Lecture 14 – Sensory Systems 2 SMELL, TASTE, HEARING AND TOUCH Flashcards
senses
- Smell (olfaction)
- Taste (gustation)
- Hearing (audition)
- Touch (mechanosensation)
- Common features of sensory systems
Human olfactory receptors
- Human olfactory receptors are GPCRs. Odorant binding leads to opening of a cyclic nucleotide-gated channel and depolarisation of olfactory receptor neurons
Human olfactory receptors are GPCR’s, with ligands being the odorants
- Recall that in the visual system, the receptors (rhodopsins) also are GPCRs, but the ligand there is light
- Here in the olfactory system the receptors are associated with a specific type of olfactory G-proteins
- Upon binding of an odorant, the G-protein is activated, and its alpha-subunit in turn activates adenylate cyclase ACIII, which leads to an increase in the concentration of cAMP
- cAMP acts as a intracellular ligand and opens the cAMP-gated cation channel, allowing Ca2+ and Na+ into the cells
- This leads to the depolarisation of olfactory receptor neurons in response to an odorant.
Combinatorial coding:
- Odorants have diverse chemical structure, and we cannot predict how something smells based on the structure on the molecule
- Each odorant binds to various receptors and activates many neurons, and in turn each neuron is activated by many odorants – combinatorial code
Taste:
Five basic tastes relate to survival:
- Bitter = avoid poisons
- Sweet = sugar & carbohydrate
Umami = l-amino acids (monosodium glutamate)
- Salty = Na+
- Sour = acids/H+
- Fat – the sixth taste?
Chemicals
- Chemicals (tastants) bind to the receptors at the tip of taste receptor cells (TRCs)
- These cells are not neurons strictly speaking, they are neuroepithelial cells and can regenerate
- Clusters of TRCs form taste buds
- Each bud contains cells that detect all types of tastants
- Taste cells release neurotransmitter that activate terminal branches of gustatory nerves
- Taste buds are localised to several kinds of papillae that are located in different parts of the tongue
Activation of receptor causes depolarisation
taste receptors
In humans taste receptors are located only on the tongue. In flies for example taste receptors are distributed throughout the body: proboscis, legs, wings, ovipositor. Serve different functions and project to different parts of the brain.
Taste receptors are different molecularly:
Gustatory receptors are of different molecular types
• First gustatory receptors were identified in the 90’s: T1R1,T1R2 and T1R3, all GPCRs. T1R2 and T1R3 together form a mammalian sweet taste receptor, that detects various sugars. T1R1 and T1R3 together detect umami stimuli (savory, glutamate). Cats don’t have T1R2 (don’t sense sweet).
• Bitter is detected by the T2R family of GPCRs. Different people have vastly different sensitivity to bitter tastants. Large family of receptors – perhaps because detecting bitter (danger) is very important. T2R receptors have very high affinity to its ligands: may be activated by very low concentrations. Different bitter receptors are co-expressed in the same receptor cell.
• Sour is detected by two TRP channels (ion channels, not GPCRs).
• Salt is detected by two systems. Low (attractive) salt concentrations are detected by epithelial Na channel (ENaC) in mice. However, humans don’t have ENaC, and mice lacking ENaC can detect high (aversive) concentrations of salt. The search for additional salt receptors continues.
Hearing: air pressure waves
¥
We detect sound as variations in air pressure (insects can detect the speed of moving particles instead – different detection mechanism)
¥ Normal hearing ranges from 20Hz to 20,000 Hz
¥ Lower frequency waves = lower pitch; lower intensity = quieter.
Sound and vestibular:
Auditory system:
. External ear
- Middle ear (bones: malleus, incus, stapes)
- Inner ear (cochlea)
Vestibular system:
- Semicircular canals (posterior, horizontal, anterior)
- Otolith organs (utricle, saccule)
Auditory system consists of the external ear, middle ear and cochlea – detect sounds
Vestibular system consists of semicircular canals and otolith organs that detect gravity, acceleration and head rotation.
Cochlea:
Hair cells transduce sound into electrical signals. Outer hair cells provide active amplification, inner hair cells send signal to the brain.
- There are three fluid-filled chambers and the Organ of Corti, which consists of hair cells, support cells and the basilar membrane
- The basilar membrane moves due to the movement of fluid, which in turn causes relative movement of the hair cells and the overlaying tectorial membrane
- The apical tips of the hair cells are embedded in the tectorial membrane, and are sheared when the basilar membrane vibrates
- There are 3 rows of outer hair cells and one row of inner hair cells in the cochlea. Outer hair cells don’t send signals to the brain, only the inner hair cells do
- Instead, the outer hair cells actively amplify minute vibration of the basilar membrane, which in turn increases the responses of the inner hair cells to quiet sounds
- The outer hair cells can change their shape due to the molecule called prestin
K+ goes into hair cells for hearing:
Stereocilia on the apical side of the hair cells are arranged like a stair case, and are connected by the tip links
- Mechanical force will pull on the channel (we don’t know what the channel is!) and physically opens it, allowing K+ in and leading to depolarization
- This direct form of transduction (mechanotransduction) is very fast
- Note: K+ goes IN, because K concentration is high in the extracellular fluid
- Hair cells will release excitatory glutamate onto the terminals of spiral ganglion neurons, which transmit signals to the brain
Tonotopy of the cochlea:
Tonotopic representation of sound, due to the properties of basilar membrane
- In the cochlea, hair cells are tuned to progressively lower frequencies from the base to the apex. This is because of the properties of the basilar membrane – it is wider and less stiff at the apex, thus resonates at lower frequencies.
Sound localisation in space:
Position of sound source is detected by comparing the time of the sound arrival to both ears (interaural time difference)
Vestibular System:
Vestibular system consists of the otolith organs and semicircular canals. Here we also have hair cells and a very similar transduction mechanism! But no sound. In utricle and saccule the inertia of otolith creates mechanical force on the hair cells. In the canals it’s the relative movement of liquid that is lagging behind.
