Smell - Olfactory System PDF

SMELL  Olfaction is the perception of gaseous odorants Olfactory organ is the nose which contains olfactory epithelium/membrane  Olfactory membrane lies within the superior part of each nostril Folding over the septum superiorly and the turbinates laterally Olfactory membrane covers...

SMELL  Olfaction is the perception of gaseous odorants Olfactory organ is the nose which contains olfactory epithelium/membrane  Olfactory membrane lies within the superior part of each nostril Folding over the septum superiorly and the turbinates laterally Olfactory membrane covers a surface area of 2.4 cm in each nostril Consists of olfactory cells, sustentacular cells, basal stem cells, and Bowman’s glands OLFACTORY CELLS About 100 million within the olfactory epithelium Bipolar nerve cells They project olfactory hairs/cilia into the nasal cavity Cilia form a dense mat in the mucus and are responsible for the detection of chemical stimuli (smell)  Bowman glands secrete mucus onto the surface of the olfactory membrane The supporting cells contain enzymes that oxidize hydrophobic volatile odorants, making them less lipid- soluble and thus less able to penetrate membranes and enter the brain. Basal stem cells produce new olfactory cells to replace those destroyed. OLFACTORY STIMULATION Odorant should be volatile in order to reach the olfactory membrane Has to be partially water soluble to pass through the mucus and reach the olfactory cilia The olfactory cell membrane is polarized prior to stimulation by odorant Odorant binds to the receptor proteins in the cilium and initiates a G protein-coupled cyclic AMP second messenger system.  The opening of sodium channels causes the depolarization, thus excitation of the cell Each G protein complex activates several molecules of adenylyl cyclase, producing a cascading effect and increasing sensitivity The signal is transmitted to the central nervous system via the olfactory nerve. Adaptation in the olfactory receptors is partial, however CNS feedback inhibition occurs through the olfactory bulb. OLFACTORY SIGNAL TRANSMISSION Signals transmitted from the olfactory cells through perforations in the cribriform plate to the glomeruli of the olfactory bulb Olfactory bulb also contains dendrites from mitral cells and tufted cells which transmit signals up to the brain Olfactory nerve continues as the olfactory tract dividing into Medial olfactory area Lateral olfactory area MEDIAL OLFACTORY AREA Also known as the primitive olfactory area Contains septal nuclei that feed into the hypothalamus and some portions of the limbic system Drives basic emotions and responses related to olfactory stimuli e.g. food LATERAL OLFACTORY AREA Also known as the less old olfactory system Composed of the prepyriform cortex, pyriform cortex and cortical portion of amygdaloid nuclei  Signals pass to almost all areas of the limbic system Some signals also pass into other areas of the cortex – the paleocortex and anteromedial portion of the temporal lobe without first going through the thalamus. Largely responsible for learned responses to olfactory stimuli. NEWER PATHWAY Passes from the hypothalamus to the dorsomedial thalamic nucleus and then to the lateroposterior quadrant of the orbitofrontale cortex Considered to help in the conscious analysis of odor. CLINICAL CONDITIONS Kallmann syndrome presents with anosmia and hypogonadism due to failure of differentiation or migration of neurons in the olfactory mucosa during the embryological period Rhino-sinusitis, sinusitis which cause inflammation of the mucosa, obstructing the olfactory cell cilia.