<P> Voltage - dependent calcium channels (VDCCs) are a group of voltage - gated ion channels found in the membrane of excitable cells (e.g., muscle, glial cells, neurons, etc .) with a permeability to the calcium ion Ca . These channels are slightly permeable to sodium ions, so they are also called Ca - Na channels, but their permeability to calcium is about 1000-fold greater than to sodium under normal physiological conditions . At physiologic or resting membrane potential, VDCCs are normally closed . They are activated (i.e., opened) at depolarized membrane potentials and this is the source of the "voltage - dependent" epithet . The concentration of calcium (Ca ions) is normally several thousand times higher outside of the cell than inside . Activation of particular VDCCs allows Ca to rush into the cell, which, depending on the cell type, results in activation of calcium - sensitive potassium channels, muscular contraction, excitation of neurons, up - regulation of gene expression, or release of hormones or neurotransmitters . VDCCs have been immunolocalized in the zona glomerulosa of normal and hyperplastic human adrenal, as well as in aldosterone - producing adenomas (APA), and in the latter T - type VDCCs correlated with plasma aldosterone levels of patients . Excessive activation of VDCCs is a major component of excitotoxicity, as severely elevated levels of intracellular calcium activates enzymes which, at high enough levels, can degrade essential cellular structures . </P> <P> Voltage - dependent calcium channels are formed as a complex of several different subunits: α, α δ, β, and γ . The α subunit forms the ion conducting pore while the associated subunits have several functions including modulation of gating . </P>

Where are the calcium-voltage gated channels located in the neuron