Download TSM34 - Chemical Senses

TSM34: CHEMICAL SENSES
13/10/08
LEARNING OUTCOMES
Describe the receptors for taste and smell
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Both taste and smell are said to be phylogenetically primitive senses – they were developed early
o Each depends on anatomically distinct systems but often ‘work together’ as senses
Anosmia is the loss of the sense of smell
Ageusia is the loss of the sense of taste (more unusual – mostly from peripheral neuropathy)
TASTE RECEPTORS
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The sensory receptors for taste are contained in discrete functional units – taste buds
o Found in various positions on the surface of papillae on the tongue
 Laterally in circumvallate and foliate papillae
 Superiorly on fungiform papillae
o Comprise a narrow pore through which chemicals can pass
o Gustatory cells are grouped below the pores with adjacent supporting cells
o Primary afferent neurones interact with the gustatory cells basally
There are four principal taste modalities to which different areas of the lateral tongue are sensitive:
o Bitterness – on the dorsum of the tongue – tested clinically using quinine
o Sourness – in the middle of the tongue – tested clinically using hydrochloric acid
o Saltiness – near the front of the tongue – tested clinically using sodium chloride
o Sweetness – on the tip of the tongue – tested clinically using sucrose
SMELL RECEPTORS
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Smell is detected by primary olfactory afferents directly (as opposed to taste, via non-neural cells)
o The cell bodies of the olfactory afferents are contained within the cribriform plate
 Large supporting cells with apical microvilli separate individual olfactory cells
 Smaller basal cells allow passage of multiple axons through gaps in the plate
o Each cell has a single descending neurite to the mucosa into which olfactory cilia project
 The actual ‘receptive units’ are contained within the olfactory cilia
o Uniquely for neural cells, olfactory afferents have a natural cycle of death and regeneration
 This makes them potentially useful in transplants – more research needed
There are at least six ‘elementary odours’ – floral, ethereal, musky, camphor, putrid and pungent
Understand the basic mechanisms of transduction of taste and smell
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In taste, gustatory receptors utilise common ion channels to generate receptor potentials:
o Molecules conveying bitterness and sweetness act via second messengers
o Molecules conveying sourness and saltiness act on ion channels directly
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In smell, receptors for all odours utilise G-protein second messengers
Describe the pathways for taste and smell
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The pathways for both taste and smell are ipsilateral
In taste, primary afferents ascend to the gustatory portion of the solitary nucleus in the medulla
o Second order neurons transmit to the ventral posterior medial nucleus (VPM) of the thalamus
o Third order neurons terminate in the primary taste cortex of the parietal lobe
In smell, primary afferents send axons to the olfactory bulb which sits on top of the cribriform plate
o Seconds order neurons transmit directly to the primary olfactory cortex of the temporal lobe
o Uniquely there is no synapse in the thalamus which demonstrates its primitiveness