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