Nerve Lecture 5 : local response , Action-Potential-Propagation, biphasic action potential.pptx

local response , Action-Potential-Propagation, biphasic action potential.

1. Electrotonic potentials and local response § The usual means for exciting a nerve in the experiment is to apply electricity to the nerve surface through two small electrodes, one of which is negatively charged (cathode) and the other positively charged (anode). § When this is done, the excitable membrane becomes stimulated at the negative electrode. § Although sub-threshold stimuli do not produce an action potential they produce _electrotonic potentials._ It is a slight change in the membrane potential that _do not propagate._

2. Electrotonus: It means the electrical and excitability changes which occur in the nerve membrane due to its stimulation by a _constant galvanic current_ of _sub- threshold intensity._ Types:

3. v Anelectrotonus: • It means the changes which occur at the region of the anode. The _RMP increases_ by addition of more +ve charges on the outer surface of the membrane i.e. localized area of _hyperpolarization._ • It is associated with decreased excitability. So, stronger stimuli (more than threshold) are needed to excite the nerve fibers. _Strong anelectrotonus can abolish completely the excitability and can cause nerve block (= anodal block)._

4. v Catelectrotonus: • It means the changes which occur at the region of the _cathode._ • The _RMP decreases_ by addition of negative charges to the outer surface of the membrane i.e. localized area of _passive depolarization._ The depolarization is less than 7mv. • It is associated with increased excitability. So, weaker stimuli (sub- threshold) can excite the nerve fibers. Figure 36: The electrical response for both cathode and anode.

5. Electrotonic Potentials - Anelectrotonus Hyperpol. ↓ Excitability >> nerve block. - catelectrotonus Depol. (passive) (below 7 mv) ↑ Excitability

6. Local Response (local excitatory change): • With stronger cathodal sub-threshold stimuli, slight _active_ changes occur and some Na+ activation gates will open and contribute to the depolarizing process, but the stimulus open a few Na+ channels not enough to reach threshold level to produce AP. • Thus, repolarization follows rapidly and the membrane potential returns to the resting level. • It is associated with increased excitability because the nerve fibers can respond to sub-minimal stimuli applied at the same time.

7. It differs from AP: § It is localized to the site of stimulation and nearby area. § During conduction, it decreases gradually with distance till it disappears (conducted with decrement), “it does not propagate”.

8. Strength of the stimulus above the threshold is coded as the frequency of action potentials

9. § Can be graded: its magnitude is proportional with the strength of the sub- minimal stimulus i.e. it does not obey the all or non- law. Also, it has no threshold. § It has no refractory period, so, it can be summated to the local excitatory state of other sub-minimal stimuli.

10. Graded potential has different names according to location Neuron cell body and dendrites Excitatory post synaptic potential (EPSP) Inhibitory post synaptic potential (IPSP) Motor end plate  End plate potential Receptor  Receptor potential Pace maker potential in GIT smooth muscle & heart Slow wave potential

11. Summation of graded potential Graded potentials occurs at soma & dendrites & travel through the neuron and they sum up and if reach a threshold level at trigger zone they can fire action potential.

12. Initiation of action potential To initiate an AP a triggering event causes the membrane to depolarize from the resting potential of -90 mvs to a threshold of-65 to – 55 mvs . At threshold explosive depolarization occurs. (positive feed back )

13. Action Potential Propagation

14. Propagation (Conduction) of the Action Potential Action potential involves only a small portion of the total excitable cell membrane; it does not remain stationary at the point of stimulation. The action potential must be propagated in order to transfer information from one place in the nervous system to another site. It conducted, or propagated throughout the entire membrane in non- decremental fashion, by regenerating itself along the axon. Once an action potential is initiated at the axon hillock, no further triggering event is necessary to activate the remainder of the nerve fiber. The magnitude of action potential does not change as it is conducted along the axon.

15. Velocity of Propagation Velocity of propagation of action potential depends upon: Myelination: Myelinated fibers conduct action potential faster than un- myelinated fibers. Diameter of nerve fiber: It is proportional to square root of the fiber diameter. The larger the fiber diameter, the faster action potentials can be propagated due to decrease resistance to local current. Arrival of action potential to terminal end of nerve fiber causes release of chemical transmitter. Figure 32: Propagation of AP.

