Selective Permeability of Membranes Some ions permitted to cross more easily than others Neuronal membranes contain ion channels >Protein tubes that span the membrane Some stay open all the time (nongated) Some open on the occasion of an action potential causing a change in the permeability of the membrane(gated)6 Selective Permeability of Membranes ➢Some ions permitted to cross more easily than others ➢Neuronal membranes contain ion channels ➢Protein tubes that span the membrane ➢Some stay open all the time (nongated) ➢Some open on the occasion of an action potential, causing a change in the permeability of the membrane (gated)