The
olfactory system is the
sensory systemA sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...
used for
olfactionOlfaction refers to the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a
main olfactory system and an
accessory olfactory systemThe Accessory olfactory system is one of the two olfactory systems commonly found in vertebrates. Like the main olfactory system, the accessory olfactory system is a chemosensory system, which transduces chemicals into neural activity. Unlike the main olfactory system, which detects volatile...
. The main olfactory system detects volatile, airborne substances, while the accessory olfactory system senses fluid-phase stimuli. Behavioral evidence indicates that most often, the stimuli detected by the accessory olfactory system are pheromones.
The olfactory system is often spoken of along with the
gustatory systemThe gustatory system is the sensory system for the sense of taste.- Importance :Humans require a way to distinguish safe food from dangerous substances. Bitter and sour foods we find unpleasant, while salty, sweet, and meaty tasting foods generally provide a pleasurable sensation...
as the
chemosensory senses because both
transduceIn physiology, transduction is the conversion of a stimulus from one form to another.Transduction in the nervous system typically refers to synaptic events wherein a mechanical/physical/etc stimulus is converted into an action potential which is transmitted along axons towards the central nervous...
chemical signals into
perceptionIn philosophy, psychology, and the cognitive sciences, perception is the process of attaining awareness or understanding of sensory information. It is a task far more complex than was imagined in the 1950s and 1960s, when it was predicted that building perceiving machines would take about a decade,...
.
The mechanism of the olfactory system can be divided into a peripheral one, sensing an external
stimulusIn physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it elicits or influences a reflex via stimulus transduction...
and encoding it as an electric signal in neurons, and a central one, where all signals are integrated and processed in the
central nervous systemThe central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of bilaterian animals—that is, all animals more advanced than sponges or jellyfish. In vertebrates, the central nervous system is enclosed in the meninges. It contains...
.
In mammals, the main olfactory system detects odorants that are inhaled through the
noseAnatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration in conjunction with the mouth....
, where they contact the main
olfactory epitheliumThe olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures about 1 square centimetre and lies on the roof of the nasal cavity about about 7 cm above and behind the nostrils...
, which contains various
olfactory receptorOlfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules. Activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain...
s.
The
olfactory system is the
sensory systemA sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...
used for
olfactionOlfaction refers to the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a
main olfactory system and an
accessory olfactory systemThe Accessory olfactory system is one of the two olfactory systems commonly found in vertebrates. Like the main olfactory system, the accessory olfactory system is a chemosensory system, which transduces chemicals into neural activity. Unlike the main olfactory system, which detects volatile...
. The main olfactory system detects volatile, airborne substances, while the accessory olfactory system senses fluid-phase stimuli. Behavioral evidence indicates that most often, the stimuli detected by the accessory olfactory system are pheromones.
The olfactory system is often spoken of along with the
gustatory systemThe gustatory system is the sensory system for the sense of taste.- Importance :Humans require a way to distinguish safe food from dangerous substances. Bitter and sour foods we find unpleasant, while salty, sweet, and meaty tasting foods generally provide a pleasurable sensation...
as the
chemosensory senses because both
transduceIn physiology, transduction is the conversion of a stimulus from one form to another.Transduction in the nervous system typically refers to synaptic events wherein a mechanical/physical/etc stimulus is converted into an action potential which is transmitted along axons towards the central nervous...
chemical signals into
perceptionIn philosophy, psychology, and the cognitive sciences, perception is the process of attaining awareness or understanding of sensory information. It is a task far more complex than was imagined in the 1950s and 1960s, when it was predicted that building perceiving machines would take about a decade,...
.
Function
The mechanism of the olfactory system can be divided into a peripheral one, sensing an external
stimulusIn physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it elicits or influences a reflex via stimulus transduction...
and encoding it as an electric signal in neurons, and a central one, where all signals are integrated and processed in the
central nervous systemThe central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of bilaterian animals—that is, all animals more advanced than sponges or jellyfish. In vertebrates, the central nervous system is enclosed in the meninges. It contains...
.
Peripheral
In mammals, the main olfactory system detects odorants that are inhaled through the
noseAnatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration in conjunction with the mouth....
, where they contact the main
olfactory epitheliumThe olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures about 1 square centimetre and lies on the roof of the nasal cavity about about 7 cm above and behind the nostrils...
, which contains various
olfactory receptorOlfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules. Activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain...
s. These olfactory receptors are membrane proteins of bipolar
olfactory receptor neuronAn olfactory receptor neuron, also called an olfactory sensory neuron, is the primary transduction cell in the olfactory system.-In vertebrates:...
s in the olfactory epithelium. Rather than binding specific ligands like most receptors, olfactory receptors display affinity for a range of odor molecules. Olfactory neurons transduce receptor activation into electrical signals in neurons. The signals travel along the
olfactory nerveThe olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity...
