FIGURE 10-20 Role of alternative splicing in the perception of sounds of different frequencies. (a) The chicken cochlea, a 5-mm-long tube, contains an epithelium of auditory hair cells that are tuned to a gradient of vibrational frequencies from 50 Hz at the apical end (left) to 5000 Hz at the basal end (right). (b) The Ca²⁺-activated K⁺ channel contains seven transmembrane α helices (S0–S6), which associate to form the channel. The cytosolic domain, which includes four hydrophobic regions (S7–S10), regulates opening of the channel in response to Ca²⁺. Isoforms of the channel, encoded by alternatively spliced mRNAs produced from the same primary transcript, open at different Ca²⁺ concentrations and thus respond to different frequencies. Red numbers refer to regions where alternative splicing produces different amino acid sequences in the various isoforms. See K. P. Rosenblatt et al., 1997, Neuron 19:1061.