AXON
We have seen that sound travels through the ear until it sets hairs into motion found in the cochlea. Each hair is attached to a nerve cell (neuron). When it moves it distorts the shape of the nerve cell. Up to this point, sound has been represented as physical movement. Now it is turned into a new form, electrical potential.
The interior of the nerve cell consists of a drop of fluid, which is usually maintained at a voltage of -70 millivolts. The outside has a different charge. This difference in the interior and exterior of the nerve cell is characteristic of all cells including plant, skin, bacteria, etc.
When the cell is distorted by sound. the electrical difference changes for a micro second. The change travels through the cell until reaching the axon, a long tentacle growth from the neuron body. The axon is basically the wire carrying info to the brain from nerve cell to nerve cell.
As a result of the distortion, sodium ions on the outside of the cell flows in , and potassium ions flow out. This creates a large number of positively charged ions and the electrical potential at the base of the axon becomes positively charged. Each positive charge travels the length of the axon and on to other axons on the way to the brain.
The axon wiring sends information in one direction so that the positive charge isn't affected by a message coming from the other direction.
The system has a threshold. It will not kick off unless the distortion and voltage change reaches a certain level. This save us from being overwhelmed with input.
SYNAPSE
At the end of the axon, the electrical pulse is near another axon, either in a bundle of nerves or in the brain. The gap between axons is called the synapse. These gaps, about one millionth of an inch across is filled with fluid. The positive charge turns into a transmitter substance and jumps the gap setting the next axon into a similar process. The synapse is like the gap in a spark plug firing off all through or nervous systems.
There are millions of neurons through out the system. A fail safe system of redundancy keeps false information from reaching the brain (like digital systems). More than one axon carries the same charge and if the information is different, it may be discarded.
An interesting feature of the synaptic gap, is that it becomes easier to pass the gap the more it is used. The synapse 'learns' repeated pulses. Thus explaining practicing piano or ear training. Also, old memories are harder to remove than new ones, as the synapses have been trained to recognize older more 'practiced' functions.
Electrical Pulse
The size of electrical pulses generated at the neurons in the ear does not change. Rather the number of pulses sent represent louder or softer sounds. A loud sound may send a stream of pulses while a soft one may send only a few spaced apart. The amount of distortion caused by the hair determines the spacing between pulses.
Brain
The brain is a metropolis of activity as information constantly arrives from all parts of the body, as well as information is sent back out (blink, breathe, move your little toe, etc.).
The information coming in through the left ear is sent to the right side of the brain and visa versa. The pulses may be interpreted in some fashion as it crosses to the other side...there are still a lot of mysteries about the brain. The interpretation of the information and how the brain interprets sound as music or noise is a fascinating area of study...but not for this class.