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How is sound frequency mapped in the primary auditory area?

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Diagram comparing action potential conduction in myelinated and unmyelinated axons, showing a neuron with labeled cell body, myelin sheath, nodes of Ranvier, and the flow of electrical signals along each type of axon. — nervous system In a myelinated axon, the myelin sheath prevents the local current (small black arrows) from flowing across the membrane. This forces the current to travel down the nerve fibre to the unmyelinated nodes of Ranvier, which have a high concentration of ion channels. Upon stimulation, these ion channels propagate the action potential (large green arrows) to the next node. Thus, the action potential jumps along the fibre as it is regenerated at each node, a process called saltatory conduction. In an unmyelinated axon, the action potential is propagated along the entire membrane, fading as it diffuses back through the membrane to the original depolarized region.

In the primary auditory area of the cerebral cortex—located in the superior transverse temporal gyri of Heschl—sound frequency is mapped in a tonotopic fashion.12

This means that different sound frequencies are represented on different, specific parts of the auditory cortex. This organization mirrors the processing that begins in the cochlea, where the basilar membrane acts as a frequency analyzer. Because the basilar membrane varies in stiffness and mass along its length, different regions vibrate most vigorously in response to specific frequencies—with high frequencies stimulating the basal end and low frequencies stimulating the apical end.23

This spatial information is preserved as it is relayed through the auditory pathways to the brain, allowing the primary auditory area to distinguish pitch based on the specific group of nerve fibres activated.13