PHYSIO Exam 4

• Sensory systems


  The ability to detect stimuli is critical to maintaining homeostasis
• Sensory systems


  It is useful for awareness of our environment
• Nervous system


  Has many ways of detecting both internal and external stimuli
• Sensory receptors


  Structures specialized to detect a stimulus
• Forms of sensory receptors

  • Bare nerve endings
  • Sense organs
• Bare nerve endings


  Used for detecting heat and pain
• Sense organs


  Nerve tissue surrounded by other tissues that enhance response to certain type of stimulus
• Sense organs


  • Taste buds for taste, and retina for vision
• Transduction


  The conversion of one form of energy to another
• Transduction


  Conversion of stimulus energy (light, heat, touch, sound, etc.) into nerve signals
• Graded potentials


  Receptors generate two types: generator potentials and receptor potentials
• Generator potentials


  Produced when the receptor is part of the sensory neuron, such as with free nerve endings. Generates action potentials if suprasthreshold.
• Receptor potentials


  Produced when the receptor is a separate cell. Changes the amount of neurotransmitter released onto the sensory neuron, which then generates graded potentials.
• Graded potentials in first order sensory neuron

  1. Must reach threshold

  2. Voltage-gated ions gates open to generate an action potential
• Sensation


  A subjective awareness of the stimulus, such as sound, touch, etc.
• Most sensory signals delivered to the CNS produce no conscious sensation, as they are filtered out in the brainstem which prevents distractions
• Modality


  The type of stimulus or the 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 issuing signals to the brain
• Receptive field


  The area that detects stimuli for a sensory neuron
• Smaller receptive fields allow for better discrimination
• Two point touch discrimination


  The ability to detect two points of touch as being separate
• If two points of touch fall in the same receptive field, then it is perceived as 1 point of touch
• Threshold


  The smallest distance of separation that allows for distinct sensations
• Intensity


  Encoded in 3 ways: which fibers are sending signals, how many fibers are doing so, and how fast these fibers are firing
• Generator (receptor) potential is proportional to size of stimulus


  After threshold is reached, increased stimulus causes increased frequency of action potentials
• Sensory adaptation


  If stimulus is prolonged, the firing of the neuron gets slower over time, and we become less aware of the stimulus
• Types of receptors


  • Phasic receptors
  • Tonic receptors
• Phasic receptors


  Generate a burst of action potentials when first stimulated, then quickly adapt and sharply reduce or stop signaling even though the stimulus continues
• Phasic receptors


  • Smell, hair movement, and cutaneous pressure
• Tonic receptors


  Adapt slowly, generate nerve signals more steadily
• Tonic receptors


  • Proprioceptors - body position, muscle tension, and joint motion
  • Pain receptors
• Receptor modalities


  • Thermoreceptors
  • Photoreceptors
  • Nociceptors (pain receptors)
  • Chemoreceptors
  • Mechanoreceptors
• Receptor origins


  • Exteroceptors - detect external stimuli
  • Interoceptors - detect internal stimuli
  • Proprioceptors - sense body position and movements
• Senses


  • General (somesthetic) senses - touch, pressure, stretch, cold, heat, and pain
  • Special senses - vision, hearing, equilibrium, taste, and smell
• Somesthetic projection pathways


  1. 1st order neuron (afferent neuron) from body/head to spinal cord/brainstem

  2. 2nd order neuron decussates to opposite side, ends in thalamus (except proprioception to cerebellum)

  3. 3rd order neuron thalamus to primary somesthetic cortex
• Pain


  Discomfort caused by tissue injury or noxious stimulation, and will typically lead to evasive action
• Pain is important as it helps protect us, but can be lost in conditions like diabetic neuropathy, leading to ignoring of injuries
• Types of pain receptors

  • Fast (first) pain - sharp, localized, stabbing
  • Slow (second) pain - longer-lasting, dull, diffuse
• Types of body pain

  • Somatic pain - from skin, muscles and joints
  • Visceral pain - from the viscera, poorly localized