The ear: hearing and balance · Biology Class 11–12 | StemPy Lab
Course 17.3·Lesson 3 of 3
The ear: hearing and balance
The ear has three parts. The external ear collects sound and the ear drum vibrates. The three ossicles of the middle ear pass the vibration to the oval window of the cochlea. In the cochlea the hair cells of the organ of Corti make the impulses. The vestibular apparatus senses the balance of the body.
about 35 min2 simulations1 practice question1 Python program5 questions to check yourself
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Two jobs
The ear has two sensory functions: hearing and the balance of the body. It has three parts: the external ear, the middle ear and the inner ear. This lesson is taught for the boards, A-Level and IB. It is not in the NEET syllabus.
The external ear
The pinna collects the sound waves.
The external auditory canal (meatus) leads to the tympanic membrane, the ear drum.
The skin of the pinna and the canal has hairs and glands that make wax.
The middle ear
The middle ear holds three small bones, the ear ossicles, joined in a chain.
The malleus is attached to the tympanic membrane.
The incus lies in the middle.
The stapes is attached to the oval window of the cochlea.
The ossicles increase the efficiency of the transmission of sound waves to the inner ear. The Eustachian tube joins the middle ear with the pharynx. It helps to equalise the pressure on the two sides of the ear drum.
A labelled section of the human ear. The external ear has the pinna, which collects sound waves, and the external auditory canal, which leads to the tympanic membrane. The middle ear holds the three ear ossicles joined in a chain: the malleus is attached to the tympanic membrane, the incus is in the middle and the stapes is attached to the oval window of the cochlea. The Eustachian tube joins the middle ear with the pharynx. The inner ear has the coiled cochlea, the round window below the oval window, and the vestibular apparatus with the three semicircular canals. The auditory nerve leaves the cochlea and goes to the brain.
You should find that the vibration passes through air, then through the bones, and then through a fluid, and that only the last step makes nerve impulses.
The inner ear
The inner ear holds a fluid filled membranous labyrinth, which lies inside a bony labyrinth. The membranous labyrinth is filled with a fluid, the endolymph. A fluid called perilymph fills the space between the two labyrinths.
The coiled part is the cochlea. Two membranes, Reissner's membrane and the basilar membrane, divide the cochlea into three chambers.
The scala vestibuli is the upper chamber. It ends at the oval window.
The scala tympani is the lower chamber. It ends at the round window, which opens to the middle ear.
The scala media is the middle chamber. It is filled with endolymph.
The organ of Corti rests on the basilar membrane. It has rows of hair cells, which are the receptors for sound. Each hair cell has many hair like processes, the stereocilia. A thin elastic tectorial membrane lies above the hair cells. The base of each hair cell touches the afferent nerve fibres.
Two panels. On the left, the cochlea drawn as if uncoiled, a long tapering tube. The scala vestibuli runs along the top from the oval window at the wide end, and the scala tympani runs along the bottom and ends at the round window. The scala media lies between them. The basilar membrane separates the scala media from the scala tympani, and the two chambers join at the narrow tip. An arrow shows the sound wave entering at the oval window. On the right, a cross section of the cochlea with the scala vestibuli on top, the scala media in the middle with Reissner's membrane above it and the basilar membrane below it, and the scala tympani at the bottom. The organ of Corti rests on the basilar membrane. It has rows of hair cells with stereocilia that touch the tectorial membrane above, and afferent nerve fibres at the base of the hair cells.
How we hear
Sound waves enter the external auditory canal and make the tympanic membrane vibrate.
The vibration passes along the three ossicles to the oval window.
It makes waves in the fluid of the cochlea.
The waves make ripples in the basilar membrane. The hair cells bend and press against the tectorial membrane.
Nerve impulses arise in the afferent neurons.
The auditory nerve carries them to the auditory cortex of the brain, where the sound is recognised.
Balance
The vestibular apparatus lies above the cochlea. It has three semicircular canals, each in a different plane, and the otolith organ, which is made of the saccule and the utricle.
The swollen base of each canal is the ampulla, with a receptor called the crista ampullaris.
The saccule and the utricle each have a projecting ridge, the macula.
The crista and the macula are the receptors for the balance of the body and for posture.
The vestibular apparatus of the inner ear. Three semicircular canals lie in three different planes. The base of each canal is swollen to form an ampulla, which holds a receptor called the crista ampullaris. Between the canals and the cochlea lie two sacs of the otolith organ, the utricle and the saccule. Each sac has a projecting ridge called the macula, with small crystals above its hair cells. A small inset shows the crista with its hair cells and a gelatinous cap. A note says that the crista and the macula are the receptors for the balance of the body and for posture.
Sound in numbers (not in the NEET syllabus)
Three standard facts are useful for A-Level and IB. NCERT does not give them.
The ear drum has an area of about 55 mm2 and the oval window about 3.2 mm2. The same force on a smaller area gives a larger pressure, by about 17 times. The lever action of the ossicles adds a factor of about 1.3. The total gain of pressure is about 22 times, which is 27 decibels.
Different parts of the basilar membrane respond to different pitches. High tones peak near the base of the cochlea, and low tones peak near the apex.
Young people hear tones from about 20 Hz to about 20 000 Hz.
A graph of the frequency of a tone in hertz, on a logarithmic scale from 20 to 20000, against the distance along the basilar membrane from the apex of the cochlea in millimetres, from 0 to 35. The curve rises steeply: low tones peak near the apex and high tones near the base. Two points are marked: 440 Hz at about 9 mm from the apex and 4000 Hz at about 23 mm from the apex. A note says that it is a model, the Greenwood function for a cochlea 35 mm long.
You should find that a tone of 1000 Hz peaks near the middle of the membrane, and that the high tones crowd near the base. This is why damage at the base of the cochlea takes away the high tones first.
Try it in Python
The program works out the pressure gain of the middle ear.
Practise with new numbers
Common mistakes
Mixing up the order of the ossicles. The order from the drum is malleus, incus, stapes.
Saying that the Eustachian tube joins the ear with the nose. It joins the middle ear with the pharynx.
Saying that the organ of Corti is in the middle ear. It is on the basilar membrane in the cochlea.
Saying that the cochlea senses balance. The cochlea senses sound. The vestibular apparatus senses balance and posture.
Calling the endolymph and the perilymph the same. The endolymph is inside the membranous labyrinth, and the perilymph lies between it and the bony labyrinth.