, which belongs to the
peripheral nervous systemThe peripheral nervous system resides or extends outside the central nervous system , which consists of the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs. Unlike the central nervous system, the PNS is not protected by bone or by the blood-brain...
. This nerve terminates in the
olfactory bulbThe olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...
, which belongs to the
central nervous systemThe central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of bilaterian animals—that is, all animals more advanced than sponges or jellyfish. In vertebrates, the central nervous system is enclosed in the meninges. It contains...
. The complex set of olfactory receptors on different olfactory neurons can distinguish a new odor from the background environmental odors and determine the concentration of the odor.
Central
AxonAn axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts electrical impulsesaway from the neuron's cell body or soma....
s from the olfactory sensory neurons converge in the
olfactory bulbThe olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...
to form tangles called
glomeruliThe glomerulus in olfaction is a structure in the olfactory bulb. It is made up of a globular tangle of axons from the olfactory receptor neurons in the olfactory epithelium and dendrites from the mitral cells, tufted cells, and other cell types.Glomeruli are important waystations in the pathway...
(singular glomerulus). Inside the glomulerus, the axons contact the
dendriteDendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...
s of mitral cells and several other types of cells. Mitral cells send their axons to a number of brain areas, including the anterior olfactory nuclues
http://en.wikipedia.org/wiki/Anterior_olfactory_nucleus, piriform cortexIn anatomy of animals, the piriform cortex, or pyriform cortex is a region in the brain. The piriform cortex is part of the rhinencephalon situated in the telencephalon....
, the medial
amygdalaThe ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans...
, and the
entorhinal cortexThe entorhinal cortex is an important memory center in the brain. The EC forms the main input to the hippocampus and is responsible for the pre-processing of the input signals. In the reflex nictitating membrane response of classical trace conditioning, the association of impulses from the eye...
.
The piriform cortex is probably the area most closely associated with identifying the odor. The medial
amygdalaThe ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans...
is involved in social functions such as mating and the recognition of animals of the same species. The
entorhinal cortexThe entorhinal cortex is an important memory center in the brain. The EC forms the main input to the hippocampus and is responsible for the pre-processing of the input signals. In the reflex nictitating membrane response of classical trace conditioning, the association of impulses from the eye...
is associated with memory, e.g. to pair odors with proper memories. The exact functions of these higher areas are a matter of scientific research and debate.
In the central nervous system, odors are represented as patterns of neural activity. These representations may be encoded by space (a pattern of activated neurons across a given olfactory region corresponds to the odor), time (a pattern of
action potentialAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
s by multiple neurons corresponds to the odor) or a combination of the two. Scientists debate whether the odor code is primarially temporal or spatial.salo
Clinical implications
Damage to the olfactory system can occur by traumatic
brain injuryTraumatic brain injury occurs when an outside force traumatically injures the brain. TBI can be classified based on severity, mechanism , or other features...
,
cancerCancer is a class of diseases in which a group of cells display uncontrolled growth , invasion , and sometimes metastasis...
, inhalation of toxic fumes, or neurodegenerative diseases such as
Parkinson's diseaseParkinson's disease is a degenerative disorder of the central nervous system that often impairs the sufferer's motor skills, speech, and other functions....
and
Alzheimer's diseaseAlzheimer's disease , also called Alzheimer disease, Senile Dementia of the Alzheimer Type or simply Alzheimer's, is the most common form of dementia. This incurable, degenerative, and terminal disease was first described by German psychiatrist and neuropathologist Alois Alzheimer in 1906 and was...
. These conditions can cause
anosmiaAnosmia is a lack of functioning olfaction, or in other words, an inability to perceive smells. It can be either temporary or permanent. A related term, hyposmia, refers to decreased ability to smell, while hyperosmia refers to an increased ability to smell. Some people may be anosmic for one...
. Doctors can detect damage to the olfactory system by presenting the patient with odors via a scratch and sniff card or by having the patient close their eyes and try to identify commonly available odors like coffee or peppermint candy.
History
Linda B. BuckLinda B. Buck, Ph.D., is an American biologist best known for her work on the olfactory system. She was awarded the 2004 Nobel Prize in Physiology or Medicine, along with Richard Axel, for their work on olfactory receptors....
and
Richard AxelRichard Axel is an American neuroscientist whose work on the olfactory system won him and Linda B. Buck, a former post-doctoral scientist in his research group, the Nobel Prize in Physiology or Medicine in 2004....
won the 2004
Nobel Prize in Physiology or MedicineThe Nobel Prize in Physiology or Medicine is awarded once a year by the Swedish Karolinska Institute. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in Physics, Chemistry, Literature, Peace, and Physiology or Medicine...
for their work on the olfactory system.
See also
- Olfactory transduction
Olfactory transduction is a series of events in which cells in the nose bind to scent-bearing molecules and send nerve signals to the brain where they are perceived as smells.- External links :* entry in the public domain NCI Dictionary of Cancer Terms...