33.1 Olfactory Epithelium

• Neuroepithelium is primarily located underneath the cribriform plate

May also be found on parts of the superior and middle turbinates and the nasal septum

May contain islands of respiratory epithelium at the peripheries

• Primary area is known as the olfactory cleft

• 1–2 mm wide with ~200 to 400 mm² of olfactory epithelium

• Proportionally, humans derive less from olfaction in the context of nasal surface area; only 15% of human nasal mucosa is designated to olfactory function compared with 40% in rats

33.1.1 Cell Types

• Bipolar sensory receptor

• Sustentacular

• Microvillar cells

• Globose basal

• Horizontal basal

• Bowman gland and duct cells

• Second-order neurons project posteriorly as the olfactory tracts to:

Thalamus

Limbic system

Orbitofrontal neocortex (secondary olfactory cortex)

33.3.2 Function

• Odour discrimination takes place in secondary cortex

• Affective responses controlled by the limbic system

• Projections of the second-order neurons to the primary olfactory cortex are direct connections with some neurons connecting in turn directly to the secondary olfactory cortex and some relaying via the thalamus between these two cortical areas

33.4 Primary Olfactory Cortex

• Anterior olfactory nucleus (AON)

• Olfactory tubercle

• Entorhinal area

• Piriform cortex

• Periamygdaloid cortex

• Corticomedial amygdala

• Anterior commissure may also carry decussating fibres from pyramidal cells of the AON to contralateral elements of the primary olfactory cortex

33.5 Vomeronasal Organ

• Present in 75 to 100% of the population

• Innervated by n terminalis (CN 0)

• In other species CN 0 would connect the VNO to the accessory olfactory bulbs (AOBs), amygdala, and hypothalamus

• Humans do not appear to possess AOBs and lack the septal organ of Masera

• Blind-ending pit of VNO contains pseudostratified columnar epithelium

• On electron microscopy: dark supporting cells and light sensory cells with neurofilaments

• Embryologically VNO is formed when the nasal mucosal epithelium invaginates to form bilateral tubular structures, which appear to be at their greatest size during the 25th week of development

33.6 Physiological Mechanisms

33.6.1 Odour Classification

• Floral

• Pungent

• Putrid

• Ethereal

• Peppermint

• Musk

• Camphoraceous

• Relies on interaction between molecules dissolved in the mucus layer and the transmembrane receptors of the cilia

• Cilia are immotile and do not function to move nasal mucus; rather they aid transduction of molecules

• Process is aided by the shunting of up to 15% of the incoming air stream towards the olfactory cleft during inhalation enabling odourant molecules to move from the deflected air stream to the largely aqueous phase of the olfactory mucus

• Turbulence provided by the turbinates mixes odours during inspiration

• Turbulent airflow during exhalation also provides odour presentation to olfactory cleft

• Orthonasal olfaction = smell odours from the outside

• Retronasal olfaction = passage of food odours from the oral cavity while eating

• 80% of food flavor is due to retronasal olfaction, not taste

• Sniffing improves the processes of increasing airflow and mixing

• Nasal mucus is produced by the Bowman glands and to a lesser degree by the sustentacular cells

• Odourant molecules reach the olfactory receptors by diffusing through the mucus, or alteratively are actively transported via odourant binding proteins

• Sustentacular cells may also deactivate some odourants and xenobiotic agents

• Function of microvilli of microvillar and sustentacular cells is not at present understood

33.6.3 Receptor Mechanism

• Odourant arrival at a G-protein coupled sevendomain receptor stimulates AP in the primary afferent fibre

• AP is increased in proportion to the odourant concentration

• Receptors appear to be located in specific groups according to the class of odours that they are sensitive to

• Arrangement of groups in different areas of the mucosa enables the CNS to receive a spatially coded signal that conveys in part the quality of the odours

• “Labelled-line” system is not dissimilar to that used in taste perception where individual gustatory cells are more sensitive to certain tastes and any particular taste perception is the result of activity of a group of gustatory cells in a group fashion

