1. Sensory Receptors
This is a sample first topic page.

2. Range from simple neurons to complex sense organs
Sensory Receptors
Range from simple neurons to complex sense organs
Types: chemoreceptors, mechanoreceptors, photoreceptors, electroreceptors, magnetoreceptors, thermoreceptors
All transduce incoming stimuli into changes in membrane potential
This is a sample first topic page.
Figure 7.1

3. Sensory Receptors
This is a sample first topic page.
Figure 7.1

4. Classification of Sensory Receptors
Based on stimulus location
Telereceptors – detect distant stimuli, e.g., vision and hearing
Exteroceptors – detect stimuli on the outside of the body, e.g., pressure and temperature
Interoceptors – detect stimuli inside the body, e.g., blood pressure and blood oxygen

5. Classification of Sensory Receptors
Based on type of stimuli the receptors can detect (stimulus modality)
Chemoreceptors – chemicals, e.g., smell and taste
Mechanoreceptors – pressure and movement, e.g., touch, hearing, balance, blood pressure
Photoreceptors – light, e.g., vision; detect photons
Electroreceptors – electrical fields
Magnetoreceptors – magnetic fields
Thermoreceptors - temperature

6. Receptors and stimulus
Location: Can distinguish the location of the stimulus (touch, light or odour)
Duration: Determine length of stimulus by responding to the stimulus for the duration of the stimulus.
Intensity: Increase in action potential frequency or increase in neurotransmitter release.

7. Sensitivity to Multiple Modalities
Adequate stimulus – preferred or most sensitive stimulus modality
Many receptors can also be excited by other stimuli, if sufficiently large, e.g., pressure on eyelid  perceive bright light
Polymodal receptors – naturally sensitive to more than one stimulus modality, e.g., ampullae of Lorenzini in sharks
Nociceptors – sensitive to strong stimuli, e.g., pain; many are polymodal receptors

8. Stimulus Encoding
All stimuli are ultimately converted into action potentials in the primary afferent neurons
How can organisms differentiate among stimuli or detect the strength of the signal?
Sensory receptors must encode four types of information
Stimulus modality
Stimulus location
Stimulus intensity
Stimulus duration

9. Dynamic Range
Action potentials code stimulus intensity through changes in frequency, e.g., strong stimuli  high frequency
Dynamic range – range of intensities for which receptors can encode stimuli
Threshold detection – weakest stimulus that produces a response in a receptor 50% of the time
Saturation – top of the dynamic range; all available proteins have been stimulated
Figure 7.4a

10. Range Fractionation
Relationships between stimulus intensity and AP frequency
Linear across large range of intensities: large change in stimulus causes a small change in AP frequency  large dynamic range, poor sensory discrimination
Linear across small range of intensities: small change in stimulus causes a large change in AP frequency  small dynamic range, high sensory discrimination
Range fractionation – groups of receptors work together to increase dynamic range without decreasing sensory discrimination
Figure 7.4b-c

11. Tonic and Phasic Receptors
Two classes of receptors that encode stimulus duration
Phasic – produce APs only at the beginning or end of the stimulus  encode changes in stimulus, but not stimulus duration
Tonic – produce APs as long as the stimulus continues
Receptor adaptation – AP frequency decreases if stimulus intensity is maintained at the same level

12. Tonic and Phasic Receptors, Cont.
Figure 7.5

13. Pain  Pain and itching are mediated by Nocireceptors
Itch comes form Nocireceptors in the skin. Higher pathways for itch are not well understood
Pain is s subjective perception

14. Chemoreception Most cells can sense incoming chemical signals
Animals have many types of chemoreceptors
Multicellular organisms typically use taste and smell
Olfaction – sense of smell
Detection of chemicals carried in air
Gustation – sense of taste
Detection of chemicals emitted from ingested food
Distinct due to structural criteria
Performed by different sense organs
Use different signal transduction mechanisms
Are processed in different integrating centers

15. The Olfactory System Evolved independently in vertebrates and insects
Vertebrate olfactory system
Can distinguish thousands of odorants
Located in the roof of the nasal cavity
Mucus layer to moisten olfactory epithelium
Odorant binding proteins – allow lipophilic odorants to dissolve in mucus
Receptor cells are bipolar neurons and are covered in cilia
Odorant receptor proteins are located in the cilia

16. Odorant Receptors are G Proteins
Each olfactory neuron expresses only one odorant receptor protein
Each odorant receptor can recognize more than one odorant
Figure 7.7

17. Pheromones Vomeronasal organ – detects pheromones
Structurally and molecularly distinct from the primary olfactory epithelium
Location
Base of nasal cavity near the septum in mammals
Palate in reptiles
Transduction
Activates a phospholipase C-based signal transduction system; adenylate cyclase-cAMP in other olfactory receptors
Figure 7.8

18. Taste Buds in Vertebrates
Group of taste receptor cells
Located on tongue, soft palate, larynx, and esophagus; external surface of the body in some fish

