Influence of ischemia on the discharge rate in motor units during a sustained submaximal contraction

Motor unit discharge patterns were observed under ischemic and non-ischemic conditions in the tibialis anterior during two sessions approximately 48 hours apart. Ten subjects completed three 5-second maximum voluntary contractions [MVCs) followed by a prolonged, submaximal, isometric contraction to induce fatigue, and three additional MVCs. This fatigue trial was completed under both ischemic and non-ischemic conditions. For the ischemic condition, a pressure cuff was placed above the knee, inflated to 180 mmHg, and arterial occlusion verified via Doppler ultrasound. During the fatigue task, a constant force of 20% MVC was maintained until endurance time. Single motor unit activity was recorded with intramuscular fine wire electrodes. Endurance time during the ischemic fatigue task (3.71 ± 0.58 min) was significantly less than the non-ischemic fatigue task (9.11 ± 0.56 min). Both tasks showed a significant decline in motor unit discharge rate. There was greater variability in the discharge rate during ischemic compared to non-ischemic conditions. Time to minimum discharge rate was greater during ischemic conditions. Overall, ischemic conditions resulted in decreased endurance time, greater rate of decline in discharge rate, and greater variability in discharge rate of motor units.Kinesiology and Health Educatio

Rates and equilibria at the acetylcholine receptor of electrophorus electroplaques. A study of neurally evoked postsynaptic currents and of voltage-jump relaxations

Kinetic measurements are employed to reconstruct the steady-state activation of acetylcholine [Ach] receptor channels in electrophorus electroplaques. Neurally evoked postsynaptic currents (PSCs) decay exponentially; at 15°C the rate constant, α, equals 1.2 ms^(-1) at 0 mV and decreases e-fold for every 86 mV as the membrane voltage is made more negative. Voltage-jump relaxations have been measured with bath-applied ACh, decamethonium, carbachol, or suberylcholine. We interpret the reciprocal relaxation time 1/τ as the sum of the rate constant α for channel closing and a first-order rate constant for channel opening. Where measureable, the opening rate increases linearly with [agonist] and does not vary with voltage. The voltage sensitivity of small steady-state conductances (e- fold for 86 mV) equals that of the closing rate α, confirming that the opening rate has little or no additional voltage sensitivity. Exposure to α-bungarotoxin irreversibly decreases the agonist-induced conductance but does not affect the relaxation kinetics. Tubocurarine reversibly reduces both the conductance and the opening rate. In the simultaneous presence of two agonist species, voltage-jump relaxations have at least two exponential components. The data are fit by a model in which (a) the channel opens as the receptor binds the second in a sequence of two agonist molecules, with a forward rate constant to 10^(7) to 2x10^(8) M^(-1)s^(-1); and (b) the channel then closes as either agonist molecule dissociates, with a voltage-dependent rate constant of 10^(2) to 3x10^(3)s^(-1)

Owning your own home: reality or myth

The focus of the white paper will be to highlight the housing challenges that people in the New England region and in the United States face. Affordability, sustainability, people needs, societal needs, environmental needs, economic incentives and impact of government policies are just a few of the topics that will be explored

Using giant scarlet runner bean embryos to uncover regulatory networks controlling suspensor gene activity.

One of the major unsolved issues in plant development is understanding the regulatory networks that control the differential gene activity that is required for the specification and development of the two major embryonic regions, the embryo proper and suspensor. Historically, the giant embryo of scarlet runner bean (SRB), Phaseolus coccineus, has been used as a model system to investigate the physiological events that occur early in embryogenesis-focusing on the question of what role the suspensor region plays. A major feature distinguishing SRB embryos from those of other plants is a highly enlarged suspensor containing at least 200 cells that synthesize growth regulators required for subsequent embryonic development. Recent studies have exploited the giant size of the SRB embryo to micro-dissect the embryo proper and suspensor regions in order to use genomics-based approaches to identify regulatory genes that may be involved in controlling suspensor and embryo proper differentiation, as well as the cellular processes that may be unique to each embryonic region. Here we review the current genomics resources that make SRB embryos a compelling model system for studying the early events required to program embryo development

Functional Stoichiometry at the Nicotinic Receptor. The Photon Cross Section for Phase 1 Corresponds to Two Bis-Q Molecules per Channel

These experiments examine changes in the agonist-induced conductance that occur when the agonist-receptor complex is perturbed. Voltage-clamped Electrophorus electroplaques are exposed to the photoisomerizable agonist trans-Bis-Q A 1-µs laser flash photoisomerizes some trans-Bis-Q molecules bound to receptors; because the cis configuration is not an agonist, receptor channels close within a few hundred microseconds. This effect is called phase 1. We compare (a) the fraction of channels that close during phase 1 with (b) the fraction of trans-Bis-Q molecules that undergo trans → cis photoisomerization. Parameter a is measured as the fractional diminution in voltage-clamp currents during phase 1. Parameter b is measured by changes in the optical spectra of Bis-Q solutions caused by flashes . At low flash intensities, a is twice b, which shows that the channel can be closed by photoisomerizing either of two bound agonist molecules. Conventional dose-response studies with trans-Bis-Q also give a Hill coefficient of two. As a partial control for changes in the photochemistry caused by binding of Bis-Q to receptors, spectral measurements are performed on the photoisomerizable agonist QBr, covalently bound to solubilized acetylcholine receptors from Torpedo. The bound and free agonist molecules have the same photoisomerization properties. These results verify the concept that the open state of the acetylcholine receptor channel is much more likely to be associated with the presence of two bound agonist molecules than with a single such molecule