Action Potential (L12&13: Physio) PDF

L12&13: Physio Action potential Definition of action potential (AP):- o It is the changes taking place in the electric potential difference between the inner & outer membrane during action (i.e as a result of activation by a stimulus). Stimulus o The stimul...

L12&13: Physio Action potential Definition of action potential (AP):- o It is the changes taking place in the electric potential difference between the inner & outer membrane during action (i.e as a result of activation by a stimulus). Stimulus o The stimulus is change in external or internal environment that provokes a response (reaction). It may be: 1- Electrical. 2- Chemical (Hormone or neurotransmitter). 3- Mechanical (Pressure). 4- Thermal (Cooling or warming). 5- Electromagnetic (Light rays). N.B.: Electrical stimuli are used experimentally to study the function of nerve and muscle because: 1- They are similar to the natural stimuli in the body. 2- They don’t injure the tissues. 3- Their intensity and duration are easily controlled. Nerve Action Potential Types: 1- Monophasic Action Potential: It is recorded by 2 microelectrodes; one is placed on the inner surface and the other on the outer surface of the membrane. 2- Biphasic Action Potential: It is recorded by 2 microelectrodes; both are placed on the outer surface of the membrane. 1|Page  L12&13: Physio Description of monophasic action potential 1- Resting (polarized) state (at – 70 mV). 2- Application of a stimulus (Stimulus artifact). 3- Latent period. 4- Spike potential; o Depolarization (rising phase = ascending limb) o Repolarization (Falling phase = descending limb). 5- Hyperpolarization. 6- Return to resting state. Different phases of Nerve AP 1) Resting membrane potential:In medium sized nerve -70 mV 2) Stimulus artifact: -It indicate the time of application of the stimulus. It is due to current leakage from the electrical stimulus to the recording electrodes. 3- Latent period: - It represents time passed between the application of the stimulus & the recording AP. During this period, the nerve impulse reaches from the point of the stimulus to recording electrodes It depends on: a. Velocity of conduction of the nerve (inversely proportional). b. The distance between the stimulus & the recording electrode (directly proportional). 2|Page  L12&13: Physio 4- The spike: It is rapid (2 m.sec) and high amplitude changes (105 m.v) It is composed of : A. Ascending limb: - it represents Depolarization, during which the membrane potential change from -70 to +35 m.v. ✓ It is due to Na+ influx as a result of activation of (opening of) voltage gated Na+ channels leading to increase Na+ permeability 1000-5000 times. ✓ The first part is slow (1st 15 m.v.)( from -70 to – 55 m.v). When the membrane potential reaches -55 m.v., the rate of depolarization increases as most voltage – gated Na channels are opened. This level is called the firing level (Threshold). B. Descending limb: - - it represents Repolarization, during which the membrane potential change from +35 to near the resting state. It is due: - ✓ Stoppage of Na + influx as a result of inactivation (closure of inactivation gates) of voltage gated Na + channels. ✓ K+ efflux as a result of activation (opening) of voltage gated K+ channels. 3|Page  L12&13: Physio 5- Hyperpolarization : ✓ They are slow (40 mSec.) and low magnitude changes. ✓ After reaching the resting level, the membrane becomes slightly hyperpolarized (more negative) then gradually returns back to the RMP level. ✓ Cause: Some of the voltage gated K+ channels remain opened for several seconds allowing excess K+ to diffuse out. 4|Page  L12&13: Physio Conduction in unmyelinated nerve- : By local circuit mechanism (current sink mechanism) The resting segment has +ve charge outside and –ve charge inside, while the activated segment has –ve charge outside & +ve charge inside. Local circuit occurs between depolarized segment and adjust resting segment of the neuronal membrane i.e. current flow from resting segment to activated segment on the outer surface and from activated segment to resting segment on the inner surface thus completing a circuit (current sink). Local circuits lead to decrease in the RMP in the resting segment, which when reach the firing level, most of voltage gated Na+ channels are opened resulting in increasing in Na+ permeability and depolarization. The newly depolarized segment depolarizes the adjacent segment in the same way and so on till the wave of depolarization reaches the end of the nerve. 5|Page  L12&13: Physio 6|Page  L12&13: Physio 2) Conduction in myelinated nerve : - By Saltatory mechanism - It is similar process to conduction in unmyelinated nerve fibers, but local circuits here occur between adjacent nodes of Ranvier (not adjacent point as unmyelinated nerve) as the myelin sheath acts as an insulator. - Thus, the wave of depolarization jumps from node of Ranvier to the adjacent one (Saltatory or jumping conduction)..‫ت َّ َح َّمل‬ ‫سائر اإّل‬ ٌ ‫ ما َوصَل‬،‫ق اإّل بالصَّبر عليه‬ ٌ ‫ما قُطِ َع طري‬ َ َ‫ وّل ّلن‬،‫ ما ذُ ِلا َل الصَّعب‬،‫بعد حرك ٍة ُمضنِيَ ٍة وتَ َعب‬ ‫ت‬ َ ‫ اإّل بعد‬،‫ وّل استقا َم المائل‬،‫الحديد‬ ٍ ‫ط َرقا‬ ‫ ث ام‬،‫ وخطوات متتا ِبعة‬،‫ت دائمة‬ ٍ ‫ وض ََربا‬،‫مستمرة‬ ‫ا‬ ،‫ ستَشفَ ُع لك كُ ال لحظ ٍة صادقة‬،‫تكون الناتيجة‬ ‫ قس ًما؛ سيكون‬،‫ستستند يو ًما على ما غرست‬..‫جابرا‬ ً ‫الفَتح‬ 7|Page  L12&13: Physio UnMyelinated nerve Myelinated nerve Type of conduction Local circuit mechanism Saltatory conduction Speed of conduction Slow Rapid Energy consumed More energy needed Less energy consumed Biphasic actiom potential It can be recorded by cathode-ray oscilloscope (CRO) through 2 microelectrodes, both are placed on the outer surface of nerve fiber. It shows the following components: 1. At rest: No potential difference is recorded Between the 2 electrodes. 2. When the nerve is stimulated and the depolarization wave reaches the near electrode near, an upward deflection (spike) is recorded. 3. When the depolarization wave reaches the area of the nerve between the 2 electrodes, the potential difference returns to zero. 4. When the depolarization wave reaches the second electrode, downward deflection (spike) is recorded. 5. When the depolarization wave leaves the second electrode, the potential difference returns again to zero. Thus biphasic action potential is composed of spikes of action potential in opposite directions separated by an isoelectric interval. 8|Page  L12&13: Physio Questions Q1:During an action potential, depolarization of nerve cell membrane is brought about by : a)Increased sodium permeability. b) increased calcium permeability. c) increased chloride permeability. d) increased potassium permeability Q2:Duration of spike of nerve action potential is: a)4 m.sec. b) 8 m.sec. c) 1 – 2 m.sec. d) 5 m.sec. e) 15 m.sec. Q3:Saltatory conduction of action potential: a)Occurs in the unmyelinated nerves. b) is slow. c) needs more energy than point to point conduction. d) is fast Regarding voltage gated Na+ channels: Q1: During rest the activation gate is ………………………. Q2: During early depolarization: the activation gate is ………………… suddenly. Q3: During late depolarization: ………………………… gate is closed. Regarding voltage gated K+ channels: Q1: It has ………………………… gate at the …………… surface. Q2: During rest: the gate is ……………………………. Q3: During Repolarization: the gate is completely ………….. & K+ …………… in large amount. 9|Page