THE OLFACTORY SYSTEM

Landmarks: midline nasal cavity, temporal lobe, anterior corpus callosum, frontal lobe, cribriform plate and olfactory epithelium.

• The cribriform plate separates the cranial vault from the nasal cavity.
• Fracture to the cribriform plate (or more commonly to the ethmoid air cells posterolateral to the cribriform plate) is a common cause of rhinorrhea — cerebrospinal fluid leak from the nasal cavity.

OLFACTORY BULB & TRACT

• Lie underneath the frontal lobe.
  • The olfactory bulb is often distinguished as the main olfactory bulb because the majority of vertebrates also have an accessory olfactory system. However, the role and existence of the accessory olfactory system (aka vomeronasal system) in humans is disputed.

THE OLFACTORY NERVE, BULB, & TRACT: ESSENTIALS

• Bipolar primary olfactory neuron: dendrites project to the olfactory epithelial surface and the centrally-mediated axon (the olfactory nerve) extends through the cribriform plate to innervate the olfactory bulb.
• Within the olfactory bulb, lie bipolar secondary olfactory cells, which connect with the olfactory nerve in the inferior olfactory bulb and also send axons down the olfactory tract: at the posterior end of the olfactory tract, lies the olfactory trigone.
• CN 1, the olfactory nerve, refers to the primary olfactory neuron/nerve, only. ## The olfactory bulb and tract are extensions of the cerebrum, itself.
• The olfactory system bypasses the thalamus as it projects to the cerebral cortex, which is unique. Auditory, visual, somatosensory, and gustatory sensory pathways all relay within the thalamus prior to synapsing in the cerebral cortex.
• The olfactory tract divides into a medial olfactory stria, which innervates the medial olfactory area in the subcallosal (aka septal) region, and a lateral olfactory stria, which innervates the primary olfactory cortex in the basal frontal and anteromedial temporal lobes.  Olfactory impulses also extend across the anterior commissure to the opposite side of the cerebrum.

THE OLFACTORY NERVE, BULB, & TRACT: CONNECTIONS

• Cilia from the apical dendrite interact with the mucus layer of the olfactory epithelial surface.

Key constituents of the olfactory epithelium include:

• The sustentacular cells, which are olfactory supporting cells.
• The basal cells, which renew the primary olfactory neurons and sustentacular cells.
• The Bowman’s glands, which secrete a serous, watery odor dissolvent.
• CN 1 comprises an unmyelinated nerve bundle to interact with bipolar secondary olfactory neurons.
• Two principal forms of secondary olfactory neuron exist: tufted cells and mitral cells.  Less notable interneurons (eg, periglomerular and granule cells) also exist within the olfactory bulb.
• The glomerular layer refers to the layer of spherical glomeruli which encompass the interaction between dendrites from the secondary olfactory neurons and primary olfactory axons within the inferior olfactory bulb.
• The secondary olfactory neurons project axons that travel either directly down the olfactory tract to synapse in the olfactory cortex or first to the anterior olfactory nucleus, which projects its axons down the olfactory tract to the olfactory cortex.

EAR CANALS: external, middle, and inner.

THE EXTERNAL EAR CANAL

• Extends through the tympanic portion of the temporal bone, just in front of the mastoid process.

THE MIDDLE EAR CANAL

• Lies mostly within the tympanic portion of the temporal bone.
• From lateral to medial, it contains three ossicles: the malleus, incus, and stapes, which are Latin for: “hammer,” “anvil,” and “stirrup,” respectively. # The stapes abuts the oval window.
• When sound is transmitted through the ossicles, the stapes pushes the oval window into the inner ear canal.
• The eustachian tube extends from the middle ear into the nasopharynx, which allows your middle ears to equilibrate with the atmospheric pressure in your nasopharynx when you swallow.
• Two important muscles exist within the middle ear canal: the tensor tympani, which is innervated by the trigeminal nerve and which acts on the tympanic membrane, and the stapedius muscle, which is innervated by the facial nerve and which acts on the stapes.

THE INNER EAR CANAL

• Lies within the petrous portion of the temporal bone.
  • The semicircular canals, which lie in superior-lateral position and serve vestibular function.
  • The cochlea, which is shaped like a snail’s shell, and lies in anterior-inferior position and serves auditory function.
  • The vestibule, which lies in between the cochlea and semicircular canals — it transmits sound waves from the oval window to the cochlea and show that it contains the otolith organs, which provide vestibular cues.

THE COCHLEA

Integral for the detection of sound.

• 3 Ducts:
  • The cochlear duct (scala media).
  • Vestibular duct (scala vestibuli) (which is continuous with the vestibule).
  • Tympanic duct (scala tympani), which ends in the round window (aka the secondary tympanic membrane).

• Membranes:
  • Reissner’s membrane separates the vestibular and cochlear ducts.
  • The Basilar membrane separates the cochlear and tympanic ducts.

• Fluids:
  • The vestibular and tympanic ducts are filled with perilymphatic fluid, which is high in Na+ and low in K+ (like extracellular fluid).
  • The cochlear duct is filled with endolymphatic fluid, which is high in K+ and low in Na+ (like intracellular fluid).

Ménière’s syndrome (bouts of vertigo, low-frequency hearing loss, and ear fullness ) is thought to be due to pathologically elevated endolymphatic sodium concentration, so it is commonly treated with salt-wasting diuretic medications.

PHYSIOLOGY OF SOUND DETECTION

• When a sound wave enters the external ear canal, it vibrates the tympanic membrane.
• The tympanic membrane, then, transmits the wave through the ossicles: the malleus, incus, and stapes, and the stapes strikes the oval window.
• When the oval window vibrates, a fluid wave passes through the vestibule and the vestibular duct.
  • The vestibular and tympanic ducts connect at the apex of the cochlea (aka the helicotrema).
  • The sound waves passes across the apex of the cochlea into the tympanic duct, through the tympanic duct, and pushes the round window into the air-filled middle ear canal.

In this process, the auditory sensory organ, the organ of Corti, which lies along the basilar membrane, is activated for sound detection.

• High-frequency sounds activate hair cells at the base of the cochlea (near the oval and round windows) whereas low-frequency sounds activate hair cells at the apex of the cochlea.
• The basilar membrane is thinnest at its base and widest at its apex.

THE MAJOR VESTIBULAR COMPONENTS OF THE EAR: THE OTOLITH ORGANS AND THE SEMICIRCULAR CANALS.

• Within the vestibule, lie the saccule, which detects vertical movement (ie, gravity), and the utricle, which detects horizontal (forward/backward) movement.
• The macula (the neuroepithelial sensory detection region) of the saccule is principally vertically oriented and its attached hair cells are horizontally oriented to detect vertical movement, whereas the macula of the utricle is principally horizontally oriented and its attached hair cells are vertically oriented to detect horizontal movement.

THE SEMICIRCULAR CANALS: HORIZONTAL, POSTERIOR, AND ANTERIOR.

• These three semicircular canals lie perpendicular to one another and detect rotational acceleration, which we address in the next tutorial.