9 Somatic sensations

9 Somatic sensations - SOMATIC SENSATION page 1 SENSATION...

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SOMATIC SENSATION AC Brown page 1 A7b SENSATION A. General Sequence 1. When stimulated, an afferent nerve ending (sensory receptor) generates one or more action potentials (1st order or primary afferent neuron) 2. These action potentials are conducted into the Central Nervous System (spinal cord and brain), where they excite adjacent nerve cells (2nd order, 3rd order, etc. neurons) 3. By this mechanism, excitation eventually reaches specialized regions of the cerebral cortex where conscious sensation occurs ; sensory pathways have a minimum of one-three synapses (two-four neurons), depending on modality 4. If this sequence is interrupted, conscious sensation is lost ( anesthesia ) DIMENSIONS OF SENSATION A. Modality 1. Define: quality of sensation 2. Basis: receptor stimulated and its adequate stimulus a. many types of stimuli can excite a given sensory receptor if sufficiently strong, but in normal circumstances, only a single type of stimulus causes excitation. This type is termed the receptor's adequate stimulus . b. the sensation evoked by stimulation of a receptor or its pathway is sensed as being caused by the receptor's adequate stimulus, no matter what the actual stimulus ("Doctrine of Specific Nerve Energies")
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SOMATIC SENSATION AC Brown page 2 A7b DIMENSIONS OF SENSATION A. Modality (continued) ADEQUATE STIMULUS EXAMPLES Mechanoreceptor mechanical distortion touch, pressure Chemoreceptor chemical concentration taste, smell, oxygen receptor Thermoreceptor temperature warm, cold Photoreceptor photons visible light Nociceptor noxious painful stimuli Proprioceptor (mechano-) body position, muscle tone joint position, tendon force B. Intensity and Time Course 1. Determined by a. Firing frequency of individual sensory nerve fibers b. Number of sensory fibers activated simultaneously -- r ecruitment , which depends on 1) distribution of sensory ending thresholds 2) relative locations of stimulus and endings c. Adaptation: decrement in sensation intensity with a maintained stimulus d. Change in receptor sensitivity due to local environment (e.g. sensitization ) e. Interaction at CNS synapses between ascending pathways or between ascending and descending pathways (e.g. gating or modulation ) C. Location 1. Basis: location of the sensory receptor and anatomical (topographic) organization of sensory pathways 2. Law of Projection : sensation is sensed as arising from (projected to) the receptor's receptive field even when it arises elsewhere (e.g. phantom limb) 3. Receptive Field : region from which application of a normal stimulus causes the afferent ending to respond 4. Acuity -- the precision of stimulus localization or the ability to distinguish fine details -- depends upon a. size of the receptive field (small field better acuity) b. innervation density (higher density better acuity) c. convergence along CNS pathways (less convergence
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This note was uploaded on 01/03/2012 for the course BIO 308 taught by Professor Acbrown during the Spring '10 term at Portland.

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9 Somatic sensations - SOMATIC SENSATION page 1 SENSATION...

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