Physiology Lecture (14) PDF

Physiology Action Potential (AP) LECTURE (14) DR. El-Sawy 0  Physiology Action Potential (AP) Definition:...

Physiology Action Potential (AP) LECTURE (14) DR. El-Sawy 0  Physiology Action Potential (AP) Definition:  It is a rapid electrical changes that occur in RMP, due to stimulation of nerve fibers by effective stimulus. Characters: Actively propagated Non decremental Obeys all or non-rule. Phases of action potential : Depolarization Repolarization  Due to Na+ influx.  Due to K+ influx.  Through voltage gated Na+ channels  Through voltage gated K+ channels DR. El-Sawy 1  Physiology Action Potential (AP) Definition:  Loss of normal polarized state of the membrane.  i.e. ↓ potential difference between the outer and inner surface of membrane. Mechanism:  stimulus ↑ the permeability of cell membrane to Na+ ions.  Na diffuse inside → gradual change in memb. potential from RMP (- 70 mV) to isoelectric line (zero) and may exceed it (reversal of polarity +35 mV) Steps: Electrotonic potentials and firing levels:  adds negative charges to the outer surface of the membrane Stimulus →↓ potential difference by about 7 mV (from – 70 mV to – 63 (cathode) mV) i.e. passive depolarization  Some voltage gated Na+ channels open allowing Na+ influx →↓ the membrane potential more → opening of more voltage- activated Na+ channels → more Na+ influx → more At – 63 mV decreased membrane potential → opening of more and more Na+ channels i.e. +ve feedback mechanism → membrane potential to – 55 mV (firing level) i.e. active depolarization  All voltage gated Na+ channels open allowing Na+ influx → rapid Na influx → the membrane potential quickly reaches At – 55 mV zero potential and then overshoots to about +35 mV, so there is reversal in polarity. DR. El-Sawy 2  Physiology Action Potential (AP) Voltage gated Na+ channels with 2 gates; m gate or activation gate and h or inactivation gate and their position during RMP, depolarization and repolarization DR. El-Sawy 3  Physiology Action Potential (AP) Def  Restoration of membrane potential to its resting value a) Stoppage of Na+ influx due to:  Closure of voltage-activated Na+ channels by closure of h gate b) Opening of voltage-activated K+ channels: 1. At threshold potential (-55 mV): Voltage-activated K+ channels open but after a slight Mechanism delay time. 2. Diffusion of K ions outside: Returns the inside of the membrane to its original -ve potential (-70 mV) i.e. restore the RMP i.e. repolarization 3. K ions continue to diffuse to outside due to delayed closure of its channels leading to hyperpolarization. Voltage gated K+ channels with its n-gate or activation gate its position during RMP, depolarization and repolarization. DR. El-Sawy 4  Physiology Action Potential (AP)  By Na-K pump which actively transports Na out and K into the cell. DR. El-Sawy 5  Physiology Action Potential (AP) Apparatus:  Cathode ray oscilloscope (CRO). Methods: B) Monophasic Action Potential  By two microelectrode. One electrode (recording) into the nerve fibers (recording) Method 2nd electrode (reference) outside away from stimulated nerve fibers 1. Latent period 2. Spike potential 3. After potential Record DR. El-Sawy 6  Physiology Action Potential (AP) a. Latent period:  Time between the stimulation of the nerve and the start of the Def action potential.  Represents time taken by impulse to travel from site of Cause stimulation to site of recording electrodes.  Affected by: Duration a) Distance between stimulus and nerve fiber. b) Velocity of nerve fibers. Importance  Calculate velocity of conduction of a nerve fiber b. Spike potential: Definition:  Large wave (105 mV from – 70 to +35 mV).  Short duration (0.5 -1msec in thick myelinated nerve fibers). It consists of :  Represents depolarization.  Due to Na+ influx which consists of 2 phases:  Slow depolarization Gradual Na influx which cause slow change in membrane potential form the resting potential (-70 mV) to threshold Ascending potential (-55 mV). limb:  Rapid depolarization (upstroke) At firing level, voltage sensitive Na channels open suddenly → rapid Na influx and rapid depolarization of membrane. With continuous Na+ influx, the membrane potential becomes +ve (+35mV) → reversal of polarity or overshoot. DR. El-Sawy 7  Physiology Action Potential (AP)  Represents major part of repolarization (70%).  Due to sudden opening of the voltage sensitive K channels with Descending limb: immediate after closure of Na channels → rapid K efflux→ rapid repolarization. c. After potentials: Definition:  Small waves with relatively longer durations. Types : Negative after potential Positive after potential Short duration (4 m sec) Long duration (40 m sec) The membrane is partially The membrane is hyperpolarized. depolarized. Due to continuous excess K+ efflux Due to slow K+ efflux. due to slow closure of K+ channels. DR. El-Sawy 8  Physiology Action Potential (AP) A) Biphasic Action Potential  By two micro-electrodes Method  Placed on the outer surface of nerve fibers  Consists of 2 waves , initial positive wave followed by negative wave and separated by isoelectric line : 1. At rest: No potential difference ( ) the 2 electrodes → zero line is recorded 2. When depolarization wave reaches 1st electrode: upward deflection is recorded. 3. When wave become () electrodes: Potential difference returns to zero. 4. When depolarization wave reaches 2nd electrode Record Downward deflection is recorded. 5. When depolarization wave leaves 2nd electrode Potential difference returns again to zero. DR. El-Sawy 9  Physiology Action Potential (AP) Action potential in Nerve Action potential in Muscle fibers Fibers RMP -70 mV -90 mV Firing level -55 mV -50 mV Magnitude of the 105 mV 130 mV spike potential Duration of the AP 0.5- 1 msec 3-5 msec Duration of after Short Long potentials Velocity of up to 120 m/sec 3-5 m/sec conduction of AP Primary hypokalemic periodic paralysis Definition  Inherited disorder.  Decreased serum K.  Recurrent attacks of muscle weakness Characters  Sometimes paralysis.  Triggered by exercise and worsened with carbohydrates eating. DR. El-Sawy 10