Action Potential Reading Material Physiology PDF

Objectives o Discuss the resting membrane potential and its genesis. o Know the ionic channels involved in resting membrane potential. o Describe the function Na+-K+ pump and the stages of action potential. o Explain the threshold Potential, local Respons...

Objectives o Discuss the resting membrane potential and its genesis. o Know the ionic channels involved in resting membrane potential. o Describe the function Na+-K+ pump and the stages of action potential. o Explain the threshold Potential, local Response and action Potentials. o Describe the electrical changes in membrane potential during the action potential, their chemical bases and excitability changes. o Describe conduction along nerve fibers, role of myelination and how nerve fibers are classified. 2  Resting Membrane Potential of Nerves (RMP): Transport properties of the resting nerve membrane for sodium and } RMP: it is the potential difference across potassium: the cell membrane during rest, without Sodium-Potassium pump (active transport) stimulation between the inner side and the K+ Leak Channels: outer side, and it is relatively –ve inside. } Measurement of RMP: using voltmeter } Normal Values : -70 in medium sized nerves and -90 mv in large nerve fibers. (inside the fiber is 90 times more negative) } During rest, the membrane is polarized (the membrane is a wall between the positive outside and negative inside) } There is high molecules of K+ inside the cell and high molecules of Na+ outside the cell. Potential: difference in charge across plasma membrane. Current: flow of charge (ions) from one point to another. 4  Causes (Origin) of Resting Membrane Potential (RMP): The important factors in the establishment of the normal resting membrane potential of -90 millivolts. } 2- Contribution of Na diffusion through the } 1- Contribution of K+ diffusion potential:- nerve membrane: The cell membrane has tendency to pump potassium (K) Very small amount of Na+ diffuses into the cell (from outside (positive charge) out, from high to low, (outflux), causing –ve to inside) down its concentration gradient. charge inside, through K leak channels, down its The membrane is only slightly permeable to Na+ through K- concentration gradient. Na leak channels. (producing energy like Niagara falls, from high to low which gives w energy to Canada) Result: Electro-positivity outside and electro-negativity inside. } 3- Contribution of the Na+-K+ pump: RMP is 100 times more permeable to K+ than Na+. This is a powerful electrogenic pump on the cell membrane. (These K+ leak channels may also leak sodium ions slightly maintains concentration gradients of K+ and Na+ between but are far more permeable to potassium than sodium) the two sides of the membrane. K diffusion contributes far more to resting It pumps 3 Na+ to outside & 2 K+ to inside, causing a net membrane potential. (most important) ‫رﻛز ﻋﻠﯾﮭﺎ اﻟدﻛﺗور‬ loss of +Ve ions from inside, returning the nerve fibre to the resting state (-4 mV). 5 Remember: Non-diffusible anions (proteins, sulphate, phosphate ions, large molecules) cannot leave the cell. Therefore, they also contribute to the negativity  Action Potential (AP): } Nerve signals are transmitted by action potentials, which are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane. Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and ends with an almost equally rapid change back to the negative potential. } Action Potential: a sudden reverse of membrane polarity (of charges) produced by a stimulus to produce a physiological effect such as: o Transmission of impulse along nerve fibers (transmission of nerve signals) o Release of neurotransmitters o Muscle contraction o Activation or inhibition of glandular secretion } Only Excitable tissue (Nerve and muscles) respond to action potential. } Firing = excitability = action potential = nerve impulse 7 Stages of Action Potential (AP): 1- Resting stage / Initiation 2- Depolarization 3- Repolarization of Action potential It is the resting membrane The membrane suddenly becomes K+ outflux through opening potential before the action permeable to Na+ ions causing Na+ Voltage gated potassium channels, potential begins. The influx to the interior of the axon this high K conductance (flow) to membrane is “polarized”. (upstroke) through opening Voltage outside, causes the normal negative gated sodium channels (positive resting membrane Initiation: The (polarized) feedback). potential.