Today Sensory receptors General properties Skin receptors

1. Today • Sensory receptors • General properties • Skin receptors

2. Sensory receptors Light • Vision • Taste • Smell • Hearing • Touch • Internal • Proprioception - limb position and movement • Baroreceptors - blood pressure • Osmoreceptors - osmolarity • Chemoreceptors – chemical concentration Chemical Mechanical

3. Sensory receptors General Properties: • Specialized structures to receive sensory information • Amplification and Transduction of sensory stimuli to electrical activity • Coding of information by the number and frequency of APs

4. Types of Sensory Neurons Spike Initiating Zone Sensory receptor Action Potentials Action Potentials

5. Spike Initiating Zone Sensory receptor Action Potentials Chemical synapse

6. Sensory Reception Cascade Stimulus reaches receptor cells Sensory Energy Activation of receptors Protein interactions & Second messengers Ion channels open or close Graded Events Generates a receptor current Changes in Vm Spread to spike initiating zone Changes in the amount of neurotransmitter release All or none APs Influence number and frequency of APs in sensory neuron Electrical Energy

7. Common Physiological Properties • Dynamic Range • The range of stimulus intensity the sensory system can respond to • Adaptation • Phasic receptors • Fire APs for only one part of stimulus • Tonic receptors • Fire APs for duration of stimulus

8. Dynamic Range Upper limit set by refractory period Frequency of APs (Hz) 1 10 100 1000 10,000 Log Stimulus Intensity Threshold for detection Sensory receptor can’t respond further Sensory receptor responds in this range

9. How to overcome limited range? • For each type of receptor, there are individual receptors specialized to respond to specific parts of the range Range of the whole sensory system Range of individual receptors Frequency of APs (Hz) 1 10 100 1000 10,000 Log Stimulus Intensity

10. Receptors of the skin Free nerve endings pain & temperature Pacinian Corpuscle Deep touch Meissner’s Corpuscle Light touch Ruffini’s Corpuscle stretching

11. Pain • Nociceptors –respond to painful stimulus • Carried by non-myelinated C fiber sensory neurons • Painful heat, acids, mechanical damage all activate non-specific cation channels

12. Pain Substances released from Damaged cells: ATP Bradykinin Substance P Painful heat Acids Mechanical damage Na+ Ca++ Free nerve ending

13. Receptors of the Skin • Tactile sensory input • Respond to pressure and movement of skin • Specialized receptors that respond to particular types of inputs Pacinian – heavy pressure, rapid vibration (300Hz) Meissner’s – light pressure, slow vibration (50 Hz) Use mechanically gated ion channels

14. Pacinian Corpuscle Nerve axon Saline bath Fluid filled layers Extracellular recording Mechanical Stimulus

15. First Node Of Ranvier axon myelin If receptor potential is large enough APs produced Receptor currents flow passively within axon Mechanical stimulus Opens ion channels

16. Pacinian Corpuscle • Example of a rapidly adapting receptor • Only gives an ‘on’ and an ‘off’ response • Epithelial layers filter out steady pressure but transfer rapid changes in pressure

17. Receptive Field The region of the skin in which a stimulus evokes a response in a single sensory neuron Discrimination depends on the density of receptors Fingers 1-4 mm Thigh 45 mm

18. skin dendrites Sensory neuron AP generated in one neuron dendrites Sensory neuron AP generated in two different neurons

19. Good Luck!