Biological data transmission

Energy efficient hearing by enforced nanoscale calcium signaling

February 24, 2015

The hair cells of the inner ear can translate atomsize vibrations into electrical nerve impulses. Within the human body, they are a cell type with exceptional capabilities. By precisely coordinated biophysical processes, they manage to translate acoustic signals into electrical impulses that are then transmitted to the brain. In order to keep the brain at any time up-to-date, they are capable of releasing neurotransmitters at rates of up to 1000 events per second. With unprecedented precision, scientists at University Medical Center Göttingen, the Max Planck Institute for Dynamics and Self-Organization and the Bernstein Center Göttingen have examined the actual path lengths over which the important messenger calcium spreads inside the cell and identified the processes that keeps the messenger on course. The energy efficiency of the cells is maximized through short distances and the comprehensive capture of calcium ions that go astray. The new findings are published today in the journal PNAS (Proceedings of the National Academy of Sciences of The United States of America).

Taking the short route: nanoscale calcium signaling at hair cell synapses

Sensory and nerve cells forward information through specialized cell junctions called synapses. Here, the information is transmitted by tiny packages (vesicles) containing neurotransmitter molecules that are released by one of the cells and that can be detected by the neighboring cell. The "sending" cell conveys the order to release the neurotransmitters by means of calcium ions. In its cell membrane, there are molecular "pores"—so-called ion channels—that sense the excitation of the cell and allow calcium ions to enter when a certain threshold is reached. In hair cells, these pores are the key translation device between the acoustic signals and the nerve impulses that are sent to the brain.

In order to fulfill its signal transmission mission, the calcium ions must quickly reach a sensor protein on the vesicles. The Max Planck researchers have calculated that this molecular sensor is located less than 20 nanometers (about 200 atomic diameters) from the ions entry point into the cell. Because they are randomly hit by the surrounding water molecules, it is physically inevitable that many of the entering ions move into wrong directions or overshoot the target location. Without special precautions these misguided ions could trigger the release of neurotransmitters outside the contact points—which would be wasteful as they would go undetected outside the synapse. Such a misplaced release would be a significant waste of energy as the vesicles and messengers must be expensively recycled after each release.

With their new results, the Göttingen researchers show that the hair cells in the inner ear use special calcium buffering proteins to capture ions that go astray. By means of a high concentration of three different calcium buffering proteins, the cells avoid the waste of energy.