Action potentials equilibrium

Immediately after the passage of an action potential, the Na ion channels close spontaneously and cannot be re-opened for a period of time. This is the refractory period. An action potential therefore cannot proceed in the opposite direction, i.e. it is unidirectional, which imposes a direction on propagation of the whole action potential. This is analogous to the means by which directionality is achieved in a metabolic pathway or a signalling sequence of reactions within either process there is at least one irreversible (non-equilibrium) reaction which provides directionality (Chapter 2). 

In any signalling process, it is essential that the signal travels only in one direction (e.g. action potential in a nerve, signalling in hormone action). To do this, non-equilibrium reactions must be included in the sequence. 

An equilibrium potential exists for each of the ions involved. This is the value of the membrane potential at which there is no net inflow or outflow of the ions concerned. For IC ions, the resting potential lies in the range of the membrane potential, while for Na" ions it is much higher at +70 mV. At the first opportunity, Na" ions will therefore spontaneously flow into the cell. The occurrence of action potentials is based on this (see B). 

To produce membrane depolarization, a current stimulus of sufficient intensity to exceed the outward K+ current must be appUed to the cell. If the depolarizing stimulus raises the membrane potential above a threshold value, sodium channels within the sarcolemmal membrane change their conformation and open their ion-selective pore, allowing Na to enter the cell driven by the electrochemical gradient. The open sodium channels raise the membrane potential toward the equilibrium potential of sodium (-f65 mV) and set into motion the intricate and precisely coordinated series of ion channel openings and closings leading to the characteristic action potential. 

Final repolarization (phase 3) of the action potential results from completion of sodium and calcium channel inactivation and the growth of potassium permeability, so that the membrane potential once again approaches the potassium equilibrium potential. The major potassium currents involved in phase 3 repolarization include a rapidly activating potassium current (Ikt) and a slowly activating potassium current (Iks)- These processes are diagrammed in Figure 14-3. 

Figure 4.10. Overview of nerve impulse transmission in chemical synapses. The action potential in the presynaptic nerve cell induces release of the nemotransmitter (e.g., acetylcholine) into the synaptic cleft. The transmitter binds to its receptor, e.g. the nicotinic acetylcholine receptor (NAR). The NAR is a hgand-gated channel it will open and become permeable to both and Na. This will move the membrane potential toward the average of the two respective equilibrium potentials however, in the process, the firing level of adjacent voltage-gated sodium charmels will be exceeded, and a full action potential will be triggered (inset).

A) On the initation of an action potential the membrane potential moves from the resting potential upward toward the Na equilibrium potential and then downward toward the equilibrium potential. 

The Action Potential. The electrical equilibrium of the resting neuron is rapidly changed upon excitation. Opening and closing of ionic pores appear to be passive processes that do not require the direct input of metabolically derived energy. This important conclusion is based on experi-... 

Simply put, the action potential is caused by a state of disequilibrium between ideal electrical potentials for two ions, sodium and potassium. The equilibrium potentials for Na" " and K can be thought of as the electrical force required to maintain the given ionic gradients across the cell membrane for each ion. For Na" ", the equilibrium potential is approximately 50 mV (with respect to the inside of the membrane) for K +, it is approximately —75 mV. (These values apply to the giant squid axon on which the early investigations on action potentials were conducted. Of course, these values... 

Fig. 2. Diagrammatic representation of oscilloscope tracing of typical action potential. Conductance (G) changes for particular ions are shown as a function of time. Equilibrium potentials for Na+ and K+ (ENa+ 4 f K+) are also depicted.

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