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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

Gustation

The scientific term for the sense of taste or the act of perceiving flavors

Five Primary Tastes

Only five primary tastes can be distinguished: salty, sour, sweet, bitter, and umami

Five Primary Tastes: Salt

Salty taste is caused by the presence of sodium ions (Na+ ) in food

Five Primary Tastes: Sour

Salty taste is caused by the release of hydrogen ions (H+ ) in food

Five Primary Tastes: Sweet

Sweet taste is caused by natural sugars (e.g., glucose, fructose, and sucrose) and by artificial sweeteners (e.g., saccharin, aspartame, and sucralose).

Five Primary Tastes: Bitter

Bitter taste is caused by a wide variety of substances (e.g., caffeine, morphine, and quinine), which produce a natural response is to spit it out, serving as a survival tool to protect you from ingesting potentially harmful substances

Five Primary Tastes: Umami

Umami taste, first reported by Japanese scientists, is described as savory, rich, or meaty flavor, caused by the presence of monosodium glutamate (MSG), as a flavor enhancer

Five Primary Tastes: Sensations

All flavors are combinations of the five primary tastes along with accompanying olfactory, tactile, and thermal sensations.

Food Odors

Odors from food can travel from the mouth into the nasal cavity and stimulate olfactory sensory neurons.

Olfaction vs Taste: Sensitivity

Olfaction is much more sensitive than taste; a given concentration of a food substance may stimulate the olfactory system thousands of times more strongly than the gustatory system.

Gustatory Receptors

Specialized sensory receptors, located in the taste buds, detect chemical compounds in food and turn them into nerve signals for the brain

Taste Bud

An oval-shaped sensory organs found mainly on the tongue, but also on the soft palate, pharynx, and epiglottis, which let you detect different tastes

Taste Bud: Specialized Cells

The taste buds consists of three kinds of epithelial cells: supporting epithelial cells, gustatory epithelial cells, and basal epithelial cells

Specialized Cells: Supporting Epithelial Cells

Supporting epithelial cells surround about 50 gustatory epithelial cells within each taste bud.

Specialized Cells: Gustatory Epithelial Cells

Specialized sensory cells located inside taste buds that detect chemical compounds dissolved in saliva

Specialized Cells: Gustatory Epithelial Cells: Gustatory Microvilli

Projections from gustatory epithelial cells that extend through the taste pore, also known as gustatory hairs

Specialized Cells: Gustatory Epithelial Cells: Taste Pore

An opening in the taste bud through which gustatory microvilli project to the external surface.

Specialized Cells: Basal Epithelial Cells

Stem cells, located near the connective tissue at the periphery of a taste bud, produce supporting epithelial cells that can develop into gustatory epithelial cells.

Specialized Cells: Gustatory Epithelial Cells: Cell Replacement

Gustatory epithelial cells have a life span of about 10 days, allowing taste receptors to recover relatively quickly after damage due to basal epithelial cells continuously produce new cells.

Specialized Cells: Gustatory Epithelial Cells: Pathway

Gustatory epithelial cells synapse with first-order sensory neurons, forming the first part of the gustatory pathway, which branch widely to receive signals from many gustatory cells across several taste buds.

Taste Buds: Lingual Papillae

Small elevations on the tongue that hold taste buds, increase surface area, and give the tongue its rough texture.

Taste Buds: Lingual Papillae: Types

Three types of lingual papillae contain taste buds: vallate papillae, fungiform papillae, and foliate papillae

Taste Buds: Lingual Papillae: Vallate Papillae

The large, dome-shaped bumps arranged in a V-shaped row at the back of the tongue, which house 100-300 taste buds

Taste Buds: Lingual Papillae: Fungiform papillae

The mushroom-shaped bumps scattered over the tongue, which house about 5 taste buds each

Taste Buds: Lingual Papillae: Foliate Papillae

The small trenches on the lateral margins of the tongue that contain many taste buds, but most of their taste buds degenerate in early childhood.

Taste Buds: Lingual Papillae: Filiform Papillae

The pointed, threadlike structures, on the entire surface of the tongue, contain tactile receptors but no taste buds

Taste Buds: Lingual Papillae: Filiform Papillae: Function

Filiform papillae increase friction between the tongue and food, making it easier for the tongue to move food in the oral cavity

Tastants

Chemicals that stimulate gustatory epithelial cells

Tastants: Input

Once a tastant is dissolved in saliva, it can make contact with the gustatory microvilli, which are the sites of taste transduction

Tastants: Output

Once the tastant contacts the gustatory microvilli, it depolarizes the receptor potential, triggers neurotransmitter release, which ultimately generates nerve impulses in first-order sensory neurons that synapse with gustatory receptor cells.

Tastants: Salt

Na+ in a salty food enters gustatory cells via Na+ channels in the plasma membrane, which ultimately depolarizes the cell to release neurotransmitter.

Tastants: Sour

H+ in a sour food enters gustatory cells via H+ channels in the plasma membrane, which ultimately depolarizes the cell to release neurotransmitter.

Tastants: Sweet, Bitter, and Umami

For sweet, bitter, and umami, tastants bind to G proteins instead of entering the gustatory cells, activating enzymes that produce IP₃ (second messenger inositol trisphosphate), which causes depolarization and neurotransmitter release.

Tastants: One Type

An individual gustatory epithelial cell responds to only one type of tastant because its membrane has either ion channels or receptors for only one primary taste.

Tastants: Many Flavors

Food tastes different because the brain interprets different combinations and patterns of signals from gustatory epithelial cells, since different tastants activate different cells in different ways.

Taste Threshold

The minimum concentration of a substance needed to produce a detectable taste sensation varies for each of the primary tastes

Taste Threshold: Bitter

Bitter substances have the lowest taste threshold, meaning bitter taste receptors are the most sensitive among the primary tastes, as they are designed to protect the body from “poisonous” substances.

