olfactory receptors

Examples of olfactory receptors in the following topics:

• Olfactory (I) Nerve

  • The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity.
  • The olfactory nerves consist of a collection of many sensory nerve fibers that extend from the olfactory epithelium to the olfactory bulb, passing through the many openings of the cribriform plate of the ethmoid bone.
  • Olfactory receptor neurons continue to emerge throughout life and extend new axons to the olfactory bulb.
  • The sense of smell (olfaction) arises from the stimulation of olfactory (or odorant) receptors by small molecules of different spatial, chemical, and electrical properties that pass over the nasal epithelium in the nasal cavity during inhalation.
  • These interactions are transduced into electrical activity in the olfactory bulb, which then transmits the electrical activity to other parts of the olfactory system and the rest of the central nervous system via the olfactory tract.

• Classification of Receptors by Stimulus

  • Sensory receptors can be classified by the type of stimulus that generates a response in the receptor.
  • During smell, olfactory receptors recognize molecular features of wafting odors.
  • Cutaneous receptors are sensory receptors found in the dermis or epidermis.
  • Encapsulated receptors consist of the remaining types of cutaneous receptors.
  • A tonic receptor is a sensory receptor that adapts slowly to a stimulus, while a phasic receptor is a sensory receptor that adapts rapidly to a stimulus.

• Taste and Smell at Birth and in Old Age

  • A diagram of the olfactory system is shown in .
  • Pregnancy and childbirth result in a high state of plasticity of the olfactory system that may facilitate olfactory learning within the mother.
  • The taste receptor cells send information detected by clusters of various receptors and ion channels to the gustatory areas of the brain via the seventh, ninth, and tenth cranial nerves.
  • The structure of taste receptors in humans is shown in .
  • Human olfactory system. 1: Olfactory bulb 2: Mitral cells 3: Bone 4: Nasal epithelium 5: Glomerulus (olfaction) 6: Olfactory receptor cells

• Overview of Sensation

  • These receptors respond to changes and stimuli in the environment.
  • The olfactory system is the sensory system used for the sense of smell (olfaction).
  • In humans, olfaction occurs when odorant molecules bind to specific sites on the olfactory receptors in the nasal cavity.
  • These receptors are used to detect the presence of smell.
  • They come together at a structure (the glomerulus) that transmits signals to the olfactory bulb in the brain.

• Classification of Receptors by Location

  • Some sensory receptors can be classified by the physical location of the receptor.
  • Our brains commonly receive sensory stimuli from our visual, auditory, olfactory, gustatory, and somatosensory systems.
  • Sensory receptors code four aspects of a stimulus:
  • Receptors are sensitive to discrete stimuli and are often classified by both the systemic function and the location of the receptor.
  • Sensory receptors are found throughout our bodies, and sensory receptors that share a common location often share a common function.

• Sensory Modalities

  • It collects and sends these vibrations to receptor cells.
  • It contains the receptors responsible for detecting molecules that are small enough to smell.
  • These receptor neurons then synapse at the olfactory cranial nerve, which sends the information to the olfactory bulbs in the brain for initial processing.
  • There are specialized receptors for cold (declining temperature) and heat.
  • The cold receptors infer wind direction, an important part in the animal's sense of smell.

• Ionotropic and Metabotropic Receptors

  • Although both ionotropic and metabotropic receptors are activated by neurotransmitters, ionotropic receptors are channel-linked while metabotropic receptors initiate a cascade of molecules via G-proteins.
  • Two types of membrane-bound receptors are activated with the binding of neurotransmitters: ligand-gated ion channels (LGICs) inotropic receptors and metabotropic G- protein coupled receptors.
  • Examples of metabotropic receptors include glutamate receptors, muscarinic acetylcholine receptors, GABAB receptors, most serotonin receptors, and receptors for norepinephrine, epinephrine, histamine, dopamine, neuropeptides, and endocannabinoids.
  • Since opening channels by metabotropic receptors involves activating a number of molecules in the intracellular mechanism, these receptors take longer to open than the inotropic receptors.
  • While ionotropic channels have an effect only in the immediate region of the receptor, the effects of metabotropic receptors can be more widespread throughout the cell.

• Blocking of Hormone Receptors

  • A receptor antagonist does not provoke a biological response upon receptor binding, but limits or dampens agonist-mediated responses.
  • A receptor antagonist is a type of receptor ligand or drug that does not provoke a biological response itself upon binding to a receptor, but blocks or dampens agonist-mediated responses.
  • Binding to the active site on the receptor regulates receptor activation directly.
  • The current accepted definition of receptor antagonist is based on the receptor occupancy model.
  • Irreversible antagonists covalently bind to the receptor target and, in general, cannot be removed; inactivating the receptor for the duration of the antagonist effects is determined by the rate of receptor turnover, the rate of synthesis of new receptors.

• Target Cell Specificity

  • Hormones target a limited number of cells (based on the presence of a specific receptor) as they circulate in the bloodstream.
  • This androgen insensitivity occurs when the receptors on the target cells are unable to accept the hormone due to an impairment in receptor shape.
  • Target cells are capable of responding to hormones because they display receptors to which the circulating hormone can bind.
  • Finally, hormone-receptor affinity can be altered by expression of associated inhibitory or co-activating factors.
  • In some instances alterations of receptor structure due to a genetic mutation can lead to a reduction in hormone-receptor affinity as in the case of androgen insensitivity.

• Adrenergic Neurons and Receptors

  • Adrenergic receptors are molecules that bind catecholamines.
  • There are two main groups of adrenergic receptors, α and β, with several subtypes. α receptors have the subtypes α1 (a Gq coupled receptor) and α2 (a Gi coupled receptor).
  • Phenylephrine is a selective agonist of the α receptor. β receptors have the subtypes β1, β2 and β3.
  • α1-adrenergic receptors are members of the G protein-coupled receptor superfamily.
  • This schematic shows the mechanism of adrenergic receptors.