GENERATION OF ACTION POTENTIAL AT NERVE TERMINAL AND THE RELEASE OF ACETYLCHOLINE
The basic property of of muscle and nerve is excitability i.e the ability to respond to a stimulus the membranes of excitable tissues are generally polarized at the resting stage Accumulation of similar charges is known as polarization there is accumulation of +ve charge on the out side of the membrane and -ve charge on the inside of the membrane the potential difference is generally -70mV this difference is due to ions sodium (Na+) ions are accumulated more outside whereas potassium(K+) Ions are accumulated more inside the membrane sodium ions are actively transported to outside by an active transport mechanism this active transport involves a carrier sodium potassium ATP pase (Na+K+ATPase) pump
Whenever the stimulus is applied a sequence of changes in potential difference occurs resulting in ionic changes within the cell these changes occurs in perfect sequence and quantity and is termed as action potential for any action potential to occur the stimulus should be sufficient to bring the resting potential to threshold potential once the threshold potential is achieved the remaining event within the cell occurs on its own (spontaneous activity ) the first change that occurs is the rapid entry (influx) of Na+ ions inside the cell resulting in increase in positive charges inside the cell this makes the potential difference less and less negative and even shooting the potential difference toward positive side this phase is termed as depolarisation phase at the end of depolarisation phase the Na+ influx stops but the excess Na+ ions inside will start exiting outside through a channel which allows a corresponding entry of Ca++ inside thus Na+/Ca++ exchange takes place the potential difference is still maintained during this phase this phase is termed as plateau phase once sufficient Ca++ ion concentration is achieved inside the cell the Ca++ entry stops and exit of K+ starts which produces net loss of positive charges the potential difference thus becomes more and more negative till the resting potential is reached this phase is termed as repolarisation in many cells the potential difference can become more negative than resting phase resulting in hyperpolarised phase later the ionic disturbances caused by the action potential is brought to normal by the Na+ /k+ ATPase pump which throws sodium ions out of the cell and bring in K+ ions the transient increase in ca++ ions during the action potential induces the vesicles of acetylcholine to fuse with the nerve ending the release of acetylcholine them occurs by the process of exocytosis
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