Olfaction and Gustation - Comprehensive Study Notes

Olfaction: an animal’s primary special sense of smell
Consists of
- Olfactory bulb
- Olfactory tract
- Lateral olfactory gyrus
- Piriform lobe
Purpose: localization of food, reflex-stimulated secretion of digestive enzymes, and detection of danger
Olfactory cells found on ethmoturbinate bones of nasal cavity

Molecules that enter nasal cavity– dissolved in fluid secreted by glands located in mucosa
Stimulate olfactory receptors
Olfactory-binding protein: protein produced by glands of nasal mucosa found in mucus that’s required for molecule binding to receptor
Olfactory cells able to distinguish a variety of odors at extremely low concentrations

Sensory neurons are bipolar– cell body present in mucosa of nasal cavity just below cribriform plate
Olfactory knob: expansion of single dendrite of neuron once it terminates in surface of mucosa
Each knob gives rise to ~10-20 cilia → spread over surface of mucosa
Each cilium covered in mucus
Have receptors needed for transduction of stimulus
Groups of neurons expressing olfactory receptors that bind same set of odors
Affinity for odorant molecules varies a lot
Fila olfactoria: a small nerve bundle of a single non-myelinated axon that emerges at opposite end of sensory neuron
Olfactory bulb: fascicles pass through foramina in plate to enter here and synapse with tufted and mitral cells to process perception of stimuli

Membrane of cilia is covered with G protein-coupled receptors
Odorant molecule binds to receptor on cilia → activates G protein (Golf) → unites with guanosine triphosphate (GTP)
2nd messenger systems– IP3 and cAMP
GTP-Golf complex activates Phospholipase C → generates IP3 → opens Ca2+ channels in membrane

In math-like terms: GTP-Golf activation leads to IP3 and cAMP signaling, enabling Ca2+ and Na+ influx and subsequent depolarization

Ciliary EPSPs travel from cilia to trigger zone of cells
When they’re strong → AP are generated and propagated along axons of cells to bulb
Impulses dispersed to wide areas of CNS, including piriform lobe for perception of smell
Unique feature– rapid adaptation to stimulus: initial discharge declines quickly to a steady-state discharge of lower amplitude

Olfactory nerve fibers terminate in ipsilateral olfactory bulb
Dendrites of tufted and mitral cells synapse with terminal ends of olfactory nerve fibers
Forms glomeruli of olfactory bulb
NT released from terminal end of axons → excite mitral and tufted cells
Activities modulated by inhibitory periglomerular interneurons

Axons of mitral and tufted cells leave bulb → reach to other central structures
Form large lateral olfactory tract
Amygdala and entorhinal cortex– central structures that receive signals
Also send info to hippocampus and frontal cortex
Also extend axons to ipsilateral septal nucleus
Efferents of cells reach contralateral bulb via medial olfactory tract and anterior commissure
Limbic system: part of amygdala that, with entorhinal cortex, hippocampal formation, and septal nuclei, process emotional reaction to smells
Autonomic response carried out by hypothalamus and periaqueductal gray of midbrain

Taste buds: contain taste receptor cells
Located in various types of papillae on tongue
- Protrusions on dorsal and lateral surface of tongue
- Fungiform: distributed throughout dorsal surface of rostral ⅔ of tongue
- Vallate: occupy caudal portion of dorsal tongue
- Foliate: present on dorsolateral part of caudal part of tongue
Dogs can sense sweet, salt, sour, and bitter
Don’t have highly sensitive salt receptors or a strong craving for salt
Filiform used for grooming (velcro feeling when licked by cats caused by these)

Chemical substances dissolve in saliva → enter taste buds through pore → bind to receptors located in membrane of microvilli
Mechanisms that generate membrane depolarization depend on taste molecules that bind to their specific receptors
Salt receptors mediated by Na+ influx through amiloride-sensitive Na+ channel (ENaC) → depolarization → theorized opening of voltage-gated Ca2+ channels → influx of Ca2+ → release of NT
Sweet receptors mediated by G protein-coupled receptors that activate phospholipase C (PLC)
PLC activation generates 2nd messengers– IP3 and diacylglycerol (DAG)
IP3 binds to Ca2+ in ER → opens Ca2+ channel → Ca2+ diffuses out of ER into cytosol → Na+ influx mediated by taste-selective cation channels (TrpM5) → generation of depolarizing receptor potential
IC Ca2+ elevation combined with membrane depolarization causes ATP release via gap junction channels in plasma membrane
Transmitter ATP acts on sensory nerve endings → induces graded potentials

Taste cells innervated by bipolar neurons that contribute axons to facial (VII) and glossopharyngeal (IX)
Cell bodies are located in geniculate ganglion of facial nerve and distal ganglion of glossopharyngeal
Peripheral axons of geniculate ganglion leave VII nerve after they leave cranium to form chorda tympani nerve → runs through middle ear cavity
Joins lingual nerve and innervate taste buds on rostral ⅔
Central axons from distal ganglion reach tongue via lingual nerve
Enters base of tongue and provides sensory fibers to taste buds in caudal portion of tongue

Central processes of bipolar neurons in geniculate and distal ganglia enter nucleus of solitary tract in medulla oblongata
Efferent fibers from nucleus of solitary tract ascend as solitariothalamic tract → terminate in ventral posteromedial nucleus of thalamus
Thalamic neurons project to ipsilateral cerebral cortex
Nucleus of solitary tract also projects to amygdala of limbic system