Lecture 20: Olfaction & Taste

Animals with a well-developed olfactory system that rely heavily on smell

macrosmatic

Animals with a relatively small olfactory system that rely less on smell

microsmatic (humans)

Complete loss of smell

anosmia

Where is the olfactory mucosa located?

superior and posterior nasal cavity

What is the structure of the olfactory mucosa?

columnar epithelium surrounding millions of olfactory neuroreceptors

What makes olfactory neuroreceptors unique?

only neurons that communicate with the external environment and undergo constant replacement

How often are olfactory neurons replaced?

approximately every 5-8 weeks

What forms the olfactory nerve?

unmyelinated axons of bipolar olfactory neurons

What is the function of the olfactory bulbs?

first processing station for olfactory information

What are the second-order neurons of the olfactory pathway?

mitral cells

Where are mitral cells located?

olfactory bulbs

What forms the olfactory tract?

axons of mitral cells

What direction do olfactory tracts travel from the olfactory bulbs?

posteriorly toward the anterior perforated substance

Where does the olfactory tract divide into 2 striae?

anterior perforated substance

What are the 2 divisions of the olfactory tract?

medial olfactory stria and lateral olfactory stria

How does the number of fibers in the medial olfactory stria compare with the lateral stria?

medial stria contains fewer fibers

where does the medial olfactory stria project?

- hypothalamus

- thalamus (DMN)

- hippocampus

- upper brainstem

Which olfactory stria is associated with learned behavior related to smell?

lateral olfactory stria

Which olfactory stria contains most olfactory tract fibers?

lateral olfactory stria

where does the lateral olfactory stria project?

- piriform cortex

- periamygdaloid cortex

- amygdala

What structures make up the primary olfactory cortex?

lateral olfactory gyrus

Where does the primary olfactory cortex project?

amygdala and olfactory association cortex

What structure contains the olfactory association cortex?

anterior portion of parahippocampal gyrus

Olfactory association cortex in the anterior parahippocampal gyrus

entorhinal cortex

Where does the entorhinal cortex project?

hippocampal formation

How do taste receptors differ from olfactory neuroreceptors?

taste receptors are specialized epithelial cells contacting peripheral nerve fibers, while olfactory receptors are neurons

What are the 4 classically defined basic tastes?

sweet, salt, sour, and bitter

Where are taste receptors located?

taste buds on the surface of tongue

How many taste receptor cells are found in each taste bud?

40-60

What is the lifespan of taste receptor cells?

1-2 weeks

Structures on the tongue containing taste buds

papillae

What papillae are found on the anterior 2/3 of the tongue?

fungiform and filiform papillae

What papillae are found along the lateral margins of the tongue?

foliate papillae

What papillae form a V-shaped line approximately 2/3 of the way back on the dorsum of the tongue?

circumvallate (vallate) papillae

What 3 cranial nerves carry peripheral taste sensations?

CN VII, CN IX, and CN X

What nerve carries taste from the anterior 2/3 of the tongue?

chorda tympani branch of CN VII (facial nerve)

Where are the first-order taste neuron cell bodies for CN VII located?

geniculate ganglion

What nerve carries taste from the posterior 1/3 of the tongue?

CN IX (glossopharyngeal nerve)

Where are the first-order taste neuron cell bodies for CN IX located?

inferior glossopharyngeal (petrosal) ganglion

What nerve carries taste from the region around the epiglottis?

superior laryngeal branch of CN X (vagus nerve)

Where are the first-order taste neuron cell bodies for CN X located?

inferior vagal (nodose) ganglion

What tract do taste afferents enter after reaching the brainstem?

solitary tract

Where do fibers of the solitary tract terminate?

nucleus of the solitary tract in medulla

Principal visceral afferent nucleus in the brainstem

nucleus of the solitary tract (NTS)

What type of input is received by the caudal NTS?

GVA inputs from CN IX and X

What type of input is received by the rostral NTS?

SVA (taste) inputs

(gustatory nucleus)

Where are the second-order neurons of the ascending taste pathway located?

nucleus of the solitary tract (NTS)

Does the ascending taste pathway from NTS to VPM cross?

no, it ascends ipsilaterally

What thalamic nucleus receives ascending taste information?

VPM nucleus

Where are third-order neurons of the ascending taste pathway located?

VPM nucleus of thalamus

Where do third-order taste neurons project?

ipsilateral taste cortex

What structure forms the taste cortex?

insular cortex

Where does the taste cortex project for higher integration of taste information?

orbital cortex and amygdala

Complete loss of taste

ageusia

Decreased sense of taste

hypogeusia

Distortion of taste

parageusia

What causes of taste disturbances are listed in the annotation?

nerve damage, medication side effects, chemotherapy, and radiation