Olfactory mucosa

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The olfactory mucosa or the olfactory epithelium in mammals ( Regio olfactoria ) contains the sensory cells of the sense of smell . It is the mucous membrane with which the uppermost area of the nasal cavity is lined. The chemoreceptors ( olfactory receptors ) of the olfactory cells are responsible for the fact that we can perceive and differentiate between a multitude of smells.

In other animal phyla, the olfactory cells can be located in completely different parts of the body - as in insects and many aquatic animals on the antennae .

Structure of the olfactory mucosa

The structure of the olfactory mucous membrane
The olfactory sense cells represent nerve cells.

The olfactory mucosa is located on the left and right in the roof of the upper nasal cavity . It has a brown color and in humans an area of ​​about 5 cm² and a height of 30–60 µm. The olfactory mucous membrane is made up of olfactory cells , supporting cells and microvilli cells and is supplemented by basal cells and serous glands . The supporting cells carrying microvilli are secretory active and contain a reddish-brown pigment, which is responsible for the deviating color of the olfactory field.

If necessary, the olfactory sensory cells can renew themselves (in the event of damage or the like) from differentiated basal cells ( stem cells ), while the old, non-functional olfactory cells perish through apoptosis . It was this amazing, continuous regeneration that led to the realization that it is based on neural stem cells, the existence of which was unknown for a long time.

Around 20–30 million olfactory sensory cells (olfactory receptor cells ) are embedded in the epithelium of the olfactory mucous membrane in humans . A dog has around 250 million olfactory cells, an eel almost a billion. From each of these cells 5–20 hairs ( cilia ) with special olfactory receptors protrude into the mucous membrane, the thin coating of which is called the mucus . The receptors respond to the scent molecules arriving there with the breath.

There are around 350 different types of them, each of which only reacts to a specific group of fragrance molecules that has to fit into the lock like a key. The groups of molecules differ both in shape and in their electricity. The combination of the addressed receptors results in an odor mixture that can have several thousand variants.

The nerve fibers ( axons ) emanating from the olfactory cells are bundled in their thousands to enable filtering and preprocessing. These axon bundles then pull through fine bone openings in the ethmoid bone ( Lamina cribrosa ) to the olfactory bulb ( Bulbus olfactorius ), which is to be regarded as an upstream part of the brain. The entire olfactory nerve is in science olfactory nerve called.

Excitation of the olfactory cells

The carriers of the odors are molecules of the respective gas, but mostly only in low concentration in the air . The odor molecules enter the upper nasal cavity through the nose or mouth to the olfactory mucous membrane, where they are dissolved and can chemically trigger excitations in the olfactory cells. These cells ( primary sensory cells ) react to the arriving molecules via their small cytoplasmic extensions (olfactory hairs), in whose membranes the olfactory receptors are located.

If the receptors are excited by olfactory molecules, an action potential arises on the axon hillock of the olfactory cell if the concentration of the molecules is sufficiently high . The electrical impulses are combined by integrating nerve fibers and passed on to the olfactory brain.

Olfactory cells and "suppression"

The molecular processes in the olfactory cells take place via G-protein-coupled receptors . They open CNG ion channels via ACIII → cAMP increase → Ca 2+ influx → depolarization. As a result of these processes, the sensory cells can adapt within a few minutes, that is, adapt to strong stimuli.

The mucus also contains some enzymes (CYP450) that may deactivate molecules that may interfere with the perception of smell, as well as transport proteins that ensure better transport of the odor molecules through the mucus to the cilia .

Olfactory bulb and transmission of stimuli

The first and only synaptic interconnection of the olfactory sense takes place in the olfactory bulb before the information reaches the relevant brain centers. So-called mitral filter and amplify the sensory stimulus by a number of olfactory cells integrate , which have in common that they are energized by the same odor molecules. Each mitral cell therefore represents a specific odor.

The nerve cords ( axons ) run from the olfactory bulb in the olfactory tract , which now divides into two: a medial cord (stria olfactoria medialis) and a lateral cord (stria olfactoria lateralis). The medial cord reaches the septal area and the olfactory tubercle , this information remains unconscious. The lateral cord leads to the prepiriform cortex (the primary olfactory cortex), the entorhinal cortex and the cortical nucleus of the amygdala . There are still connections to the hypothalamus and (partly after interconnection in the mediodorsal core of the thalamus ) to the orbito-frontal association cortex ( seat of personality ).

The olfactory function protects the respiratory organs and the entire organism from harmful influences, e.g. B. toxic (mostly foul-smelling) gases (cave: carbon monoxide is odorless). Pleasant smells trigger secretion reflexes , e.g. For example, when you smell delicious food, you “get your mouth watering”. Unpleasant smells, on the other hand, can cause nausea. So there is a close connection between olfactory sensations and the unconsciously working part of the nervous system ( vegetative nervous system ). So could z. B. McClintock and Russell show that menstrual cycle synchronization in community women is based on olfactory perception.

Web links

Commons : Olfactory receptor neurons  - collection of images, videos and audio files

See also

Individual evidence

  1. a b Steffen Schaal, Konrad Kunsch, Steffen Kunsch: The human being in numbers: A data collection in tables with over 20,000 individual values . 4th edition. Springer, Berlin 2015, ISBN 978-3-642-55399-8 , pp. 178 .
  2. LC Junqueira, J. Carneiro: Histology: Textbook of cytology, histology and microscopic human anatomy . 2nd Edition. Springer, Berlin 2013, ISBN 978-3-662-07782-5 , pp. 587 .