Sixth lecture

Monophasic recording of the action potential A) Latent period: It is the time between the stimulation of the nerve and the start of the action potential. It’s duration is affected by the distance between the stimulating and recording electrodes and by the velocity of conduction of the nerve fibers. B) Spike potential: It is a large wave of a short duration (0.5-1 m.sec in thick myelinated nerve fibers). It consists of ascending limb due to Na+ influx occurs in two stages: 1- Slow until the threshold potential. 2- Rapid after the threshold potential. Descending limb: which represents the major part of the process of repolarization. Repolarization is caused by K+ efflux.

C) After potentials: These are small waves with relatively longer durations and consists of: 1- Negative after potential: The outer surface of the membrane still have some negative charges (negative after potential) which are gradually neutralized by the outward diffusion of K+ ions at the end of this phase. 2- Positive after potential (after hyperpolarization): The excess K+ ions return back again inside the nerve fiber which makes the membrane potential to return to the normal resting level.

Excitability Depends On 1- Strength of stimulus ( a maximum strength ) is essential. 2 -Duration of stimulus, the stimulus must continue for a certain minimum period, which varies inversely as the strength. ( High stimulus / Shorter lime, and vice versa) 3 - The direction of the current; If the current is passed transversely across the nerve,no effect will be produced ; Which it pass along the length of the nerve, there is a maximum chance of stimulation.

4 - Frequency of impulse; Usually one stimulus generate one nerve impulse, but, if the stimulus is too strong, more than oik: impulse may follow. Ca++ lack increase this tendency of multiple response. 5 - Injury ; Excitability is increased near the site of injury, but later on, it becomes depressed.

Action potential recorded from a group of nerve fibers, e.g. seiatie nerve or nerve trunk is called Compound action potent. as it is suinmiled action potentials from different types of.cp fibers having different conduction velocities.(myelin and non thick and thin etc.) with large shock the action potential appears splitted into 3 waves A, B, and C. The A wave is divided into <x, 1,# sections.this A wave represents the activity of myelinated axon. The C wave represents that of unmyelinaled axon, where the » wave represents both types.

CONDUCTIVITY The nerve impulse is conducted along the nerve fibers. Conductivity show the following characteristics: a) impulse is propagated along a nerve fiber in both direction but in normal condition in one way, motor nerve to (rcsponding'igan, and sensory nerve to the center due to the action of synapses). b) Velocity of nerve impulse: The nerve impulse is propagated with a definite speed. The conduction velocity depends on : i - The diameter of nerve fibers (thick & high) ii - The presence or absence of myelination. iii - Tcmpraturc (in cold blooded animal conduction is lower then those of worm blooded ).

Factors Affecting Conductivity And Exclibility I- Temperature (cooling diminishes and warming increases these properties). II- Mechanical pressure depresses these processes. III- Blood supply; If blood supply is cut off both properties is lost. IV- Chemicals; CO2 and narcotics diminish and finally abolish both properties. V- Hydrogen ion concentration; in alkalinity they increase an-in acidity they decrease. VI -H fleet of ions ; lack and excess ofCa1' have the same el'l as rise and lull of I'11 respectively, Change in K1 exert opposite effect. Na+ and MgH similar to Kf but vvilli less degree. thus Na '*", K1 and Mg ''" arc neuroexitibility, while Ca'"h is ncurosedativc. VII- Oxygen lack; depresses and if continue abolish these properties.

ALL or NON LOW If a stimulus be adequate, a single nerve will alawys give a maximum response.If the duration or strength of the stimulus be further no alteration in the response will lake place. (In the whole nerve this property differ,but is true for a singl nerve). SUMMATION In a nerve fibre summation of two subminiinal stimuli is possible. ADAPTATION The nerve fibre is quickly adapt itself, thus gradual changes to excite, but sudden alteration cause excitation.

ACCOMODATION If the stimulus, even with stronger strength is applied very slowly to a nerve, these may have no response only due to l of attaining the threshold strength. JNDIFAT1GABIL1TY In the nerve muscle preparation, if the nerve is stimulated repeatedly, then after a certain period llic muscle fail to give response., Now, if the nerve is isolated from the muscle and placed in fresh muscle, then stimulated it will excited the muscle; as the nerve is not fatigue. REFRACTORY PERIOD When the nerve fiber is once excited, it will not responds to second stimulus for a brief period, this is called Refractory period.