force to one-half that value), normalized maximum contraction and normalized maximum relaxation rates (peak positive and negative time derivatives of the force signal normalized to twitch force), and the times at which these maximum rates occurred. 3. For different units, the directions of resultant forces were approximately evenly distributed between thumb abduction and flexion. At the onset of the experiment, initial twitch forces ranged from 3 to 34 mN, contraction times from 35 to 80 ms, and one-half relaxation times from 25 to 108 ms. 4. Resultant twitch forces were positively correlated to normalized maximum relaxation rates, but not to other rate indexes or to conduction velocity. The various contraction rate measures were correlated to each other, but generally not to relaxation rates. 5. After the first test involving tetanic stimulation, the twitches of most units were potentiated and slowed, especially their relaxation phase. However, the extent of these changes varied considerably between units. In general, units with weak initial forces potentiated most, some up to three-fold. These changes in twitch properties were denoted posttetanic twitch potentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Axons↗
PubMed1990

A comparison of human thenar motor-unit properties studied by intraneural motor-axon stimulation and spike-triggered averaging.

1. Measurements of twitch contractile properties of human motor units recorded by spike-triggered averaging may be distorted by partial fusion between twitches, because motor units seldom fire at rates below 8-10 Hz. The effects of this fusion were examined by comparing the responses of 27 human thenar motor units when their motor axons were stimulated at 1, 8, and 10 Hz. 2. Resultant forces were calculated from the abduction and flexion force components, together with various contraction and relaxation rate indexes as reported previously. Values for single twitches were compared to measurements made from the unfused force fluctuations ("apparent twitches") of the same units recorded during 8 and 10 Hz stimulation. 3. For all units, stimulation at 8 and 10 Hz caused partial twitch fusion. At 10 Hz, mean values for "apparent twitch" forces, contraction times (CT), and one-half relaxation times (1/2RT) were reduced to 44, 76, and 52% of the corresponding values measured from separate twitches evoked by 1 Hz stimulation. Similar but smaller reductions were seen at 8 Hz. 4. Slow units, with initial twitch CT greater than 60 ms, showed significantly more distortion of all "apparent twitch" parameters when stimulated at both 8 and 10 Hz, compared to fast units (less than 50 ms). 5. The potentiated abduction force component data were compared with abduction forces obtained previously by spike-triggered averaging from the same muscle group. Mean force obtained by spike-triggered averaging ("STA twitch" force: 21 mN) was significantly larger than that measured in abduction in response to either 1 or 10 Hz motor-axon stimulation (14 mN, 6 mN, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Axons↗
PubMed1988

Coordinated isometric muscle commands adequately and erroneously programmed for the weight during lifting task with precision grip.

Small objects were lifted from a table, held in the air, and replaced using the precision grip between the index finger and thumb. The adaptation of motor commands to variations in the object's weight and sensori-motor mechanisms responsible for optimum performance of the transition between the various phases of the task were examined. The lifting movement involved mainly a flexion of the elbow joint. The grip force, the load force (vertical lifting force) and the vertical position were measured. Electromyographic activity (e.m.g.) was recorded from four antagonist pairs of hand/arm muscles primarily influencing the grip force or the load force. In the lifting series with constant weight, the force development was adequately programmed for the current weight during the loading phase (i.e. the phase of parallel increase in the load and grip forces during isometric conditions before the lift-off). The grip and load force rate trajectories were mainly single-peaked, bell-shaped and roughly proportional to the final force. In the lifting series with unexpected weight changes between lifts, it was established that these force rate profiles were programmed on the basis of the previous weight. Consequently, with lifts programmed for a lighter weight the object did not move at the end of the continuous force increase. Then the forces increased in a discontinous fashion until the force of gravity was overcome. With lifts programmed for a heavier weight, the high load and grip force rates at the moment the load force overcame the force of gravity caused a pronounced positional overshoot and a high grip force peak, respectively. In these conditions the erroneous programmed commands were automatically terminated by somatosensory signals elicited by the start of the movement. A similar triggering by somatosensory information applied to the release of programmed motor commands accounting for the unloading phase (i.e. the parallel decrease in the grip and load forces after the object contacted the table following its replacement). These commands were always adequately programmed for the weight.

Adolescent↗
PubMed1988

Programmed and triggered actions to rapid load changes during precision grip.

