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Illuminations

POSTSYNAPTIC POTENTIAL SUMMATION AND ACTION POTENTIAL INITIATION: FUNCTION FOLLOWING FORM

Cátedra de Fisiología, Facultad de Ciencias Veterinarias, Universidad Nacional de La Plata, 60 y 118 (B1900AVW) La Plata, Argentina E-mail: mauriciog{at}fcv.unlp.edu.ar

The mechanisms and physiological significance of postsynaptic potential summation and action potential initiation are among the most difficult concepts for students to grasp. To help students understand these concepts, we emphasize that neurons have three distinct functional zones: 1) the "input," 2) the "integrative," and 3) the "conductive" zones (Fig. 1). The input zone, which consists of the dendritic and somatic domains, contains ligand-gated ion channels that are activated by neurotransmitters (ligands) secreted by presynaptic terminals. Activation of the input zone creates a postsynaptic potential. The integrative zone, which consists of the axon hillock domain, summates the postsynaptic potentials and initiates an action potential. Importantly, action potentials depend on the activation of voltage-gated ion channels. The conductive zone, which consists of the axon domain, propagates the action potential. Thus postsynaptic potentials require activation of ligand-gated ion channels located on the postsynaptic membrane, whereas action potentials require activation of voltage-gated ion channels located at very high concentrations along the axon hillock and at lower concentrations along the remainder of the axon. It is important to note that the input zone, dendrites, lack voltage-gated ion channels, whereas the soma contains low concentrations of voltage-gated ion channels. The different location and concentration of ligand-gated vs. voltage-gated ion channels provide the basis for understanding the differences between postsynaptic potentials and action potentials (Table 1) (2).

View larger version (34K): [in this window] [in a new window]   FIG. 1. Features of a motor neuron: ion channel distribution, domains, functional zones, and input and output signals.

We also emphasize that a motor neuron receives over 1,000 presynaptic terminals (1), and each postsynaptic potential is below threshold (Table 2). Thus postsynaptic potentials must summate to reach the depolarization threshold. Summation of postsynaptic potentials occurs when a presynaptic neuron fires repeatedly at a high rate ("temporal summation") or when several presynaptic terminals fire at the same time ("spatial summation") or from a combination of temporal and spatial summation. When the threshold for voltage-gated sodium channels activation is reached (at the hillock) an action potential occurs. This is an all-or-none process, like flushing a toilet: it either occurs or it does not. Finally, we compare a neuron with a circuit of the nervous system. In this analogy, dendrites and the soma are the afferent limb, and the axon is the efferent limb. The axon hillock is the integrative center, gathering information received by the dendrites and soma and deciding to fire an action potential or not. This approach to understanding postsynaptic potential summation and action potential initiation is an excellent example of function following form that students appreciate.

	REFERENCES

TOP REFERENCES

 

1. Bear MK, Connors BW, and Paradiso MA, Editors. Neuroscience: Exploring the Brain. Baltimore, MD: Williams & Wilkins, 1996, p. 111–119.
2. Kandel ER, Schwartz JH, and Jesell TM, Editors. Principles of Neural Science. New York: McGraw-Hill, 2000, pp. 25–35, 222–228.

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