Ch. 18-Anatomy

• Sensory receptor


  A specialized cell or cell process that monitors conditions in the body or external environment
• Sensation


  Sensory information arriving at the brain
• Perception


  A conscious awareness of a sensation
• General senses


  • Temperature
  • Pain
  • Touch
  • Pressure
  • Vibration
  • Proprioception (body position)
• General senses arrive at primary sensory cortex or somatosensory cortex via pathways
• Special senses


  • Smell (olfaction)
  • Taste (gustation)
  • Balance (equilibrium)
  • Hearing
  • Vision
• Sense organ


  Eye or ear
• Receptor specificity


  Results from the structure of the receptor cell itself or from the presence of accessory cells or structures that shield it from other stimuli
• Free nerve endings


  Simplest receptors are dendrites of sensory neurons
• Receptive field


  Area monitored by a single receptor cell
• Larger the receptive field


  The poorer our ability to localize a stimulus
• Labeled line


  Connection between receptor & cortical neuron that carries information concerning a specific sensation from receptors in a specific part of the body
• Sensory coding


  Provides information about the strength, duration, variation, & movement of stimulus
• Types of receptors


  • Tonic (always active)
  • Phasic (normally inactive but become active for a short period of time)
• Peripheral (sensory) adaptation


  When the receptors or sensory neurons alter their levels of activity
• Fast-adapting receptors


  Respond strongly but thereafter the activity along the afferent fiber gradually declines in part due to the synaptic fatigue
• Slow-adapting receptors


  Show little peripheral adaptation
• Central adaptation


  Involves the inhibition of nuclei along a sensory pathway
• Humans do not have receptors for every possible stimulus</b>
• Our receptors have characteristic ranges of sensitivity
• A stimulus must be interpreted by the brain. Our perception of a particular stimulus is an interpretation & not always a reality
• Types of general senses

  • Exteroceptors (provide information about the external environment)
  • Proprioceptors (monitor body position)
  • Interoceptors (monitor conditions inside the body)
• Types of stimuli that excite receptors

  • Nociceptors (respond to a variety of stimuli usually associated with tissue damage)
  • Thermoreceptors (respond to change in temperature)
  • Mechanoreceptors (stimulated or inhibited by physical distortion, contact, or pressure)
  • Chemoreceptors (monitor chemical composition of body fluids)
• Nociceptors


  Especially common in the superficial portions of the skin, joint capsules, within the periostea of bones, around the wall of blood vessels
• Types of nociceptors


  • Receptors sensitive to extremes of temperature
  • Receptors sensitive to mechanical damage
  • Receptors sensitive to dissolved chemicals
• Fast pain


  Produced by deep cuts or similar injuries
• Slow pain


  Same types of injuries as fast pain just begin later & persist longer
• Referred pain


  Sensations are often perceived as originated in more superficial regions that are innervated by the same spinal nerves
• Thermoreceptors


  Found in the dermis of the skin, skeletal muscles, liver, hypothalamus
• Cold receptors are more numerous than warm receptors
• Thermoreceptors are very active when temperature is changing
• Thermoreceptor sensations are conducted along the same pathways that carry pain sensations
• Thermoreceptor sensations are sent to the reticular formation, thalamus, & primary sensory cortex
• Mechanoreceptors


  Sensitive to stimuli that stretch, compress, twist, or distort their plasmalemma
• Types of mechanoreceptors


  • Tactile receptors (provide sensations of touch, pressure, vibration)
  • Baroreceptors (detect pressure changes in the walls of blood vessels, digestive, reproductive, & urinary tracts)
  • Proprioceptors (monitor the positions of joints & muscles)
• Fine touch pressure receptors

  Provide detailed information about a source of stimulation, including its exact location, shape, size, texture, movement
• Crude touch pressure receptors

  Provide poor localization & little additional information about the stimulus
• Free nerve endings


  Common in the papillary layer of the dermis
• Tactile disc


  A sensory neuron across a vesiculated synapse that involves an expanded nerve terminal (Merkel's disc)
• Root hair plexus


  Free nerve endings that monitor distortions & movements across the body surface