Everything about The Olfactory System totally explained
The
olfactory system is the
sensory system used for
olfaction. Most mammals and reptiles have two distinct parts to their olfactory system: a
main olfactory system and an
accessory olfactory system. The main olfactory system detects volatile, airborn 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 system as the
chemosensory senses because both
transduce chemical signals into
perception.
Function
The mechanism of the olfactory system can be divided into a peripheral one, sensing an external
stimulus and encoding it as an electric signal in
neurons, and a central one, where all signals are integrated and processed in the
central nervous system.
Peripheral
In mammals, the main olfactory system detects odorants that are inhaled through the
nose, where they contact the main
olfactory epithelium, which contains various
olfactory receptors. These can distinguish a new odor from the background environmental odors and determine the concentration of the odor.
These olfactory receptors are connected to
olfactory receptor neurons in the olfactory epithelium, which transduce receptoractivation into electrical signals in
neurons. The signals travel along the
olfactory nerve, which belongs to the
peripheral nervous system. This nerve terminates in the
olfactory bulb, which belongs to the
central nervous system.
Central
Axons from the olfactory sensory neurons converge in the olfactory bulb to form tangles called
glomeruli (singular glomerulus). Inside the glomulerus, the axons contact the
dendrites of
mitral cells and several other types of cells. Mitral cells send their axons to a number of brain areas, including the
piriform cortex, the medial
amygdala, and the
entorhinal cortex.
The piriform cortex is probably the area most closely associated with identifying the odor. The medial
amygdala is involved in social functions such as mating and the recognition of animals of the same species. The
entorhinal cortex is associated with memory, for example 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 potentials by multiple neurons corresponds to the odor) or a combination of the two. Scientists debate whether the odor code is primarially temporal or spatial.
Clinical implications
Damage to the olfactory system can occur by traumatic
brain injury,
cancer, inhalation of toxic fumes, or neurodegenerative diseases such as
Parkinson's disease and
Alzheimer's disease. These conditions can cause
anosmia. 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. Buck and
Richard Axel won the
2004 Nobel Prize in Physiology or Medicine for their work on the olfactory system.
Further Information
Get more info on 'Olfactory System'.
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