Olfactory+System

=**Olfactory System**=


 * 1. Describe the major pathways of olfactory system.**

Primary sensory neurons in the nasal cavity olfactory epithelium project through the cirbiform plate and to the olfactory bulb. Secondary neurons in the olfactory bulb project to olfactory cortex regions including the anterior olfactory nucleus, piriform cortex, olfactory tubercle, cortical nucleus of the amygdale, and enteorhinal cortex.

Neurons form entorhinal cortex project to hippocampus. Olfactory tubercle and piriform cortex (endopiriform nucleus) neurons project to medial dorsal nucleus of the thatamus, which then projects to orbitofrontal cortex (conscious perceptions). Neurons in the amygdale and piriform cortex project to hypothalamus and midbrain tegmentum (affective responses). Centrifugal pathways descend to the olfactory bulb, including the anterior olfactory nucleus which descends contralaterally through the anterior commissure, and the basal forebrain and olfactory cortex which descend ipsilaterally.


 * 2. State the steps and timing of olfactory receptor cell regeneration.**

Receptorcells continuously degenerate and are replaced by new ones every 60 days. Basal cells develop into receptor cells.


 * 3. Explain how information about odors is converted to electrical activity in the olfactory epithelium.**

The olfactory system uses as many as 950 different odorant receptor genes that express chemoreceptors on the olfactory epithelium. These receptors all use G-protein (G-olf) coupled second messenger intracellular signaling (cAMP, IP3, etc.). G-protein cascade ultimately results in Na+ and Ca2+ entry and Cl- exit which results in depolarization of CN I and generation of action potentials.

Different groups of glomeruli are activated by different odorants, but not in any simple pattern. Glomeruli are the functional units for processing of odor information. Olfactory receptor neurons having the same receptor project to the same glomerulus.


 * 4. Describe the organization of the olfactory bulb and neurotransmitters involved.**

Receptor cells terminals synapse with mitral cell dendrites in glomeruli – complex oval structures containing thousands of synapses. Mitral cells are located in a specific layer in the olfactory bulb and have dendrites that project superficially to glomeruli and axons that project to olfactory cortex.

Periglomerular cells are interneurons located near glomeruli and have inhibitory effects on signal conduction, using DA and GABA neurotransmitters. Granule cells are also interneurons but //lack axons//, making inhibitory dendrodendritic synapses with mitral cells via GABA. These cells act as local feedback mechanisms on mitral cells.

Tuff cells are located deep to the glomeruli and have dendrites that project to glomeruli (glutamine and aspartate neurotransmitters) and axons projecting to olfactory cortex.

Centrifugal innervation comes from the basal forebrain and olfactory cortex ipsilaterally, and the anterior olfactory nucleus through the anterior limb of the anterior commisure contralaterally. The ipsilateral pathways make the biggest inputs and use ACh neurotransmitters.


 * 5. Describe the organization of the olfactory cortex.**

The olfactory cortex has 3-4 layers (as opposed to 6 layers in neocortex). The main ascending inputs project to the superficial part of layer I (instead of mainly layer IV). Pyramidal cells are in layer II and apical dendrites are oriented perpendicular to the olfactory cortex layers. Inputs from olfactory bulb end on the distal parts of the apical dendrites. Association inputs from other cortical regions end on the proximal parts of apical dendrites. Because association fibers end more proximally to the apical dendrites, it suggests that association fibers from other cortical regions have a greater influence on the perception of smell than the actually primary sensory fibers themselves.


 * 6. State possible causes and effects of impaired sense of smell.**

Anosmia is the loss of sense of smell. Hyposmia is the decreased sense of smell. Possible causes include:

(1) mechanical blockage of airway (2) nasal infection and increased mucus (3) chemical interference with olfactory receptors (e.g., zinc sulfate damage) (4) tumor affecting olfactory tract or olfactory cortex (5) head trauma with damage to olfactory nerve (6) loss of receptors, olfactory bulb glomeruli, and mitra cells during aging

Loss or decreased sense of smell can result in loss of interest in eating because of changes in food taste and unawareness to environmental smells which might warn of danger.