Dominic Stone
The ossicles, also known as auditory ossicles, are three small bones in the middle ear of humans and other mammals. They are called the malleus, incus, and stapes, which translate to hammer, anvil, and stirrup in Latin. The ossicles are responsible for transmitting and amplifying sound vibrations from the outer ear to the inner ear. This process is facilitated by two muscles, the tensor tympani and the stapedius, which contract in response to loud noises to dampen the vibrations and protect the inner ear. The tensor tympani muscle attaches to the malleus, while the stapedius muscle is connected to the stapes. These muscles play a crucial role in regulating the movement and vibrations of the ossicles, ensuring proper sound transmission and protecting the inner ear from damage.
| Characteristics | Values |
|---|---|
| Number of muscles controlling ossicles | 2 |
| Names of the muscles | Tensor tympani, Stapedius |
| Location of tensor tympani | Inside a bony canal superior to the pharyngotympanic (Eustachian) tube |
| Tensor tympani's function | Dampens vibrations caused by loud sounds to protect the internal ear |
| Stapedius's function | Dampens excessive vibrations caused by loud noises |
| Stapedius's nerve | Facial nerve |
| Stapedius's size | Smallest muscle in the body |
| Ossicular chain | Malleus, incus, stapes |
| Ossicular chain discontinuity | Fusion of ossicles, loss of free movement, hypersensitivity to loud noises |
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What You'll Learn
Tensor Tympani Muscle
The tensor tympani muscle is a small but long paired muscle of the middle ear. It is one of two muscles that control the ossicles, the other being the stapedius muscle. The tensor tympani muscle is located in a bony canal found superior to the osseous part of the auditory tube (also known as the pharyngotympanic or Eustachian tube). The tensor tympani muscle passes through this canal and ends in the tympanic cavity as a slim tendon that connects to the handle of the malleus, one of the three auditory ossicles.
The tensor tympani muscle is innervated by the nerve to medial pterygoid, which arises from the mandibular division of the trigeminal nerve (CNV3). It is vascularized by the superior tympanic branch of the middle meningeal artery, which also supplies the superior tympanic artery. The tensor tympani is supplied by the tensor tympani nerve, a branch of the mandibular branch of the trigeminal nerve. As it is supplied by motor fibres of the trigeminal nerve, it does not receive fibres from the trigeminal ganglion, which has sensory fibres only.
The tensor tympani muscle functions to dampen loud sounds, such as those produced from chewing, shouting, or thunder. It does this by pulling the handle of the malleus medially, which tenses the tympanic membrane and reduces the amplitude of its oscillations. This, in turn, reduces sound transmission to the vestibular window and lowers the perceived amplitude of sounds. This process is known as the tympanic reflex, an evolutionary adaptation to protect the inner ear from excessively loud noises, which also aids speech coordination. The tensor tympani muscle contracts in response to the perception or anticipation of loud sounds, and this lowered reflex threshold for contraction is called tonic tensor tympani syndrome (TTTS).
While the tensor tympani muscle is important for protecting the inner ear, it is not fast enough to prevent hearing damage caused by sudden loud noises, like explosions or gunshots. However, some individuals have voluntary control over the muscle and can tense it preemptively.
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Stapedius Muscle
The stapedius muscle is a tiny slender muscle in the middle ear that is responsible for stabilising the smallest bone in the human body, the stapes or stirrup bone. The stapedius muscle is attached to the neck of the stapes bone, which is located in the tympanic cavity in the middle ear. The stapedius muscle is considered the smallest muscle in the human body, measuring approximately 6 millimetres in length.
The stapedius muscle is a crucial component of the acoustic middle ear reflex, which helps protect the hearing apparatus when exposed to loud sounds. It contracts in response to loud noises, dampening the vibrations of the stapes bone and reducing the amplitude of sound waves transmitted to the inner ear. This protective mechanism is essential for safeguarding the auditory system from damage caused by sudden impulsive sounds, such as explosions or gunshots.
The stapedius muscle is innervated by the stapedial branch of the facial nerve (CN VII). When the incoming sound is loud enough to stimulate the receptor cells in the inner ear, afferent signals travel to the cochlear nucleus in the brainstem via the vestibulocochlear nerve (CN VIII). The brainstem then sends efferent signals to the middle ears, triggering the contraction of the stapedius muscle.
Paralysis of the stapedius muscle can occur due to damage to the facial nerve or its branches. This condition, known as hyperacusis, results in hypersensitivity to everyday sounds, as the auditory system loses one of its key protective mechanisms. Without the stapedius muscle's ability to attenuate external sounds, individuals with hyperacusis can experience complete deafness.
