Olfaction: Sense of Smell

Olfaction

The scientific term for the sense of smell or the act of perceiving odors

Olfactory Receptors

Specialized sensory receptors, located in the olfactory epithelium of the nose, detect smell molecules in the air

Olfactory Epithelium

A specialized layer of tissue that occupies the roof of the nasal cavity, covering the inferior surface of the cribriform plate and the superior nasal concha.

Olfactory Epithelium: Specialized Cells

The olfactory epithelium consists of three kinds of cells: olfactory sensory neurons, supporting epithelial cells, and basal epithelial cell

Specialized Cells: Olfactory Sensory Neurons

First-order neurons are bipolar neurons with an exposed, knob-shaped dendrite and an axon projecting from the cribriform plate to the olfactory bulb.

Specialized Cells: Olfactory Sensory Neurons: Olfactory Cilia

Hair-like structures projecting from the dendrite of an olfactory sensory neurons that serves as sites of olfactory transduction, converting stimulus energy into graded potential in a sensory neuron

Specialized Cells: Olfactory Sensory Neurons: Olfactory Receptor

Specialized proteins in the plasma membranes of olfactory cilia detect and bind inhaled chemicals (odorants) to stimulate olfactory sensory neurons.

Specialized Cells: Supporting Epithelial Cells

Columnar epithelial cells of the mucous membrane lining the nose that physically support, nourish, and insulate olfactory sensory neurons and detoxify chemicals that contact the olfactory epithelium.

Specialized Cells: Basal Epithelial Cells

Stem cells located between the supporting epithelial cells divide to continuously produce olfactory sensory neurons, replacing older neurons every two months.

Olfactory Glands

Mucus-secreting gland that moisten the olfactory epithelium and dissolves odorants, allowing for transduction; also known as the Bowman’s glands

Olfactory Glands: Stimulation

Parasympathetic neurons of the facial (VII) innervate these glands to produce tears and a runny nose when inhaling irritating substances (e.g., pepper, ammonia, and smoke)

Olfaction: Input

Odor molecules bind to receptors on the olfactory cilium, activating a G protein, which activates the enzyme adenylyl cyclase to produce cAMP, a second messenger that helps trigger the nerve signal.

Olfaction: Output

When cAMP opens cation channels, Na⁺ and Ca²⁺ enter the cytosol, causing depolarization, and if it reaches threshold, a nerve impulse (action potential) is generated

Olfaction: Type

The nose has about 10 million olfactory receptors with 400 types, and each olfactory sensory neuron has only one receptor type that responds to a specific group of odorants.

Olfaction: Brain

The ability to recognize and discriminate odors comes from different combinations of these receptor cells being activated and creating unique patterns of brain activity.

Odor Threshold

Only a few molecules of certain substances need to be present in air to be perceived as an odor

Odor Threshold: Adaptation

Olfactory receptors quickly adapt to odors, becoming about 50% less sensitive within the first second and sometimes completely insensitive after about a minute, partly because the central nervous system also adapts.

Olfactory Pathway

A route that carries olfactory information from the olfactory sensory neurons to the brain, where it becomes consciously perceived as smell.

Olfactory Pathway: Step 1

Both right and left olfactory (I) nerves have 40 bundles of olfactory sensory neurons that pass through the cribriform foramina of the cribriform plate of the ethmoid bone

Olfactory Pathway: Step 2

The olfactory nerves then extend to the olfactory bulbs in the brain, which contain ball-like arrangements called glomeruli

Olfactory Pathway: Step 3

Each glomerulus receives input from only one type of olfactory sensory neuron and passes information about specific groups of odorants to mitral cells, the second-order neurons

Olfactory Pathway: Step 4

Mistral cells then form the olfactory tract that project to the olfactory cortex in the temporal lobe of the cerebrum, where conscious awareness of smell occurs

Olfactory Pathway: Note

Note that olfactory sensations are the only sensations that reach the cerebral cortex without first synapsing in the thalamus

Olfactory Pathway: Limbic System

Other axons of the olfactory tract project to the limbic system, contributing to emotional responses to odors.

Olfactory Pathway: Step 5

Sensory information from the olfactory cortex then travels through the thalamus to the orbitofrontal cortex, where odors are identified and distinguished, in the frontal lobe

Olfactory Pathway: Damage

People who suffer damage in this area have difficulty identifying different odors