Figure 5.3. Cell membrane with ion-channel proteins.Present throughout the nerve cell membrane are various ion-channel
proteins (Fig. 5.3). These are polypeptides made of hundreds of amino
acids folded up into secondary and tertiary structures, with multiple
subunits grouped together into quaternary structures, having pores
or holes through their interiors of the right size and structure to allow
specific kinds of ions to pass through. There are many kinds of ion
channels, including Na channels, K channels, Ca** channels, and Cl-