Extracellular Cytoplasmic COOH Diagrammatic representation of the Ca2+ cha pore-forming subunits of three ion channels The a subunit of the Nat and Ca2+ channels traverse the membrane 24 COOH times in four repeats of six membrane spanning units. Each repeat has a"P loop between membrane spans 5 and 6 that does not traverse the membrane K+ channel These p loops are thought to form the pore. Note that span 4 of each repeat is colored in red, representing its net“+” charge. The k channel has only a single repeat of the six spanning regions and P COOH loop. Four K subunits are assembled for a functional K channel anong'sReviewofMedicalPhysiologywwrw.accessmedicine.com opyright s The McGraw-Hill Companies, Inc. All rights reserved.Diagrammatic representation of the pore-forming subunits of three ion channels. The α subunit of the Na+ and Ca2+ channels traverse the membrane 24 times in four repeats of six membrane￾spanning units. Each repeat has a “P” loop between membrane spans 5 and 6 that does not traverse the membrane. These P loops are thought to form the pore. Note that span 4 of each repeat is colored in red, representing its net “+” charge. The K+ channel has only a single repeat of the six spanning regions and P loop. Four K+ subunits are assembled for a functional K+ channel