Slide 1 Sensory Receptors

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Slide 2 Sensory Receptors

Range from simple neurons to complex sense organsTypes: chemoreceptors, mechanoreceptors, photoreceptors, electroreceptors, magnetoreceptors, thermoreceptorsAll transduce incoming stimuli into changes in membrane potential

Figure 7.1

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Slide 3 Sensory Receptors

Figure 7.1

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Slide 4 Classification of Sensory Receptors

Based on stimulus locationTelereceptors – detect distant stimuli, e.g., vision and hearingExteroceptors – detect stimuli on the outside of the body, e.g., pressure and temperatureInteroceptors – detect stimuli inside the body, e.g., blood pressure and blood oxygen

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Slide 5 Classification of Sensory Receptors

Based on type of stimuli the receptors can detect (stimulus modality)

Chemoreceptors – chemicals, e.g., smell and tasteMechanoreceptors – pressure and movement, e.g., touch, hearing, balance, blood pressurePhotoreceptors – light, e.g., vision; detect photonsElectroreceptors – electrical fieldsMagnetoreceptors – magnetic fieldsThermoreceptors - temperature

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Slide 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.

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Slide 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

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Slide 8 Stimulus Encoding

All stimuli are ultimately converted into action potentials in the primary afferent neuronsHow can organisms differentiate among stimuli or detect the strength of the signal?Sensory receptors must encode four types of information

Stimulus modalityStimulus locationStimulus intensityStimulus duration

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Slide 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

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Slide 10 Range FractionationRelationships 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

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Slide 11 Tonic and Phasic ReceptorsTwo 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

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Slide 12 Tonic and Phasic Receptors, Cont.

Figure 7.5

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Slide 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

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Slide 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

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Slide 15 The Olfactory SystemEvolved independently in vertebrates and insectsVertebrate 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

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Slide 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

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Slide 17 PheromonesVomeronasal organ – detects

pheromonesStructurally 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

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Slide 18 Taste Buds in Vertebrates

Group of taste receptor cellsLocated on tongue, soft palate, larynx, and esophagus; external surface of the body in some fish

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Slide 19 Taste Buds in Vertebrates50 to 150 taste cellsEpithelial cells that have apical and basal sides and joined by tight junctionsLife span of 10-14 daysBasal stem cells divide to regenerate taste cellsMicrovilli on its apical surface that project into the mucus of the tongueTaste receptor proteins are found in the microvilliChemicals are soluble and diffuse to the bind to their receptorsDifferent 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

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Slide 20 Taste buds and peripheral innervation

Figure 7.11c-d

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Slide 21 A generic taste cell.

Apical surface: both channels and G-protein-coupled receptors that are activated by chemical stimuliBasolateral surface: voltage-gated Na+, K+, and Ca2+ channels, as well as all the machinery for synaptic transmission mediated by serotoninThe increase in intracellular Ca2+ is either by the activation of voltage-gated Ca2+ channelsor via the release from intracellular stores causes synaptic vesicles to fuse and release their transmitter onto receptors on primary sensory neuronsEach 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

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Slide 22 A generic taste cell…cont.

The response to the chemical is mediated by the expression of receptors for that chemical in the microvilliThe response is a depolarization of the cell sometimes enough to generate an action potentialThe 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.

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Slide 23 Transduction mechanisms

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Slide 24 G-Protein-Coupled Receptors

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Slide 25 G-protein and adenylate cyclase

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Slide 26 The inositol-phospholipid signaling pathway

You don’t have to memorize this ☺ weeeeeeebut be aware of it and know which taste is transmitted using this pathway ie bitter

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Slide 27 Salt taste

The Na+ enters into the cell through the passive amiloride-sensitive Na+ channelThese proteins are found in frog skin and kidneyAmiloride will block Na+ salt taste receptionEntry of Na+ into the cell of course causes the cell to depolarizeNeed a large concentration of Na+ to trigger a sufficient depolarization to signal to the post-synaptic sensory neuron

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Slide 28 Salt taste

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Slide 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.

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Slide 30 Sour taste

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Slide 31 Sweet taste There are specific membrane receptors for different sweeteners and sugarsThese receptors are not ligand gated ion channels but rather are metabotropic receptorsThese 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 sweetenersThese 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 cellThis pathway leads to a depolarization and threshold is reached to trigger an action potential

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Slide 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

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Slide 33 Sweet taste

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Slide 34 Bitter taste Different cells have different mechanisms of bitter taste transduction1. In mammals the bitter receptor is a metabotropic receptor called T2R.

There are about 30 different subtypes in mammalsThese signal through a G protein called gustducin to PLC and thus generate IP3Like 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

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Slide 35 Bitter taste

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Slide 36 Amino acid taste cellsIn some animals (catfish) there are a high number of amino acid taste cellsThere appears to be multiple ways that animals respond to amino aicds

1. In fish and other amphibians, amino acids such as L-arginine and L-proline bind to specific receptors which are ligand gated ion channels

2. In mammals there are taste cells that respond to L-glutamate. In these cells L-glutamate activates a metabotropic receptor glutamate receptorlinked to a G protein. Glutamate binds to many different metabotropicreceptors and in taste cells it is the mGluR4 that is responsible for thetaste transduction

3. 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.

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