Chapter 9 - Chapter 9 1 Afferent Fibers Convey Somatic...

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Chapter 9 1. Afferent Fibers Convey Somatic Information to the Central Nervous System a. Pseudounipolar: Peripheral and central components are continuous, attached to the cell body in the ganglia in a single process. b. A stimulus alters the permeability of cation channels in the afferent nerve endings, generating a depolarizing current known as a receptor potential. If sufficient in magnitude, the receptor potentials in the afferent giber; the resulting rate of AP firing in roughly proportional to the magnitude of the depolarization. c. Afferent fibers are often encapsulated by specialized receptor cells that help tune the afferent fiber to particular features of somatic stimulation. d. Free nerve endings: Afferent fibers that lack specialized receptor cells. e. Afferents that have encapsulated ending generally have lower threshold for AP generation and are thus are more sensitive to sensory stimulation that free nerve endings. 2. Somatic Sensory Afferents Exhibit Distinct Functional Properties a. Ia: Supply the sensory receptors in the muscle b. Aβ: Information subserving touch c. d. Receptive field: Area of the skin surface over which stimulation results in a significant change in the rate of AP. The size of the receptive field is largely a function of the branching characteristics of the afferent within the skin; smaller arborization results in smaller receptive fields. There are systematic regional variations in the average size of afferent receptive fields that reflect the density of afferent fibers supplying the area. The receptive fields in regions with dense innervations are relatively small compared to those in the forearm or back that are innervated by a smaller number of afferent fibers. e. Two-point discrimination: Minimum interstimulus distance required to perceive two simultaneously applied stimuli as distinct. f. Rapidly adapting afferents (those that become quiescent in the face of continued stimulation) are thought to be particularly effective in conveying information about changes in ongoing stimulation such as those produced by stimulus movement g. Slowly adapting afferents are better suited to provide information about the spatial attributes of the stimulus, such as size and shape. h. Afferents encapsulated within specialized receptor cells in the skin respond vigorously to mechanical deformation of the skin surface, but no to changes in temperature or the presence of mechanical forces or chemicals that are known to elicit painful sensations. 3. Mechanoreceptors Specialized to Receive Tactile Information a. Haptics: Active touching b. Stereognosis: Manipulating an object with the hand can provide enough information to identify the object. c.
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Chapter 9 - Chapter 9 1 Afferent Fibers Convey Somatic...

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