Potensi tindakan fisiologi

Potensi tindakan, sebentar (kira-kira seperseribu saat) pembalikan polarisasi elektrik membran sel saraf (neuron) atau sel otot. Di neuron, potensi tindakan menghasilkan impuls saraf, dan dalam sel otot menghasilkan pengecutan yang diperlukan untuk semua pergerakan. Kadang-kadang disebut potensi disebarkan kerana gelombang pengujaan secara aktif disebarkan di sepanjang saraf atau serat otot, potensi tindakan dilakukan pada kecepatan antara 1 hingga 100 meter (3 hingga 300 kaki) sesaat, bergantung pada sifat serat dan persekitarannya.

sistem saraf: Potensi tindakan

Oleh kerana ia berbeza-beza dalam amplitudo, potensi lokal dikatakan dinilai. Semakin besar kemasukan muatan positif - dan, akibatnya, depolarisasi

Sebelum rangsangan, sel neuron atau otot mempunyai polarisasi elektrik yang sedikit negatif; iaitu bahagian dalamnya mempunyai muatan negatif berbanding dengan cecair ekstraselular. Keadaan terpolarisasi ini dihasilkan oleh kepekatan ion natrium bermuatan positif di luar sel dan kepekatan tinggi ion klorida bermuatan negatif (serta kepekatan kalium bermuatan positif yang lebih rendah) di dalam. Potensi rehat yang dihasilkan biasanya mengukur sekitar −75 milivolt (mV), atau −0.075 volt, tanda tolak yang menunjukkan cas negatif di dalamnya.

In the generation of the action potential, stimulation of the cell by neurotransmitters or by sensory receptor cells partially opens channel-shaped protein molecules in the membrane. Sodium diffuses into the cell, shifting that part of the membrane toward a less-negative polarization. If this local potential reaches a critical state called the threshold potential (measuring about −60 mV), then sodium channels open completely. Sodium floods that part of the cell, which instantly depolarizes to an action potential of about +55 mV. Depolarization activates sodium channels in adjacent parts of the membrane, so that the impulse moves along the fibre.

If the entry of sodium into the fibre were not balanced by the exit of another ion of positive charge, an action potential could not decline from its peak value and return to the resting potential. The declining phase of the action potential is caused by the closing of sodium channels and the opening of potassium channels, which allows a charge approximately equal to that brought into the cell to leave in the form of potassium ions. Subsequently, protein transport molecules pump sodium ions out of the cell and potassium ions in. This restores the original ion concentrations and readies the cell for a new action potential.

The Nobel Prize for Physiology or Medicine was awarded in 1963 to Sir A.L. Hodgkin, Sir A.F. Huxley, and Sir John Eccles for formulating these ionic mechanisms involved in nerve cell activity.