Gross Anatomy base material 5

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Innervation of the tympanic cavity

  • The mucosa is supplied by the tympanic branch of the glossopharyngeal nerve. Thus, pain from a middle ear infection is carried in CN IX.

 

Figure 8. Relationships of the tympanic cavity. Superior view. The petrous temporal bone is transparent.

Auditory ossicles

Three tiny bones extend across the tympanic cavity, covered in mucous membrane, from the tympanic membrane to the oval window. From lateral to medial these are the malleus, incus, and stapes.

Malleus (“hammer”)
  • Head is round and articulates with the incus.
  • Handle passes downward to attach to the tympanic membrane.
  • Lateral process is attached to the tympanic membrane (producing the mallear folds).
Incus (“anvil”)
  • Body is rounded and articulates anteriorly with the head of the malleus.
  • Long process projects inferiorly, behind and parallel to the handle of the malleus (where its shadow can sometimes be seen on otoscopic examination). Articulates with the head of the stapes.
  • Short process projects posteriorly and is attached to the posterior wall of the tympanic cavity by a ligament.

Figure 29.9 WWW. ANATOMYBOX. COM.

Stapes (“stirrup”)
  • Head is very small and articulates with the long process of the incus—receives the attachment of the stapedius muscle.
  • Two limbs attach the neck to the base (footplate).
  • The edges of the base attach to the margins of the oval window by a ring of connective tissue called the annular ligament.

 

Given that the tympanic membrane has a much larger surface area than the base of the stapes, the ossicles act as levers to amplify the movements of the tympanic membrane (sources vary on this, but the amplification may be as much as 20 times), so that a substantial force can be produced at the oval window by smaller movements of the tympanic membrane.

Figure 10.

Muscles of the ossicles

The tensor tympani and stapedius muscles attenuate (reduce) the movements of the ossicles (and tympanic membrane) in response to loud noises, which might otherwise cause damage to the delicate sensory apparatus of the internal ear.

Tensor tympani
  • Origin: from the bony wall of its canal (opens on the anterior wall of the tympanic cavity) as well as from the cartilage of the pharyngotympanic tube.
  • Insertion: its tendon turns around a bony shelf (pulley) to insert on the handle of the malleus.
  • Action: Reduces the vibrations of the malleus and movements of the tympanic membrane by pulling the malleus and tympanic membrane medially (thus tensing the tympanic membrane).
  • Innervation: a branch of the mandibular nerve (V3).

Figure 11. Tensor tympani and stapedius muscles. Lateral wall of tympanic cavity is seen—viewer is looking from medial to lateral. GRAY’S ANATOMY FOR STUDENTS, FIGURE 8.120.

 

Stapedius
  • Origin: from the pyramidal eminence on the posterior wall of the tympanic cavity.
  • Insertion: near the head of the stapes.
  • Action: Pulls the stapes posteriorly and tightens up the annular ligament to prevent excessive movement of this tiny bone.
  • Innervation: a branch from the facial nerve.

Nerves associated with the tympanic cavity

Facial nerve
  • Leaves the cranial cavity and enters the petrous part of the temporal bone through the internal acoustic meatus along with the vestibulocochlear nerve (CN VIII). At the lateral end of the meatus, the facial nerve enters the facial canal.
  • Within the facial canal, the nerve passes between the cochlea and vestibule, which are both parts of the internal ear.
  • Lateral to the cochlea, the facial canal turns posteriorly, producing a sharp bend in the nerve = the genu (Latin = “knee”, because it is bent like a flexed knee). This is the location of the sensory ganglion of the facial nerve (geniculate ganglion).

 

Figure 12. Course of the facial nerve. The petrous temporal bone has been rendered transparent. The view is looking from medial to lateral.

  • Distal to the genu, the facial nerve moves posteriorly in the facial canal midway between the tympanic cavity and the cochlea. Here the bony prominence of the facial canal is located in the upper medial wall of the tympanic cavity.
  • Posterior to the tympanic cavity, the facial canal bends inferiorly and descends between the tympanic cavity and mastoid air cells to the stylomastoid foramen, where it exits the skull and enters the parotid gland.

 

Chorda tympani (a branch of the facial nerve)
  • Arises from the facial nerve in the facial canal just above the stylomastoid foramen and runs superiorly through the temporal bone to enter the tympanic cavity from its posterior wall.
  • Runs across the internal surface of the tympanic membrane from posterior to anterior, where it passes between the handle of the malleus and the long process of the incus.
  • It leaves the tympanic cavity through a tiny slit in its anterior wall, the petrotympanic fissure, to enter the infratemporal fossa, where it joins the lingual nerve.
  • Chorda tympani carries taste information from the anterior 2/3 of the tongue (special sensory) and preganglionic parasympathetic fibers to the submandibular ganglion (visceral motor).
Nerve to stapedius
  • Arises from the facial nerve as it descends in the facial canal and supplies the stapedius muscle while it is within the pyramidal eminence.
Greater petrosal nerve (a branch of the facial nerve)
  • Branches from the genu of the facial nerve and travels anterior.
  • Leaves the petrous temporal bone through a bony fissure and enters the middle cranial fossa. Traverses the middle cranial fossa under the dura, then enters the foramen lacerum.
  • Carries preganglionic parasympathetic fibers to the pterygopalatine ganglion. Postganglionic fibers innervate the lacrimal gland and mucous glands in the nasal cavity.
  • Greater petrosal also carries taste fibers from taste buds in the palate.

