Nerve Excitation - Lecture Notes PDF

The Excitable By Dr. Nahed El Sokkary tissues Assistant professor of Medical Physiology Excitable tissues Nerve and muscle are called excitable tissues. They respond to stimul...

The Excitable By Dr. Nahed El Sokkary tissues Assistant professor of Medical Physiology Excitable tissues Nerve and muscle are called excitable tissues. They respond to stimuli (electrical, mechanical or chemical) when they are excited. Functions of excitable tissues Nerve: is an excitable tissue which receives stimuli, processes them, and transmits signals to target tissues to integrate the functions of the whole body. Muscle : is an excitable tissue which contracts in response to specific stimuli. Its function is to produce force and cause motion, either locomotion or movement of internal organs. Ø The unit structure of the nervous system is the nerve cell or neuron. Nerve trunk Resting Membrane Potential (RMP) All the points on the outer surface of the membrane of a resting excitable tissue {nerve or muscle} are equipotential or isopotential Resting membrane potential (RMP) There is always an electric potential difference between the outer and inner surfaces of a living cell membrane, called The RMP. RMP : is the potential difference created across the cell membrane by the metabolic processes of the fiber during rest. Resting membrane potential a current flows from outside to the inside of the fiber at rest Indicating that the outside of the fiber is positively charged relative to the inside i. e. the membrane is in the polarized state RMP of nerve fiber = -70 mV RMP of muscle fiber = -90 mV Causes of RMP unequal distribution of electrically charged ions on both sides of the membrane with prevalence of cations at the outer surface and anions at the inner surface. Factors involved in the production of RMP 1. Selective permeability across the cell membrane 2-Sodium –potassium pump Factors involved in the production of RMP 1. Selective permeability across the cell membrane On the outer surface : Na+, Cl- et HCO3- with small quantity of K+ & proteins. On the inner surface: K+ & proteins with small quantity of Na+, Cl- & HCO3-. RMP The permeability of the membrane to K+ ions is 50-100 times more than its permeability to Na+ ions. Factors involved in the production of RMP 2-Sodium –potassium pump Active Transport 1. Primary active transport Na+ - K+ pump Active Transport Factors involved in the production of RMP 2-Sodium –potassium pump Importance of Na+ - K+ pump: 1. Establishes a negative electrical potential inside the cells, which helps to transmit signals throughout the nervous system. 2. Maintains Na+ and K+ conc gradients across the cell membrane 3. Maintenance of normal level of intracellular K+ which is necessary for protein metabolism. 4. keeps osmotic equilibrium to maintain the cell volume. Ø Properties of nerves (1) Excitability (2) Conductivity (3) All or none law (4) Accommodation (5) Infatiguability  Changes occuring in the nerve during its activity 1. Electric changes (Action potential). 2. Excitability changes. 3. Metabolique changes. 4. Thermique changes (production of heat). 1. Electrical changes (Action potential ) Definition : The electrical changes accompanying the propagation of excitation wave Registered by: Causes of action potential 1. During the resting state : ØAll voltage – gated Na+ and K+ channels are closed. 1. Electrical changes (Action potential ) Subdivided into 3 phases: Latent period Spike potential After depolarization Action Potential 1. Latent period Represents time taken by excitation wave to travel from site of stimulation to recording electrodes. No change in the membrane potential occurring until the excitation wave reaches the recording electrodes. About 1-3 msec.  Action potential 2. Spike potential : is synchronous with the passage of excitation wave along the nerve fibre. consists of : (a) Ascending limb (b) Descending limb Spike potential (a) Ascending limb Represents the process of depolarization. Opening of Na+ channels (Na+ influx) membrane potential becomes less than -70 mv. Rate of depolarization gradually increases from -70mv to -55 mv. (Firing Level) Followed by rapid complete depolarization of the membrane = potential difference = 0 (iso-potentiel) Followed by reversal of polarity (overshoot) Spike potential (a) Ascending limb Reversal of polarity (overshoot): Ø Membrane potential is reversed Ø outerside of membrane becomes –ve & innerside +ve Ø potential difference is + 35mv. Ø magnitude of action potential is –70+35 =105 mV. Spike potential (b) Descending limb Represents the process of repolarisation. Repolarisation : Restoration of the normal resting polarized state of the membrane. Opening of K+ channels (K+ efflux or outflux) Action potential ØN.B: Re-excitation of the membrane cannot occur unless the membrane is repolarized. ØN.B: Repolarization restores the resting electrical conditions of the neuron, but does not restore the resting ionic conditions. Ø Ionic redistribution is accomplished by the sodium-potassium pump following repolarization. Action potential  Cell Action Potential 1. Resting State ~ -70mV Na++ K+ 2. Depolarization Na+ + ++ 3. Repolarization ++ + Compound action potential When a mixed nerve trunk is stimulated→ the spike potential will be made up of several waves. Each fiber in a nerve trunk conducts at different velocities → impulses propagated by them are separated according to their relative velocities→compound action potential. Compound action potential Larger the diameter →greater velocity of conduction & shorter duration &bigger magnitude of the spike 3 main waves A, B, and C. Each wave belongs to a group of fibers Faster conduction group A > B > C. Group A is further subdivided into a, b, g, d.  