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Biomedical subjects

J A Stephens

Publications and source records attributed to J A Stephens.

At least 91 records · Page 5Linked to original sources
PubMed1975

The tendon organs of cat soleus: static sensitivity to active force.

The static force sensitivity of soleus tendon organ (Ib) afferents has been studied by noting their responses to graded force development produced by isometric contractions of either the whole muscle or single motor units. Data included responses of 23 Ib afferents to contraction of 8 whole muscles (8 experiments) and 16 Ib afferents to contraction of 30 motor units (5 experiments). Tendon organ responses of varying magnitude to contraction of the whole muscle or several of its individual motor units could be explained by differences in the number of muscle fibers that insert into each receptor's capsule and by differences in the contraction strength of these fibers. This finding suggests that soleus tendon organs have similar absolute sensitivities to static force development. An estimate was made of this absolute sensitivity and the value obtained (314 pps/g of force actually coupled to the receptor) is 2 orders of magnitude greater than those previously reported indices that simply relate Ib firing rate to force as measured at the tendon. The relationship between force exerted on a tendon organ's capsule and Ib firing rate is whown to be curvilinear and in keeping with a possible saturation effect that reduces the receptor's responsiveness to active contractions at relatively long muscle lengths.

Animals↗
PubMed1975

The motor units of cat medial gastrocnemius: problem of their categorisation on the basis of mechanical properties.

The mechanical properties of 126 motor units from medial gastrocnemius muscle have been studied in 12 adult cats. Units with long contraction times (greater than 45 msec) were non fatigable (24 out of 26 units) and small (25 out of 26 units with less than or equal to 0.3% of the parent whole muscle tetanic tension) thus forming a very homogeneous population. In contrast, fast twitch units (contraction time less than or equal to 45 msec) exhibited a very broad range of tetanic tensions and fatigability. Significant correlations were found, however, within the fast contracting population which indicate a tendency for the more fatigable units to develop more tetanic tension and to be faster contracting. These findings are discussed in relation to the problems associated with using the interrelationships between twitch contraction time, tetanic tension and fatigue resistance to classify motor units into subpopulations sharing similar mechanical properties.

Animals↗
PubMed1975

The motor units of cat medial gastrocnemius: speed-size relations and their significance for the recruitment order of mortor units.

The interrelationships between axonal conduction velocity, tetanic tension, twitch contraction time and rate of force development during a tetanus have been studied in 126 motor units from 12 cat medial gastrocnemius muscles. While the range of axonal conduction velocities for slow- (contraction time greater than 45 msec) and fast-twitch units overlapped, the mean conduction velocity for the slow-twitch group was significantly lower. No difference could be found between the axonal conduction velocities of the fast non-fatiguing and fast fatiguing units. Within individual experiments, few significant correlations were found between conduction velocity and tetanic tension or contraction time for the fast- and slow-twitch units. Some correlations did appear when data from these populations were pooled, but such results are shown to be misleading. Some weak correlations were found between motor unit contraction strength and twitch contraction time. The rate of rise of isometric force development was found to be most strongly related to tetanic tension and only weakly related to contraction time. The ordering of motor units according to contraction strength reveals the association of motor unit mechnical properties to be ideally suited for the dual role of medial gastrocnemius as a postural and powerful phasic muscle.

Animals↗
PubMed1975

The motor units of cat medial gastrocnemius: electrical and mechanical properties as a function of muscle length.