 

Figure 13. Course of the facial nerve. The medial wall of the tympanic cavity is rendered transparent. The view is looking lateral to medial.

Tympanic nerve plexus

Located atop the promontory on the medial wall of the tympanic cavity.

Nerve fibers derived from these nerves form the tympanic plexus:

  • Tympanic nerve (Jacobsen’s nerve): Arises from the glossopharyngeal nerve in the jugular foramen and enters the tympanic cavity through its floor to join the tympanic plexus. The tympanic nerve carries sensory fibers that innervate the mucosa of the tympanic cavity and preganglionic secretomotor fibers that form the lesser petrosal nerve.
  • Postganglionic sympathetic fibers from the internal carotid plexus enter the tympanic cavity through its anterior wall and join the plexus. These supply smooth muscle in blood vessels.

Figure 14. Cut away view of tympanic cavity. The view is looking from anterior to posterior.

 

These nerve fibers leave the tympanic plexus:

  • Sensory branches (from the tympanic nerve of CN IX) supply the mucosa of the tympanic cavity, inner aspect of the tympanic membrane, mastoid air cells, and the pharyngotympanic tube.
  • The lesser petrosal nerve leaves through the roof of the tympanic cavity. It contains preganglionic parasympathetic fibers from CN IX that synapse in the otic ganglion in the infratemporal fossa. From here, postganglionic parasympathetic fibers innervate the parotid gland.

 

Internal ear ( optional reading)

The internal ear contains the organs specialized for (1) reception of sound, and (2) detection of the position and movement of the head—information needed to maintain balance and equilibrium. Here we will only discuss the basics of internal ear anatomy to get you up to speed in preparation for more heavy-duty information in your neuroscience course.

The internal ear is made up of a series of cavities carved out of the petrous part of the temporal bone (bony labyrinth) containing a network of membrane-lined ducts and sacs (membranous labyrinth).

Bony labyrinth

Interconnected hollow cavities within the petrous temporal bone lined by a periosteum and filled with a fluid called perilymph.

 

Components of the bony labyrinth:

  • Three semicircular canals (anterior, posterior, and lateral), oriented at right angles to one another.
  • Vestibule: A spherical cavity with the oval window in its lateral wall. The vestibule communicates with the semicircular canals and the cochlea.
  • Cochlea: A spiral canal shaped like a snail’s shell. It makes two and one-half turns around a central bony core called the modiolus. The round window is located in the lateral wall of the base (first turn) of the cochlea.

 

Figure 15. Position of bony labyrinth within the petrous part of the temporal bone.

Membranous labyrinth

Interconnected series of ducts and sacs constructed of dense connective tissue and lined internally by an epithelium. The membranous labyrinth is suspended within the bony labyrinth and separated from its bony walls by the perilymph.

 

The membranous labyrinth is filled with a fluid called

endolymph.

Parts of the membranous labyrinth:

  • Three semicircular ducts lie inside the semicircular canals. Where the ducts meet there are enlargements called ampullae. Receptors within the ampullae monitor angular acceleration of the head (side-to-side head rotation).
  • Utricle and saccule are within the vestibule of the bony labyrinth. Receptors within the utricle and saccule monitor

(1) static equilibrium (detecting the orientation of the head with respect to the ground when the head is stationary), and (2) linear acceleration (sensing when the body moves in a straight line).

Figure 16. The parts of the membranous labyrinth shown within the bony labyrinth.

 

  • Stimulation of receptor cells in the vestibular organs mentioned above produces action potentials in neurons of the vestibular division of the vestibulocochlear nerve (CN VIII). These are bipolar neurons with cell bodies in the vestibular ganglion (Scarpa’s ganglion), located within the internal acoustic meatus.
  • The cochlear duct occupies a central position within the

bony cochlea. It has a roof called the vestibular membrane and a floor called the basilar membrane. Because of its central location, the cochlear duct (containing endolymph) subdivides the cavity of the cochlea into two perilymph- filled channels: the scala vestibuli above and the scala tympani below. The cochlear duct contains the spiral organ (organ of Corti), where the special receptor cells for hearing are housed.

  • Stimulation of receptors in the Organ of Corti produces action potentials in neurons of the cochlear division of the vestibulocochlear nerve (CN VIII). These are bipolar neurons with cell bodies in the cochlear ganglion (spiral ganglion), located in the modiolus.

Figure 17. Cross section of one turn of the cochlea. The scala vestibuli and tympani belong to the bony labyrinth, while the cochlear duct is part of the membranous labyrinth.