BIO 3324 1nd Edition Lecture 10Outline of Last LectureI. Sensory integration and processing II. Basic Behavioral PatternsIII. How memory worksIV. HippocampusV. Receptors Outline of Current Lecture I. Receptor signalsII. Somatosensory pathwaysIII. SmellIV. TasteV. PainVI. EarVII. visionCurrent LectureReceptor signals:• Converting diverse physical stimuli into electrical signals is called transduction– Common: Stimulation of a receptor results in the altering of membrane permeability by opening non-selective small ion channels• Typically results in inward flux of Na+ resulting in local depolarizationThese notes represent a detailed interpretation of the professor’s lecture. Grade Buddy is best Used as a supplement to your own notes, not as a substitute.– Less Common: signal transduction and 2nd messengers• Each receptor is specific for a particular form of energy (stimulus)• The stimulus must meet a threshold (minimum amount of stimulation) in order to activate the receptor• Results in a graded potentials called a receptor potential – must initiate an AP to transmit the signal– Results in an AP in a sensory neuron – or – – Results in the release of NT that acts on an associated sensory neuronReceptive fields:• Specific physical area where a sensory neuron detects a stimulus• One receptive field is associated with one primary sensory neuron which synapses with a secondary sensory neuron resulting in a larger secondary receptive field– Receptive fields can overlap– Often exhibit convergence enabling subthreshold stimuli to sum at the postsynaptic (secondary) neuron• The size of the secondary receptive field determines the sensitivity to a given stimulusDistinguishing between stimuli:• All stimuli are converted into action potentials & so the CNS must preserve the attributesof the stimulus– Modality – dependent on which type of receptors are stimulated and which pathways are activated• Stimulation of a specific receptor will always result in perception of that stimulus– Location – dependent on which receptive fields are activated, coded by the organization of the CNS. Exception is sound which is dependent on timing differences between the left & right ear– Intensity & Duration – determined by the number of receptors activated and the frequency of the APs coming from those receptorsLateral inhibition:• Occurs in the CNS and results when the most strongly activated signal pathway from an excited area inhibits the less excited pathways• Occurs at lateral interneurons between ascending fields • Mechanism to increase contrasts between activated receptive fields• End result: increases perception of localization and strength of stimuliAdapting to a prolonged stimulus:• Adaptation – diminished response to sustained stimulus• Receptor no longer responds to a stimulus to the same degree• Two types of receptors– Tonic• Do not adapt at all or adapt slowly• Present where maintaining information about the stimulus is important• Examples: – Stretch receptors (skeletal muscles)– Proprioreceptors (posture & balance)– Phasic• Rapidly adapting• When stimuli stops – slight depolarization called an off response• Important where changes in stimulus intensity are detected• Example: – Touch receptor in the skinSomatosensory pathways:• Discrete chains of neurons that synaptically interconnect and at each level get processedwith greater sophistication– First order neuron = peripheral afferent receptor to the spinal cord or medulla• Fine touch receptors and proprioceptors synapse in the medulla• Pain, temperature and coarse touch synapse in the spinal cord– Second order neuron = spinal cord or medulla to the thalamus• All decussate at some point• Also may synapse with neurons to cerebellum– Third order neuron = thalamus to the sensory cortex– And so on to the defined region of the cortex• In this way specific information is kept separate from other specific information so that itcan be properly decoded (type of signal & location of signal)Touch receptors:• Mechanoreceptors that respond to various types of stimuli• Found in the skin (at varying depths) and in deeper tissues• Can be free nerve endings or encapsulated– Encapsulation increases the receptive fieldNociceptors:• Three types of nociceptors– Mechanical nociceptors – mechanical stimuli (cutting, crushing, pinching)– Thermal nociceptors – thermal stimuli– Polymodal nociceptors – responds to all kinds of stimuli including chemical• They do not adapt to sustained or repetitive stimuli• Modulated by a variety of chemicals which lower the activation threshold: either activating them or sensitizing them– Substance P, histamines, serotonin, prostaglandins• Signals from nociceptors are transmitted in either:– Ab fibers• Large, myelinated• Specific mechanical stimuli– Ad fibers• Small, myelinated• Fast pain (sharp initial response)• Specific mechanical or cold stimuli– C fibers• Small, unmyelinated• Slow pain (dull persisting ache)• Specific mechanical; heat and cold stimuli• Activate two pathways– Reflexive protective responses at the level of the spinal cord– Ascending pathways to the cortex that become conscious sensationsPain:• Protective mechanism bringing about conscious awareness that tissue damage is occurring or is about to occur• Accompanied by motivated behavioral response & an emotional response• Influenced by experienceAnalgesia:• Suppression of the pain response• Two regions are known to be a part of descending pathway– Periaquaductal gray matter– Stimulation of the reticular formation• Blocks the release of substance P• Dependent on opiate receptors – bind endogenous opiates (endorphins, enkaphalins, dynorphin)• Inhibit the presynaptic terminal of afferent terminal (presynaptic inhibition)Smell:• Olfactory mucosa – three types of cells– Supporting cells – secrete mucus– Basal cells – Give rise to new olfactory receptors cells– Olfactory receptors cells – an afferent neuron• Possesses a bulbous receptor end with protruding cilia• Multiple bindings sites for odorants• Each receptor responds to only one discrete component of the odor (odorant molecule) rather than the whole odor• The afferent fibers synapse in the olfactory bulb within the glomeruli (neural junctions) which contains the terminals of mitral cells• Olfactory