Oral Presentation Dec 10th 2009

Karthikeyan Subramanian

This presentation discusses the idea that the human auditory system would evolve in the distant future to protect the inner hair cells in the cochlear subsystem from the impact of continuous loud low-frequency environmental noise through a closed-loop negative feedback mechanism. The proposed mechanism would employ a set of muscle fibers between the cochlear structure and the ossicles to increase loading of the ossicle subsystem. This in turn would reduce the vibration levels in the cochlear fluid, thus protecting the inner hair cells from damage. The muscle fibers would be activated by an excitatory neuron activated by the movement of the round window beyond a threshold level.

Agenda1. Current Design of the Auditory System2. Issues with Current Design3. Proposed redesign4. Theory of Operation5. Benefits of the New Design6. Limitations and Assumptions7. Possible Evolutionary Path8. Conclusion

Current Design of the Auditory System

OSSICLE SUB-SYSTEM

Issues with Current Design Continuous loud low-frequency noise suspected to

be a leading cause of deafness in the modern world. Current design of the auditory system evolved in a

pre-industrial environment. Not suitable for current environmental conditions with

high levels of continuous noise due to industrial activity.

Built-in mechanism of Attenuation reflex is an open-loop inhibition mechanism and not particularly effective against continuous loud noise . What is needed is a closed-loop negative feedback

inhibitory mechanism

Proposed Redesign

Feedback Neuron

Muscle Fibers

Hypothetical Activation/Response

Theory of Operation Feedback neuron will fire action potentials that will

cause muscle contraction. Loading of the ossicle subsystem will be proportional

to the noise level (negative feedback mechanism). Feedback circuit will be activated only beyond a

threshold noise level (round window movement) Time constant of the feedback neuron activation

response would ideally be long. Recovery of the feedback neuron when noise

ceases would also take long.

Benefits of the New Design Protect the inner hair cells in the cochlear system

from damage caused by sustained large vibrations. Reduce the risk of deafness caused by exposure to

continuous loud noise.

Limitations and Assumptions Reduced hearing when the feedback system is in the

activated state. More moving parts in the system increasing the risk

of mechanical failure and pathological effects. Assuming that the excitatory neuron in the feedback

circuit is not subject to habituation effects.

Possible Evolutionary Path Feedback excitatory neuron with mechanically gated

ion channels connected to round window and activated by its longitudinal movement (30,000 yrs)

Muscle fibers connected to feedback neuron and incus-stapes junctions (20,000 yrs)

Optimization/Tuning of the redesigned negative feedback system (10,000 yrs)

Conclusion Modification of the auditory system will benefit

human beings by protecting the auditory system from the impact of continuous loud environmental noise.

As human auditory system is already highly evolved, proposed idea would be considered only a minor improvement over existing design.

Hope that nature will consider the proposed redesign of the auditory system as part of its long-term evolutionary plans.

References Principles of Neural Science, Kandel et al, 4th

Edition. Textbook of Medical Physiology, Guyton and Hall,

11th Edition.