Peripheral nerve features

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  1. Sensory receptors or transducers
    1. Pacinian corpuscles – mechanoreceptors located in the skin and deep tissues and mediate pressure or vibration; rapidly adapting
      1. Shaped like an onion; responds quickly and returns to baseline
      2. Rapidly adapting receptors discharge initially to the presence of a stimulus and then to any stepwise change in intensity
        1. Compare slowly adapting receptors that responds continuously during the presence of the stimulus
    2. tonic mechanoreceptors – evoked potential is maintained for the duration of the stimulus with only slight adaption
      1. firing frequency codes for stimulus strength
    3. photoreceptors
      1. rods – used in dim light; achromatic; greater numbers of rods than cones in the human retina
        1. contains rhodopsin consisting of a small vitamin A derivative (retinine) attached to a large protein (opsin)
        2. light changes the isomerization of the retinine portion and leads to a breakdown of rhodopsin; rhodopsin is activated when light converts bound 11-cis-retinal to all-trans retinal; activation of rhodopsin activates a G protein; activation of cGMP phosphodiesterase increases hydrolysis of cGMP to 5’-GMP; the decreased concentration of cGMP results in decreased current through the Na channel and subsequent hyperpolarization
        3. both rods and cones are hyperpolarized by light; in the dark, Na ions flow into photoreceptors and depolarize the cell; in light rods and cones are hyperpolarized and the release of neurotransmitter (including dopamine, glycine, taurine, and acetylcholine) is inhibited
      2. cones – used in bright light; sensitive to different parts of the light spectrum
        1. higher resolution with shorter integration time and more rapid response than rods; fovea contains only cones
        2. fovea lies lateral to the optic disk (a.k.a. optic papilla, a.k.a. blind spot)
      3. rods and cones are arranged in an annular organization where the center and peripheray are mutually antagonistic
    4. Meissner’s corpuscles – rapidly adapting touch receptors in the fingertips (dermis) and lips, A-beta (type II) myelinated fibers
    5. Merkel’s discs – slowly adapting touch and pressure sensors located in the dermis; innervated by single A-beta (type II) fiber; small receptive field
    6. Ruffini end organs – subcutaneous, slowly adapting for heavy touch and pressure
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