A test object (grip apparatus) was held at its upper part using a precision grip. Small balls were dropped into a target cup at the bottom of the apparatus. The grip force, the load force (vertical lifting force) and the vertical movement were measured. Electromyographic activity (e.m.g.) was recorded from four antagonist pairs of hand/arm muscles primarily influencing the grip force or the load force. The balls were dropped either by the subject during a bimanual task, or unexpectedly by the experimenter. When the subject dropped the ball, preparatory actions occurred before the rapid increase in the vertical load caused by the impact. These actions appeared ca. 150 ms prior to the impact and involved a grip force increase and a lifting movement of the grip apparatus. The e.m.g. activity increased in all eight of the hand and arm muscles, indicating a general stiffening of the hand/arm system prior to the impact. Furthermore, the preparatory actions were programmed adequately for the size of the load force step at the impact, i.e. an adequate safety margin to prevent slips was preserved during the critical period of the impact. Thus, variations in this step caused by changes in (i) the weight of ball, (ii) the weight of the grip apparatus and (iii) the length of the drop were adequately taken into account during the programming of these actions. In addition, the frictional condition between the skin and the grip surface was also taken into account. The relevant sensory information apparently was obtained during the handling of the ball and the grip apparatus prior to the drop. There were also task-related automatic muscle responses triggered by the impact. These responses, which also served to stiffen the hand/arm system, were most pronounced during unexpected load changes, but they appeared too late to prevent slips. However, if no overall slip occurred, the triggered responses were functional in the sense that they helped to quickly restore the safety margin and the vertical position of the object.

Adult↗
PubMed1987

Responses in glabrous skin mechanoreceptors during precision grip in humans.

Impulses in single tactile units innervating the human glabrous skin were recorded percutaneously from the median nerve using tungsten electrodes. The units were classified as belonging to one of the four categories: fast adapting with small receptive fields (FA I), fast adapting with large receptive fields (FA II), slowly adapting with small fields (SA I), and slowly adapting with large fields (SA II). A small test object was lifted, positioned in space and replaced using the precision grip between fingers and thumb. The grip force, the load force (vertical lifting force), the vertical movements of the object and vibrations (accelerations) in the object were recorded. After being virtually silent between lifts, the FA I units whose fields contacted the object became highly active during the initial period of grip force increase (initial response). This was also true for most SA I units. Accordingly, most of the skin deformation changes took place at low grip forces (below ca. 1 N). Later, while the load and grip forces increased in parallel during isometric conditions, the FA I and SA I units continued firing but generally at declining impulse rates. As long as the object was held in the air, the SA I units generally maintained firing with a tendency to adaptation. A minority of the FA I unit also discharged, especially during periods of pronounced physiological muscle tremor. The SA I units usually became silent when the grip and load forces in parallel declined to zero during isometric conditions after the object had contacted the table. However, during the very release of the grip the FA I units and some SA I units showed brief burst discharges (release response). The FA II units responded distinctly to the mechanical transients associated with the start of the vertical movement and especially with the sudden cessation of movement at the terminal table contact. FA II units whose end organs were remotely located in relation to the skin areas in contact with the object also responded. Most FA II units also discharged at the initial touch and at the release of the object, albeit less reliably than the type I units. In addition to weak dynamic responses during the phase of isometric force increase, the SA II units showed comparatively strong tonic responses while the object was held during static conditions. High firing rates also were maintained during long-lasting lifts. Moreover, it was established that the signals in SA II afferents were related to the three dimensional force profile in the grip.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1987

Signals in tactile afferents from the fingers eliciting adaptive motor responses during precision grip.

While human subjects lift small objects using the precision grip between the tips of the fingers and thumb the ratio between the grip force and the load force (i.e. the vertical lifting force) is adapted to the friction between the object and the skin. The present report provides direct evidence that signals in tactile afferent units are utilized in this adaptation. Tactile afferent units were readily excited by small but distinct slips between the object and the skin revealed as vibrations in the object. Following such afferent slip responses the force ratio was upgraded to a higher, stable value which provided a safety margin to prevent further slips. The latency between the onset of the a slip and the appearance of the ratio change (74 +/- 9 ms) was about half the minimum latency for intended grip force changes triggered by cutaneous stimulation of the fingers. This indicated that the motor responses were automatically initiated. If the subjects were asked to very slowly separate their thumb and the opposing finger while the object was held in air, grip force reflexes originating from afferent slip responses appeared to counteract the voluntary command, but the maintained upgrading of the force ratio was suppressed. In experiments with weak electrical cutaneous stimulation delivered through the surfaces of the object it was established that tactile input alone could trigger the upgrading of the force ratio. Although, varying in responsiveness, each of the three types of tactile units which exhibit a pronounced dynamic sensitivity (FA I, FA II and SA I units) could reliably signal these slips. Similar but generally weaker afferent responses, sometimes followed by small force ratio changes, also occurred in the FA I and the SA I units in the absence of detectable vibrations events. In contrast to the responses associated with clear vibratory events, the weaker afferent responses were probably caused by localized frictional slips, i.e. slips limited to small fractions of the skin area in contact with the object. Indications were found that the early adjustment to a new frictional condition, which may appear soon (ca. 0.1-0.2 s) after the object is initially gripped, might depend on the vigorous responses in the FA I units during the initial phase of the lifts (see Westling and Johansson 1987). The role of the tactile input in the adaptation of the force coordination to the frictional condition is discussed.

