ACTION POTENTIAL pathophysiology by aashish.pptx

action potential description

1. ACTION POTENTIAL AASHISH GHO SHRESTHA MPT 1 st year

2. CONTENT Membrane potential Resting membrane potential Action potential Stages

3. MEMBRANE POTENTIAL Potential is defined as the distribution of the electric charges across the cell. The electrical state of a cell membrane is known as membrane potential. It is measured in mV (milli volts) The membrane potential is present due to the difference in concentration of ions present inside and outside the cell.

4. continues… The concentration of ions in extra cellular and intra cellular fluid is largely balanced with a net neutral charge. Slight difference in charge will occurs at the membrane surface both internally and externally.

5. RESTING MEMBRANE POTENTIAL Generally, there are sodium ions, calcium ions and chlorine ions in the extracellular fluid whereas there is presence of potassium ions and negatively charged anion inside the cell. So, if this is overviewed, the distribution of the ions provides the information that the cell is negatively charged (compared to the outside of the cell) and the net negative charge being close to -70mV. Hence, the potential during rest or steady state of a cell is called Resting Membrane Potential.

6. During rest all the ion channels are closed. The cystole contains a high concentration of anions(- ve ), in the form of phosphate ions and negatively charged proteins. According to the various cell, it varies. During rest all the gated channels are closed, but the resting membrane potential is maintained by the Leakage channel/ligand gated channels. NOTE: Ligand gated channel is the gated-channel which responds to the ligand and in this case neurotransmitter is the ligand.

7. ACTION POTENTIAL Action potential is defined as the series of electrical changes that occurs in the membrane potential when a nerve or muscle is stimulated. It can also be defined as the electrical signal that races down the axon at the rate of 100m/s. It starts with the opening of gated channels.

8. ACTION POTENTIAL

9. Series of action potential Stimulus Depolarization Repolarization Hyperpolarization Resting stage

10. Stimulus: The exchange of the charged ions in the junction between the dendrites leads to conversion of the chemical signals to the electrical signals. Multiple dendrites get the input from the surrounding leading to change in the charge of the cell and hence it triggers the action potential. The change in the charge is the stimulus which will cause rapid changes in the potential. Strength of the stimulus is very important to initiate an action potential. NOTE: Neuron uses neurotransmitter as signal to communicate with one another and they use action potential to propagate signal within the axon

11. Depolarization It is caused due to rapid rise in membrane potential. Due to the opening of ligand gated channels in the cell membrane, there is influx and exit of the positive ions. Na+ and Ca++ are the positively charged ion leading to change the voltage immediately inside the cell relative to outside.(the exit of potassium ions also occurs) Hence there is net gaining of the positive ions and this phenomenon is known as depolarization.

12. Release of the neurotransmitter ↓ Binds to the receptors in the dendrites ↓ Opening of the ligand gated ion channel( eg. Ligand gated sodium channel) ↓ Entry of the ions( eg Na ions) ↓ Cell becomes less negative (depolarized)

13. Hence at the end if the net influx of positive charge is greater then it is called an Excitatory Post-synaptic potential and it causes the increase in the potential and it may reach threshold value(-55mV). Threshold value will activate the voltage gated sodium channel at the starting of the axon, called “ axon hillock.” This procedure continues throughout the length of the axon, and the change in the charges can be called as the firing of the neuron. Note: voltage gated channel is the one which opens with the change in the voltage in the cell The membrane potential reaches about +30mV, the the voltage gated sodium channel goes in the inactivation phase but activating the inhibition gate.

14. Repolarization There is opening of the voltage gated K+ channels after the inactivation of the voltage gated sodium channel so that the K+ ions move outside the cell, taking positive charge with it. It responds slowly to the change in the voltage. The membrane potential moves back towards its resting voltage as it stays open for longer period and this process is known as repolarization, i.e. membrane potential begins to move back towards the -70mV resting voltage.

15. Hyperpolarization During repolarization, the cell relays on the sodium potassium pump which carries 3 sodium ions outside the cell and carries 2 potassium ions inside the cell. This as a result causes the further functioning of the sodium potassium pump and voltage gated potassium channel. But it actually overshoots the K+ ions to reach equilibrium potential (when the membrane voltage is below -70mV) ,so hyperpolarization occurs while K+ channels are open. Those K+ channels are slightly delayed in closing, accounting for this short overshoot.

16. Hyperpolarization is the lowered membrane potential caused by the efflux of K+ ions and closing of the K channels.

17. Resting stage When the membrane potential returns to the resting voltage that occurred before the stimulus comes.

18. Propagation of AP There is presence of the myelin sheath at the different place of the axon. The myelin sheath are the fatty myelin formed of Schwann cell or oligodendrocytes. They works as insulator since they do not have the voltage gated channels. So the ions cannot simply flow from the cell and occur only in the space between them called node of Ranvier. Hence charges jumps from one NOR to another NOR. The charges inside the myelin sheath moves really fast and hence is called saltatory conduction.