Sensory systems: Smell and taste - the chemical senses [WIP]

Lecture 8

Olfaction

Detection of airborne chemical stimuli called odorants that are encoded into electrical signals

Olfactory epithelium

Odorants interact with olfactory receptor neurons (ORNs) found in an epithelial sheet, the___ ___

The transduction of olfactory information that results in the conscious sense of smell begins here

→ Sheet of neurons and supporting cells that lines approximately half of the surface of the nasal cavity

olfactory bulb

Axons project through the cribriform plate directly to neurons in the ___ ___

Olfactory Tract

The olfactory bulb sends projections to the pyriform cortex and other forebrain structures via the ____ ___

thalamic relay

Unique among the sensory systems, the olfactory system does not include a ___ ___ from primary receptors to the cortical region that processes the sensory information

anosmia

The inability to detect one or more odors

Respiratory epithelium

Lines the remaining surface of the nasal cavity

• Maintains appropriate temperature and moisture for inhaled air

• Provides an immune barrier that protects the nasal cavity from irritation and infection

Mucus layer

• Secreted by Bowman’s glands

• Contains enzymes and antibodies to protect exposed portions of cells to avoid passing infection to brain

• Controls ionic milieu for olfactory cilia

• When thickens → significantly

Olfactory receptor neurons (ORNs)

• Bipolar with unmyelinated axons

• Apical surface → knob that extends olfactory cilia into mucus layer

Olfactory cilia

The primary site of odorant transduction

Basal cells

The stem cells of the adult olfactory epithelium

Sustentacular (supporting) cells

detoxify potentially dangerous chemicals

Olfactory Neurogenesis

Because olfactory receptor neurons are particularly exposed, they are:

a) protected by mucus and b) continuously regenerated

The olfactory epithelium contains stem cells (in the layer of basal cells) that regenerate olfactory neurons throughout life

olfactory nerve

Olfactory receptor neuron axons form a large # of bundles that together make up the ___ ___ (cranial nerve I)

Glomeruli

__ in the olfactory bulb are the synaptic target of the primary olfactory axons

→ Individual ___ respond to different odorants and concentration of odor increases their activity

mitral cells

Within the glomeruli, ORN axons contact apical dendrites of ___ ___

→ Excitatory glutamatergic synapses

These cells are the principal projection neurons of the olfactory bulb

Granule cells

These cells synapse primarily on basal dendrites of mitral cells

→ Establish local inhibitory circuits and participate in synaptic plasticity

Glomerular layer

Layer of the Olfactory Bulb that contains:

– Dendritic tufts of mitral cells

– ORN axon terminals

– Periglomerular cells that define the margins of the glomerulus

External plexiform layer

Layer of the Olfactory Bulb that contains:

– Lateral dendrites of mitral cells

– Cell bodies and lateral dendrites of tufted cells

– Dendrites of granule cells that make dendrodendritic synapses with the other dendritic elements

Mitral cell layer

Layer of the Olfactory Bulb that contains:

– Cell bodies of mitral cells

Internal plexiform layer

Layer of the Olfactory Bulb that contains:

– Mitral cell axons

Granule cell layer

Layer of the Olfactory Bulb that contains:

– Granule cell bodies

pyriform cortex

Main target of mitral cell projections → neurons show broad responses and may integrate different odors (function is unclear)

This area projects to higher areas

orbitofrontal cortex

A higher area of the brain responsible for reward processing

Receives projections from the pyriform cortex

amygdala

A higher area that is responsible for emotional salience and memory

Receives projections from the pyriform cortex

vomeronasal system

The second division of the olfactory system present in carnivores and rodents

The primary function of this system is to encode and process info about odorants (such as pheromones or kairomones) and bring about sexual, reproductive, homeostatic, and/or aggressive responses.

Pheromones

Chemical signals released by an animal and used for communication within a species (conspecifics)

Kairomones

Chemical signals released by an animal and used for communication between species (Predator or prey)

The gustatory system

The system responsible for taste

papillae

3 types → Fungiform, Foliate, and Circumvallate

Human Taste Nerves

Axons run in cranial nerve VII, cranial nerve IX, and cranial nerve X

nucleus of solitary tract

Cranial nerves project to __ ___ ___ ___ (gustatory nucleus)

Taste cells

__ ___ are clustered in taste buds

taste pore

Taste cells are clustered around a __ ___

basal cells

Taste cells are regenerated every 2 weeks from ___ ___, similar to ORNs

5 tastes

Sour, bitter, salty, sweet, and MSG (umami (savory))

salty and sour

____ tastes are elicited by ionic stimuli like positively charged ions in salts (Na+ ) or protons in acids (H+ )

→ Ion channels

Sweet and savory

These taste receptors are G-protein-coupled receptors to PLC IP3 signaling opening TRP channels

Bitter

This taste receptors are also G-protein-coupled receptors with a specific G-protein called gustducin (role remains unclear)

Flehmen response

When animals open their mouth to take in pheromones