Chapter 16 (1)

• Sensory receptor


  A structure specialized to detect a stimulus
• Sensory receptors

  • Some are bare nerve endings
  • Others are true sense organs: nerve tissue surrounded by other tissues that enhance response to a certain type of stimulus
• Transduction


  The conversion of one form of energy to another
• Receptor potential

  Small local electrical change on a receptor cell brought about by a stimulus
• Sensation

  A subjective awareness of the stimulus
• Sensory receptors transmit four kinds of information

  • Modality
  • Location
  • Intensity
  • Duration
• Modality


  Type of stimulus or sensation it produces
• Labeled line code

  All action potentials are identical. Each nerve pathway from sensory cells to the brain is labeled to identify its origin, and the brain uses these labels to interpret what modality the signal represents
• Location


  Encoded by which nerve fibers are firing
• Receptive field


  Area within which a sensory neuron detects stimuli
• Sensory projection

  Brain identifies site of stimulation
• Projection pathways

  Pathways followed by sensory signals to their ultimate destinations in the CNS
• Ways the brain can distinguish stimulus intensity

  • Which fibers are sending signals
  • How many fibers are doing so
  • How fast these fibers are firing
• Duration


  How long the stimulus lasts
• Sensory adaptation


  If a stimulus is prolonged, firing of the neuron gets slower over time
• Phasic receptors


  Adapt rapidly: generate a burst of action potentials when first stimulated, then quickly reduce or stop signaling even though the stimulus continues
• Tonic receptors


  Adapt slowly: generate nerve signals more steadily throughout presence of stimulus
• Classification of receptors by modality

  • Thermoreceptors
  • Photoreceptors
  • Nociceptors
  • Chemoreceptors
  • Mechanoreceptors
• Classification of receptors by origin of stimuli

  • Exteroceptors: detect external stimuli
  • Interoceptors: detect internal stimuli
  • Proprioceptors: sense body position and movements
• Classification of receptors by distribution

  • General (somesthetic) senses: widely distributed
  • Special senses: limited to head
• Receptors for the general senses

  • Relatively simple in structure and physiology
  • Consist of one or a few sensory nerve fibers and a spare amount of connective tissue
• Unencapsulated nerve endings


  Lack connective tissue wrappings
• Encapsulated nerve endings


  Wrapped by glial cells or connective tissue
• Encapsulated nerve endings

  • Tactile (Meissner) corpuscles
  • Krause end bulbs
  • Lamellar (pacinian) corpuscles
  • Bulbous (Ruffini) corpuscles
• Pain


  Discomfort caused by tissue injury or noxious stimulation, and typically leading to evasive action
• Nociceptors


  • Two types providing different pain sensations
  • Fast pain travels myelinated fibers at 12 to 30 m/s
  • Slow pain travels unmyelinated fibers at 0.5 to 2 m/s
• Types of pain


  • Somatic pain: from skin, muscles, and joints
  • Visceral pain: from the viscera
• Referred pain


  Pain in viscera often mistakenly thought to come from the skin or other superficial site
• Endogenous opioids


  Internally produced opium-like substances
• Spinal gating


  Stops pain signals at posterior horn of spinal cord
• Pathway for pain blocking (modulation)


  1. Signals from hypothalamus and cerebral cortex feed into central gray matter of midbrain

  2. Midbrain relays signals to certain nuclei in the reticular formation of the medulla oblongata

  3. Medulla issues descending, serotonin-secreting analgesic fibers to the spinal cord via the reticulospinal tract

  4. Fibers terminate in the posterior horn at all levels of the spinal cord

  5. Descending analgesic fibers synapse on short spinal interneurons

  6. Interneurons secrete enkephalins and inhibit the second-order neuron (postsynaptically)

  7. Some fibers from the medulla also exert presynaptic inhibition by synapsing on the axons of nociceptors and blocking the release of substance P
• Gustation (taste)


  Sensation that begins with action of chemical stimulants on taste buds
• Taste buds


  • 4,000 found mainly on tongue
  • Some found inside cheeks, and on soft palate, pharynx, and epiglottis
• Lingual papillae


  • Filiform: no taste buds
  • Foliate: weakly developed
  • Fungiform: a few taste buds
  • Vallate (circumvallate): at rear of tongue in a "V", contains up to one-half of all taste buds
• Taste cells


  • Have tuft of apical microvilli (taste hairs) that serve as receptor surface for taste molecules
  • Taste pores: pit into which the taste hairs project
  • Synapse with and release neurotransmitters onto sensory neurons at their base
• Basal cells


  Stem cells that replace taste cells every 7 to 10 days
• Supporting cells


  Resemble taste cells without taste hairs, synaptic vesicles, or sensory role
• Five primary taste sensations

  • Salty
  • Sweet
  • Sour
  • Bitter
  • Umami
• Taste is influenced by food texture, aroma, temperature, and appearance
• Mouthfeel


  Detected by branches of lingual nerve in papillae