Contraction propagated

The electrical impulse for contraction (propagated action potential p. 136) originates in pacemaker cells of the sinoatrial node and spreads through the atria, atrioventricular (AV) node, and adjoining parts of the His-Purkinje fiber system to the ventricles (A). Irregularities of heart rhythm can interfere dangerously with cardiac pumping func-tioa... 

Following manual stimulation of active contraction in the apices of the top two cells to initiate the simulation, the apices of the neighbouring active cells are stretched and so triggered, automatically, to contract. This leads to a wave of contraction propagated, domino-like, from cell to cell. These coordinated changes in cell shape lead to an overall change in tissue form similar to that seen in neural tube formation (Fig. 2). 

Excitability refers to the capacity of nerves and other tissues (e.g. cardiac), as well as individual cells, to generate and sometimes propagate action potentials, signals that serve to control intracellular processes, such as muscle contraction or hormone secretion, and to allow for long- and short-distance communication within the organism. Examples of excitable cells and tissues include neurons, muscle and endocrine tissues. Examples of nonexcitable cells and tissues include blood cells, most epithelial and connective tissues. 

Muscle contraction is initiated by a signal from a motor nerve. This triggers an action potential, which is propagated along the muscle plasma membrane to the T-tubule system and the sarcotubular reticulum, where a sudden large electrically excited release of Ca " into the cytosol occurs. Accessory proteins closely associated with actin (troponins T, I, and C) together with tropomyosin mediate the Ca -dependent motor command within the sarcomere. Other accessory proteins (titin, nebulin, myomesin, etc.) serve to provide the myofibril with both stability... 

Of all the requirements that have to be fulfilled by a manufacturer, starting with responsibilities and reporting relationships, warehousing practices, service contract policies, airhandUng equipment, etc., only a few of those will be touched upon here that directly relate to the analytical laboratory. Key phrases are underlined or are in italics Acceptance Criteria, Accuracy, Baseline, Calibration, Concentration range. Control samples. Data Clean-Up, Deviation, Error propagation. Error recovery. Interference, Linearity, Noise, Numerical artifact. Precision, Recovery, Reliability, Repeatability, Reproducibility, Ruggedness, Selectivity, Specifications, System Suitability, Validation. 

Each muscle fiber is innervated by a branch of an alpha motor neuron. The synapse between the somatic motor neuron and the muscle fiber is referred to as the neuromuscular junction. Action potentials in the motor neuron cause release of the neurotransmitter acetylcholine. Binding of acetylcholine to its receptors on the muscle fiber causes an increase in the permeability to Na+ and K+ ions. The ensuing depolarization generates an action potential that travels along the surface of the muscle fiber in either direction that is referred to as a propagated action potential. This action potential elicits the intracellular events that lead to muscle contraction. 

Action potentials are generated in single-unit smooth muscle. Simultaneous depolarization of 30 to 40 smooth muscle cells is required to generate a propagated action potential the presence of gap junctions allows this to occur readily. Because single-unit smooth muscle is self-excitable and capable of generating action potentials without input from the autonomic nervous system, it is referred to as myogenic. In this muscle, the function of the autonomic nervous system is to modify contractile activity only. Input is not needed to elicit contraction. 

Normal cardiac contraction depends on the conduction of electrical impulses through the myocardium in a highly coordinated fashion. Any abnormality of the initiation or propagation of the impulse is referred to as an arrhythmia. These disorders are the most common clinical problem encountered by a cardiologist. There is a wide range of types of arrhythmias with multiple etiologies and a variety of symptoms. In this section, two types of cardiac tachyarrhythmias are discussed. The most common treatment for these conditions is drug therapy. 

Intestinal mechanical clearance thus consists of both reflex-mediated contractions (peristalsis) elicited by the stimulatory effect of luminal contents and of periods of spontaneous contractile activity (e.g. the migrating motor complex). During fasting about 50% of intestinal transit has been attributed to phase III of the migrating motor complex, the remaining mostly to the propulsive contractions and motor patterns during phase II [ 108]. Luminal flow can also occur in the absence of propagating contractions of the circular muscle layer, so far considered the motor event mainly responsible for flow in the small intestine. 

In conclusion, gastric microflora is altered in patients with severe forms of intestinal pseudoobstruction due to frequent duodenogastric reflux episodes caused by abnormal retrogradely propagating contractions. 

The long duration of an individual uterine contraction in human (60—90 s) is difficult to explain using a single signalling mechanism. Myometrium clearly demonstrates action potential propagation, but this rapid (5 cm/s) mechanism... 

Young In any calculation you allow for a number of variables in terms of trying to model the system of action potential propagation. We know that action potentials have to travel within a fasciculus, which is a macroscopic 1—2 mm connective tissue grouping of bundles. Those do not follow a very tortuous route, at the most they wind once or twice over 30 cm, but not much larger than that. The best simulation that s been done using action potential alone used around nine parameters and was totally unrealistic. I probably didn t point it out but the dashed line that overlayed my clinical contraction was where I actually assumed a 350 /tm size bundle. I assumed an intracellular Ca2+ wave and cell activity for 20 s, and I was able to fit it except for the foot at the end (Young 1997). 

Young RC 1997 A computer model of uterine contractions based on action potential propagation and intercellular calcium waves. Obstet Gynecol 89 604-608... 

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