Transcript Lect13
• Today – Sensory receptors • General properties – Skin receptors Sensory receptors • • • • • Vision Taste Smell Hearing Touch Light Chemical Mechanical • Internal – – – – Proprioception - limb position and movement Baroreceptors - blood pressure Osmoreceptors - osmolarity Chemoreceptors – chemical concentration Sensory receptors General Properties: 1. Specialized structures to receive sensory information 2. Amplification and Transduction of sensory stimuli to electrical activity 3. Coding of information by the number and frequency of APs Types of Sensory Neurons Sensory receptor Spike Initiating Zone Action Potentials Action Potentials Spike Initiating Zone Sensory receptor Action Potentials Chemical synapse Sensory Reception Cascade Sensory Energy Stimulus reaches receptor cells Activation of receptors Protein interactions & Second messengers Ion channels open or close Generates a receptor current Changes in Vm Spread to spike initiating zone Graded Events Changes in the amount of neurotransmitter release Electrical Energy Influence number and frequency of APs in sensory neuron All or none APs Common Physiological Properties • Dynamic Range – The range of stimulus intensity the sensory system can respond to • Adaptation 1. Phasic receptors • Fire APs for only one part of stimulus 2. Tonic receptors • Fire APs for duration of stimulus Frequency of APs (Hz) Dynamic Range Upper limit set by refractory period 1 Threshold for detection 10 100 1000 10,000 Log Stimulus Intensity Sensory receptor responds in this range Sensory receptor can’t respond further How to overcome limited range? Frequency of APs (Hz) • 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 1 10 100 1000 10,000 Log Stimulus Intensity Receptors of the skin Free nerve endings pain & temperature Pacinian Corpuscle Meissner’s Corpuscle Ruffini’s Corpuscle Deep touch Light touch stretching • 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 Pain Painful heat Acids Mechanical damage Substances released from Damaged cells: ATP Bradykinin Substance P Na+ Ca++ Free nerve ending 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 Pacinian Corpuscle Nerve axon Saline bath Fluid filled layers Extracellular recording Mechanical Stimulus First Node Of Ranvier axon Mechanical stimulus Opens ion channels myelin Receptor currents flow passively within axon If receptor potential is large enough APs produced 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 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 skin dendrites Sensory neuron AP generated in one neuron dendrites Sensory neuron AP generated in two different neurons • Good Luck!