The Nervous System, Ear, Hearing and Balance — Page 60, Lesson 95

I want to walk you through the nervous system and the ear, starting with a quick recap of the autonomic nervous system, or ANS. As we've seen, breathing is regulated by the autonomic system, and it also controls a range of other functions. Those include arterial pressure, gastrointestinal motions, urinary output, sweating, body temperature, and something called General Adaptation Syndrome, or GAS. GAS is sometimes known as the fight or flight response. The GAS syndrome relies on the sympathetic and parasympathetic systems, which are both parts of the ANS. We'll discuss that syndrome in detail in Chapter 7 on Stress. The ANS itself is a biological control system that is neurohormonal. That means it works through both nerves and hormones. Like other control systems in the body, it is highly self-regulating under normal circumstances or environments. Now let's move to the sense organs. In aviation, the two most important senses are sight and hearing. In this chapter and the next, we'll consider the ear and the eye. These two organs provide us with the majority of the information essential to appreciating our position in space and our sense of balance. They are of particular concern to you as an aviator because, in an environment for which evolution has not adapted us, they may present incorrect or misleading information. That's a critical point: your senses can lie to you in flight. Let's look at the ear and its purpose. The ear performs two quite separate functions. First, it receives vibrations in the air — that is, sounds. Second, it acts as a balance organ and an acceleration detector. I want you to look at Figure 4.1, which shows the construction of the ear. The ear is divided into three sections: the outer ear, the middle ear, and the inner ear. Starting with the outer ear: the outer ear directs sounds that are collected by the pinna — that's the visible part of your ear — through the auditory canal, which is also called the meatus, and onto the eardrum. The sound waves cause the eardrum to vibrate. Now the middle ear. The eardrum, also called the tympanum, separates the outer ear from the middle ear. Connected to the eardrum is a linkage of three small bones called the ossicles. Their names are the malleus, the incus, and the stapes. These three bones transmit the vibrations across the middle ear. The middle ear is filled with air. The vibrations travel to the inner ear, which is filled with liquid. The last of these bones, the stapes, is attached to the oval window of the inner ear. At the oval window, a diaphragm sets in motion the fluid of the cochlea, which is the hearing organ of the inner ear. There's also the eustachian tube. It vents to the mouth and nose, allowing pressure to equalize across the eardrum. This is crucial for you as a pilot because pressure changes during flight can affect your ears. The phenomenon of otic barotrauma, which is associated with the eustachian tube, is discussed in detail in Chapter 6 on Health and Flying.