16. Propagation in Un-myelinated Nerve Fiber Action potential originated at one location on the axon, acts as a stimulus for production of a new action potential on the adjacent regions.

17. Mechanism of Contiguous Propagation During the reversal of polarity (over shoot) of the action potential, an electrical potential difference is generated between the active area and adjacent resting area. Active area has negative outside and positive charges inside (depolarized) while adjacent area still in polarized state has positive charges outside and negative charges inside. Positive charges flow passively (attracted) to the area of negativity on both outer and inner surfaces of the membrane and across the cell membrane to complete a closed loop of current flow (local passive circuit). The adjacent area becomes depolarized. If the adjacent area reaches threshold level a new action potential is generated, while the active area returns to its resting level. The depolarization of the membrane produced by the new action potential spreads passively and the entire process is repeated. The original action potential does not propagate along the nerve fiber, but rather results in the sequential generation of identical action potentials. The propagation of the action potential in un-myelinated nerve fibers is called contiguous propagation (means touching or next to in sequence). Every part of nerve fiber undergoes depolarization, slow speed of impulse conduction and more energy consumption.

18. Propagation in Myelinated Nerve Fiber Myelinated regions of axon are electrically insulated and action potentials occur only at un-myelinated regions; nodes of Ranvier. Figure 34: Myelinated nerve fiber Propagation of action potential in myelinated axons is the same as in un- myelinated axons but it conducts impulses faster than un-myelinated. 1 The action potentials are generated only at the nodes. So it becomes the stimulus for the generation of an action potential at the adjacent node. 2 Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction . 3 The speed of propagation in myelinated nerve fiber is proportionated to the diameter of the axon. As the diameter of the axon increases the inter- nodal distance increases. Figure 35: The propagation of AP in myelinated nerve fiber.

19. Importance of Saltatory Conduction 1 Increased Velocity It increases the velocity of conduction of the nerve impulse up to 50 fold. 2 Energy Conservation It conserves energy for the axon, the energy needed for the Na+-K+ pump restricted to the nodes of Ranvier. Myelinated fibers use about 1% of the energy used by un-myelinated fibers.

20. Types of action potential

21. Types of Action Potential Monophasic AP: 2 microelectrodes : 1 on the outer surface (indifferent), 1 dipped inside the axon (recording). (connected to CRO ). Biphasic AP: 2 recording microelectrodes on the outer or inner surface of the axon. Compound AP: 2 recording microelectrodes inside a nerve trunk (many nerve fibers with different velocities of conduction).

22. Biphasic action potential Monophasic action potential

23. Biphasic action potential It is measured by putting the two microelectrodes on the outer surface. As the depolarization wave reaches the electrode near the stimulator , the electrode becomes negative relative to the other electrode & a downward deflection is recorded . When the wave passes to the part of the nerve fiber between the two electrodes , the potential difference returns to zero .

24. When the wave reaches the second electrode , it becomes negative relative to the first electrode & an upward deflection is recorded . When the depolarization wave leaves the second electrode , the potential difference returns again to zero. Therefore , the record shows a downward deflection followed by an isoelectric interval & then an upward deflection .

25. Biphasic action potential Depolarization In the nearby electrode Depolarization In the distant electrode Voltage Duration

26. Compound action potential: -In nerve trunk….(different diameters, threshold of stimulation, velocities)

27. nerve conduction velocity is an application of diphasic action potential

28. Types of Nerve Fibers: A Fibers Diameter The fiber of this group has a diameter between 2-20 microns. Conduction Rate Their rate of conduction ranges from 20-120 m/sec. Subdivisions The fibers are subdivided into alpha (a), beta (b), gamma (g) and delta (d) fibers.

29. Types of Nerve Fibers: B and C Fibers B fibers: The fibers of this group have a diameter between 1-5 microns. Their rate of conduction ranges from 5-15m/sec. The preganglionic autonomic nerve fibers belong to this group. C fibers: The fibers of this group have a diameter less than one micron. Their rate of conduction ranges from 0.5-2 m/sec. The postganglionic autonomic nerve fibers belong to this group.