Sensory Receptors Sensory Receptors Range from simple neurons

Sensory Receptors

Sensory Receptors í Range from simple neurons to complex sense organs í Types: chemoreceptors, mechanoreceptors, photoreceptors, electroreceptors, magnetoreceptors, thermoreceptors í All transduce incoming stimuli into changes in membrane potential Figure 7. 1

Sensory Receptors Figure 7. 1

Classification of Sensory Receptors íBased on stimulus location íTelereceptors – detect distant stimuli, e. g. , vision and hearing íExteroceptors – detect stimuli on the outside of the body, e. g. , pressure and temperature íInteroceptors – detect stimuli inside the body, e. g. , blood pressure and blood oxygen

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

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

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

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

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. 4 a

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. 4 b-c

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

Tonic and Phasic Receptors, Cont. Figure 7. 5

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

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

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

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

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 Cbased signal transduction system; adenylate cyclasec. AMP in other olfactory receptors Figure 7. 8

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

Taste Buds in Vertebrates í 50 to 150 taste cells í Epithelial cells that have apical and basal sides and joined by tight junctions í Life span of 10 -14 days í Basal stem cells divide to regenerate taste cells í Microvilli on its apical surface that project into the mucus of the tongue í Taste receptor proteins are found in the microvilli í Chemicals are soluble and diffuse to the bind to their receptors í Different cells in the same bud can detect Na. Cl, sucrose, H+ and quinine (bitter) í Taste cell forms a chemical synapse with a sensory neuron that projects to the brain from the tongue

Taste buds and peripheral innervation Figure 7. 11 c-d

A generic taste cell. íApical surface: both channels and G-proteincoupled receptors that are activated by chemical stimuli íBasolateral surface: voltage-gated Na+, K+, and Ca 2+ channels, as well as all the machinery for synaptic transmission mediated by serotonin íThe increase in intracellular Ca 2+ is either by the activation of voltage-gated Ca 2+ channels or via the release from intracellular stores causes synaptic vesicles to fuse and release their transmitter onto receptors on primary sensory neurons íEach cell contains the standard complement of neuronal proteins including Na+/K+ ATPase at the basal level, voltage-gated Na+ and Ca 2+ channels, leak K+ channel

A generic taste cell…cont. íThe response to the chemical is mediated by the expression of receptors for that chemical in the microvilli íThe response is a depolarization of the cell sometimes enough to generate an action potential íThe signaling of the cell to the sensory neuron depends on a sufficient depolarization to open the voltage-gated Ca 2+ channels necessary for vesicle fusion and neurotransmitter release.

Transduction mechanisms

G-Protein-Coupled Receptors * * * * *

G-protein and adenylate cyclase * * * *

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

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

Salt taste * * *

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.

Sour taste * *

Sweet taste í There are specific membrane receptors for different sweeteners and sugars í These receptors are not ligand gated ion channels but rather are metabotropic receptors í 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 T 1 R 2/T 1 R 3 receptors have a response to sugars and sweeteners í These receptors stimulate a G protein (Gp) which in this case activates phosopholipase C (PLC) í PLC breaks down PIP 2 (phosphatidylinositol 4, 5 -bisphosphate) into IP 3 (inositol triphosphate) and DAG í IP 3 will bind to and activate a ion channel (TRP channel called TRPM 5) which allows Ca 2+ to influx into the cell í This pathway leads to a depolarization and threshold is reached to trigger an action potential

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 c. AMP 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

Sweet taste * * *

Bitter taste Different cells have different mechanisms of bitter taste transduction 1. In mammals the bitter receptor is a metabotropic receptor called T 2 R. There about 30 different subtypes in mammals 2. These signal through a G protein called gustducin to PLC and thus generate IP 3 3. Like sweet receptors the IP 3 activates a TRPM 5 channel to open and allow Ca 2+ to influx into the cell. 4. 2. Some bitter chemicals such as quinine bind to and block specific K+ channels and thus result in depolarization of the cell

Bitter taste * * *

Amino acid taste cells In some animals (catfish) there a high number of amino acid taste cells There appears to be multiple ways that animals respond to amino aicds 1. In fish and other amphibians, amino acids such as L-arginine and Lproline 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 receptor linked to a G protein. Glutamate binds to many different metabotropic receptors and in taste cells it is the m. Glu. R 4 that is responsible for the taste transduction 3. In mammals there also two metabotropic receptors T 1 R 1/T 1 R 3 that combine to respond to the standard 20 amino acids. This combination signals through G protein activation of PLC and the generation of IP 3 and the activation of the TRPM 5 channel.