Question

Describe, in detail, an action potential including which ions open and when, and at what point the action potential is generated. Include the various stages of the action potential as well (ex. refractory periods)

Answer 1

ANSWER -

ACTION POTENTIAL is an electrical signal that propagates along the surface of the neuronal membrane, due to movement of ions (sodium and potassium ) between the interstitial fluids and inside of the neuron , through specific ion channels in its plasma membrane .

An electrical potential difference across the membrane is known as Membrane potential .

Resting membrane potential (RMP) develops because of the presence of negative ions in the cytosol along the inside of the membrane, and an equal build up of positive ions in the extra cellular fluid.

RMP in neurons ranges between - 40 to  - 90 mV ( Typically -70mV ) . A cell exhibiting membrane potential is said to be polarised .

Generation of Action Potential -

Action potential is a sequence of rapidly occuring events that decreases and rfeverses the membrane potential , eventually restoring the resting state .

2 main phases -

1. Depolarizing phase
2. Repolarizing phase

DEPOLARIZING PHASE - When the membrane potential of axon reaches threshold , the Na+ channel activation gates open. As Na+ ions move through these channels into the neuron, a build up of positive charge is formed along inside of the membrane .and the membrane becomes depolarized .Inward movement of Na+ changes membrane potential from -55 mV to + 30 mV.

REPOLARIZING PHASE -

Pottasium (K+) channels are open , K+ outflow slowing of Na+ inflow .Acceleration of K+outflow causes membrane potential to change from +30 mV to -70 mV.

After Hyperpolarizing phase -

Voltage gated K+ channels open

Membrane potential becomes even more negative ( - 90 mV)

As the voltage gated K+ channels close , the membrane potential returns to a resting level of -70 mV.

REFRACTORY PERIOD- The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus.

• Absolute refractory period- even a very strong potential cannot initiate second action potential .
• Relative refractory period- second action potential can be initiated, but only by a larger than normal stimulus.