Chapter 9 - vestibular system, Taste, and Smell

Explain how accelerations and changes in the position of the head are transduced into sequences of action potentials

Semicircular canals:

Head rotation causes fluid flow in corresponding canal

Fluid movement bends stereocilia in ampulla

Bending of stereocilia excites vestibular neurons

Utricle and saccule:

Detect linear acceleration and deceleration

Otolithic membrane lags slightly during head movement

Lag bends stereocilia of nearby hair cells

Bending stimulates hair cells to track straight-line motion

Signal integration:

Three semicircular canals provide accurate tracking of head rotation

Utricle and saccule provide information on linear movement

Combined signals allow brain to calculate position and movement in 3D space

What are the main vestibular projections to the brainstem?

Vestibular part of vestibulocochlear nerve (cranial nerve VIII); Most fibers synapse in vestibular nuclei of brainstem; Some fibers bypass to cerebellum

What are the functional importance of vestibular projections?

Planning body movements; Maintaining balance against gravity; Directing sensory organs (eyes, ears) while body is in motion; Vestibulo-ocular reflex (VOR)

What is the Vestibulo-ocular reflex (VOR) and how does it function?

Maintains gaze on fixed point during head movement; Uses high-speed brainstem network; Functions accurately even with eyes closed

What are some examples of vestibular dysfunction or abnormal stimulation?

Motion sickness: Caused by strong vestibular stimulation; Sensory conflict theory: contradictory sensory messages; Virtual reality devices can induce motion sickness; Somatogravic illusion (false-climb illusion): Misinterpretation of linear acceleration as upward tilt; Can be dangerous for pilots in low visibility conditions

What are the three types of papillae on the tongue?

Circumvallate, foliate, and fungiform

What is the function of papillae on the tongue?

House taste buds; Circumvallate and foliate papillae contain many taste buds in their sides, while fungiform papillae contain about six taste buds each

Where are circumvallate, foliate, and fungiform papillae located on the tongue?

Circumvallate: Few in number, located at the back of the tongue; Foliate: Located on the sides of the tongue; Fungiform: Hundreds distributed across the tongue surface, resembling button mushrooms

What are taste buds and where are they located?

Clusters of 50-150 taste receptor cells located within papillae; Have a taste pore at the surface; Taste cells extend cilia into the taste pore to contact tastants; Each taste cell is sensitive to one basic taste; Lifespan of 10-14 days, constantly replaced

Identify the five basic tastes, and explain how each is transduced by specialized mechanisms of the tongue

Five basic tastes:

Salty

Sour

Sweet

Bitter

Umami

Salty taste transduction:

Primary mechanism: Sodium (Na+) enters taste cells through sodium channels

Secondary mechanisms: TRPV1 variant (detects other cations), TMC-1 (candidate receptor)

Chloride (Cl-) detection mediates aversive properties of high salt concentrations

Sour taste transduction:

Proton (H+) influx through OTOP1 channels

OTOP1 is highly selective for protons

Directly depolarizes sour taste cells

Sweet taste transduction:

T1R2 + T1R3 heterodimer receptor (G protein-coupled receptor)

Different recognition sites for various sweet molecules

Possible additional sweet-sensing mechanisms

Bitter taste transduction:

T2R family of G protein-coupled receptors (about 30 members)

Each bitter-sensing taste cell expresses most or all T2R types

Broadly tuned to detect various bitter substances

Umami taste transduction:

Variant of metabotropic glutamate receptor

T1R1 + T1R3 heterodimer receptor

Responds to amino acids, particularly glutamate

What are the cranial nerves involved in gustatory information transmission?

Facial nerve (VII), Glossopharyngeal nerve (IX), Vagus nerve (X)

Where do gustatory fibers synapse with second-order neurons in the brainstem?

Brainstem

Which nucleus do second-order gustatory neurons project to in the thalamus?

Ventral posterior medial nucleus

Where is the cortical taste area located?

Located in the insula, anterior to the central sulcus

What is the debate in taste encoding regarding how the brain processes taste information?

Pattern coding: brain extrapolates from relative activity across axon ensembles vs. Labeled lines: five distinct axon types, one for each taste

Where are extra-oral taste receptors located?

Soft palate, Stomach and intestines, Lungs and airways, Brain, Gonads (testes), Skin

What are the functions of taste receptors in the digestive system?

Organize appropriate digestive responses, Mobilize insulin and digestive hormones, Alter food transit time, Activate antimicrobial immune responses, Regulate appetite and nutrient absorption

What are the functions of taste receptors in the respiratory system?

Regulate protective reflexes (sneezing, coughing), Induce immune responses against noxious substances and infectious agents, Mediate bronchial inflammation (potential for new asthma treatments)

What are the functions of taste receptors in the testes, skin, and brain?

