Nerve Physiology Lecture Notes PDF

PHYSIOLOGY OF THE NERVE -Lecture 2 By Merhan M. Ragy Prof. of Physiology –Minia UNIVERSITY  NERVE IMPULSE It is a physicochemical disturbance produced by a stimulus of threshold intensity or more, and is propagated in the form of a wave along the nerve fib...

PHYSIOLOGY OF THE NERVE -Lecture 2 By Merhan M. Ragy Prof. of Physiology –Minia UNIVERSITY  NERVE IMPULSE It is a physicochemical disturbance produced by a stimulus of threshold intensity or more, and is propagated in the form of a wave along the nerve fiber. It is accompanied by: 1.Electric changes. Action potential 2.Excitability changes. 3.Metabolic changes. 4.Thermal changes Electric Changes Recorded during Action Potential: 1. The Stimulus Artifact (SA): moment time of application of the stimulus. Cause: current leakage (electrons of stimulus ) from the stimulating electrode to the recording electrode along the extracellular fluid. 2. The Latent Period (LP): It is an isopotential period (i.e. base line) following the stimulus artifact. Significance: It represents the time (t) consumed in the propagation of the impulse from the stimulating electrode to the recording electrode (through the nerve). If this time (t) is estimated and the distance (d) between the stimulating and recording electrodes is measured, so the velocity (v) of conduction along the nerve fiber can be calculated: V = d/t.  K OUTFLUX Na influx  3-The Spike Potential DEPOLARIZATION= ascending limb 1-CAUSE opening voltage-gated Na+ channels=Na+ influx 2-=It is a decrease in membrane potential from –start gradual from 70 mV to -55 mV then rapidly( ALL Na channel open ) PASS 2 PONTS 1-ZERO POTENTIAL (ISOPOTENTIAL) 2- Reversal of polarity + ve inside till +35 The firing level (FL): is the membrane potential at which the rate of depolarization changes from gradual to rapid. It is -55 mV in the nerve. Repolarization descending limb CAUSE maximum opening of voltage-gated K+ channels results in K+ outflux = is rapid at first (until the membrane is 70% repolarized), Gradual slow repolarization in the last 30% of repolarization) due to delayed outflux of K.. The SPIKE POTENTIAL: is the rapid depolarization followed by the rapid repolarization When the RMP is reached, the membrane potential overshoots in the direction of hyperpolarization. This is called AFTER HYPERPOLARIZATION CAUSES hyperpolarization due to prolonged K+ outflux. N.B VOLTAGE GATED SODIUM CHANNEL IS RAPID VOLTAGE GATED POTASSIUM CHANNEL IS SLOW When the membrane reaches the RMP after hyperpolarization , the Na+- K+ pump will restore the normal ionic distribution to keep the Na+ concentration more outside and the K+ concentration more inside. Excitability Changes (Phases) during the Action Potential:  Absolute refractory period: Excitability is completely lost. It corresponds to the ascending and early part of the descending limb of the spike.  Relative Refractory Period: Excitability is starting to return to normal, but it is still below normal. It corresponds to the last 2/3 of the descending limb of the spike.  Supernormal Phase of Excitability: Excitability is greater than normal and subthreshold stimuli can produce a response when applied during this phase. It corresponds to the l.  Subnormal Phase of Excitability: Excitability is less than normal. A stronger stimulus (more than the threshold) is needed to excite. It corresponds to the after hyperpolarization Excitability Changes (Phases) during the Action Potential:  Absolute refractory period: Excitability is completely lost. It corresponds to the ascending and early part of the descending limb of the spike.  Relative Refractory Period: Excitability is starting to return to normal, but it is still below normal. It corresponds to the last 2/3 of the descending limb of the spike.  Supernormal Phase of Excitability: Excitability is greater than normal and subthreshold stimuli can produce a response when applied during this phase. It corresponds to the 30% of repolarization (slow repolarization  Subnormal Phase of Excitability: Excitability is less than normal. A stronger stimulus (more than the threshold) is needed to excite. It corresponds to the after hyperpolarization 1. Nerve impulse It is a physicochemical disturbance produced by a stimulus of threshold intensity or more, and is propagated in the form of a wave along the nerve fiber. 2. Latent period an isopotential period (i.e. base line) following the stimulus artifact. It represents the time (t) consumed in the propagation of the impulse from the stimulating electrode to the recording electrode (through the nerve) used to estimate the nerve velocity 3. The firing level (FL): is the membrane potential at which the rate of depolarization changes from gradual to rapid. It is -55 mV in the nerve 4. The SPIKE POTENTIAL: is the rapid depolarization followed by the rapid repolarization 1. Absolute refractory period: Excitability is completely lost. It corresponds to the ascending and early part of the descending limb of the spike. 2. Relative Refractory Period: Excitability is starting to return to normal, but it is still below normal. It corresponds to the last 2/3 of the descending limb of the spike. 3. Supernormal Phase of Excitability: Excitability is greater than normal and subthreshold stimuli can produce a response when applied during this phase. It corresponds to the 30% of repolarization ( slow repolarization) 4. Subnormal Phase of Excitability: Excitability is less than normal. A stronger stimulus (more than the threshold) is needed to excite. It corresponds to the after hyperpolarization PHYSIOLOGY OF THE NERVE -Lecture 3 By Merhan M. Ragy Prof. of Physiology –Minia UNIVERSITY  Factors Increases the Nerve Excitability (Any condition that causes depolarization (i.e., membrane potential is closer to the firing level) → the nerve becomes more excitable). 1- Warming & alkalinity → increases excitability. 2-Effect of ions: A- High K+ concentration in the extracellular fluid → decrease K+ outflux (by decreasing its conc. gradient) → ↓ the RMP (i.e., depolarization) → increases the excitability. B-Low Ca++ concentration in the extracellular fluid →↑ the permeability to Na+ → increases the excitability. N.B. The active form of calcium is the ionized form. Ionization of calcium is increased in an acidic medium (i.e., ↓pH) and vice versa. Therefore, changes in the pH of ECF affect nerve excitability indirectly by changing the rate of ionization of calcium (which guards the Na+ channels on the nerve fiber).  : Factors that Decrease the Excitability= nerve stabilizers 1. Local anesthetics: e.g., cocaine which decreases the permeability of the nerve membrane to Na+ → decreases the excitability. 2. Effect of ions: A- Low K+ concentration in the extracellular fluid → increases the RMP (i.e., hyperpolarization) → decreases excitability (e.g., as in a hereditary disease called familial periodic paralysis). B- High Ca++ concentration in the extracellular fluid → decreases the permeability to Na+ → decreases excitability. 3-Others Cooling, ↓blood supply, O2 lack, and acidity: ↓ excitability.