1 6 1 Sensory Systems BIO 361T Fall 2014 1 The figure below illustrates a weak stimulus in the center of Neuron A s receptive field and a large stimulus at the periphery of Neuron A s receptive field Will the animal be able to tell that the two stimuli are occurring in different locations Explain your answer Possibly the weak stimulus will cause A to fire a low frequency of APs to its target The strong stimulus will cause A to fire a low frequency of APs as it will not hit many receptors but also will cause B to do the same The CNS could integrate these signals and determine that the strong stimulus is on the periphery of A where it overlaps with B 2 When neurons with overlapping receptive fields laterally inhibit each other each of their receptive fields can have a central excitatory region and a peripheral inhibitory region Alter the figure below to demonstrate how this could happen Imagine A and C both laterally inhibit B If B is stimulated in the center only B will fire If B is stimulated on the periphery then both A and B will fire A will laterally inhibit B so only A will pass on a signal Thus stimulating B on its periphery actually inhibits it If B inhibited A and C stimulating B in the center would inhibit A but be irrelevant Stimulating B on the periphery would also activate A but because B inhibits A only B would fire This would not create an inhibitory peripheral region If everyone inhibited all neighbors stimulating B in the center would work as previous example Stimulating B on the periphery would also cause A to fire A would inhibit B and B would inhibit A Neither of those terminals would fire so neither A nor B would pass on a signal 2 3 Get two toothpicks and hold them very close together With your groupmate looking away gently push the two toothpicks against his or her arm simultaneously He or she will perceive that there is only one point of stimulation Now move them apart very slightly and push again Repeat this process moving the toothpicks slightly farther apart each time until your groupmate can feel two different points of pressure Note the approximate distance at which this occurs a Why did the two stimuli feel like one when they were close together You are within the receptive field of a single neuron where it does not overlap with any others b Would the distance at which the two points are individually perceived be larger or smaller on your groupmate s fingertip c Test your hypothesis by repeating the process on someone s fingertip Was spatial discrimination better or worse Should be better d How do you think the size of the receptive fields in your fingertip compare to that of your arm Receptive fields in the fingertip are smaller which also increases acuity in addition to overlapping receptive fields and lateral inhibition 4 Sensory receptors vary in their sensitivity This is described by the minimum stimulus intensity necessary to elicit an action potential 50 of the time or threshold intensity which varies between cells a In what way does the magnitude of a sensory receptor cell s response change In other words what actually changes when the receptor is exposed to a stronger stimulus Frequency coding stronger stimulus causes larger graded depolarization which causes higher frequency of action potentials to be fired b This range of response or dynamic range has an upper limit Come up with three possible explanations for why a sensory receptor cell has a maximum possible response Specify whether each explanation refers to a sensory afferent or a cell that synapses onto a sensory afferent Sensory afferent refractory period maximum ion channels open Other receptor proteins saturated maximum ion channels open ion responsible for receptor potential has reached equilibrium 3 5 What determines the stimulus intensity an animal will perceive and is this always the same as actual stimulus intensity Explain your answer Frequency of action potentials fired by the sensory afferent No Range of stimulus intensities that are detectable and perceivable is much wider than dynamic range of sensory cell receptor Wide range of stimuli is encoded by multiple populations of receptors that all respond to the same stimulus but have different ranges to which they are sensitive The CNS integrates all of these responses 6 The sensations of brightness loudness and weight all have something in common regarding actual vs perceived stimulus intensity Think about these stimuli and how you perceive them What do they all have in common They all have a logarithmic relationship between stimulus intensity and response magnitude at small changes in stimulus intensity response AP frequency changes rapidly When stimulus intensity is high response does not change much Allows for broad dynamic range but also fine discrimination at low intensities You can tell the difference between no light and very little light but if a brightly lit room changes the same magnitude you cannot tell You can tell the difference between holding nothing and holding a very light book but if you are carrying a pile of books and add that one to the top you won t notice a difference
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