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Chapter 10

Chapter 10, Kee

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University of Toronto St. George
Michelle French

Chapter 10 General Principles of Sensory Physiology receptors detect stimuli in the external environment visceral receptors + arise within the body examples + chemoreceptors in major blood vessels that monitor O2, CO2, and H+ levels in the blood + baroreceptors in certain blood vessels that monitor blood pressure + mechanoreceptors in the gastrointestinal tract that monitor the degree of stretch of distension sensory systems that enable us to perceive the external environment include the somatosensory system and the special system + somatosensory system necessary for perception of sensations associated with receptors in the skin (somesthetic sensation + proprioception, the perception of the position of the limbs and the body depends on specific proprioceptors in muscles and joints and on more generalized receptors in the skin + special senses necessary for senses of vision, hearing, balance, and equilibrium, taste, and smell Receptor Physiology sensory receptors specialized neuronal structures that detect a specific form of energy in either the internal or external environment + energy form of a stimulis modality - ie: light waves, sound waves, pressure, temperature, chemicals + law of specific nerve energies a given sensory receptor is specific for a particular modality special cells in the eye called photoreceptors detect light waes, but not sound waves + modality to which a receptor responds best adequate stimulus Sensory Transduction function of sensory receptors transduction, the conversion of one form of energy into another sensory transduction + receptors convert the energy of a sensory stimulus into changes in membrane potential receptor potentials/generator potentials + receptor potentials resemble postsynaptic potentials in that they're graded potentials caused by opening/closing of ion channels + greater the strength of stimulus, greater the change in membrane potential + receptor potentials triggered by sensory stimuli sensory receptors exist in two basic forms + 1. a sensory receptor is a specialized structure at the peripheral end of an afferent neuron - when depolarized to threshold, an action potential is generated in the afferent neuron and propagated to the CNS transmitting information about the stimulus + 2. the sensory receptor is a separate cell that communicates through a chemical synapse with an associated afferent neuron when sensory receptor is a separate cell, changes in the receptor cell's membrane potential cause the release of a chemical messenger or transmitter + greater the excitatory stimulus, greater the amount of transmitter released + transmitter then binds to receptors on the afferent neuron and causes changes in the membrane potential of that cell Receptor Adaptation some receptors continue to respond to a stimulus for as long as the stimulus is applied + most receptors adapt to the stimulus their response declines with passage of time receptor adaptation + a decrease over time in the magnitude of the receptor potential in the presence of a constant stimulus slowly adapting/tonic receptors + show little adaptation and can therefore function in signalling the intensity of a prolonged stimulus rapidly adapting/phasic receptors + adapt quickly function best in detecting changes in stimulus intensity + respond at the onset of a stimulus, then adapt + some show a second, smaller response upon the termination of a stimulus off-response Sensory Pathways specific neural pathways that transit information pertaining to a particular modality labeled lines, and each sensory modality follows its own labeled line activation of a specific pathway causes perception of the associated modality, regardless of the actual stimulus that activated the pathway pressure on the eye may trigger light flashes because the pressure generates signals in pathway for perception of light + pathways for different modalities terminate in different sensory areas of the cerebral cortex sensory unit + a single afferent neuron and all the receptors associated with it + all the receptors associated with a given afferent neuron are of the same type + activation of any of the associated receptors may trigger action potentials in the afferent neuron area over which adequate stimulus can produce a response (excitatory/inhibitory) in the afferent neuron is receptive field of that neuron + corresponds to the region containing receptors for that afferent neuron afferent neuron that transmits information from periphery to the CNS first-order neuron + a single first-order neuron may diverge within the CNS and communicate with several interneurons may receive converging input from several first-order neurons + some of the interneurons transmit the information to the thalamus, the major relay nucleus for sensory input second-order neurons + in thalamus, second-order neurons form synapses with third-order neurons that transmit information to the cerebral cortex where sensory perception occurs receptive field of any neuron in a sensory pathway is the area in which an adequate stimulus can induce a response in that neuron Sensory Coding Coding for Stimulus Type coded by the receptor and pathway activated when the stimulus is applied our perception of stimuli + brain must integrate information from different sensory systems Coding for Stimulus Intensity coded by the frequency of action potentials (frequency coding) and the number of receptors activated (population coding) in frequency coding, a stronger stimulus results in a larger receptor potential + as long as graded potential exceeds threshold, stronger depolarizations can overcome therelative refractory period of an action potential and generate a second action potential more quickly than can weaker depolarization stronger stimuli produce a higher frequency of action potentials in population coding, a stronger stimulus activates or recruits a greater number of receptors may be associated ith a single afferent neuron in which the receptor potentials that are generated at the individual receptors sum and produce a greater frequency of action potentials in that neuron + a stimulus may also recruit receptors associated with different afferent neurons in which case more afferent neurons transmit signals to the CNS concerning the presence of the stimulus a greater frequency of action potentials is transmitted to the CNS in response to the stimulus Coding for Stimulus Location basis for coding the locations of most stimuli is receptive fields + when an adequate stimulus is applied within a particular receptive field, it activates receptors associated with a specific afferent neuron precision with which the location of a stimulus is perceived acuity + depends on size and number of receptive fields, amount of overlap between receptive fields, and lateral inhibition if a specific afferent neuron is activated, then the stimulus must be located within that neuron's receptive field + sizes of receptive fields vary considerably over the body + localization of a stimulus is better in areas served by neurons with a smaller receptive fields + information from one afferent neuron alone doesn't provide precise localization of a stimulus because the stimulus could be anywhere within its receptive field localization improved by the overlapping of the receptive fiel
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