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How Ossicles Transmit Sound
The ossicles, also known as the auditory ossicles, are three small bones in the middle ear of humans and other mammals. They are the smallest bones in the human body and are called the malleus ("hammer"), incus ("anvil"), and stapes ("stirrup"). The auditory ossicles serve as a kinematic chain to transmit and amplify sound vibrations collected from the air by the eardrum to the fluid-filled labyrinth (cochlea).
Sound waves enter the ear and strike the eardrum, causing it to vibrate. These vibrations pass from the eardrum to the ossicles, which are attached to the eardrum. The ossicles further amplify the sound vibrations. The malleus then transmits the vibrations via the incus to the stapes and ultimately to the membrane of the fenestra ovalis (oval window), the opening to the vestibule of the inner ear.
The ossicles give the eardrum a mechanical advantage via lever action and a reduction in the area of force distribution. This results in stronger vibrations that don't move as far. The ossicles increase the force of vibrations but decrease the amplitude as the vibrations are transmitted across each ossicle. This process allows sound to be transformed into electrical signals that the brain can interpret as sound.
The extent of the movements of the ossicles is controlled and constricted by two muscles attached to them: the tensor tympani and the stapedius. These muscles can contract in response to loud noises to dampen the vibration of the ossicles and protect the inner ear. This protective function is not instantaneous, as the loop between the eardrum and brain must first be stimulated by a loud noise before constriction occurs. Age can also slow down this loop, making older individuals more susceptible to hearing damage.
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Ossicular Chain Discontinuity
The ossicles, or auditory ossicles, are three irregular bones in the middle ear of humans and other mammals: the malleus (hammer), incus (anvil), and stapes (stirrup). They are among the smallest bones in the human body. The ossicles transmit and amplify sound vibrations collected from the air by the eardrum to the fluid-filled labyrinth (cochlea).
The tensor tympani and stapedius muscles control the movements of the ossicles. In response to loud noises, these muscles contract to lessen the movement of the ossicles and dampen vibrations, protecting the structures of the inner ear. Paralysis of these muscles, caused by damage to the nerves that supply them, results in an inability to reduce amplitudes and oscillations created by loud sounds, leading to hypersensitivity to loud noises (hyperacusis).
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Ossicular Muscles and Hearing Loss
The ossicles, also known as the auditory ossicles, are three small bones in the middle ear of humans and other mammals. They are the malleus ("hammer"), incus ("anvil"), and stapes ("stirrup"), and they are among the smallest bones in the human body. The auditory ossicles serve as a kinematic chain to transmit and amplify sound vibrations from the eardrum to the fluid-filled labyrinth (cochlea) of the inner ear.
The ossicles are controlled by two muscles attached to them: the tensor tympani and the stapedius. These muscles can contract in response to loud noises, reducing the movement and vibration of the ossicles, and thus protecting the structures of the inner ear. This protective mechanism is particularly important because air vibrations from loud noises can easily damage the inner ear. The tensor tympani muscle lies inside a bony canal superior to the pharyngotympanic (Eustachian) tube, and the stapedius muscle is the smallest muscle in the body, originating from the mesenchyme of the second branchial or pharyngeal arch.
Ossicular chain discontinuity occurs when the auditory ossicles are not articulating correctly due to issues such as trauma, infection, or otosclerosis (abnormal growth of the ossicles). This can result in moderate to severe conductive hearing loss. Treatment options include hearing aids or ossicular chain reconstruction surgery. Additionally, paralysis of the tensor tympani or stapedius muscle can lead to hyperacusis, an increased sensitivity to loud noises. This paralysis is caused by damage to the nerves that supply these muscles, which can be due to trauma or infection.
The absence or pathology of the auditory ossicles can also result in moderate to severe conductive hearing loss. For example, genetic mutations can lead to the partial or total absence of one or more ear bones. Furthermore, otosclerosis, a condition where the auditory bones fuse together, can cause progressive hearing loss and may require surgery.
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Frequently asked questions
Ossicles, or auditory ossicles, are three small bones in the middle ear of humans and other mammals. They are called the malleus, incus, and stapes, or "hammer, anvil, and stirrup".
The ossicles are controlled by two muscles: the tensor tympani and the stapedius.
The tensor tympani and stapedius muscles contract in response to loud noises, reducing the movement and vibrations of the ossicles to protect the structures of the inner ear.