Type of Site Diameter Velocity of nerve fibers conduction Group A In somatic 3-20µ 15-120m/sec myelinated fibers Including small Group B myelinated, 1-3µ 5-13m/sec Preganglionic autonomic fibers Including small non- Group C myelinated, Somatic 0. 3-1. 3µ 0. 5-3m/sec and postganglionic autonomic fibers All or None Rule Stimulation of a single nerve fiber by a stimulus of threshold (minimal) intensity or over gives a maximal response and no more. Subthreshold or subminimal stimulus gives no response at all So, the single nerve fiber either responds maximally or not at all according to the intensity of the stimulus The all or non rule can be applied also to: the single skeletal muscle fiber, cardiac muscle and certain types of smooth muscles with gap junctions. All or None Rule Not applicable on the mixed nerve trunk and the whole skeletal muscle The gradual increase in the response in case of nerve trunk and the whole skeletal muscle depends on the number of the stimulated fibers N.B: single fiber inside the nerve trunk or the whole skeletal muscle obeys the all or non rule.  Local response Local excitatory state (Graded potential) Subthreshold stimuli are not able to produce an action potential, but they don’t pass without any effect , they lead to: (a) a slight decrease in the RMP (below the firing level) (b) slight increase in the excitability. (c) application of multiple subthreshold stimuli can be summated to give a response called temporal summation (d) Its magnitude and duration are proportionate to the strength of the stimulus = graded response. (e) Die out over short distances ( the spread of graded potential gradually decreases). Excitability changes during the action potential The conduction of nerve impulse alters the excitability of the fiber in the following order: (a) The absolute refractory period (ARP) (b) The relative refractory period (RRP)  Excitability changes during the action potential (a) The absolute refractory period (ARP): During which the excitability is abolished, No stimulus whatever its strength may be, can excite the nerve fiber. As the membrane is in the depolarized state and it must be repolarized before it can respond to a second stimulus. Coincides with the duration of the ascending limb of the spike and the upper third of the descending limb. Excitability changes during the action potential (b) The relative refractory period (RRP): During this period the excitability is being recovered, but still below the normal level A stronger stimulus is needed to excite the fiber during this period And the response generated (AP) is of smaller magnitude and lower velocity Nerve block It means failure of conduction of nerve impulse from one place to another, or loss of excitability of the nerve fiber i.e there is no generation or propagation of nerve impulses. Properties of the nerve The function of nerve is the Conduction of nerve impulses Nerve fibers can be excited at any part along their length. can conduct impulses in both directions Action potential does not decrease in strength with distance. Main properties of the nerve are: (1) Excitability. (2) Conduction. (3) Adaptation. (4) Infatiguability.  Conductivity Propagation of action potential on an excitable membrane The action potential produced at the first location in the axon membrane (the initial segment of the axon) thus serves as the depolarization stimulus for the next region of the axon membrane, which can then produce the action potential.  Conduction of action potential Conduction of an action potential excites adjacent portions of the membrane. Types: 1. Conduction in unmyelinated nerve fibers (propagation conduction) 2. Conduction in myelinated nerve fibers (saltatory conduction) Un-myelinated The “wave” travels (the repeated action potential), at different locations along the axon membrane) Local circuits of Current Myelinated: Axons can conduct depolarizations over only a very short distance (1-2 mm), the nodes of Ranvier cannot be separated by more than this distance. Importance of saltatory conduction: 1. It increases velocity of conduction along nerve fiber by the process of jumping. The velocity of conduction is 3-120 m/sec. 2. Depolarization is limited to the nodes of Ranvier and so leakage of Na+ ions is minimum to the inside of the fiber. This saves the energy required by the sodium pump to extrude sodium ions to the outside.