The effects of changing muscle length on the mechanical properties of 89 motor units from adult cat medial gastrocnemius have been studied in eight experiments. Few differences were found between the effects of length on tetanic tension, twitch tension, twitch-tetanus ratio, twitch contraction time, twitch half relaxation time, rate of force development and electrical activity for fast contracting (twitch contraction time less than or equal to 45 msec) and slowly contracting (greater than 45 msec) units. Those differences that did appear did not persist when these two groups were matched by tetanic tension. It is concluded that the biophysical mechanisms responsible for the changes in mechanical and electrical properties with length must be similar for fast and slow twitch units and not related to potential differences in their muscle fiber type. The effects of changing muscle length on the mechanical properties of the eight whole muscles suggest that changes in force output with length are of minor importance during normal movements as the muscle is found to be electrically active over a relatively narrow range of lengths close to the optimum length for tetanus of the whole muscle. The very shortest muscle lengths at which there is only minimal force development are not used in natural movements, while the declining limb of the length tension curve is at muscle lengths beyond the maximum in situ length.

Animals↗
PubMed1975

The motor units of cat medial gastrocnemius. Twitch potentiation and twitch-tetanus ratio.

Twitch potentiation and the associated changes in contraction time and 1/2 relaxation time have been studied in a sample of 78 medial gastrocnemius motor units from 8 cats. Potentiation was produced by repetitive stimulation of the motor units every 10 sec with a brief tetanus followed 2 sec later by a twitch. Fast twitch fatigue resistant units were found to potentiate more strongly than either slow twitch (contraction time greater than 45 msec) or fast twitch fatigable units. It is concluded that Type C muscle fibres are more susceptible to potentiation than either Type A or Type B fibres. In a sample of 88 motor units from the same experiments, values for twitch/tetanus ratio were compared amongst units sharing similar mechanical properties. Slow contracting units developed small tetanic tensions and had small twitch/tetanus ratios. Fast twitch non fatigable units had intermediate values for contraction strength and twitch/tetanus ratio overlapping the ranges found for both the slow twitch and the more powerful fast twitch fatigable units. It is concluded that differences in twitch/tetanus ratio for medial gastrocnemius motor units are primarily related to motor unit contraction strength rather than differences in muscle fibre type.

Animals↗
PubMed1975

The tendon organs of cat medial gastrocnemius: significance of motor unit type and size for the activation of Ib afferents.

1. Histological and histochemical studies suggest that each tendon organ in a mixed mammalian muscle should be particularly responsive to the contraction of a discrete number of motor units (ca. ten to fifteen), each with differing mechanical properties. This report describes physiological experiments that demonstrate this arrangement for the tendon organs of cat medial gastrocnemius. 2. No correlations could be found between the intensity of discharge of a single tendon organ and the contraction strengths of motor units whose contraction excited the receptor. Tendon organs were found to be as responsive to contraction of small slow twitch units as they were to contraction of larger fast twitch units. Taking the data as a whole, the apparent sensitivity of the receptors during motor unit contractions (pps/force recorded at the tendon) was inversely related to the contraction strengths of the motor units. 3. These findings are discussed in relation to recent evidence on the territory of single motor units in medial gastrocnemius and the force producing capabilities of their individual muscle fibres. It is concluded that in general each motor unit, whose contraction excites a given receptor, contributes one muscle fibre to the receptor capsule. Further, it appears that the various excitatory effects of those muscle fibres inserting into a given receptor capsule are not simply related to their relative contraction strengths but also depend on the details of the mechanical coupling between each fibre and the Ib afferent receptor endings. 4. The results of an ensemble analysis show that despite the lack of correlation between the intensity of tendon organ discharge and the force developed at the tendon during contraction of different motor units, a correlation does appear when the responses of several tendon organs and the forces developed by the motor units which excite them are summed progressively. This finding has implications for the recruitment order of motor units in that the profile of the collective Ib response is shown to differ according to whether motor unit forces are summed randomly or in order of increasing contraction strengths.

Action Potentials↗
PubMed1975

Tendon organs of cat medial gastrocnemius: responses to active and passive forces as a function of muscle length.

