Cranial Nerves 7, 9, and 10 (Gustation)

6th tastant - oleogustus

Fatty acids are a chemical stimulus for a specific receptor However, perception is creaminess, richness, et. which are texture qualities Many think it is a combination of texture and taste together

Taste Buds

Found primarily on tongue papillae Total of 2-8,000 taste buds Each taste bud contains 50-100 receptor cells

Impact of smell/taste disorders on daily life

Decreased enjoyment of food, difficulties cooking, risk of failure to perceive fire or gas leaks, problems with personal hygiene, problems with social life

Distortion or perversion

Dysosmia Dysguesia

Damage to the nucleus ambiguus

Dysphagia Dysphonia Deviation of the uvula toward contralateral side

Simple NST/DMV reflex circuit

Ex) as you are eating your mechanoreceptors stretch as stomach fills 1. Afferent vagus nerve senses stimulation from mechanoreceptors 2. As vagus nerve enters the caudal medulla, it splits into two neurons. 3. A)One neurons synapses onto a cell body of the solitary nucleus, and travels up to the hypothalamus to relay satiety B) Second neuron is an interneuron that synapses on a preganglionic parasympathetic nucleus. (connects to efferent motor neurons) Info travels to the postsynaptic ganglion and eventually increases acid secretion. (4 order neuron process)

Solitary tract is adjacent and medial to the trigeminal nucleus

H is solitary tract nucleus

Medullary reticular formation

Has a number of CPGs and autonomic centers

Nucleus ambiguus and dorsal motor nucleus of the vagus

Hypoglossal nucleus is most centrally Dorsal motor nucleus is in the middle Nucleus ambiguus is most laterally

Reduced perception

Hyposmia Hypoguesia

MYTH: Different regions of the tongue respond to different taste qualities

Taste cells containg receptors for all 5 taste qualities are found all over the surface of the tongue and throughout the oral cavity AND WITHIN THE SAME TASTE BUD Detection of different tastes within the same taste bud

Conditioned taste aversion

When a person associated the taste of a certain food with symptoms caused by a toxic substance (useful for poisons) This can be problematic when nausea and vomiting are caused by drugs (cancer chemotherapy drugs) and then associated with a non-toxic food

Increased perception

hyperosmia hyperguesia

Caudal medulla key points

1. Hypoglossal nerve (CN VII) is now more ventral compared to the rostral slides 2. Presence of the Area Postrema (can be found at the obex point on the anterior view) 3. Presence of the Gracile nucleus

Integration of Flavor Cortex

1. Olfactory Cortex (piriform) --> MD thalamus of smell 2. Insula of gustation 3. Somatosensory cortex of texture 1 + 2 + 3 = all to the orbitofrontal cortex for flavor processing

Efferent connections from caudal NST

1. Perception - VPM of the thalamus synapses at the somatosensory cortex 2. Homeostasis/regulation of food intake - Reciprocal connections with hypothalamus 3. Nausea and pain - amygdala and other cortical regions 4. Reflexes and CPGs involving NA and DMV

5 accepted groups of tastants

1. Sweet - Sugars 2. Savory (umami) - Monosodium glutamate 3. Sour - Acids (H+) 4. Salt - Sodium 5. Bitter - Alkaloids 6. Olegustus - rancid fats

Percent of adults with a taste/smell disorder

15% and incidence increases with age

VAGUS NERVE IS PRIMARILY AN AFFERENT NERVE

80% of vagus nerve is afferent 15% is preganglionic parasympathetic Most of the Vagus nerve is travelling back to the Solitary nucleus of the NST

Extended Gag reflex

Afferent sensory information is conveyed by cutaneous stimulation of Aδ and C fibres on the caudal base of the tongue and/or the caudal soft palate. Information is further relayed up the afferent limb of CN IX (cell bodies in the superior ganglia) which terminates in the spinal nucleus of V either directly or through interneurons. The spinal nucleus of V then projects bilaterally to the nucleus ambiguus. Efferent stimulation occurs from the nucleus ambiguus through both CN IX and X to the sylopharyngeous muscle (via CN IX) to the pharyngeal constrictor muscles and to the muscles that move the palate (CN X). Because there are bilateral constrictor pharyngeal muscles, constriction of muscles ipsilateral to the stimulus is considered the direct response, whereas contriction on the contralateral side is considered the consensual response. Therefore, in response to irritation in the caudal oral cavity, the pharynx constricts and elevates in an attempt to expel or occlude the foreign object. Discomfort is perceived through the spinal trigeminal tract of the face— which also project to the nucleus ambiguus — can induce a similar response.

Arnold's reflex

An automatic cough, chest pressure or nausea caused by touching the Vagus Nerve (X Cranial) Stimulation of the vagus nerve branch that innervates the external ear canal can cause the coughing Idiopathic cough may be due to sensory neuropathy of the vagus nerve

Complete absence of smell/taste

Anosmia Aguesia

There is no clear boundary that separates vagal from glossopharyngeal rootlets.

