Excitatory (Excitability) refers to Excitable tissue Excited or when the cells are stimulated ( action potential The ability or characteristic). The relationship between organism and environment not only in material and energy metabolism Also, caused by changes in the activities of the body when the change of environmental conditions, the organisms continuously active To adapt to the environment to survive.

Excitability concept

Myocardium The excitement of the
As with other myocardial excitable tissue excitability and usually its excitability level threshold As a measure index.

Excitability Exciting period

Cyclical changes in excitability
Myocardial cell And nerve Cells Similar excitability is variable. When myocardial cells are stimulated to produce an excitement, excitement has also undergone a series of periodic changes, and these changes membrane potential Is closely related to the change of channel, function. Changes in excitability can be divided into the following periods:
(1) Absolute refractory period and Effective refractory period The absolute refractory period is equivalent to: Myocardium Once excited, from action potential The 0 stage began to phase 3 depolarization repolarization within the membrane potential is about -55mV this time, if you give it no matter how strong stimulation, stimulation, myocardial cells are not excited again. Therefore, this period is called the absolute refractory period. The membrane potential is very small, the Na+ channel is in an inactive state, decreased excitability of cardiac muscle cells to zero. From the membrane potential -55mV to -60mV the repolarization period, if given the suprathreshold stimulus, Muscle membrane Can have a local depolarization ( Local excitation ), but still can not produce action potentials from the action potential depolarization to -60mV this time, called the effective refractory period. The local depolarization is because Na+ channel is just beginning to rise.
(2) Relative refractory period Effective refractory period is completed, from the 3 phase of the membrane potential of -60mV to -80mV during this period, with the Suprathreshold stimulus Can cause action potential That is called the relative refractory period. This shows that myocardial excitability has been gradually restored, but still lower than the normal, the reason is that the Na+ channel activity recovery.
(3) Supernormality From the 3 membrane potential -80mV repolarization period to -90mV repolarization during this period, with subthreshold stimulation can cause cardiac action potential, indicating myocardial excitability than normal, so called supernormality. In the meantime, Myocardial cell The membrane potential Has been basically restored, the Na+ channel has been basically raised to the standby state can be activated; while the membrane potential is lower than the absolute value Resting potential Distance. Threshold potential The smaller the gap, the excitement was higher than the normal level.
Factors affecting excitability
The level of myocardial excitability except for threshold As a measure, between the resting potential and threshold potential gap and ion channel The characters can also affect the excitability.
(1) the resting potential absolute value of resting potential increases from the threshold potential gap to increase excitement required Stimulus threshold Increased excitability decreased; on the contrary, the absolute value of resting potential is reduced, then increased excitability.
(2) the threshold potential: the threshold level shift, and increase the gap between the resting potential, the myocardial excitability decreased; whereas threshold level down, then increased excitability.
(3) Na+ channel: Na+ channel character refers to the state of myocardial cells are excited by a Na+ can be activated as a precondition. The Na+ channel has three functions, namely, activation, inactivation and standby. Na+ channel is in one state, depending on the time of the membrane potential The level and Time course That is, Na+ channel activation, deactivation and resurrection is voltage dependent and time compliance.
Response form (excitatory and inhibitory): the excitement is refers to the organization after receiving stimulation, activities produce or enhance the inhibition is refers to the organization to accept the stimulation, the stop or reduce