Physiology of NERVE IMPULSE and action potential

These slides explain about nerve impulse generation and action potential

1. Physiology of NERVE IMPULSE By Ms . Keerti H Dept of OBG Nursing

2. NERVE IMPULSE/ ACTION POTENTIAL A stimulus is a change in the environment with sufficient strength to initiate a response . The nerve impulse causes a movement of ions across the cell membrane of the nerve cell. Excitability is the ability of a neuron to respond to the stimulus and convert it into a nerve impulse. When a nerve is stimulated, the resting potential changes . Examples of such stimuli are pressure, electricity, and chemicals, etc.

3. Different neurons are sensitive to different stimuli (Although most can register pain). The stimulus causes sodium ion channels to open. The rapid change in polarity that moves along the nerve fiber is called the "action potential.'

4. Resting potential: * Resting potential may be defined as when there is variance in voltage between the inside and outside of the cell as measured across the cell membrane. * When a neuron is not being stimulated, it maintains a resting potential Ranges from -40 to -90 millivolts (mV ) Average about -70 mV.

5. Two major forces act on ions in starting the resting M embrane is potential : * Electrical potential produced by unequal distribution of charges. * Concentration gradient produced by unequal concentrations of molecules from one side of the membrane to the other.

6. Action Potential : A neuron receives information from other neuron through a chemical action called a N eurotransmitter . If this input is strong, the neuron forwards the signal to downstream neurons . The transmission of a signal in the neuron (In one direction, from the dendrite to the axon terminal) occurs through the opening and closing of voltage-controlled ion channels that cause a brief inversion of the membrane in forming an A ction potential.

7. When an action potential transfer down the axon, P olarity modifications occurs across the membrane . Once the signal reaches the axon terminal , it stimulates other neurons.

8. Steps of the formation of an action potential : Step 1- A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential Step 2- If the threshold of excitation is reached all Na+ channels open and the membrane depolarizes.

9. Step 3 : At the peak action potential, K+ channels are opened and K-+ begins to leave the cell. At the same time, Nat channels are closed . Step 4 : The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire Step 5 : The K+ channels close and the Na+/K+ transporter restores the resting potential.

10. Phases of action potential: From the aspect of ions, an action potential is caused by temporary changes in membrane permeability for diffusible ions. These changes cause ion channels to open and the ions to decrease their concentration gradients . The value of threshold potential depends membrane permeability, intra- and extracellular concentration of ions, and the properties of the cell membrane.

11. An action potential comprised of six phases: 1. Stimulus and Hypopolarization . 2. Depolarization. 3. Repolarization. 4. Hyperpolarization. 5. Resting state.

12. 1. Stimulus and Hypo-polarization : Stimulus starts the rapid change in voltage or action potential. In patch- clamp mode, sufficient current must be administered to the cell in order to raise the voltage above the threshold voltage to start membrane depolarization.

13. 2. Depolarization Phase : Depolarization is caused by a rapid rise in membrane potential opening of sodium channels in the cellular membrane, resulting in a large influx of sodium ions . During depolarization, the inside of the cell becomes more and more electropositive, until the potential gets closer the electro chemical equilibrium for sodium of +61 mV. This is called t he overshoot phase.

14. 3. Repolarization phase: After the overshoot, the sodium permeability suddenly decreases due to the closing of its channels. The overshoot value of the cell potential opens voltage-gated potassium channels, which causes a large potassium efflux, decreasing the cell's electro-positivity.

15. 4. Hyperpolarization phase: Hyperpolarization is a lowered membrane potential caused by the efflux of potassium ions and closing of the potassium channels.

16. 5. Resting state: Resting state is when membrane potential returns to the resting voltage that occurred before the stimulus occurred.