Action potential

7 July 2016

The ionic events consists of four main stages which are resting potential, repolarization, depolarization and hyperpolarization. The resting potential is due to the sodium potassium pump where 3 Na+ ion move outside the membrane and 2 K+ ion move into the cell ( and this causes the negative potential inside the cell membrane. Besides that, the potassium sodium “leak channel” which is more permeable to K+ than Na+ also lead to the resting membrane potential. Since, potassium is more permeable, the K+ ion will move out from the cell rather than Na+ enter the cell. This also will lead to negative membrane potential or the membrane is said to be polarized (-70 to -90 mV).

This, negative potential exist mainly because the movement of positive ion outside the cell due to the effect of sodium potassium pump and potassium sodium “leak channel” and the remaining of the large negative protein molecule inside the cell. This negative potential is the resting membrane potential of the cell. This is the resting membrane potential before the action potential begins. In response to a depolarizing stimulus, some of the voltage-gated Na+ channels become active and membrane potential become less negative until it reaches threshold potential. Any stimulus if strong enough to reach the threshold potential will lead to depolarization stage. When the threshold potential is reached, more voltage-gated Na+ channels are open and membrane suddenly becomes very permeable to Na+, allowing tremendous numbers of positively charged Na+ to diffuse to the interior of the axon. More sodium ions move into the nerve cells via voltage-gated Na channel (Resting to activated state). This will lead to membrane potential become more positive or less negative.

The membrane potential rising rapidly in the positive direction. This is called depolarization. Within a few millisecond after the membrane becomes highly permeable to Na+, the Na+ channels rapidly enter a closed state (inactivated state) and k+ channel will enter its active state. Opening of voltage-gated K+ channels causes rapid diffusion of K+ to the exterior re-establishes the normal negative resting membrane potential. The repolarization stage is when the voltage-gated K channel open. Potassium ion move out from the nerve cells. This will establish normal negative resting membrane potential. Finally, slow return of potassium ion gated channel to the closed state, causing an excessive efflux of potassium ions. This efflux causes hyperpolarization of the membrane. Hyperpolarisation ensure the action potential move in one direction as it create a refractory period for the membrane. Than the membrane potential will return to the resting membrane potential.

Describe the steps in neuromuscular transmission

An action potential is initiated and propagates along motor neuron to the presynaptic terminals. The presynaptic terminal is depolarized causes voltage gated. Ca2+ channels in the presynaptic terminal to open. Ca2+ enter into the terminal causes the synaptic vesicles to fuse with the presynaptic membrane, resulting in the release of Acetylcholine (Ach) into the synaptic cleft by exocytosis. Ach diffuses across the synaptic cleft and binds to nicotinic receptors on the motor end plate. This binding cause the ligand-gated channel open and the flow of ions occurs.

The Na+ flow into the cell and K+ flow out of the cell.Depolarization accour bcz more na+ move in than K+ move out. This flow of ions generate the end plate potential (EPP). When EPP had achieved certain membrane potential, the voltage-gated sodium channel open and cause the flow of Na+ into the cell. Muscle action potential is generated. Generation of EPP, local currents depolarize the adjacent muscle cell membrane to its threshold potential Generates an AP that propagates over the muscle fiber surface and into the fiber along t-tubules. This couses the sarcoplasmic reticulam to release ca2+ in which initaite the muscle contraction. Results in muscle contraction

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