• Functional units exist because each olfactory neuron expresses a specific receptor gene, of which there are thousands, and all neurons with the same expression project to the same glomeruli of the olfactory bulb

• Interaction between odour molecules and receptor is debated to be either due to shape theory or due to electron tunnelling (vibrational) theory

33.6.4 Cellular Mechanism in Receptor Cells

• Guanine nucleotide-binding protein (G_olf) activates the enzyme adenyl cyclase to induce production of the second messenger cAMP or with some odourants cGMP

• These products enable cellular depolarization by opening cyclic nucleotide-gated ionic channels and Ca²⁺-dependent Cl^– or K⁺ channels having diffused through the cytoplasm

• G_olf is one of 13 G proteins present in receptor cells

• Cellular mechanisms allow the human nose to detect ~10,000 odours (a bloodhound can detect 40,000)

33.6.5 Cortical Activity

• Concentration dependency in odour detection has been demonstrated as being relevant to cerebellar activation whereby postero-lateral areas of the cerebellum have been shown to be stimulated with functional imaging studies

• Anterior cerebellar activation evident with sniffing alone

• Probably because the size of sniffing is modified via the cerebellum in proportion to the odour intensity

33.7 Vomeronasal Organ Physiology

• Serous glands present suggest a gustatory feature innervated autonomically

• Blood vessels with autonomic innervation run alongside the VNO suggesting a vasomotor pump to actively enhance stimulant uptake by the organ (“flehmen”)

• In animals this leads to activation of the hypothalamus via the AOBs and amygdala

• However, the use of a vomeropherin has been shown to cause changes of autonomic function:

Pulsatile release of luteinizing and folliclestimulating hormones (in males)

Decreased respiratory frequency

Increased cardiac frequency

Event-related changes of electrodermal activity and EEG pattern

• Animal studies suggest that the VNO enables hamsters to differentiate different smells

33.7.1 Gustatory System Anatomy Lingual Papillae

• Taste buds are located in papillae in the following locations:

Tongue

Palate

Oropharynx

Larynx (epiglottis)

Upper oesophagus

• Bulb-shaped structures comprising 50 to 120 bipolar cells

• Microvilli protrude from the cells into a mucusfilled taste pit

• Vallate papillae (48%)

Lie anterior to sulcus terminalis and extend in a V-shaped line across the tongue root

CN IX innervation

• Foliate papillae (28%)

Located along posterolateral margins of the tongue surface

CNs IX and VII (chorda tympani) innervation

• Fungiform papillae (24%)

Seen easily as the pink elevations on the anterior tongue

3½ taste buds per papilla—1120 fungiform taste buds

• Filiform papillae—do not contain taste buds

Taste Bud Cell Types

• I—Insulating cells to envelop axons

• II—Possible secretory function

• III—Gustatory sensory cells

• IV—Undifferentiated stem cells

• V—Marginal cells

Gustatory Innervation

• Gustatory (chorda tympani) and somatosensory fibres run in the lingual nerve to fungiform (± foliate) papillae on anterior two-third of tongue

• Greater petrosal nerve supplies taste buds on soft palate

• Glossopharyngeal nerve supplies vallate and foliate papillae and pharyngeal taste buds

• Superior laryngeal branch of the vagus supplies taste buds on epiglottis, larynx, and oesophagus

Central Processing

• Structures involved include:

Nucleus of the solitary tract

Thalamic taste area

Insular-opercular (primary) taste cortex

Caudolateral orbitofrontal secondary cortical taste area

Amygdala

Hypothalamus

Basal ganglia

33.7.2 Gustatory System Physiology

Taste Detection

• Taste modalities detected by receptors for:

Umami—L-glutamate and nucleotide enhancers

Sweet—sugars, artificial sweeteners, D-amino acids, glycine, sweet proteins

Bitter—cyclohexamide, denatonium, salicin, PTC, saccharin, quinine, strychnine, atropine

Salt—NaCl and sodium salts

Sour—acids

Carbonated drinks