Adaptation, Physiological↗
PubMed1987

Significance of cutaneous input for precise hand movements.

During precision manipulation of small objects a refined coordination of forces exerted on the object by the tips of the fingers and thumb is required. An essential feature of this coordination is that the grip forces change in parallel with the load forces produced to overcome various forces counteracting the intended manipulation. The balance between the grip forces and load forces tending to cause slips is adapted to the friction between the skin and the object, providing a relatively small safety margin to prevent slips. This adaptation, which is dependent on tactile afferent input from the fingers, works highly automatically. Tactile afferent signals account for an initial adjustment of the force balance to the current frictional condition, taking place soon after the object is initially touched. Moreover, brief bursts of action potentials elicited in dynamically sensitive units by small slips can trigger a change in this balance, resulting in an increased safety margin for the prevention of further slips. This change includes a rapid reflex response (latency 60-80 msec) together with updating of a sensorimotor memory maintaining the new force balance.

Action Potentials↗
PubMed1984

Roles of glabrous skin receptors and sensorimotor memory in automatic control of precision grip when lifting rougher or more slippery objects.

To be successful, precision manipulation of small objects requires a refined coordination of forces excerted on the object by the tips of the fingers and thumb. The present paper deals quantitatively with the regulation of the coordination between the grip force and the vertical lifting force, denoted as the load force, while small objects were lifted, positioned in space and replaced by human subjects using the pinch grip. It was shown that the grip force changed in parallel with the load force generated by the subject to overcome various forces counteracting the intended manipulation. The balance between the two forces was adapted to the friction between the skin and the object providing a relatively small safety margin to prevent slips, i.e. the more slippery the object the higher the grip force at any given load force. Experiments with local anaesthesia indicated that this adaptation was dependent on cutaneous afferent input. Afferent information related to the frictional condition could influence the force coordination already about 0.1 s after the object was initially gripped, i.e. approximately at the time the grip and load forces began to increase in parallel. Further, "secondary", adjustments of the force balance could occur later in response to small short-lasting slips, revealed as vibrations in the object. The new force balance following slips was maintained, indicating that the relationship between the two forces was set on the basis of a memory trace. Its updating was most likely accounted for by tactile afferent information entering intermittently at inappropriate force coordination, e.g. as during slips. The latencies between the onset of such slips and the appearance of the adjustments (0.06-0.08 s) clearly indicated that the underlying neural mechanisms operated highly automatically.

Adaptation, Physiological↗
PubMed1984

Factors influencing the force control during precision grip.

A small object was gripped between the tips of the index finger and thumb and held stationary in space. Its weight and surface structure could be changed between consecutive lifting trials, without changing its visual appearance. The grip force and the vertical lifting force acting on the object, as well as the vertical position of the object were continuously recorded. Likewise, the minimal grip force necessary to prevent slipping, was measured. The difference between this minimal force and the employed grip force, was defined as the safety margin to prevent slipping. It was found that the applied grip force was critically balanced to optimize the motor behaviour so that slipping between the skin and the gripped object did not occur and the grip force did not reach exceedingly high values. To achieve this motor control, the nervous system relied on a mechanism that measured the frictional condition between the surface structure of the object and the fingers. Experiments with local anaesthesia indicated that this mechanism used information from receptors in the fingers, most likely skin mechanoreceptors. In addition to friction, the control of the grip force was heavily influenced by the weight of the object and by a safety margin factor related to the individual subject. The frictional conditions during the previous trial could also, to some extent, influence the grip force.

Adolescent↗