Action potentials initiation

Semyanov, A. and Kullmann, D. M. (2001) Kainate receptor-dependent axonal depolarization and action potential initiation in intemeurons. Nat. Neurosci. 4,718-723. 

Depolarisation of the membrane of the cardiomyocyte, resulting from the action potential, initiates contraction in cardiac as in skeletal muscle. This depolarisation arises in the sinoatrial node, a small group of cells in the right atrium, and then spreads through the heart causing, first, the muscles in the atria to contract and then the muscles in the ventricles to contract. 

Dionaeae muscipula (Venus flytrap 2 action potential-initiating stimulus events required before trap... 

An important general result demonstrated in Figure 19.4 is that action potential initiation has a threshold behavior. That is, stimuli producing transmembrane voltages above a threshold value initiate action potentials, while those below do not. This response is called all or none, although (as Figure 19.4 shows) there is a variation in response for near-threshold stimuli. Threshold values are not constcint. The threshold value for a particular stimulus varies depending on factors such as the location of the electrodes relative to the tissue that is excited, the stimulus duration, and the amount of membrane affected. 

Koch, C., O. Bernander, and R.J. Douglas (1995). Do neurons have a voltage or a current threshold for action potential initiation J. Comput. Neurosci. 2,63-82. 

FIGURE 30.5 Action potential initiation by extracellular stimulation of CNS neurons by cathodic and anodic stimuli. Each trace shows transmembrane voltage as a function of time for different sections of the neuron, (a) Stimulation with a monophasic cathodic stimulus pulse from an electrode positioned f mm over a node of Ranvier of the axon. Depolarization occurs in the node directly beneath the elertrode (solid arrowhead) and hyperpolarization occurs in the adjacent nodes of Ranvier (open arrowhead). Action potential initiation occurs in the node of Ranvier directly under the electrode (arrow) and the action potential propagates in both directions, (b) During threshold stimulation with an electrode positioned 1 mm over the cell body, action potential initiation occurs at a node of Ranvier of the axon. With cathodic stimuli (duration 0.1 msec) action potential initiation occurred at the second node of Ranvier from the cell body (arrow), (c) With anodic stimuli (duration 0.1 msec) action potential initiation occurred in the third node of Ranvier from the cell body (arrow). 

During stimulation over the axon with a cathodic current the axon is depolarized immediately beneath the electrode, and hyperpolarized in regions lateral to the electrode (arrowheads Figure 30.5a). Action potential initiation occurs in the most depolarized node of Ranvier, immediately beneath the electrode (arrow) and then propagates in both directions. 

Neural Excitation of Muscle. Voluntary contraction of human muscle initiates in the frontal motor cortex of the brain, where impulses from large pyramidal cells travel downward through corticospinal tracts that lead out to peripheral muscles. These impulses from the motor cortex are called action potentials, and each impulse is associated with a single motor neuron. The principle structure of a motor neuron is shown in Fig. 6.IS. The action potential initiates in the cell body, or soma, and travels down a long efferent trunk, called the axon, at a rate of about 80 to 120 m/s. The action potential waveform is the result of a voltage depolarization-repolarization phenomenon across the neuron cell membrane. The membrane ionic potential at rest is distuibed by a surrounding stimulus, and Na ions are allowed to momentarily rush inside. An active transport mechanism, called the Na —K+ pump, quickly returns the transmembrane potential to rest This sequence of events, which lasts about 1 ms, stimulates a succession of nerve impulses or waves that eventually reach muscle... 

As shown in Fig. 19B, SA spikes identical to those initiated by direct current injection are also observed on the depolarizing after-potentials which follow full-blown action potentials initiated by antidromic or orthodromic activation. The SA spikes superimposed on depolarizing afterpotentials can give rise to bursts of full-blown action potentials. Occasionally, the full-blown action potentials (Fig. 20) evoked by perforant path... 

FIGURE30.8 Central nervous s)rstem (CNS) stimulation results in direct effects and indirect effects on CNS neurons, (a) Two-dimensional maps of thresholds for indirect (synaptic) and direct activation of neurons in the red nucleus [from Baldissera et al., 1972]. (b) Complex polyphasic changes in the firing rate of a cortical neuron in response to extracellular stimulation [firom Butovas and Schwarz, 2003]. (c) Transmembrane potential in the axon (top trace) and cell body (bottom trace) of a model thalamocortical neuron before, during (black bar at bottom), and after extracellular stimulation [from McIntyre et al., 2004]. Extracellular stimulation results in simultaneous inhibition of the cell body, as a result of activation of presynaptic terminals and subsequent indirect effects, and excitation of the axon, as a result of direct action potential initiation in a node of Ranvier. (d) Firing rate in the cell body and axon during extracellular stimulation of a model thalamocortical neuron [modified from McIntyre et al., 2004]. The firing rate in the cell body is lower than that in the axon, as a result of simultaneous indirect synaptic effects on the soma, and direct excitation of the axon. 

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