These characteristics were shown to be a consequence of the conformational change or a phase transition in the lipid molecules that accompany the sound wave. Remarkably, even in such a minimalistic system that is devoid of any proteins and macromolecules other than lipids, these waves behave strikingly similar to nerve impulses in a neuron, including the solitary electromechanical pulse propagation, the velocity of propagation and all-or-none excitation. We showed that sound or compression waves can indeed propagate within a molecular thin film of lipid molecules, mimicking action potentials in the plasma membrane. Therefore, experimental evidence for such a phenomenon was crucial, which was provided by us in 2014. Moreover, sound waves are generally not associated with such characteristics, rather sound waves are known to spread out, disperse, dissipate, superimpose and interfere, which is counter-intuitive given the properties of nerve impulses. As a wave phenomenon, nerve pulse propagation has remarkable properties, such as a threshold for excitation, non-dispersive (solitary) and all-or-none propagation, and annihilation of two pulses that undergo head-on collision. The suggestion has been highly controversial because of the well-accepted and widely successful nature of the electrical basis of nerve pulse propagation in spite of its few inconsistencies. Thus the characteristics of the wave are derived from the principles of condensed matter physics and thermodynamics, unlike the emphasis on molecular biology in the electrical theory. In this framework, the electromechanical nature of the nerve impulse, also known as an action potential, emerges naturally from the collective properties of the plasma membrane, in which the sound or the compression wave propagates.
To address these inconsistencies, researchers had previously proposed that nerve pulse propagation results from the same fundamental principles that cause the propagation of sound in a material and not the flow of ions or current. Furthermore, several studies have reported reversible temperature changes that accompany a nerve impulse, which is inconsistent with the electrical understanding from a thermodynamic standpoint. However, for as long as the electrical theory has been around, scientists have also been measuring various other physical signals that are equally characteristic of a nerve impulse, such as changes in the mechanical and optical properties that propagate in sync with the electrical signal. Nerve impulses are believed to propagate in a manner similar to the conduction of current in an electrical cable. The findings, published in the Journal of Royal Society Interface, provide the experimental evidence that sound waves propagating in artificial lipid systems that mimic the neuron membrane can annihilate each other upon collision – a remarkable property of signals propagating in neurons that was considered to be inaccessible to an acoustic phenomenon. The research was conducted in partnership with Professor Matthias F Schneider at the Technical University in Dortmund, Germany. Rarefaction is a region in a longitudinal wave where the particles are furthest apart.Shamit Shrivastava, a post-doctoral researcher in the Department of Engineering Science, writes about a recent finding that has far-reaching consequences for the fundamental understanding of the physics of the brain. Figure 2 - A simple sine wave, shown as a transverse waveĬompression happens in the region in a longitudinal wave where the particles are closest together. Think of the way a slinky behaves if two people are holding each end and one person quickly sends a number of vibrations down it.
In contrast, longitudinal waves have vibrations along the same axis as the direction in which the wave is traveling. Transverse waves vibrate at 90 degrees to the direction of the wave. Most waves are transverse, including light and the ripples we see on water. Sound waves are longitudinal and should not be confused with transverse waves. Figure 1 - Longitudinal sound wave showing compression (squeezing-high pressure) and rarefaction (spreading-low pressure) of air particles Each individual molecule only moves a small distance as it vibrates, which causes the adjacent molecules to vibrate in a rippling effect all the way to the ear. It's the vibrating air molecules that cause the human eardrum to vibrate, which the brain then interprets as sound.Īir molecules do not travel from the noise source to the ear. air, as it propagates away from its source.Īs sound passes through the air, the air particles move left and right due to the energy of the sound wave passing through it. A sound wave is a pressure vibration caused by the movement of energy traveling through a medium e.g.
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