(Negative inside) resting membrane potential The membrane charges reverse (more (Na+ channels begin to close and rises from -90 to 0. (Gradual positive inside) the K+ channels open.) depolarization) due to Membrane potential rises from 0 to + 35 This channel is slow, which leads to threshold stimulus. mv, so all Na channels begin to close the next step. suddenly. à (Depolarization ends) This channel ends fast. A state of Hyperpolarization follows repolarization. *Will be explained later* 8  Excitation: *The process of eliciting the action potential* } Basically any factor that causes sodium ions to begin to diffuse inward through the membrane in sufficient numbers can set off opening of sodium channels. This opening of the sodium channels can result from mechanical disturbance of the membrane, chemical effects on the membrane, or passage of electricity through the membrane Threshold for excitation and “Acute local potentials” Threshold stimulus: The membrane potential at which occurrence of the action potential is inevitable. When a stimulus is strong enough to move RMP from its resting value (-90) to the range of -70mV to -55mV (-65 to 55) which leads to production (start) of an AP or depolarization. Subthreshold stimulus / Acute subthreshold potential: Stimulus that results in local depolarization. (local action potential) (does not propagate or move along). When stimulus is below the threshold. All or nothing principle: When threshold value for excitation is reached, a full Action Potential is produced, so its intensity can not be increased by increasing stimulus intensity (suprathreshold). 9 Types of Transport Channels Through the Nerve Membrane } There are two types: The voltage gated Na+ channel has two gates: 1. Voltage gated Na+ channels 2. Voltage gated K+ channels. } What opens the voltage gated channels? A stimulus Has 2 gates strong enough to depolarize them to threshold. Activation gate Inactivation gate on the outer side of on the inner side of membrane membrane 10  First: The Na+ Voltage-Gated Channel: Has three states. 1.Resting state 2.Activated state 3.Inactivated state oThe activation gate is Threshold Depolarizing Stimulus: moves the MP from its resting value (-90 mV ) to its A few milliseconds after the activation closed in the resting cell, gate opens, the channel becomes when the MP* = RMP* is: threshold value (-65 to -55mV) inactivated: At the peak of AP the o-70 to -90 mV. This opens the activation gate. inactivation gate will close the inactivation ( NB in this case BOTH the activation gate & gate will NOT open by a second stimulus üThis prevents entry of à & the cell becomes Na+ to the interior of the inactivation gate are open ) à permeability to Na+ becomes increased 500 to 5000 times à Refractory( ‫ )ﻣﻣﺎﻧﻌﺔ‬to another cell through this gate stimulation Na+ influx *Na+ flows into the cell in large amounts* *This goes on until the MP has gone back to its resting ( RMP) level ( -70 to - 90mV)* *The activation gate is still open the inactivation gate is closed* 11 What Happens After Action Potential? video } Refractory period: few milliseconds Time during which can’t stimulate neuron a second time Happens until recovery of resting potential } Two stages: 1. Absolute refractory period: The period during which a second action potential cannot be elicited, even with a strong stimulus. (No new action potential possible). ” ‫“ﯾﻌﻧﻲ ﻣﮭﻣﺎ ﻋطﯾﻧﺎ ﻣﺣﻔز ﺛﺎﻧﻲ ﻣﺎﯾﺳﺗﺟﯾب‬ 2. Relative refractory period: Can trigger new action potential if stimulus is very strong. 17 Summary of Action Potential : “EXTRA” When membrane potential (mV) reaches - 70 Na gates will open , and the gates will close when (mV) reaches + 30. K gates will be open when (mV) reaches +30 , and close when it reaches -90. 18  The Neuron } Definition: Unit of function of the central nervous system. 1- Soma: cell body 2- Dendrites: carry nerve impulses from surroundings to soma. 3- Axon hillock: start of action potential (because it has a lot of voltage gated Na channels) 4- Axon and axon terminal. Axon with myelin sheath Axon without myelin sheath Only in male slides: Myelinated axons diameter: >1um Unmyelinated axon diameter: An Excitable tissue is the tissue Schwann cells deposit