Jeremy Payne

Abstract There is increasing evidence that the architectural design and arrangement of the fibers within a motor unit have important physiological and developmental ramifications. Limited data, however, are available to directly address this issue. In the present study the physiological properties of one motor unit in each of seven cat tibialis anterior (TA) muscles were determined. Each of these units then was repetitively stimulated to deplete the glycogen in all muscle fibers within the unit. Subsequently, the length, type of ending, and spatial distribution of fibers sampled from these physiologically and histochemically typed motor units were determined. Four fast fatigable (FF), one fast, fatigue resistant (FR), and two slow (S) motor units (MU) were studied. The samples consisted of all those glycogen‐depleted fibers (9–27) contained within a single fascicle or a circumscribed area of each of the motor unit territories. The mean fiber lengths for the two slow motor units were 35.9 and 45.5 mm. The mean fiber lengths for the fast motor unit samples ranged from 8.8 to 48.5 mm. Some fibers of both the fast and slow units reached lengths of 58 mm. Most of the fibers in the slow units extended the entire distance between the proximal and distal musculotendinous planes, had relatively constant cross‐sectional areas, and terminated at the tendon as blunt endings. In contrast, the majority of the fibers in the fast units terminated intrafascicularly at one end, and the cross‐sectional area decreased progressively along their lengths, that is, showed a tapering pattern for a significant proportion of their lengths. Therefore, the force generated by units that end midfascicularly would appear to be transmitted to connective tissue elements and/or adjacent fibers. All fibers of a fast unit within a fascicle were located at approximately the same proximo‐distal location. Thus, developmentally the selection of muscle fibers by a motoneuron would seem to be influenced by their spatial distribution. The architectural complexities of motor units also have clear implications for the mechanical interactions of active and inactive motor units. For example, the tension capabilities of a motor unit may be influenced not only by the spatial arrangement of its own fibers, but also by the level of activation of neighboring motor units.

1. We recorded single unit activity from individual primary electrosensory afferent axons in the posterior branch of the anterior lateral line nerve of gymnotid weakly electric fish, Apteronotus leptorhynchus. We analyzed the responses of P-type (probability-coding) afferent fibers to externally applied amplitude step changes in the quasi-sinusoidal transdermal potential established by the fish's own electric organ discharge (EOD). 2. In response to AM step increases in transdermal potential, the firing rate of P-type afferents exhibited an abrupt increase followed by an initially rapid and subsequently more gradual decay back toward the baseline level. Afferent responses continued to adapt slowly throughout the duration of prolonged step stimuli lasting > 100 s. The time course of sensory adaptation was similar for all units tested. 3. We introduce a new functional form for describing the time course of sensory adaptation in which the change in firing rate delta r decays logarithmically with time: delta r(t) = A/[B In (t) + 1]. This logarithmic form accurately describes the adaptation time course of P-type afferents over five decades in time, from milliseconds to hundreds of seconds, with only two free parameters. Using a nonlinear least-squares fitting technique, we obtained a mean value of the parameter B, which characterizes the adaptation time course, of 0.149 +/- 0.028 (mean +/- SD, n = 49). 4. We compare logarithmic fits with traditional multiexponential and power law forms and demonstrate that the logarithmic form yields a better characterization of P-type afferent responses. This analysis helps explain the variability in previously reported adaptation time constants, which have ranged from 0.2 to 3.4 s, in gymnotid P-type afferents. 5. We tested the linearity of P-type afferent responses using positive and negative AM steps of varying amplitudes. Aside from nonlinearities associated with rectification (firing rates cannot be negative) and saturation (firing rates cannot exceed the EOD frequency), we found that P-type afferent responses scaled linearly with stimulus amplitude. 6. Based on the observed linearity, we predict the frequency domain response characteristics of P-type afferents and find that the predicted gain and phase are in good agreement with experimental measurements using sinusoidal AM stimuli over a range of AM frequencies from 1 to 100 Hz. Thus the logarithmic parameterization of the step appears to accurately capture the response dynamics of P-type afferents over a wide range of behaviorally relevant AM frequencies. 7. We conclude that the temporal filtering properties of pyramidal cells in the medullary electrosensory nucleus, the electrosensory lateral line lobe (ELL), need to be reevaluated in light of the logarithmic adaptation time course in the periphery, and we discuss implications for the role of P-type afferents in driving a feedback gain control mechanism that regulated ELL pyramidal cell responsiveness.

BACKGROUND: Stroke is a leading cause of long-term disability. Greater rehabilitation therapy after stroke is known to improve functional outcomes. This study examined therapy doses during the first year of stroke recovery and identified factors that predict rehabilitation therapy dose. METHODS: Adults with new radiologically confirmed stroke were enrolled 2 to 10 days after stroke onset at 28 acute care hospitals across the United States. Following an initial assessment during acute hospitalization, the number of physical therapy, occupational therapy, and speech therapy sessions were determined at visits occurring 3, 6, and 12 months following stroke. Negative binomial regression examined whether clinical and demographic factors were associated with therapy counts. False discovery rate was used to correct for multiple comparisons. RESULTS: Of 763 patients enrolled during acute stroke admission, 510 were available for follow-up. Therapy counts were low overall, with most therapy delivered within the first 3 months; 35.0% of patients received no physical therapy; 48.8%, no occupational therapy, and 61.7%, no speech therapy. Discharge destination was significantly related to cumulative therapy; the percentage of patients discharged to an inpatient rehabilitation facility varied across sites, from 0% to 71%. Most demographic factors did not predict therapy dose, although Hispanic patients received a lower cumulative amount of physical therapy and occupational therapy. Acutely, the severity of clinical factors (grip strength and National Institutes of Health Stroke Scale score, as well as National Institutes of Health Stroke Scale subscores for aphasia and neglect) predicted higher subsequent therapy doses. Measures of impairment and function (Fugl-Meyer, modified Rankin Scale, and Stroke Impact Scale Activities of Daily Living) assessed 3 months after stroke also predicted subsequent cumulative therapy doses. CONCLUSIONS: Rehabilitative therapy doses during the first year poststroke are low in the United States. This is the first US-wide study to demonstrate that behavioral deficits predict therapy dose, with patients having more severe deficits receiving higher doses. Findings suggest directions for identifying groups at risk of receiving disproportionately low rehabilitation doses.