Muscles and Nerves Physiology Resting membrane potential



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Almustansiryia college of Dentistry Muscle and Nerve Physiology 2017-2018

Assistant prof. Dr. omar Msc. Ph D. Medical Physiology




Muscles and Nerves Physiology
Resting membrane potential: The equilibrium or resting potential across the biological membrane is the point at which the forces of concentration gradient and electrical gradient balance. Electrical potentials exist across the membrane of essentially all cells of the body. During rest or inactivity, the electrical potential across the membrane is called resting membrane potential which is vary between –100 mv to -10 mv on the inside relative to the outside of the membrane depending on the type and the size of tissues. This mean an excess of negative ions (anions) accumulates immediately inside the cell membrane along its inner surface and excess of positive ions (cations) accumulates immediately outside the membrane.
The genesis and the magnitude of resting membrane potential: In humans the genesis and the magnitude of the normal resting membrane potential is mainly due to:

(1) Passive outward diffusion of K ions (diffusion potential) which contribute far more to the membrane potential than will the inward diffusion of Na ions. This is because the permeability of the membrane to K ions is far more than for Na ions (100 times more). This outward diffusion will create a state of electropositivity outside the membrane and electronegativity on the inside (because of negative anions that remain behind). Passive K diffusion alone creates -86 mv membrane potential. The diffusion potential can be generated only if the membrane is permeable to the ion and the size of it depends on the size of the concentration gradient. The out word diffusion of K ions will create electrochemical differences between the inner and outer side of the membrane that exactly counterbalances further diffusion of K ions down its concentration gradient. This counterbalance force is called K equilibrium potential.

(2) Electrogenic pump (Na-K pump) which pumps three Na ions out of for every two K ions pumped in. this pump utilizes energy for its action, which is derived from ATP. Thus, for every cycle of the pump the inside of the excitable cell losses one positive charge a process that leads to an excess of positive charges outside. Electrogenic pump creates only -4 mv giving a total resting membrane potential of -90 mv.

The excitable cell and tissue: Define as those cells and tissues that are capable of generating action potential by themselves at their membranes and transmit these action potentials along their membrane such as nerves and muscles.
The action potential of the nerve and skeletal muscle fiber:

(spike potential)

[1] Resting stage: a (circle 1 page 4)

[2] Initiation of an action potential:

threshold level (circle 3) at which action potential will be generated. Therefore, the threshold level can be defined, as the level of membrane potential required to cause an action potential, which is between -50 to –70 mv (about –65 mv in large nerve fiber). The latent period corresponds to the time it takes the impulse to travel along the axon from the site of stimulation to the recording electrode.

[3] Depolarization stage: When the membrane potential reaches the threshold level, the potential across the membrane rises suddenly and rapidly in the positive direction approaching zero or may overshoots and become positive and the normal polarized state of -90 mv is lost (circle 4). This initial, very large sudden rapid change in membrane potential is also called depolarization. The genesis of this depolarization is due to sudden opening of special type of membrane channels for Na ions which their opening depend on the voltage across the membrane (threshold level). Therefore, they are called voltage-gated Na channels and consequently increase the Na permeability of the membrane substantially allowing Na to pass from exterior to intracellular fluid (inward Na+ current). This process will continue in a positive-feedback vicious circle until all of these channels have become totally activated (opened). After the voltage-gated Na channel has remained open for short times, it suddenly closes, and Na ions can no longer pour to the inside of the membrane (circle 5). At this point the membrane potential begins to recover back toward the resting membrane state, which is repolarization process. A very important characteristic of the voltage-gated Na channel is that the gate will not reopen again until the membrane potential returns either to on nearly to the original resting membrane potential level.

[4] Repolarization stage: in which the normal resting membrane-polarizing state is re-established (circles 6, 7, 8). The causes of repolarization are:

[A] Closure of voltage-gated Na channels preventing further inflow of Na ions inside the nerve

fiber.


[B] Opening of voltage-gated K channels which allow the passage of K ions from intracellular fluid (ICF) to extracellular fluid (ECF) (outward K+ current).

[C] The electrogenic pump.

Action potential fails to occurs if:

[A]: the membrane potential rises very slowly the voltage-gated Na channels will then have time to close while the other gates (i.e. voltage-gated K channels) will have the time to open.

[B]: the stimulus is subthreshold in magnitude.

Action potential wave occurs with a constant amplitude and the strength of the stimulus if the stimulus is at or above threshold intensity. The action potential is therefore all or none in character and is said to obey the all or none law.










At the end of repolarization (i.e. end of action potential), the return of the membrane potential to the negative state causes the voltage-gated K channel to close back to their original status after a short delay. This delay in closure of K gates allows excess K ions to diffuse out the nerve fiber, leaving an extra deficit of positive ions on the inside, which mean more negativity. This is called hyperpolarization (circle 7).



initiation and conduction of action potentials in nerve fibers applies equally well to skeletal muscle fiber except that the duration of action potential in skeletal muscle about five times longer and velocity of conduction is much slower than in the large myelinated nerve fiber.
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