The Nervous System, Ear, Hearing and Balance

Introduction

Before considering the ear and the eye it is first necessary to consider the nervous system. Our knowledge of the outside world is gained through our sense organs. Information from our eyes and ears provide the majority of information but there are other sources.

Our senses of taste and smell both give information to our brain and special nerve cells in our skin can inform us of touch sensations, temperature changes and also detect pain. Within our muscles are pressure sensitive cells that can assist our appreciation of the relative position of our limbs and can detect the effects of gravity.

The Nervous System

General

As the most complex of the systems in the human body, the nervous system is responsible for sending, receiving and processing nerve impulses. It serves as the body control centre and its electrical-chemical communications network. It integrates countless pieces of information and generates reactions by sending these electrochemical impulses through the nerves to trigger organs such as muscles or glands. All of the body’s muscles and organs rely upon the nervous impulses to function.

The nerve cells are called neurons and the connection between two neurons is the synapse. When a nerve impulse (electrical) travels across a neuron to the synapse, it causes a release of chemicals which carry the signal to the next neuron. Thus messages are sent through the nervous system by both electrical and chemical means (electrochemical).

Three systems work together to carry out the mission of the nervous system:

Central Nervous System (CNS)

Encased in bone, the CNS is responsible for issuing nerve impulses and analysing sensory data. It consists of the brain and the spinal cord. The brain weighs an average of 1.4 kg and comprises 97% of the entire nervous system. Nerve pathways extend from the brain to virtually every tissue and structure of the body.

Peripheral Nervous System (PNS)

The PNS is made up of a series of specialised cells that both pass information received from the body organs and muscles back to the CNS through sensory nerves. It also is responsible for passing information from the CNS directly to the organs and muscles themselves through motor nerves. Thus it is not always necessary for the brain to be involved in the reaction to stimulus. Reflex actions, such as quickly withdrawing one’s hand from a painful stimulus, will only involve a loop between the hand and the spinal cord. Because fewer neurones are involved, reflexes are rapid.

Autonomic (Vegetative) Nervous System (ANS)

A special autonomic (independent) nervous system manages the glands of the body and the involuntary muscles of the internal organs and blood vessels. Although the autonomous nerves have connections with the CNS, we are not aware of the autonomic system working and have no conscious control over it.

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As we have seen, breathing is regulated by the autonomic system and other functions include:

•Arterial pressure.

•Gastrointestinal motions.

•Urinary output.

•Sweating.

•Body temperature.

•General Adaption Syndrome (GAS).

This is sometimes known as the Fight or Flight Response. The GAS Syndrome is reliant upon the Sympathetic and Parasympathetic Systems which are part of the ANS. This Syndrome is discussed in Chapter 7 - Stress).

The ANS is a biological control system which is neurohormonal and, like others, is highly selfregulating in normal circumstances or environments.

The Sense Organs

The two most important of our senses in aviation are sight and hearing. In this and the following chapter we shall be considering the ear and the eye which provide us with the majority of the information essential to the appreciation of our position in space and our sense of balance. They are of particular concern to the aviator as, in an environment for which evolution has not adapted us, they may present incorrect or misleading information.

The Ear - Purpose

The ear performs two quite separate functions. Firstly it is used to receive vibrations in the air (sounds), and secondly it acts as a balance organ and acceleration detector. See Figure 4.1 for the construction of the ear.

The ear is divided into three sections, the outer, middle, and inner ear.

Outer Ear

The outer ear directs sounds, which are collected by the pinna, through the auditory canal (Meatus) and onto the eardrum. The sound waves will cause the ear drum to vibrate.

The Middle Ear

The ear drum - or tympanum - separates the outer and middle ear. Connected to the ear drum is a linkage of three small bones the ossicles (the malleus, incus and stapes) which transmit the vibrations across the middle ear, (filled with air) to the inner ear which is filled with liquid. The last of the bones (the stapes) is attached to the oval window of the inner ear where a diaphragm sets in motion the fluid of the cochlea of the inner ear. The eustachian tube vents to the mouth and nose allowing pressure to equalize across the ear drum. The phenomena of otic barotrauma which is associated with the eustachian tube is discussed in detail in Chapter 6 (Health and Flying).

The Inner Ear

The diaphragm attached to the stapes causes the fluid in the cochlea to vibrate. Inside the cochlea there is a fine membrane covered with tiny hair-like cells. The movement of these small cells will be dependent on the volume and pitch of the original sound. The amount and frequency of displacement is detected by the auditory nerve which leads directly to the cortex of the brain where the tiny electrical currents are decoded into sound patterns. It is possible for hearing also to bypass the ear drum and ossicular system and for the transmission of sound to pass through the bone. This is because the cochlea is embedded in a bony cavity within the temporal bone. Vibrations of the entire skull can cause fluid vibrations in the cochlea itself. Therefore, under certain conditions, a tuning fork or vibration device placed on the skull causes the person to hear the note/sound.

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