Fifth lecture

Sodium and potassium pump Na+ & K+ pump Na+ ions are not allowed to accumulate inside the nerve cell and pumped outside by an active process. In the same time K+ ions are pumped from outside to inside the nerve cells. The pumping of these ions is linked together because they are both transported by the Na+ - K+ ATP as in the membrane.

Mechanism of Na+ - K+ pump The pump is formed of a carrier formed of a large phospholipid molecule in the cell membrane which has ATPase property. At rest, this carrier molecule has a 3 Na+ binding site at the inner surface and a K+ binding site at the outer surface of the cell membrane. the carrier molecule undergoes transformation which results in splitting of ATP release of Na+ outside and release of K+ inside the cell. If Na – K pump inhibited by cooling, Na+ ions will accumulate inside the cell, the resting membrane potential becomes progressively lost.

Action potential Electrical changes which occur in the resting membrane potential as a result of stimulation by an effective stimulus. These electrical changes propagate to the effector organ producing the response or action.

(A) Depolarization Stimulus increasing the permeability of the membrane to sodium. If the stimulus is suffiently strong, it results in an action potential. The stimulus increases the permeability of the cell membrane to Na+ ions, which diffuse inside causing gradual change in the membrane potential from the resting potential (-70 m.v) to the threshold potential or the firing level (-55 m.v). At that level, the gates of the voltage activated sodium channels open and Na+ ions flow into the cell (Na influx). Na+ ions pass into the cell until another type of gates closes the channels. The membrane potential quickly reaches zero, potential and then overshoots to about +35 m.v, there is a momentary reversal in polarity. The area of depolarization then spreads down the length of the axon, till it reaches the effector organ which produces response.

1-Repotarization results from closure of sodium gates. (B) Repolarization At the threshold potential (-55 m.v), the voltage sensitive K+ channels also open. These channels begin to act after a slight delay time, and stay open until the membrane reaches the resting potential. 1-Repotarization results from closure of sodium gates. 2-Opening of the potassium gates, allowing K+ ions outside returns the inside of the membrane to its original negative potential (-70) i.e. restores the resting membrane potential which means repolarization.

Redistribution of ions inside (C) Redistribution of ions inside and outside the membrane After passage of an action potential, the ionic composition is slightly disturbed. Redistribution of Na+ and K+ ions is established by the Na – K pump.

Propagation of the action potential (conductivity) An action potential is iniated, it propagates along the axon from the region of the initial segment down to the terminal ending.

A) Propagation in unmyelinated nerve fibers (Continuous conduction) During the action potential the stimulated membrane becomes depolarized (membrane potential becomes +35 m.v), this creates a potential difference between the depolarized (active) area and the adjacent polarized (resting) area (-70 m.v). Because of this potential difference local current flows between the two areas causing the polarized (resting) area to become depolarized to the threshold level. This generates an action potential at the resting area, which by turn becomes the stimulus for generating another action potential in the next area along the nerve fiber. This type of conduction relatively slow 0.5-2.0 meters/sec.

Propagation in myelinated fibers (Sultatory conduction) In myelinated nerve fibers, the myelin acts as an effective insulator around the axon except at the nodes of Ranvier, where the membrane of the axon is exposed and contains large number of Na+ and K+ channels.

Stimulation of a myelinated nerve fiber leads to generation of an action potential at the nearest node of Ranvier. During action potential, the first node of Ranvier, becomes depolarized, this creates a potential difference between the depolarized (active) and the next polarized (resting) node of Ranvier. Because of this potential difference charges jump from one node to another leading to depolarization of the next polarized node. The greater the distance between nodes of Ranvier, the greater the velocity of conduction of the action potential. It is faster (may reach up to 120 mer/sec). Salutatory conduction and myelin sheath has the following significance: a- It increases the velocity of conduction. b- It decreases the energy needed for the Na+ - K+ pump