Chemical Senses Flashcards
What are the volatile airborne chemical stimuli which interact with the olfactory epithelium called?
Odorants
What are the different cell types found in the olfactory organ? State the function of each.
Olfactory receptor neurons (ORNs) - bind odorants molecules
Supporting/sustentacular cells - digest potentially damaging chemicals
Basal cells - maintain a population of stem cells which continually divide to replace ORNs
Bowman’s gland - secretes mucus to protect the exposed neurons, respiratory epithelial cells and supporting cells
Describe what is meant by olfactory discrimination.
Each olfactory sensory neuron expresses one type for olfactory receptor. The axons from olfactory sensory neurons with the same type of olfactory receptors extend to the olfactory bulb and form one glomerulus. The signalling is transferred to the brain from the glomerulus through mitral cells
Outline how olfactory transduction occurs.
- The odourant binds to a G-coupled protein receptor
- This causes activation of adenylate cyclase
- Which converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP)
- cAMP activates gated sodium channel and increases sodium influx which depolarises the cell
- The olfactory neuron transmits action potential to the central nervous system via the olfactory nerve
Describe the central pathway of the olfactory nerve.
Axons leaving olfactory epithelium collect into 20 or more bundles and pass through the cribriform plate of the ethmoid bone to reach the olfactory bulb of the cerebrum where the first synapse occurs.
Fibres from bulb leave as the olfactory tract - these target the pyriform cortex, olfactory tubercle, amygdala and entorhinal cortex. Projections from here then to the orbitofrontal cortex, thalamus and hypothalamus.
Pyriform cortex has direct fibres to the orbitofrontal cortex and the entorhinal cortex has direct fibres to the hippocampal formation
List the different abnormalities of olfaction. State the cause of each of these.
General anosmia (complete lack of olfactory sensation) & Hyposmia (diminished sense of smell)
- caused by respiratory infection
Chronic anosmia/hyposmia (caused by damage to the olfactory epithelium)
- caused by infections
- head trauma that severs the olfactory nerves passing through holes in cribriform plate which then become blocked by scar tissue
- neurodegenerative disease such as Parkinson’s and Alzheimer’s
Cacosmia (olfactory hallucination of repugnant smells)
- consequences of epileptic seizures as they originate from brain areas associated with the olfactory cortex
Describe the structure of taste papillae and its contents.
Papillae are defined by multicellular protuberances surrounded by local invaginations in the tongue epithelium.
- taste buds are distributed along the lateral surfaces of the papillae protuberances as well as in the trench walls
- they consist of specialised neuroepithelial cells (taste cells), supporting cells and occasional basal cells
In which regions of the oral cavity are taste buds distributed?
Throughout surface of tongue, palate, epiglottis and oesophagus
List the different types of lingual papillae and where they can be found.
Filiform papilla - anterior 2/3 of tongue
Fungiform papilla - only found on anterior 2/3 of tongue
Vallate/circumvallate papilla - 9 of these form a chevron at rear of tongue
Foliate papilla - found on posterolateral tongue
Which type of lingual papillae has the most abundant taste buds in humans?
Circumvallate papillae
Which type of lingual papillae does not contain any taste buds?
Filiform papillae
- provides friction
List the categories of basic taste in gustatory discrimination.
Sour, bitter, salty, sweet and umami (savoury and meaty taste)
List the different types of molecules that each taste modalities are sensitive.
Sweet receptors
- sugars, saccharine, some amino acids
Sour receptors
- acids
Bitter receptors
- alkaloids, toxins
Salty receptors
- sodium in salt and metal ions
Umami
- glutamate and other amino acids
Outline the sensory transduction in the taste cell.
- Taste molecules binds to taste receptors or pass through transmembrane channels
- This either causes direct opening of depolarising channels or does so via a signalling cascade initiated by G proteins
- Neurotransmitter are released from the cells
- These bind to the sensory nerve fibres at the synapses
- Action potential is generated
- The signal is carried along the nerve fibers to cranial nerves 7 and 9 depending on the location of the taste bud on the tongue
Outline the central pathways of gustation.
Primary neuron axons from gustatory cells extend to the CNS from the tongue through paired cranial nerves 7 and 9
- CN 7 - info from anterior 2/3 of tongue
- CN 9 0 info from posterior 1/3 of tongue
Primary neurons synapse in the nucleus solitarious of the brainstem
Secondary neurons travel and synapse in the thalamus
Tertiary neurons travel form thalamus and terminate in the primary gustatory cortex in the insula
