An action potential’s threshold and shape are governed by the distribution and subunit composition of voltage-gated sodium and potassium channels in the axon. To learn how differences in subunit expression might contribute to the exact site of action potential initiation, Lorincz and Nusser examined the distribution of four potassium and sodium channel subunits (Nav1.1, Nav1.6, Kv1.1, and Kv1.2) in the axon initial segment (AIS) of neurons in several regions of adult rat brain. The expression pattern was surprisingly heterogeneous across cell types and brain regions. For example, only inhibitory interneurons expressed Nav1.1, and in some neurons, it was expressed along the entire AIS, whereas in others it was restricted to the proximal AIS. Likewise, expression of other subunits was uniform or graded depending on cell type. In Purkinje cells — in which action potential generation occurs in the first node of Ranvier rather than the AIS — neither potassium channel subunit was expressed in the AIS.
Channel Subunits Are Heterogeneously Expressed in the AIS
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