Biology – Organ Of Hearing {The Ear}

 Biology

Topic: Organ of Hearing

The Ear

The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear, middle ear and the inner ear.

Sound funnels through the pinna into the external auditory canal, a short tube that ends at the eardrum (tympanic membrane).

Sound causes the eardrum and its tiny attached bones in the middle portion of the ear to vibrate, and the vibrations are conducted to the nearby cochlea. The spiral-shaped cochlea is part of the inner ear; it transforms sound into nerve impulses that travel to the brain.

The outer ear consists of the pinna and the ear canal. Since the outer ear is the only visible portion of the ear in most animals, the word “ear” often refers to the external part alone. The middle ear includes the tympanic cavity and the three ossicles. The inner ear sits in the bony labyrinth, and contains structures which are key to several senses: the semi-circular canals, which enable balance and eye tracking when moving; the utricle and saccule, which enable balance when stationary; and the cochlea, which enables hearing. The ears of vertebrates are placed somewhat symmetrically on either side of the head, an arrangement that aids sound localisation.

The middle ear lies between the outer ear and the inner ear. It consists of an air-filled cavity called the tympanic cavity and includes the three ossicles and their attaching ligaments; the auditory tube; and the round and oval windows. The ossicles are three small bones that function together to receive, amplify, and transmit the sound from the eardrum to the inner ear.

The inner ear sits within the temporal bone in a complex cavity called the bony labyrinth. A central area known as the vestibule contains two small fluid-filled recesses, the utricle and saccule. The inner ear structurally begins at the oval window, which receives vibrations from the incus of the middle ear. Vibrations are transmitted into the inner ear into a fluid called endolymph, which fills the membranous labyrinth.

The ear has two major functions:

• Hearing: detection of vibrations, their frequency (pitch) and amplitude (loudness).
• Balance: detection of direction of motion, acceleration and head position related to gravity.

The middle ear includes:

• eardrum
• cavity (also called the tympanic cavity)
• ossicles (3 tiny bones that are attached)
  • malleus (or hammer) – long handle attached to the eardrum
  • incus (or anvil) – the bridge bone between the malleus and the stapes
  • stapes (or stirrup) – the footplate; the smallest bone in the body

The inner ear includes:

• oval window – connects the middle ear with the inner ear
• semicircular ducts – filled with fluid; attached to cochlea and nerves; send information on balance and head position to the brain
• cochlea – spiral-shaped organ of hearing; transforms sound into signals that get sent to the brain
• auditory tube – drains fluid from the middle ear into the throat behind the nose

 

When the stapes moves, it pushes the oval window, which then moves the cochlea. The cochlea takes the fluid vibration of sounds from the surrounding semicircular ducts and translates them into signals that are sent to the brain by nerves like the vestibular nerve and cochlear nerve.

Inner Ear Functions

1. The vibrations set the liquid contained in the inner ear, in motion.
2. The ciliate cells located in the liquid amplify the sound vibrations and categorize them by frequency. They transform mechanical energy into nerve signals.
3. The auditory nerve collects all the pulses emitted by the ciliate cells.
4. The brain registers, analyses and interprets all the information.

Ear Structure

• Pinna: This is said to collect sound waves and channel them into the external auditory canal.
• External Auditory Canal: Earwax and hairs prevent foreign material from entering the ear.
• Eardrum (tympanum): vibrates with the same frequency as the sound waves beating against it and it also separate the outer ear from the middle ear.
• Ear Ossicles (hammer, anvil, stirrup): transmit and amplify the vibrations of the eardrum to the inner ear.
• Eustachian Tube: When it opens, it equalizes the air pressure on both sides of the eardrum allowing the eardrum to vibrate freely so accurate sound sensations are generated.
• Oval Window: a flexible partition allowing transfer of vibrations to inner ear.
• Round Window: a flexible partition to deaden the pressure changes in the inner ear.
• Cochlea: conversion of sound waves into nerve impulses — ‘hearing’ apparatus.
• Auditory Nerve: carries impulses from the cochlea to the brain’s hearing centre.
• Semicircular Canals: detection of direction of motion and acceleration — balance.
• Utricle and Saccule: detection of acceleration and head position related to gravity — balance.
• Vestibular Nerve: carries impulses from the balance organs to the cerebellum.

Correction of a Hearing Defect

In children middle ear inflammation, called otitis media, is a common cause of hearing loss.

• Cause: food allergy or bacterial infection.
• Treatment: antibiotics or avoidance of the allergen.

The Function of hearing

• Motor and reflex effects: position of the eyes and head. Maintaining balance.
• Effects on higher functions: thought, memory, language, body language, emotions.
• Sensory effects: position and movement of the head.
• Sensorial effects: perception of sound.
• Effects on state of consciousness and vigilance: triggering of sleeping and waking phases. Maintaining alertness.

Mechanism of Hearing

Sound waves are converted to nervous impulses in the following stages:

1. The waves are picked by pinna, passed down to the ear tube (auditory canal) which makes it vibrate.
2. The vibrations on the ear drum set up further vibrations of the ossicles. They act as levers by magnifying sound waves at the end of the stapes.
3. The stapes vibrate against the oval window, thereby setting up vibration in the perilymph. This passes down into the cochlea.
4. The vibration is further transmitted into the endolymph, from where it gets to organ of corti where nervous impulses are triggered off along the auditory nerve to the brain and back, and we are made to hear.

Sound circuit

1. The sound arrives in the auditory canal
2. The sound causes the eardrum to vibrate
3. The malleus and incus transmit the vibrations
4. The inner ear decodes the sound and sends it to the auditory nerve
5. The auditory nerve conveys the sound to the brain
6. The brain analyses and interprets the sound

Mechanism of Balancing

Certain mechanisms are involved in the maintenance of balance. It all begins with the head. Head movement in any direction affects the fluid or endolymph in the corresponding semicircular canal which are at right angle to each other. This forces the sensory cells in the ampulla to set up impulses through the auditory nerve to the brain and intern for interpretation. The brain relays impulses to the body muscles for balancing or determining the position of the body.  

Care of the Ear

1. Avoid loud noise
2. Clean with cotton buds and avoid using sharp objects as it can damage the ear drum.
3. Seek medical advice in case you suspect any infection.