LEC 4,Properties of action potential - Copy.pptx

Properties of action potential

1. Effect of ions on Action Potential & Properties of Action Potential

2. Roles of Other Ions During the Action Potential Sodium and potassium ions do play vital role in generating the action potential Other types of ions, negative anions, calcium & potassium ions must be considered as well

3. Impermeant Negatively Charged Ions (Anions) Inside the Nerve Axon Inside the axon are many negatively charged ions that cannot go through the membrane channels ( protein molecules, organic phosphate compounds & sulfate compounds). These negatively charged maintain negativity iniside the cell

4. Calcium ions Calcium ion concentration is more (10,000 times) in the extracellular fluid than in the intracellular fluid. Tremendous diffusion gradient and electrochemical driving force for the passive flow of calcium ions into the cells Cells do have Calcium pump which pumps calcium ions out of cell Some cells (cardiac muscle and smooth muscle) have Voltage-gated calcium channels (slow channels) When the channels open in response to a stimulus, calcium ions flow to the interior of the cell cause sustained depolarization .

5. Role of calcium in membrane stability Calcium (normal levels) stabilizes the membrane: Inner side of sodium channels is highly negatively charged. calcium ions appear to bind to the exterior surfaces of the voltage gated sodium channel protein. Positive charges of these calcium ions, alter the electrical state of the sodium channel protein → altering the voltage level required to open the sodium gate. The concentration of calcium ions in the extracellular fluid also has a profound effect on the voltage level at which the sodium channels become activated

6. Normal levels of calcium in plasma Total calcium -2.4 mmol /L (9.4mg/dl)

7. Hypocalcemia Increased Permeability of the Sodium Channels When There Is a Deficit of Calcium Ions When there is a deficit of calcium ions in ECF, the sodium channels become activated by very little increase of the membrane potential from its normal level. Only 50% below normal value of calcium → nerve fiber becomes highly excitable, sometimes discharging repetitively without provocation → muscle “ tetany”

8. Tetany (pathophysiology) Decreased calcium in ECF ↓ No complete closure of activation gates of sodium channels at rest ↓ Sodium ions leak into membrane from ECF ↓ Membrane potential becomes less negative & near to threshold ↓ On slight stimulation action potential (tetany).

9. Effect of hyperventilation Hyperventilation ↓ Alkalosis ↓ Calcium Binds with proteins ↓ Decreased ionized Calcium ↓ Hypocalcemia ↓ Tetany

10. Inhibition of Excitability—Stabilizers and Local Anesthetics Local Anesthetics : procaine and tetracaine Mechanism of action: act directly on the activation gates of the sodium channels, making it much more difficult for these gates to open and thereby reducing membrane excitability. Nerve impulses fail to pass along the anesthetized nerve

11. Hypercalcemia can decrease excitability (high extracellular fluid calcium ion concentration decreases membrane permeability to sodium ions and simultaneously reduces excitability. Hypercalcemia ↓ Nervous system depression, cardiac arrhythmias, lethargy.

12. Effect of potassium on membrane excitability

13. Effect of hypokalemia Hypokalemia : decrease in plasma potassium level below 3.5 mmol /L. Hypokalemia ↓ RMP more – ve ↓ Membrane hyperpolarized ↓ Less excitable.

14. Effect of hyperkalemia Hyperkalemia: serum potassium concentration greater than approximately 5.0-5.5 mEq /L Hyperkalemia → RMP less – ve → initially membrane hypopolarized and is excitable, if more and more hyperkalemia → loss of RMP or intracellular negativity → membrane not excitable → death.

15. Properties of Action potential Sudden abrupt onset Limited amplitude (AP from -90 to+ 35-40 mv) Short duration 0.2- 3.0 msec in nerve fiber 2-5 msec in skeletal muscle, longer in cardiac (200 to 300 msec )and even longer in smooth muscle fibers Self propagative Obeys all or none law : action potential is produced with its maximum amplitude, if subthreshold stimulus not produced at all Has refractory period . (when there wont be response to 2nd stimulus).

16. Stimulus A sudden change of the (internal or external) environmental condition of the cell. Includes chemical, mechanical stimulus. inside the body. The electrical stimulus is often used for the physiological Lab and research. outside the body Threshold (intensity): the lowest or minimal intensity of stimulus to elicit an action potential. Threshold stimulus Subthreshold stimulus Suprathreshold stimulus :

17. What is local/graded potential? Occurs in small, specialized region of excitable cell membranes Magnitude of graded potential varies directly with the magnitude of the triggering event The Stronger a triggering event, the larger the resultant graded potential. Graded Potential spread by passive Current flow. Graded potentials die over short distances Decremental : gradually decreases If strong enough, graded potentials trigger action potentials.

18. Examples of graded potential Motor end plate Receptor potential Excitatory post synaptic potential Inhibitory post synaptic potential

19. Refractory period Absolute refractory period: Tissue does not respond to 2nd stimulus what ever the strength of stimulus During depolarization & first 2/3rd of repolarization Due to inactivation of sodium channels. (closure of inactivation gates of Na channels, will not open till potential reaches resting value.) Relative refractory period : Tissue may respond to 2nd stimulus if it is of larger intensity First 2/3rd of repolarization to the beginning of after depolarization.

20. All-or-None Principle If a stimulus is threshold or suprathreshold, action potential is produced with its maximum amplitude, if subthreshold stimulus not produced at all

21. Properties of Nerve Fibers Conductivity Excitability All or None law Refractory period Infatiguability Summation

22. Conductivity Conductivity is the ability of nerve fibers to transmit the impulse from the area of stimulation to the other areas Action potential is transmitted through the nerve fiber as nerve impulse

23. Excitability Property of Nerve Fiber due to which it respond to stimulus by generating nerve signal. Stimulus – Mechanical, Electrical, Chemical or Thermal

24. Strength Duration Curve A graph between electrical stimuli of different intensities and recording the time needed by each stimulus to start the response

25. Rheobase Minimum intensity of stimulus applied for adequate time produces response

26. Utilization Time Minimum time that a current equal to rheobase must act to induce an AP is called utilization time Stimulus having current strength less than rheobase will not reach threshold value even if applied for unlimited time

27. Chronaxie Minimum duration for which intensity of double the Rheobase should applied to produce response Nerve fibers have shorter chronaxie than muscle tissue means they are more excitable

28. Significance of Strength Duration Curve Chronaxie is an important parameter to determine the condition of nerve fiber Clinically, the damage of nerve fiber is determined by measuring the chronaxie Lesser is the chronaxie more is the excitability Nerve fibers have a low threshold for excitation than the other cells

29. Infatigability Nerve fiber cannot be fatigued, even if it is stimulated continuously for a long time. The reason is that nerve fiber can conduct only one action potential at a time . At that time, it is completely refractory and does not conduct another action potential

30. Summation When one subliminal(weak) stimulus is applied, it does not produce any response in the nerve fiber if two or more subliminal stimuli are applied within a short interval of about 0.5 millisecond, the response is produced. The subliminal stimuli are summed up together to become strong enough to produce the response

31. Compound Action Potential It is recording of AP from mixed nerve. Mixed Nerve – which contains different types of nerve fibers with different diameters So it’s the algebraic summation of AP of many axons