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Sensation

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Is Sensation Different from Perception?

  • Sensation is any stimuli the body is aware of
    • What are we not aware of?
      • X-rays, ultra high frequency sound waves, UV light
    • We have no sensory receptors for those stimuli
    • Sensory receptors transduce stimuli into membrane potentials
  • Perception is the conscious awareness & interpretation of a sensation.
    • precisely localization & identification
    • memories of our perceptions are stored in cortex

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Sensory Modalities

  • Different types of sensations
    • touch, pain, temperature, vibration, hearing, vision
  • Each type of sensory neuron can respond to only one type of stimuli
  • Two classes of sensory modalities
    • general senses
      • no structural specializations in free nerve endings that provide us with pain, tickle, itch, temperatures
      • some structural specializations in receptors for touch, pressure & vibration
    • special senses
      • very complex structures---vision, hearing, taste, smell, & balance

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Process of sensation

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Process of Sensation

  • Sensory receptors demonstrate selectivity
    • respond to only one type of stimuli
  • Events occurring within a sensation
    • stimulation of the receptor
    • transduction (conversion) of stimulus into a graded potential
      • vary in amplitude and are not propagated
    • generation of impulses when graded potential reaches threshold
    • integration of sensory input by the CNS

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Properties of Receptors

  • Information about stimulus that can be conveyed
    • modality or type of stimulus or sensation
    • location of stimuli
      • each sensory receptor receives input from its receptive field
      • brain identifies site of stimulation = sensory projection
    • intensity (frequency, numbers of fiber & which fibers)
    • duration = change in firing frequency over time
      • phasic receptor - burst of activity & quickly adapt (smell & hair receptors)
      • tonic receptor - adapt slowly, generate impulses continually (proprioceptor)

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Receptive Fields

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Adaptation of Sensory Receptors

  • Change in sensitivity to long-lasting stimuli
    • decrease in responsiveness of a receptor
      • bad smells disappear
      • very hot water starts to feel only warm
    • potential amplitudes decrease during a maintained, constant stimulus
  • Receptors vary in their ability to adapt
    • Rapidly adapting receptors (smell, pressure, touch)
      • adapt quickly; specialized for signaling stimulus changes
    • Slowly adapting receptors (pain, body position)
      • continuation of nerve impulses as long as stimulus persists

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Classification of Sensory Receptors

  • Structural classification
  • Type of response to a stimulus
  • Location of receptors & origin of stimuli
  • Type of stimuli they detect

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Structural Classification

  • Compare free nerve ending, encapsulated nerve ending and sensory receptor cell

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Structural Classification of Receptors

  • Free nerve endings
    • bare dendrites
    • pain, temperature, tickle, itch & light touch
  • Encapsulated nerve endings
    • dendrites enclosed in connective tissue capsule
    • pressure, vibration & deep touch
  • Separate sensory cells
    • specialized cells that respond to stimuli
    • vision, taste, hearing, balance

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Classification by Response to Stimuli

  • Generator potential
    • free nerve endings, encapsulated nerve endings & olfactory receptors produce generator potentials
    • when large enough, it generates a nerve impulse in a first-order neuron
  • Receptor potential
    • vision, hearing, equilibrium and taste receptors produce receptor potentials
    • receptor cells release neurotransmitter molecules on first-order neurons producing postsynaptic potentials
    • PSP may trigger a nerve impulse
  • Amplitude of potentials vary with stimulus intensity

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Classification by Location

  • Exteroceptors
    • near surface of body
    • receive external stimuli
    • hearing, vision, smell, taste, touch, pressure, pain, vibration & temperature
  • Interoceptors
    • monitors internal environment (BV or viscera)
    • not conscious except for pain or pressure
  • Proprioceptors
    • muscle, tendon, joint & internal ear
    • senses body position & movement

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Classification by Stimuli Detected

  • Mechanoreceptors
    • detect pressure or stretch
    • touch, pressure, vibration, hearing, proprioception, equilibrium & blood pressure
  • Thermoreceptors detect temperature
  • Nociceptors detect damage to tissues
  • Photoreceptors detect light
  • Chemoreceptors detect molecules
    • taste, smell & changes in body fluid chemistry