Physiology Lecture (15) PDF

Physiology Propagation or Conduction of Action Potential LECTURE (15) DR. El-Sawy 0  Physiology Propagati...

Physiology Propagation or Conduction of Action Potential LECTURE (15) DR. El-Sawy 0  Physiology Propagation or Conduction of Action Potential Continuous (Sweeping) Saltatory ( jumping ) Site  Unmyelinated nerve (C).  Myelinated nerve (A&B) Speed  Slow (0.5 – 2 m / sec)  Fast (Up to 120 m/sec) Energy  More energy consumption  Less (1% of continuous)  Stimulation of nerve makes  Stimulation of nerve makes membrane depolarized nearest node of Ranvier (+35 mV) at site of depolarized (+35 mV) stimulation  Potential difference  Potential difference between between depolarized depolarized (active) node (+ (active) area (+ 35 mv) & 35 mv) & adjacent polarized adjacent polarized (resting) (resting) node (- 70 m.v)  area (- 70 m.v)  makes a makes a local current flows Mechanism local current flows between between the 2 nodes causing the 2 areas causing the the polarized (resting) node polarized (resting) area to to become depolarized to the become depolarized to the threshold level (- 55mV)  threshold level (- 55mV)  action potential & so on action potential & so on  Significance : 1. ↑ velocity of conduction. 2. Conservation of energy. DR. El-Sawy 1  Physiology Propagation or Conduction of Action Potential 1. The speed of conduction of action potential depends upon : a) Internal diameter of nerve fibers: ↑ diameter → ↓ internal resistance →↑ conduction velocity b) Degree of myelination: ↑ myelin sheath thickness → ↑ membrane resistance to current → charge will jump from one node to another and increase the conduction velocity 2. Orthodromic and antidromic conduction:  In the normal direction this is called orthodromic conduction  in the opposite direction is called antidromic conduction DR. El-Sawy 2  Physiology Propagation or Conduction of Action Potential Type A fibers Type B fibers Type C fibers Type  Thick myelinated  Thin myelinated  Thin unmyelinated  Somatic sensory  Autonomic  Somatic motor & carrying slow pain Site preganglionic sensory nerves  Autonomic nerves postganglionic Diameters  2-20 μm  1-3 μm  0.5-1 μm Speed  10-120 m/sec  3-15 m/sec  0.5-2 m/sec  Prolonged deep Sensitive  O2 lack (Hypoxia)  Local anesthetics to pressure  alpha, beta, delta Subtypes ----------------- ------------------ and gamma. DR. El-Sawy 3  Physiology Propagation or Conduction of Action Potential  In the form of spike potential and after potentials (as before).  The nerve contains: Enzymes responsible for glycolysis, citric acid (Kreb’s) cycle and electron transport (cytochrome oxidase). Thus the nerve can generate and store energy in the form of ATP.  Nerve fiber spends energy to maintain RMP. During rest  Energy utilized is derived from ATP.  Metabolic reactions are ↑↑ to about double the resting state.  The increased metabolic reactions are manifested by: During passage of 1. Increase in co2 production. nerve impulse 2. Increase in glucose utilization. 3. Increase in heat production.  The nerve impulse is accompanied by an increase in heat production.  Appears in two phases: Initial heat  Due to generation and propagation of nerve impulse.  30 times the initial heat & remains for 30 minutes. Delayed heat  Due to metabolic reactions needed to reform the ATP utilized during the action potential. DR. El-Sawy 4  Physiology Propagation or Conduction of Action Potential  During conduction of a nerve impulse, the excitability of the nerve fibers varies.  It passes in the following phases: a) Temporal rise of excitability: Associated with the local response (local depolarization) in nerve fiber before firing level. b) Absolute refractory period (ARP) c) Relative refractory period (RRP) d) Supernormal phase of excitability e) Subnormal phase of excitability DR. El-Sawy 5  Physiology Propagation or Conduction of Action Potential Absolute refractory Relative refractory Supernormal phase Subnormal phase period (ARP) period (RRP) of excitability of excitability  The excitability of nerve  The excitability of nerve is  The excitability is  The excitability is fiber is completely lost. partially recovered (but above normal. below normal.  The nerve is absolutely 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 potential (after firing  Late part of descending Correspond level) limb of spike potential till  Negative after  Positive after to  Early part of descending Firing level. potential. potential. limb (initial 1/3 of repolarization). DR. El-Sawy 6  Physiology Propagation or Conduction of Action Potential 1. During ascending limb of  Membrane is partially  Membrane is still  Membrane is spike: repolarized & Strong partially hyperpolarized Gates of voltage stimuli can reopen many depolarized & & away from activated Na+ channels (not all) of the gates of near to threshold the threshold are already opened (by Na+ channels. level. level. first stimulus). If 2nd stimulus is applied, it  Leads to depolarization of cannot have any effect membrane and production Mechanism (the gates are opened). of 2nd action potential. 2. During early part of  Magnitude of 2nd action descending limb: potential is less than Gates are just closed. normal (not all Na+ gates Need sufficient period of are opened). repolarization to be re- opened.  Limits number of impulses Significance that can be produced and conducted by nerve fibers. DR. El-Sawy 7  Physiology Propagation or Conduction of Action Potential Def : Failure of initiation (excitability) and propagation (conductivity) of nerve impulses from one place to another. Causes & types: a. Thermal : Sever cooling. Physical causes b. Mechanical : Deep pressure.  Local anesthetic drugs (lidocaine). Chemical causes  ↑ Ca (membrane  ↓ Na stabilizers) :  ↓K DR. El-Sawy 8  Physiology Propagation or Conduction of Action Potential Definition  Autoimmune disease due to both genetic and environmental factors.  In MS, autoantibodies attack myelin, causing inflammation and Cause injury to the sheath and eventually the nerves that it surrounds.  Women : men = 2:1. Incidence  20 and 50Y.  Muscle weakness, fatigue, diminished coordination  Slurred speech Clinical  Blurred or hazy vision picture  Bladder dysfunction  Sensory disturbances  Steroids : Treatment 1. Suppress immune system. 2. Decrease antibodies formation. DR. El-Sawy 9