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LESSON 21

Middle ear muscles

Year 1
Fall semester
Anatomy I
Ear anatomy
Duration: 8:57

Two tiny muscles in the middle ear attach to the ossicular chain: the stapedius and the tensor tympani. In this lesson we cover where they lie and attach, the embryological reason they are supplied by the facial and trigeminal nerves, the acoustic reflex arc, and the clinical importance of these muscles.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will meet the two small muscles of the middle ear: the stapedius and the tensor tympani.
We will learn where they attach, which nerves drive them, and how they relate to the acoustic reflex.
Two small skeletal muscles in the middle ear attach to the ossicular chain.
The first is the musculus stapedius, the stapedius muscle; it attaches to the stapes.
The second is the musculus tensor tympani, the muscle that tenses the eardrum; it attaches to the malleus.
Both muscles lie inside a bony canal; only their tendons reach into the cavity.
When the muscles contract, the ossicular chain and the tympanic membrane become tense, and the chain stiffens.
This stiffening changes how the middle ear transmits sound, especially low-pitched sound.
The stapedius is the smallest skeletal muscle in the human body.
It lies in a bony canal inside the pyramidal eminence on the posterior wall of the middle ear.
Its tendon leaves the tip of the eminence and attaches to the back of the neck of the stapes.
When it contracts, it pulls the stapes backward and limits the movement of the footplate in the oval window.
This reduces the energy delivered to the inner ear, especially at low frequencies.
The tensor tympani is a longer muscle that runs in a bony canal just above the Eustachian tube.
The muscle arises from the cartilaginous part of the Eustachian tube and the surrounding bone.
As its tendon enters the middle ear, it turns at a right angle around a small bony process on the medial wall.
It then crosses the cavity from medial to lateral and attaches to the upper part of the handle of the malleus.
When it contracts, it pulls the malleus inward and tenses the tympanic membrane, which gives the muscle its name.
The two muscles are driven by different cranial nerves, because they differ in embryological origin.
The stapedius is supplied by the seventh cranial nerve, the nervus facialis, or facial nerve.
The tensor tympani is supplied by the mandibular branch of the fifth cranial nerve, the trigeminal nerve.
The stapedius develops from the second pharyngeal arch, whose nerve is the facial nerve.
The tensor tympani develops from the first pharyngeal arch, whose nerve is the trigeminal nerve.
In humans, the main muscle of the acoustic reflex to sound is the stapedius.
The tensor tympani contracts mainly in the startle response and to non-acoustic stimuli, such as a puff of air on the face.
The facial nerve enters the temporal bone through the internal auditory canal, together with the auditory nerve.
It then passes above the oval window on the medial wall of the middle ear and descends in the posterior wall.
In this descending part, it gives a small branch to the stapedius muscle.
The nerve then gives off the chorda tympani and leaves through a foramen in the skull base to reach the facial muscles.
This long course means the stapedius reflex can help estimate the site of a facial nerve problem.
A loud sound makes the stapedius muscle contract in both ears; this is called the acoustic reflex.
The reflex starts in the cochlea, and the signal travels along the auditory nerve to the brainstem.
The first stop is the cochlear nucleus.
From here the signal is passed to the superior olivary complex on both sides.
It then reaches the facial motor nuclei on both sides and, through the facial nerves, makes the stapedius muscles contract.
This is why a sound given to one ear normally triggers the reflex in both ears.
The reflex is measured with the tympanometry probe as a small change in admittance in the ear canal.
In ipsilateral, or same-side, measurement, the stimulus and the recording are in the same ear.
This measurement assesses the part of the arc that does not cross to the other side of the brainstem.
In contralateral, or opposite-side, measurement, the stimulus goes to one ear and the contraction is measured in the other.
This measurement also tests the pathway that crosses to the other side in the brainstem.
The two measurements complement each other, and the results are interpreted together.
The lowest stimulus level that triggers the reflex is called the acoustic reflex threshold.
In people with normal hearing, this threshold for pure tones is usually seventy to one hundred decibels HL, around eighty five on average.
With broadband noise, the reflex appears at lower levels.
Even a small conductive problem in the probe ear often prevents the reflex from being recorded.
We will cover detailed measurement and interpretation rules in the immittance lessons.
The best-known role of the muscles is to give the inner ear some protection from loud, long-lasting low-pitched sounds.
But the reflex works with a delay, so it cannot catch the first moment of a sudden blast such as a gunshot.
By reducing low-pitched sounds, it also reduces how much low-pitched noise masks the high-pitched sounds of speech.
So the reflex is thought to help with understanding speech in noise.
The stapedius also contracts just before we start to speak, helping our own voice sound softer in our ears.
In facial nerve palsy, if the lesion lies before the stapedius branch, the muscle on that side does not work and the reflex is absent.
During this period some patients report that sounds feel uncomfortably loud; this is called hyperacusis.
The stapedius tendon may be cut during stapes surgery; in that case the reflex cannot be measured in the operated ear.
In otosclerosis the stapes footplate is fixed, so contraction causes no measurable change and the reflex is often absent.
In the rare middle ear myoclonus, the muscles contract involuntarily and rhythmically; the person may hear a clicking sound.
These contractions can sometimes be seen as rhythmic ripples on the curve during tympanometry.
A tip from practice: whichever ear gets the stimulus, the reflex is always recorded in the ear with the probe.
So the measured contraction belongs to the stapedius muscle on the side where the probe is placed.
In facial palsy, to assess the muscle on the affected side, place the probe in that ear and present the stimulus to each ear in turn.
This small detail prevents reflex results from being recorded on the wrong side.
The stapedius runs from the pyramidal eminence to the neck of the stapes and is driven by the facial nerve.
The tensor tympani runs from above the Eustachian tube to the malleus and is driven by the mandibular branch of the trigeminal nerve.
In humans, the main muscle of the acoustic reflex to sound is the stapedius.
A loud sound given to one ear normally triggers the reflex in both ears.
Test yourself: Which cranial nerve supplies the tensor tympani muscle?
The correct answer is B: the trigeminal nerve.
The tensor tympani comes from the first pharyngeal arch, so it is driven by that arch's nerve: the mandibular branch of the trigeminal.
In the next lesson we will examine the Eustachian tube, which runs just below the tensor tympani, with its structure and functions.
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Prerequisites

LESSON 20 The Ossicular Chain : Malleus, Incus and Stapes

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