Physiology Delta Lecture (5) PDF

Physiology Factors Affecting Excitability LECTURE (5) Factors affecting excitability DR. El-Sawy 0  Physiology...

Physiology Factors Affecting Excitability LECTURE (5) Factors affecting excitability DR. El-Sawy 0  Physiology Factors Affecting Excitability Absolute refractory Relative refractory Supernormal phase Subnormal phase period (ARP) period (RRP) of excitability of excitability  The excitability of nerve  The excitability of nerve  The excitability is  The excitability is fiber is completely lost. is partially recovered above normal. below normal.  The nerve is absolutely (but still below normal) Definition resistant (refractory) to further stimulation.  0%  0-100%.  ˃100%  ˂100%  No other stimulus  Stronger stimuli (more  Weaker stimuli  Stronger stimuli Stimuli whatever its strength can than threshold) needed to (below minimal) can needed to excite excite nerve. excite nerve. excite nerve. nerve.  Ascending limb of spike  Late part of descending potential (after firing level) Correspond  Early part of descending limb of spike potential till  Negative after  Positive after to Firing level. potential. potential. limb (initial 1/3 of repolarization). DR. El-Sawy 1  Physiology Factors Affecting Excitability  Cause of relative refractory period: ① During this time, some of the sodium channels still have not been reversed from their inactivation state. ② Potassium channels are usually wide open at this time causing greatly excess flow if positive potassium ion charges to the outside of the fiber. DR. El-Sawy 2  Physiology Factors Affecting Excitability A. Physical Factors :  Warming: ↑ by ↑ metabolic reactions needed for Na - K pump. Thermal factors  Cooling : ↓ by ↓ metabolic reactions needed for Na – K pump. Mechanical  Deep pressure, trauma and injury → ↓ factors B. Chemical factors : Local anesthesia (cocaine , Novocaine and xylocaine ):  They block Na channels → ↓ permeability to Na. Ions : ↑ ↓  ↑ depolarization → ↑ excitability  ↓ depolarization → ↓ excitability Na and conductivity and conductivity  in extra-cellular fluid → ↓ K efflux  in extra-cellular fluid → ↑ K efflux K → ↓ RMP→ ↑ excitability. → ↑ RMP → ↓ excitability.  ↓ excitability & conductivity.  ↑ excitability & conductivity Ca due to ↓ Na+ permeability by due to ↑ Na permeability. binding to Na+ channels protein PH changes : Alkalosis : → ↓ free Ca → ↑ excitability. Acidosis : → ↑ free Ca → ↓ excitability. Hypoxia , ↑Co2 and ischemia : ↓ excitability of the nerve fibers. DR. El-Sawy 3  Physiology Factors Affecting Excitability C. Electrical factors Electrotonic potentials (electrotonus): Changes which occur in nerve as a result of stimulation by constant galvanic current of sub threshold intensity. An-elecrtotonus Cat-electrtonus Site  Anode  Cathode  Application of positive  Application of negative charges on outer surface → charges on outer surface → Mech. ↑ potential difference across ↓ potential difference across the membrane the membrane  Hyper-polarization →  De -polarization → Result ↓ Excitability ↑ Excitability DR. El-Sawy 4  Physiology Factors Affecting Excitability Causes :  Mechanical factors: Pressure, injury & trauma. Physical  Thermal factors: severe cooling. 1. Local anesthetic drugs. Chemical 2. Increased Ca ions. (membrane 3. Decreased Na ions. stabilizers) : 4. Decreased K ions. Electrical  Severe an-electrotonus. Use of vasoconstrictors with local Anesthesia :  ↓ local blood flow  Slows the rate of absorption of Importance local anaesthetic  Prolongs its local effect.  When adrenaline is included in an injection of lidocaine the Concentration final concentration should be 1 in 20 000. DR. El-Sawy 5