Discharge Behaviors of Trapezius Motor Units During Exposure to Low and High Levels of Acute Psychosocial Stress

Abstract

This study investigated the effects of acute psychosocial stress on trapezius single motor unit discharge behaviors. Twenty-one healthy women performed feedback-controlled isometric contractions under conditions of low and high psychosocial stress in the same experimental session. Psychosocial stress was manipulated using a verbal math task combined with social evaluative threat which significantly increased perceived anxiety, heart rate, and blood pressure (P<0.001). Motor unit discharge behaviors including the threshold and discharge rate at recruitment (7.7 (5.7) %MVC and 7.3 (6.8) pps, P>0.121, N=103) and derecruitment (6.0(4.4) %MVC and 6.5(4.1) pps, P>0.223, N=99), the mean (11.3 (2.3) pps, P=0.309, N=106) and variability (2.5 (0.91) pps, P=0.958, N=106) of discharge rate, and the proportion of motor units exhibiting double discharges (21%, P=0.446) did not change across stress conditions. Discharge rate modulation with changes in contraction intensity was highly variable and similar across stress conditions (P>0.308, N=89). Rate-rate modulation of concurrently active motor units was also highly variable (r=−0.84-1.00, N=75). Estimates of ΔF for motor unit pairs with rate-rate modulation ≥0.7 were positive and similar across stress conditions (4.7(2.0) pps, P=0.405, N=16). Results indicate that acute psychosocial stress does not alter trapezius motor unit discharge behaviors during a precisely controlled motor task in healthy women.

Effects of persistent inward currents, accommodation, and adaptation on motor unit behavior: a simulation study

“…Such low values of assigned PICs might partly account for the relatively small values of ⌬F (ϳ2 imp/s) in simulations during which PICs were enabled. Experimentally, average values of ⌬F in human subjects are ϳ4 imp/s (Gorassini et al 2002a;Mottram et al 2009;Stephenson and Maluf 2010). It should be noted, however, that when multiple nonlinearities were included in the model (Fig.…”

“…Furthermore, we restricted our focus in this study to the intriguing ⌬F phenomenon, which is seen experimentally (Gorassini et al 2002a(Gorassini et al , 2002b(Gorassini et al , 2004Kiehn and Eken 1997;Mottram et al 2009;Oya et al 2009;Powers et al 2008;Revill et al 2010;Revill and Fuglevand 2009;Stephenson and Maluf 2010;Udina et al 2010) and has been accounted for as primarily arising due to one of these intrinsic properties, namely, to PICs (Gorassini et al 2002a;Oya 2009). Our results indicate that a positive ⌬F can arise only by the inclusion of nonlinear intrinsic properties, and as such, previous models of motor unit populations (Fuglevand et al 1993;Heckman and Binder 1991) do not account for this physiological feature of motor unit behavior.…”

“…An expanding number of reports consider positive ⌬F measurements (i.e., higher firing rate of control unit at recruitment compared with at derecruitment of a test unit) as prima facie evidence for PIC activation in motor units (ElBasiouny et al 2010;Gorassini et al 1998Gorassini et al , 2002aGorassini et al , 2002bGorassini et al , 2004Heckman et al 2005Kiehn and Eken 1997;Mottram et al 2009;Oya et al 2009;Powers et al 2008;Stephenson and Maluf 2010;Udina et al 2010). It seems feasible, however, that other nonlinear properties of motor neurons, such as accommodation or adaptation, might also lead to positive ⌬F values.…”

Characterization of motor units in behaving adult mice shows a wide primary range

“…It seems unlikely that the mice are under much stress as they are acclimatized to the environment and to handling, and also have enclosures during testing. Also, recent work indicates that discharge rate of motor units is not effected by stress (Stephenson and Maluf 2010).…”

Short interspike intervals and double discharges of anconeus motor unit action potentials for the production of dynamic elbow extensions

“…These functions include absorbing the resting slack of the muscle to enable linear isometric force production, increasing series elastic stiffness for greater force transmission (Binder-Macleod and Kesar 2005;Parmiggiani and Stein 1981;Wilson and Larimer 1968), or rapid summation of MU twitch tensions to increase the rate of isometric torque production (Desmedt and Godaux 1978;Mrǒwczyński et al 2011;Van Cutsem et al 1998). Although DDs have been observed periodically in long trains of MU action potentials (Bawa and Calancie 1983;Kudina and Alexeeva 1992;Westad et al 2004) and at termination of isometric force production (Kudina and Alexeeva 1992;Kudina and Churikova 1990;Stephenson and Maluf 2010), they are more often found at or near minimal firing rates (Bawa and Calancie 1983;Calvin and Schwindt 1972) when MUs are first recruited during low isometric torque contractions (Bawa and Calancie 1983;Denslow 1948;Kudina 1974;Kudina and Churikova 1990), or preceding fast ballistic isometric contractions (Bawa and Calancie 1983;Desmedt and Godaux 1977). Similarly, short ISIs have been noted preceding moderately fast (Ͻ250°/s) dynamic contractions in 97% of triceps brachii MUs recorded (Griffin et al 1998).…”