Special Senses - Olfactory

Special Senses ● Sensory receptors are within large, complex sensory organs in the head. ● Smell in olfactory organs ● Taste in taste buds ● Hearing and equilibrium in ears ● Sight in eyes OLFACTORY Olfactory Receptors   A.     B.    C.   Chemoreceptors Respond to chemicals dissolved in liquids Olfactory Epithelium (Lower Part of Image) Located in the nasal cavity, it contains olfactory receptor neurons (yellow structures with cilia). These neurons have cilia (olfactory hairs) that detect odor molecules. Supporting cells provide structural and metabolic support. Basal cells function as stem cells to replace damaged olfactory neurons. Olfactory Nerve Pathways Odor Molecules ↓ Dissolve in Nasal Mucus ↓ Blind to Olfactory Receptors on Cilia ↓ Olfactory Sensory Neurons (Generate Action Potential) ↓ Olfactory Nerves (CN I) ↓ Olfactory Bulb (Processing in Glomeruli) ↓ Olfactory Tracts ↓ -Olfactory Cortex (Conscious Odor Perception) -Limbic System (Emotion & Memory Processing) ↓ Adaptation and Olfactory Coding Olfactory Bulb (Upper Part of Image) The first processing center of olfactory signals in the brain. Contains glomeruli, where synapses form between incoming sensory neurons and mitral cells. Mitral cells transmit signals to the olfactory cortex for further processing. Cribriform Plate (Thin Bone Layer) A porous part of the ethmoid bone that allows the passage of olfactory neuron axons. Provides structural support while enabling olfactory signal transmission to the brain. Function of This System:  Odor molecules bind to receptors on olfactory neurons.  Signals travel through the olfactory bulb via the cribriform plate.  The olfactory bulb processes the signals and sends them to the olfactory cortex, amygdala, and hippocampus for interpretation and emotional association.  The olfactory pathway is responsible for detecting, processing, and interpreting smells. Unlike other sensory pathways, olfaction bypasses the thalamus and has direct connections to the limbic system, which is involved in emotions and memory. 1. Odor Detection Odor molecules dissolve in the mucus layer within the nasal cavity.  They bind to olfactory receptors on the cilia of olfactory sensory neurons. 2. Olfactory Nerve Activation (Olfactory Nerve CN I)  The binding of odor molecules generates an action potential.  The impulse travels along the axons of olfactory neurons, which collectively form the olfactory nerves (CN I). 3. Olfactory Bulb Processing  The olfactory nerves synapse with mitral cells and tufted cells in the olfactory bulb.  Glomeruli in the olfactory bulb help in initial processing and sorting of odor signals. 4. Signal Transmission via Olfactory Tracts  Processed signals leave the olfactory bulb through the olfactory tracts.  These tracts relay the information to higher brain areas. 5. Odor Perception and Emotional Response  The olfactory signals reach two primary regions: o Olfactory Cortex (Temporal Lobe) → Conscious odor perception and identification. o Limbic System (Amygdala, Hippocampus, Hypothalamus) → Emotional and memoryrelated responses to smell. 6. Adaptation and Olfaction coding: allow detection of a vast range of odors  Olfactory Stimulation 1. Threshold & Sensitivity - Olfactory receptors can sense even small amounts of odor molecules (low threshold), though sensitivity differs per person. 2. Adaptation & Fatigue - We stop noticing constant smells quickly due to desensitized receptors and central inhibition. 3. Neural Processing - The olfactory bulb processes signals and sends them to the limbic system, connecting smell with emotions and memories. 4. Olfactory Code - Different smells trigger specific receptor patterns, helping us identify thousands of scents with around ~400 receptor types; brain interprets different odors based on the pattern of activation across multiple receptor types Clinical Application Anosmia (loss of smell) Inability to detect odor due to receptor, nerve, or brain damage -Post-viral anosmia (e.g., after COVID19) -Head trauma damaging olfactory pathways -Nasal polyps blocking airflow Hyperosmia (Heightened smell) Increased sensitivity to odors, often due to hormonal, neurological factors or medication effects. -Pregnancy-related nausea from strong smell -Migraines causing sme;ll hypersensitivity -Addison’s disease leading to heightened olfactory perception Phantosmia (Olfactory hallucinations) Perceiving odors that aren’t present,linked to neurological conditions due to abnormal activity of olfactory cortex. -Smelling burnt toast before a seizure (epilepsy) -Migraines causing phantom floral or metallic scents -Alzheimer’s patients detecting non-existent smoke Synesthesia (Mixed-up senses) Cross-activation of sensory pathways, leading to intertwined perceptions. -Associating names with specific smells (e.g., “Emma” smells like cinnamon –Seeing colors when smelling spices -Tasting flavors when hearing musical notes