Physiology of Excitable tissue L 2 Stimulation AP

Physiology of Excitable tissue L 2 Stimulation AP, Nerve signal & Impulse Prof. Fakhir Al-Ani fakeralani 2000@yahoo. com

2. Stimulation Excitable Tissue or (membrane) can be stimulated by: 1. Electrical stimuli. 2. Chemical stimuli. 3. Mechanical stimuli. 4. Thermal stimuli. The best is the electrical: 1. Similar to the natural stimulus. 2. Can be easily started, controlled & ended. 3. Dose not harm the nerve.

Responses to Stimulation Nerve Impulse Nerve impulse: Changes that occurs in the excitable cell & membrane. (Electrical, Mechanical, Thermal, Chemical) One of the changes on nerve stimulation is the electrical changes

Nerve signals, Impulse & Action potential Nerve signals: Information are transmitted by Nerve Impulses: These are the changes that occur in the nerve that lead to the transmission of signals. They include Electrical, Chemical, Thermal, …. Action Potential: The electrical changes in the nerve that occur during transmission of signal.

Nerve signals, Impulse & Action potential Nerve signal Information . lse er pu Th Im , Ch e, El Ele= AP

Electrical changes = A. P. The electrical changes in all the excitable membrane is of the same principle Whether the membrane is related to a N. or M. But The difference may be in the magnitude

Information (Nerve signals) are transmitted by A. P. Action Potential: It is a rapid changes in the membrane potential that spread along the membrane of the nerve fiber. Or: It is a wave of negativity that travels rapidly along the surface of the membrane. From interior it begins with sudden change from normal resting negative membrane potential to a positive potential & then back to negative Within 1/10, 000 of a second

Stages of A. P. 1. Resting stage. 2. Depolarization Stage. 3. Repolarization Stage.

1. Resting Stage It is the R. M. P before the A. P. begins. RMP in nerve is ranging between (-70 to -90 m. V) The membrane is said to be polarized during resting stage The Interior is negative & the exterior is Positive Na channels are Closed

2. Depolarization Stage Started by opening of voltage gated Na channels So The membrane suddenly becomes very permeable to Na+. So Na+ influxes So Change the normal "polarized" state (-90 m. V) to a neutralized & even overshoot to a positive value (+35 m. V) as in large n. f. In some smaller fibers & many central nervous system neurons, the potential merely approaches the zero level & does not overshoot to the positive state.

Mechanism of Na channels opening As the R. M. P reaches a value between (-55 m. V firing level) Conformational changes cause opening of the get So Na+ permeability 500 -5000 X

Voltage gated Sodium channels The channel has two gates: 1. Activation gate: outside. 2. Inactivation gate: inside. RMP = -70 to - 90 m. V. Firing level = – 50 m. V. 10. 000 of a sec. Repolarizing Not opened till reach resting

Repolarization Stage Within a few 10, 000 of a sec. , after depolarization, Na channels begin to close & K channels open more than normal. Rapid diffusion of K ions to the exterior through the voltage gated K channels Re-establishes the normal negative resting membrane potential (-90 m. V).

Stages of Action Potential 1. Resting stage. 2. Depolarization Stage. 3. Repolarization Stage.

Action Potential Spike Depolarization Na+ Influx by Chemical & Elec. gradient Firing level- 55 m. V. R. M. P. (-90 m. V. ) Repolarization K+ Efflux due to Electrical gradient R. M. P. Hyperpolarization

Channels that play a role in A. P. 1. Voltage gated Na channels: Depolarization & repolarization. 2. Voltage gated K channels: Fasten membrane repolarization. 3. Na+-K+ pump: Repolarization. 4. Na+-K+ leakage channels. Depolarization & repolarization.

Voltage gated K+ channels During rest: Channels are closed. On excitation: As membrane potential change from – 90 m. V to Zero K voltage changes are opened. Due to the delay in opening of K channels they opens at the beginning of closer of Na channels.

Action Potential. A. P. : Rapid changes in membrane potential that spread along the membrane of the nerve fiber. Positive Negative then Positive again Within 1/10, 000 of a sec. It is a wave of negativity that travels rapidly along the surface of the membrane.

3. Threshold It is the minimum intensity of the stimulus needed to excite the tissue The intensity that shift the R. M. P. to the firing level (Intensity needed to open Na channels) High excitable need lower intensity to be excited so has low threshold value

Threshold for Initiation of the A. P. A sudden rise in membrane potential of 15 - 30 m. V usually cause opening of Na channels So membrane potential in a large nerve changed from -90 m. V up to about -55 m. V Causing opening of Na channels 15 to 30 m. V is said to be threshold for stimulation. But A. P. will not occur until there is vicious cycle. When large number of Na+ entering the fiber becomes greater than the number of K+ ions leaving the fiber.

Threshold

Roles of other ions during A. P. Anion = negatively charged ions 1. Protein. 2. Organic phosphate compound. 3. Organic sulfate compound. Protein can not leave the excitable tissue So It is responsible for the negativ charge inside the excitable tissue.

Roles of other ions during A. P. Calcium ions (Ca++): Calcium action is nearly similar to that of sodium. There is Calcium pump which push Ca ions from inside to outside. So Ca++ conce. outside is about 10 -3 while Ca concentration inside is about 10 -7 There is voltage gated Ca++ channels which are also permeable to sodium so called Ca++ - Na + channels

Calcium ions Calcium channels are present in: - Cardiac muscle. - Smooth muscle. - Neuro-muscular Junction (NMJ). - Skeletal muscle sarcoplasmic reticulum. In some types of smooth muscle & Pacemaker cells The fast Na channels are hardly present So that the A. P. are caused almost entirely by activation of slow Ca++ channels.

How Ca++ affect the Na channels Ca ++ appear to bind to the surfaces of the Na channel protein molecule. The positive charges of these Ca++ in turn alter the electrical state of the channel protein itself. So altering the voltage level required to open the sodium gate

4. Site of excitation Excitation occurs under the cathode ? When does the Na channel opened? + +++++++++ -------------

Local Change Produced by impulse < threshold can not be propagated Ø They are passive changes: on membrane surface. Occurs by: adding/ subtraction of charge on the surface. Ø These changes are disappeared spontaneously: by rearrangement of electrolyte on membrane surface. Ø They are of two types: - Cat-electro-tonic : Under the cathode. - An-electro-tonic : Under the anode.

Local Change Cat- Electrotonic Potentials Shift the membrane Potential toward the firing level (Less value) so induces A. P. An- Electrotonic Potentials Shift the membrane Potential away from the firing level (More value) So can not induces A. P.

Local response Electro-tonic Potentials An-electrotonic potential Firing level + +++++++++ ------------- -55 -60 -65 -70 m. V Cat-electrotonic Potential