Sensory Physiology

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Sensory Physiology

• Al Mina, M.D.
• Erskine College

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Sensory information

• Anything that is picked up and processed by a
  sensory system
• Sensation information reaches consciousness
• Perception info understood and has meaning

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Receptors

• Transform stimuli into graded potentials
• Either ending of axon, or separate cells
• Sensory transduction change stimulus to signal
• Highly tuned to specific stimulus, but can be
  activated by others if stimulus strong enough
  however, signal will be same

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Receptor classes

• Mechanoreceptors pressure/stretch
• Thermoreceptors temp
• Photoreceptors light
• Chemoreceptors substances
• Nociceptors - pain

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Signal tranduction

• Starts with ion channels open when stimulated
  (directly or 2nd messenger)
• Creates graded potential receptor potential
• Current flows to area with voltage gated channels
• Action potential can then be generated

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Signal strength

• Each action potential is all or none, but
  frequency of APs is influenced by graded
  potential magnitude
• Magnitude influenced by
• Stimulus strength
• Summation
• Rate of change in strength
• May also be influenced by adaptation

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Coding

• Transferring stimulus into signal
• Begins at receptor sites (receptor field)
• Influenced by type, intensity, and location of
  stimulus
• Receptor fields of different sensory units
  usually overlap

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Stimulus type

• Modality what kind of stimulus
• All receptors of a single axon sensitive to same
  type, but may overlap with fields of different
  types

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Stimulus intensity

• Increased intensity increased receptor
  potential
• Also increased signal will affect adjacent
  branches, increasing summation affect, which
  increases AP frequency
• Recruitment other sensory units involved

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Location

• Labeled lines specific pathway traveled by AP
  from a receptor
• Precision influenced by size, density, and
  overlap of sensory units in area.
• More receptors greater precision.
• Smaller receptive field greater precision

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Lateral inhibition

• Further enhances ability to locate
• Strongest signals will use inhibitory neurons to
  dampen the response in surrounding neurons
• Not all pathways use equally
• Prominent in retina, pressure
• Less relevant in temperature/pain

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Duration

• Reaction to constant stimulus
• Adaptation stopping or decreasing signal in
  spite of constant stimulus

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Neural pathways

• Ascending pathways bundles of chains of sensory
  pathways.
• Run parallel to each other, deliver information
  to specific location on cerebral cortex

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Ascending

• Usually carry same type of information (specific
  ascending pathways)
• Synapse in CNS two or more times
• Cross from one side of body to opposite cortex
  specific locations (somatosensory cortex, visual,
  auditory, etc.)
• Signal then goes to cortical association areas
  for further processing and integration

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Nonspecific pathways

• May send multiple types of signals
• Usually signal ends in brainstem or thalamus
• Nonspecific, but signals help in alertness

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Processing of perceptions

• Association areas are outside sensory areas
• Integrate multiple stimuli
• Also add input from learning, memory, emotion

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Influences on perception

• Adaptation
• Emotion, memory, experience, etc.
• Preventing overload
• Inability to receive stimulus (infrared)
• Damaged neural networks
• Chemical/disease altering perceptions

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Somatic sensation

• Receptors respond to variety of stimuli
• Sense temperature, pressure, vibration, position,
  touch, pain
• Visceral sensations use same type of receptors

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Touch, pressure

• Mechanoreceptors nerve endings linked to
  collagen fibers and encapsulated
• Fibers transmit stimulus to ion channels,
  activate
• Variable adaptation rates
• Sensitivity varies

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Posture, movement

• Muscle-spindle stretch receptors
• Mechanoreceptors
• Respond to rate and magnitude of stretch
• Assisted by vision, balance organs

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Temperature

• Separate cold and warm thermoreceptors
• Open nonselective cation channels at certain
  temperatures
• Also respond to certain chemicals (ethanol,
  menthol)

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pain

• Nociceptors free nerve endings
• Respond to deformity, heat, chemicals (typically
  those released by damaged cells)
• On synapsing with interneurons, may induce
  referred pain, especially from visceral organs

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Contd

• After stimulus, may get increased or decreased
  sensitivity to signal
• Hyperalgesia increased pain after signal
• May be influenced by emotions, past experience

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• Analgesia reduction of pain stimulus
• Suppression by higher cortical function down
  descending pathway
• Also release endogenous opioids
• Can be stimulated with accupuncture
• Stimulation of local touch receptors can also
  inhibit pain receptors

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Pathways

• Anterolateral pathway conduit for pain and
  temperature
• Crosses in spinal cord
• Dorsal column pathway conduit for touch,
  pressure, proprioception
• Crosses in brainstem
• Both synapse in thalamus, then in somatosensory
  cortex

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Somatosensory cortex

• Receptors go to specific areas on cortex
• Common areas grouped together
• Highest density areas have larger areas
• Homunculus
• Can modify in case of injury