Taste Adaptation

A specific taste may decrease sensitivity within about 1–5 minutes of continuous stimulation— a result of the changes in taste receptors, olfactory receptors, and neurons of the gustatory pathway in the CNS

Gustatory Pathway

A route that carries gustatory taste information from the gustatory epithelial cells to the brain, where it becomes consciously perceived as taste.

Gustatory Pathway: Step 1

Three cranial nerves carry taste signals: the facial (VII) nerve from the anterior 2/3 of the tongue, the glossopharyngeal (IX) nerve from the posterior 1/3, and the vagus (X) nerve from the throat and epiglottis.

Gustatory Pathway: Step 2

Taste information travels from gustatory epithelial cells in the taste buds through cranial nerves to the gustatory nucleus in the medulla, where some signals go to the limbic system and hypothalamus, while others go to the thalamus.

Gustatory Pathway: Step 3

The conscious perception and discrimination of taste happen when taste information from the thalamus travel to the gustatory cortex in the insula of the cerebrum.

Vision

The special sense of sight, defined as the process where the eyes and brain work together to detect light patterns reflecting off objects and interpret them into visual images

Ophthalmology

The science that deals with the eyes and their disorders.

Electromagnetic Radiation

The energy in the form of waves that radiates from the sun

Electromagnetic Spectrum

The complete range of all types of electromagnetic radiation ordered by frequency and wavelength (e.g., gamma rays, x-rays, UV rays, visible light, infrared radiation, microwaves, and radio waves)g

Wavelength

The distance between two consecutive peaks of an electromagnetic wave

Wavelength Range

Wavelengths range from short to long; for example, gamma rays have wavelengths smaller than a nanometer, and most radio waves have wavelengths greater than a meter.

Visible Light

The small part of the electromagnetic spectrum that the human eye can see with wavelengths ranging from about 400 to 700 nm.

Visible Light: Color

The color of visible light depends on its wavelength from red to violet; light that has a wavelength of 400 nm is violet, and light that has a wavelength of 700 nm is red

Visible Light: The Color of an Object

An object can absorb certain wavelengths of visible light and reflect others; the object will appear the color of the wavelength that is reflected (e.g., an apple appears red because it mostly reflects red light, while absorbing most other color wavelengths)

Visible Light: White and Black

An object appears white because it reflects all wavelengths of visible light; an object appears black because it absorbs all wavelengths of visible light.

Eyelids

The folds of thin skin and muscle that shade the eyes during sleep, protect the eyes from excessive light and foreign objects, and lubricating over the eyeballs, known as the palpebrae

Eyelids: Levator Palpebrae Superioris

The upper eyelid is more movable than the lower eyelid and contains the levator palpebrae superioris muscle, which helps raise it.

Eyelids: Eye Twitch

An involuntary, minor spasm of the eyelid muscles is usually harmless and temporary, and is often associated with stress and fatigue.

Eyelids: Palpebral Fissure

The open space between the upper and lower eyelids when the eyes are open

Eyelids: Lateral Commissure

The outer corner of the eye where the upper and lower eyelids meet, which is narrower and closer to the temporal bone

Eyelids: Medial Commissure

The inner corner of the eye where the upper and lower eyelids meet, which is broader and nearer the nasal bone.

Eyelids: Lacrimal Caruncle

The small, pink, fleshy mound located at the inner corner of each eye, which contains sebaceous glands and sudoriferous glands.

Eyelids: Components

From superficial to deep, each eyelid consists of epidermis, dermis, subcutaneous tissue, fibers of the orbicularis oculi muscle, a tarsal plate, tarsal glands, and conjunctiva

Eyelids: Tarsal Plate

The thick band of dense connective tissue inside each eyelid that gives the lid its shape and structural support

Eyelids: Tarsal Plate: Tarsal Glands

Embedded in each tarsal plate is a row of elongated modified sebaceous glands that keep the eyelids from adhering to each other

Eyelids: Chalazion

A small tumor or cyst that forms on the eyelids when the tarsal glands becomes infected.

Eyelids: Conjunctiva

A thin, clear mucous membrane that covers the front of the sclera and lines the inside of the eyelids, containing nonkeratinized stratified squamous epithelium with goblet cells.

Eyelids: Palpebral Conjunctiva

Thin mucous membrane that lines the inner, posterior surface of the upper and lower eyelids

Eyelids: Bulbar Conjunctiva

Thin, clear membrane that covers the sclera (white of the eye) but does not cover the cornea, which is the transparent front surface of the eyeball.

Eyelids: Bloodshot Eyes

Bloodshot eyes occur when the blood vessels of the bulbar conjunctiva dilate and become congested due to irritation or infection.

Eyelashes

The short, curved hairs that grow along the edge of an upper or lower eyelid

Eyelashes: Sebaceous Ciliary Glands

Sebaceous glands at the base of the hair follicles of the eyelashes that release a lubricating fluid into the follicles.

Eyelashes: Sty

A painful, red, pimple-like bump on the edge of your eyelid caused by a bacterial infection in the sebaceous ciliary glands

Eyebrows

Lines of hair above each eye that help keep sweat and dust out and protect the eyes from foreign objects, sunlight, and perspiration.

Lacrimal Apparatus

The structural system that produces, circulates, and drains tears (lacrimal fluid) through a process called lacrimation.

Lacrimal Apparatus: Lacrimal Glands

An almond-shaped exocrine structure located in the upper outer corner of each eye orbit that produces lacrimal fluid

Lacrimal Apparatus: Excretory Lacrimal Ducts

The 6 to 12 small tubes that transport lacrimal fluid from the lacrimal gland onto the surface of the conjunctiva of the upper lid