Olfaction

OLFACTION

ODOUR RECEPTORS:

• Located in olfactory mucous membrane/neuro-epithelium, in roof of nasal cavity near septum.
• Protected location.
• Hence, not directly exposed to inspired air flow.
• Receptors are “Neuron” themselves.
• Main receptor part – “Projecting cilia” into nasal mucosa.
• Acts as dendrites of neruon/receptor cell.
• Axons form olfactory nerve → Pass through cibriform plate→ Terminates in olfactory bulb.

OLFACTORY BULB:

Inside bulb:

• Axons of olfactory N. synapse with dendrites of mitral cells forming “Olfactory glomeruli”.

Cellular content – 

• Mitral & Peri-glomerular cells.

1. “Mitral cell” –

• Principal output neurons of olfactory bulb.
• Their axons form “Olfactory tract”.

2. “Peri-glomerular cells” –

• Inhibitory neurons & granule cells without synapse.
• Make reciprocal synapses with mital & tufted cells.

• Olfactory tract projects to “Primitive brain parts” & “Neocortex”. 

1. Pathway I (To primitive brain):

• Principally to,
• Pyriform area/olfactory cortex.
• Amygdala.
• Entorhinal cortex.
• In-turn to hippocampus.

2. Neocortex pathway:

• Relay in olfactory tubercle → Thalamus → Finally “Orbitofrontal cortex”.

OLFACTORY TRANSDUCTION:

• Odour receptors – Type of “G-protein coupled” receptors.

Receptor-ligand interaction:

• Activated by “Adsorption of odorant molecules to receptor cilia”.
• Opens Na2+channels via IInd messenger cAMP.
• Na2+influx → Generates AP.

Note: 2 different theories – Activates Calcium/Sodium channels.