Testes: Essential for reproductive function (knockout leads to sterility in mice), Skin: Possibly sensing environmental chemicals, Brain: Function not yet clear

What is the significance of taste receptors in tissues other than the tongue?

Suggests taste transduction mechanisms may play a more fundamental role in cellular signaling beyond gustation, Opens new avenues for understanding body-wide chemosensation, Potential for developing new therapeutic approaches (e.g., asthma treatments)

What are the three cell types found in the olfactory epithelium?

Supporting cells, basal cells, and olfactory receptor neurons

How many olfactory receptor neurons do humans have?

About 6 million

Where does the apical dendrite of olfactory receptor neurons extend to?

To the mucosal surface

Where is the olfactory bulb located?

At the anterior end of the brain

What is the main function of mitral cells in the olfactory bulb?

They synapse with olfactory receptor neuron axons

Which brain regions do mitral cell axons from the olfactory bulb extend to?

Prepyriform cortex, entorhinal cortex, amygdala, and hypothalamus

Through which structure do olfactory receptor neuron axons pass to reach the olfactory bulb?

Cribriform plate

What is unique about olfaction in terms of sensory modalities?

It can synapse directly in the cortex without passing through the thalamus

What are the steps of olfactory transduction?

1. Odorants dissolve in the mucosal layer

2. Interact with olfactory receptors on cilia and dendritic knobs

3. Olfactory receptors are G protein-coupled receptors (GPCRs)

4. Activation triggers synthesis of second messengers (e.g., cAMP)

5. cAMP opens cation channels, leading to depolarization

6. Depolarization generates action potentials

What is the role of olfactory receptors?

1. Part of a large superfamily of GPCRs

2. Use a specific G protein called Golf

3. Humans have about 1000 olfactory receptor genes, but only about 400 are functional

4. Each olfactory receptor neuron expresses only one type of receptor protein

How does olfactory receptor diversity contribute to odor discrimination?

1. Humans can distinguish thousands to potentially trillions of different odors

2. Odor recognition likely involves pattern coding (activation of combinations of receptors)

3. Individual differences: 30% variation in olfactory receptor makeup between people

What is the olfactory receptor distribution pattern?

1. In rodents, receptor subfamilies are synthesized in separate bands of the epithelium

2. Topographic organization in the olfactory bulb (glomerular map)

Which species have sharper sense of smell than humans?

Many mammals such as cats, mice, and dogs.

What is the olfactory receptor neuron numbers for humans, dogs, and mice?

Humans: ~6 million, Dogs: 100-200 million (bloodhounds: ~300 million), Mice: ~2 million.

What is an example of dogs' olfactory sensitivity?

Dogs can detect certain odors at extremely low concentrations, like 2 parts per trillion.

What can humans distinguish in terms of odors?

Humans can distinguish thousands to potentially trillions of odors.

How do differences in olfactory abilities reflect evolutionary importance?

Differences in olfactory abilities reflect evolutionary importance for survival and reproduction.

What has happened to humans over evolutionary time regarding olfactory receptor genes?

Humans have lost functionality in many olfactory receptor genes over evolutionary time.

Vomeronasal system structure

Present in most terrestrial mammals, amphibians, and reptiles

Vomeronasal organ (VNO) contains receptors near the olfactory epithelium

Connects to the accessory olfactory bulb, which projects to the medial amygdala and hypothalamus

Vomeronasal receptors

Two major families in rodents: V1R and V2R

GPCRs, different from main olfactory system receptors

Can be sex-specific and distributed differently in males and females

V2Rs are sensitive to major histocompatibility complex (MHC) molecules

Function of the vomeronasal system

Specialized in detecting pheromones

Highly sensitive to low concentrations of pheromone molecules

Important for reproductive behavior and kin recognition in many animals

Vomeronasal system in humans

Debatable whether humans possess a functional VNO

Most genes for vomeronasal receptors and associated channels have become nonfunctional pseudogenes in humans

Evidence for human pheromone responses

Some behavioral evidence exists (e.g., menstrual cycle synchronization, effects of female tears on male arousal)

If present, likely accomplished through the main olfactory epithelium rather than VNO

What are trace amine-associated receptors (TAARs) and where are they found?

TAARs are found in the main olfactory epithelium and respond to volatile sex-specific compounds that may be pheromones.

How do trace amine-associated receptors (TAARs) challenge the dichotomy between odor and pheromone detection systems?

TAARs suggest that the separation between odor and pheromone detection systems may be oversimplified.

What does accumulating evidence suggest about the importance of odor in human social behavior?

Accumulating evidence indicates that odor plays a significant role in human social behavior.

What is the current understanding regarding human response to pheromones?

The current understanding is that while odor is important for human social behavior, further research is needed to determine if true pheromonal communication exists in humans.

What remains unclear about the mechanism for potential human pheromone detection?

The mechanism for potential human pheromone detection is still not fully understood.