At the level of the brainstem, only very small lesions would affect the fibers of IX but not X Sections between CN VII, IX, X are not well defined but overlap significantly in the nucleus of the solitary tract

Where do axons synapse from CN X in the solitary tract?

Axons enter the solitary tract and travel rostrally (upward) to synapse in the solitary nucleus

Type II taste cell

Bitter sweet, umami Taste receptor linked to a Gustducin (G protein) leading to calcium release and sodium influx. Increase in sodium leads to production of ATP released out of the cell

Gustatory pathway

CN 7, 9, 10 --> Solitary nucleus of the brainstem --> 1. VPM of thalamus --> insula & frontal cortex 2. Hypothalamus 3. Amygdala

Afferent fibers derivation of CN IX and CN X

CN IX - arises from Brainstem, tympanic membrane, tonsils, and tongue Vagus is much widely distributed through GI, liver, kidney, spleen, small intestine, colon heart, lungs, throat and feed into ganglia in neck Each take information from these areas and relay it to the nucleus of solitary tract

Extremely unpleasant perception

Cacosmia Cacoguesia

Caudal inputs into the NST (cardiorespiratory nucleus)

Chemo and mechano receptor info from the carotid bodies (IX) and the aortic bodies (X) Chemo, mechano, and nociceptors in the Heart, Pharynx, Airways, and lungs Somatosensory information from the middle ear and a little from the external ear (VII, IX, X)

Autonomic and vasomotor centers

Collections of interneurons that control involuntary functions Respiratory, vomiting, sneezing, coughing Controllers can be modulated by higher centers and by sensory input, however their outputs are not entirely fixed/rigid

Constipation in Parkinson's disease

Constipation can be an early, non-motor symptoms of Parkinson's disease. This type of constipation is thought to be due to damage to cells in the dorsal motor nucleus of the vagus

Neuroepithelial Problem (smell/taste)

Damage or absence of receptor cells, their processes, or any aspect of the transduction mechanism Olfactory: Head trauma, drugs Gustatory: Burned tongue

Central Problem (smell/taste)

Damage or malfunction in any of the central pathways Can't tell stimuli apart - hallucinations and loss of discrimination Mix up things that are there with things that are not and vice versa

Facial Nerve

Innervates anterior 2/3 of tongue Lingual branch of the Trigeminal nerve (V) has another branch - Chorda tympani - that merges with the Facial nerve (VII) to go to the geniculate ganglion and then the salvitory nucleus Cell bodies found in the geniculate CT through petrotympanic fissure and exits the internal auditory meatus as CN VII

Vagus Nerve

Innervates the pharynx coming through the jugular foramen Branch off as the Pharyngeal nerve going to the tongue Cell bodies in the nodose

Glossopharyngeal Nerve

Innervates the posterior 1/3 of tongue coming through the jugular foramen Lingual nerve is a branch of the glossopharyngeal nerve going to the tongue Cell bodies found in the Petrosal (inferior) found just outside the jugular foramen

Taste cell atrophy

Like many receptor cells, taste cells atrophy if their innervation is destroyed. Damage to afferent neuron will cause the taste receptor cells to die as well. Ex) volume of saliva is decreased then shut down receptor cells and don't get replaced by stem cells. So if patient is taking anticholinergic medications this could be a problem (lose taste sensation)

Brainstem Reticular Formation

Loose collection of cells outside the major nuclei and tracts In midbrain, pons, and medulla Critical in many REGULATORY PATHWAYS --> interneurons in reflexes and more complex behaviors OR send axons rostrally to modulate cortical function (reticular activating system)

Intensity Coding in Gustatory system

Lower thresholds for bitter, higher thresholds for sweet and salt People can differ in -Number of taste buds --> super tasters (more taste buds) vs chemotherapy patients -Changes in gene regulation --> downregulation of sweet receptors by leptin (anti-hungry hormone)? Changes in taste perception following bariatric surgery

Nucleus Ambiguss

Motor nucleus for CN IX and CN X CN IX - Voluntary control of ipsilateral skeletal muscles of the upper airway CN X - voluntary control of ipsilateral skeletal muscles of the esophagus (soft palate, pharynx, and larynx) Important n swallowing, gag reflex, and speech Preganglionic parasympathetic neurons of the heart and lungs

Dorsal motor nucleus of the vagus

Motor nucleus for CN X Preganglionic parasympathetic neurons of the GI tract

Gustatory cortex lateral view

Must fold back the lateral fissure Note insula Olfactory on the bottom portion where knife is holding it back

Central Pattern Generators

Neural circuits that produce rhythmic outputs in the absence of rhythmic input and generally control multiple muscles or muscle groups (skeletal or smooth) Blinking, breathing, chewing

Nucleus ambiguus and dorsal motor nucleus of the vagus nerve

Nucleu ambiguus is very hard to find and does not have definitive borders

Nucleus of the Solitary Tract

Nucleus of the medulla that receives information from visceral organs and from the gustatory system Primary visceral sensory relay