The responses of 13 Golgi tendon organs to graded force development of 29 motor units in medial gastrocnemius of the cat have been studied in five experiments. Of the 13 tendon organs, 11 were responsive to passive stretch within the physiological range of muscle length and 5 were "spontaneously" active at very short lengths where no passive tension could be recorded. The relationship between passive force and the firing rates of the various afferents ranged from a linear one to a power relation (Y = Axb + c) with b, a widely varying exponent. Results support the general conclusion that although many Ib afferents respond to passive force within the physiological range of muscle stretch, this form of stimulus is not a particularly effective one. The statis responses of Golgi tendon organs to active force development produced by single motor units was studied at different muscle lengths. In all cases the apparent sensitivity (change in firing rate per active force developed) decreased as muscle length approached Lo. The static responses of Golgi tendon organs to force developed by single motor units were also studied during fatiguing contractions. The data suggest a sigmoid relationship between force developed at the tendon and the Ib response. The collective response of all 13 tendon organs to active and passive forces at different muscle lengths was also examined. This analysis offered further support for the viewpoint that active motor unit contractions provide themost significant excitatory input to tendon organs and that changes in passive force during muscle stretch have comparatively little effect on the collective tendon organ response. The interaction between active and passive force inputs to the Golgi tendon organs is discussed in relation to the complicated nature of the relationship between forces measured at the tendon and those acting within the receptor capsule. When these complications were taken into account it was possible to explain the differences in responsiveness of a given tendon organ to active contraction of several motor units and to passive force in terms of a single force-firing rate curve for the receptor. It is concluded that changes in the force of contraction of single motor units result in relatively small changes in Ib afferent firing and that during normal muscle contractions, changes in the number of motor units acting on a single receptor must produce far more significant changes in firing rate than changes in the amount of force developed by any single unit. Changes in dynamic Ib sensitivity to single motor unit contractions are also shown to depend on length and in a similar way to the changes in static Ib sensitivity. During fatiguing contractions, a sigmoid relation was found between the dynamic Ib response and the rate of force development by single motor units.

Animals↗
PubMed1975

Interface unit for on-line measurements of motor unit properties with a small laboratory computer.

Some simple circuitry is described which has been successfully used in conjunction with a small laboratory computer for "on-line" measurements of the force of contraction of single motor units in cat gastrocnemius and soleus muscles. Our approach emphasizes use of a sample and hold device to allow high amplification of small forces developed during single motor unit contractions which are often superimposed on relatively large whole muscle passive forces. Two digitally controlled selectable gain amplifiers are used to ensure proper modulation of the computer's ADC and to enable automatic scaling of the measured variables. An analog peak detector is incorporated into the unit such that peak force measurements can be made with the minimum of computer software. The principles governing the present design are directly applicable to the measurement of intracellularly recorded postsynaptic potentials and a variety of other physiological variables which require accurate measurement of a small transient signal superimposed on a large "control" or "resting" signal.

Animals↗
PubMed1972

Fatigue of maintained voluntary muscle contraction in man.

1. The mechanism of fatigue has been studied in maintained maximal voluntary contractions of the first dorsal interosseous muscle of the hand.2. Fatigue occurs in two phases. In the first, lasting 1 min, force falls to about 50%. The smoothed rectified e.m.g. (s.r.e.) falls with the same time course and the normal linear relation between s.r.e. and force of unfatigued muscle is preserved.3. In the second phase, force falls relatively faster than s.r.e.4. Arterial occlusion does not affect the first phase, but in the second phase causes force to fall to zero, whereas without occlusion it tends to stabilize at about 25%.5. The size of the synchronous muscle action potential evoked by ulnar nerve stimulation falls to about 65% of normal, most of this fall occurring in the first phase.6. During recovery after prolonged fatigue, the relation between force and s.r.e. is changed for weak voluntary contractions much more than for strong ones, such that force is less for a given s.r.e. than normal.7. These results are interpreted as evidence that, in a maximal voluntary contraction, neuromuscular junction fatigue is most important at first, but later, contractile element fatigue increases, particularly when the blood supply is obstructed.8. Neuromuscular junction fatigue is believed to be most marked in high threshold motor units, while contractile element fatigue more especially affects low threshold units.

Action Potentials↗