19. Taste Buds in Vertebrates
50 to 150 taste cells
Epithelial cells that have apical and basal sides and joined by tight junctions
Life span of days
Basal stem cells divide to regenerate taste cells
Microvilli on its apical surface that project into the mucus of the tongue
Taste receptor proteins are found in the microvilli
Chemicals are soluble and diffuse to the bind to their receptors
Different cells in the same bud can detect NaCl, sucrose, H+ and quinine (bitter)
Taste cell forms a chemical synapse with a sensory neuron that projects to the brain from the tongue

20. Taste buds and peripheral innervation
Figure 7.11c-d

21. A generic taste cell.
Apical surface: both channels and G-protein-coupled receptors that are activated by chemical stimuli
Basolateral surface: voltage-gated Na+, K+, and Ca2+ channels, as well as all the machinery for synaptic transmission mediated by serotonin
The increase in intracellular Ca2+ is either by the activation of voltage-gated Ca2+ channels or via the release from intracellular stores causes synaptic vesicles to fuse and release their transmitter onto receptors on primary sensory neurons
Each cell contains the standard complement of neuronal proteins including Na+/K+ ATPase at the basal level, voltage-gated Na+ and Ca2+ channels, leak K+ channel

22. A generic taste cell…cont.
The response to the chemical is mediated by the expression of receptors for that chemical in the microvilli
The response is a depolarization of the cell sometimes enough to generate an action potential
The signaling of the cell to the sensory neuron depends on a sufficient depolarization to open the voltage-gated Ca2+ channels necessary for vesicle fusion and neurotransmitter release.

23. Transduction mechanisms

24. G-Protein-Coupled Receptors* * * * * * * * * *

25. G-protein and adenylate cyclase* * * * * * * * * * * * * *

26. The inositol-phospholipid signaling pathway
You don’t have to memorize this  weeeeeee
but be aware of it and know which taste is transmitted
using this pathway ie bitter

27. Salt taste
The Na+ enters into the cell through the passive amiloride-sensitive Na+ channel
These proteins are found in frog skin and kidney
Amiloride will block Na+ salt taste reception
Entry of Na+ into the cell of course causes the cell to depolarize
Need a large concentration of Na+ to trigger a sufficient depolarization to signal to the post-synaptic sensory neuron

28. Salt taste * * *

29. Sour taste
Taste response produced by acids, excess protons (H+). These positive ions enter the cell through a H+, cation specific ion channel and in turn depolarize the cell to threshold for an action potential.

30. Sour taste * * * *

31. Sweet taste
There are specific membrane receptors for different sweeteners and sugars
These receptors are not ligand gated ion channels but rather are metabotropic receptors
These receptors belong to the family of seven transmembrane domain proteins that are linked to signaling cascades through G proteins. In mammals a combination of the T1R2/T1R3 receptors have a response to sugars and sweeteners
These receptors stimulate a G protein (Gp) which in this case activates phosopholipase C (PLC)
PLC breaks down PIP2 (phosphatidylinositol 4,5-bisphosphate) into IP3 (inositol triphosphosphate) and DAG
IP3 will bind to and activate a ion channel (TRP channel called TRPM5) which allows Ca2+ to influx into the cell
This pathway leads to a depolarization and threshold is reached to trigger an action potential  

32. Sweet taste
In other animals sugars also appear to bind to receptors that stimulate G proteins (Gs) that activate adenylate cyclase
This results in an increase in cAMP in the cell that activates a protein kinase (PKA) which in turn phosphorylates a K+ channel to close the channel
Once the K+ channel is close the cell will depolarize
Both these signaling cascades are used in multiple biological systems
In the nervous system neurotransmitter binding to specific metabotropic receptors can trigger these cascades
Photoreceptor and olfactory neurons also use parts of these cascades for their sensory transduction

33. Sweet taste * * * * * *

34. Bitter taste
Different cells have different mechanisms of bitter taste transduction
In mammals the bitter receptor is a metabotropic receptor called T2R. There are about 30 different subtypes in mammals
These signal through a G protein called gustducin to PLC and thus generate IP3
Like sweet receptors the IP3 activates a TRPM5 channel to open and allow Ca2+ to influx into the cell.
2. Some bitter chemicals such as quinine bind to and block specific K+ channels and thus result in depolarization of the cell

35. Bitter taste * * * * *

36. Amino acid taste cells
In some animals (catfish) there are a high number of amino acid taste cells
There appears to be multiple ways that animals respond to amino aicds
In fish and other amphibians, amino acids such as L-arginine and L-proline bind to specific receptors which are ligand gated ion channels
In mammals there are taste cells that respond to L-glutamate. In these cells L-glutamate activates a metabotropic receptor glutamate receptor linked to a G protein. Glutamate binds to many different metabotropic receptors and in taste cells it is the mGluR4 that is responsible for the taste transduction
In mammals there are also two metabotropic receptors T1R1/T1R3 that combine to respond to the standard 20 amino acids. This combination signals through G protein activation of PLC and the generation of IP3 and the activation of the TRPM5 channel.