Disorders of olfaction and gustatory

Olfactory = -osmia Gustatory = -guesia Agnosia = can't identify the smell or taste

Connection between Gustatory and Olfactory

One of olfactory tract pathways: OB --> Olfactory tract --> piriform Cortex --> MD thalamus --> Orbitofrontal cortex --> conscious perception of flavor

Orbitofrontal cortex

Orbitofrontal cortex (OFC) plays an important role in representing taste, flavor, and food reward Gustation coming in from CN VII and CN IX Somatosensation coming inn from trigeminal Olfaction coming in from CN I All meeting in the orbitofrontal cortex

Rostral cross section of medulla

Pyramidal tracts - lower, dark Olivary nucleus - weird squiggles Inferior cerebellar peduncle - dorsal lateral pop out Know rostral because fourth ventricle is very wide Nucleus of Solitary Tract is light and surrounds the tract Solitary tract is dark

Divisions of the nucleus of solitary tract

Rostral (upper) part has CN VII Middle part has CN IX Caudal (lower) part has CN X

Which contributes more to flavor: smell or taste?

Smell contributes more to flavor Patients who complain of a taste disorder are often ultimately diagnosed with a smell disorder Patients with Anosmia (can't smell) usually complain of both anosmia and aguesia

Solitary nucleus versus Nucleus of the Solitary Tract

Solitary nucleus = middle portion of the brainstem mostly relating to CN IX Nucleus of Solitary Tract = whole nucleus of the brainstem, includes gustatory nucleus, solitary nucleus, and cardiorespiratory nucleus

Type III taste cell

Sour (H+), salty (Na+) Binding of sour/salty particles stimulates the ion channel that causes depolarization of the taste cell. This leads to a calcium influx. Then there is conventional neurotransmitter release

Peripheral Problem (smell/taste)

Stimulus can't reach receptors OR are altered on the way to the receptor Olfactory: too much mucous (infection), inflammation, foreign body (tampon for nose bleeds) Gustatory: Dry mouth, thick saliva, thrush

Innervation of the tongue

THREE SENSORY nerves (compared to one for olfactory CN V) Anterior 2/3 - CN VII - Facial Posterior 1/3 - CN IX - Glossopharyngeal Epiglottis - CN X - Vagus

Three components to perception of flavor

Taste - primary gustatory cortex to flavor cortex Smell Texture - somatosensory component, come in through trigeminal nerve --> somatosensory cortex near insula and project forward into the orbitofrontal cortex where it will be combined with gustatory flavor cortex fibers Close in front of the Insula, Insula is the curved portion next to the tongue

Conscious perception of taste

Taste bud release NT --> signal primary afferent neurons (7,9,10) --> cell bodies of primary afferent neurons (geniculate, petrosal, nodose) --> brainstem --> solitary nucleus tract, second order neurons --> VPM thalamus --> fourth order neurons going to primary gustatory cortex (frontal lobe, insula)

Gustatory sensitivity declines with age.

This can be problematic in patients with hypertension or diabetes if the decreased sensitivity leads to addition of more salt and sugar to foods in order to taste them

Taste receptors (TR) are not just on receptor cells in taste buds

Tissues expressing taste receptors: Brain GI tract Lymphocytes Liver Pancreas Taste receptors on the surface of endocrine cells in the stomach will release NT that stimulate the vagus nerve and send messages to your brain Implications of these receptors elsewhere in the body help explain diseases link diabetes, obesity, asthma

After leaving the Solitary tract nucleus...

Transduction in the taste bud travels via primary afferent neurons. Then info undergoes modulation in nucleus of Solitary tract. As leaving the Solitary Nucleus fibers it can go 1/3 ways 1. Ventral Posteromedial thalamus and then on to the gustatory cortex (frontal/Insula) for conscious perception of taste 2. Hypothalamus/amygdala for homeostasis/visceral reactions 3. Reticular formation for reflex pathways

Types of Taste cells

Type I: structural supporting cells, detecting cells Type II: receptor cell Type III: receptor cells Type IV: Stem cells, replace damaged taste buds ONLY TYPE III HAS CONVENTIONAL SYNAPSES WITH PRIMARY AFFERENT NEURONS Average life span of a taste bud is 10 days

Baroreceptor reflex

produced by a group of mechanoreceptors that are found w/in walls of the heart. the reflex is activated when pressure rises w/in the large arteries above 60 mm Hg. peak in activity at approx 180 mm Hg. Results in vasodilation secondary to inhibition of the vasomotor centers w/in the medulla as well as a decrease in heart rate and strength of contraction secondary to vagal stimulation Look in Step One

Gag reflex

test from CN 9 and 10 afferent receptor in the pharyngeal mucosa travel on CN 9 and 10 and goes to NST, then nucleus ambiguus and efferent CN 9 and 10 to the palate, pharynx, and larynx