Today

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• Today
• Sensory receptors
• General properties
• Skin receptors

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Sensory receptors
Light

• Vision
• Taste
• Smell
• Hearing
• Touch
• Internal
• Proprioception - limb position and movement
• Baroreceptors - blood pressure
• Osmoreceptors - osmolarity
• Chemoreceptors chemical concentration

Chemical
Mechanical
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Sensory receptors

• General Properties
• Specialized structures to receive sensory
  information
• Amplification and Transduction of sensory stimuli
  to electrical activity
• Coding of information by the number and frequency
  of APs

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Types of Sensory Neurons
Spike Initiating Zone
Sensory receptor
Action Potentials
Action Potentials
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Spike Initiating Zone
Sensory receptor
Action Potentials
Chemical synapse
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Sensory Reception Cascade
Stimulus reaches receptor cells
Sensory Energy
Activation of receptors
Protein interactions Second messengers
Ion channels open or close
Graded Events
Generates a receptor current
Changes in Vm Spread to spike initiating zone
Changes in the amount of neurotransmitter release
All or none APs
Influence number and frequency of APs in sensory
neuron
Electrical Energy
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Common Physiological Properties

• Dynamic Range
• The range of stimulus intensity the sensory
  system can respond to
• Adaptation
• Phasic receptors
• Fire APs for only one part of stimulus
• Tonic receptors
• Fire APs for duration of stimulus

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Dynamic Range
Upper limit set by refractory period
Frequency of APs (Hz)
1 10 100 1000 10,000
Log Stimulus Intensity
Threshold for detection
Sensory receptor cant respond further
Sensory receptor responds in this range
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How to overcome limited range?

• For each type of receptor, there are individual
  receptors specialized to respond to specific
  parts of the range

Range of the whole sensory system
Range of individual receptors
Frequency of APs (Hz)
1 10 100 1000 10,000
Log Stimulus Intensity
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Receptors of the skin
Free nerve endings pain temperature
Pacinian Corpuscle Deep touch Meissners
Corpuscle Light touch Ruffinis
Corpuscle stretching
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• Pain
• Nociceptors respond to painful stimulus
• Carried by non-myelinated C fiber sensory neurons
• Painful heat, acids, mechanical damage all
  activate non-specific cation channels

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Pain
Substances released from Damaged
cells ATP Bradykinin Substance P
Painful heat Acids Mechanical damage
Na Ca
Free nerve ending
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Receptors of the Skin

• Tactile sensory input
• Respond to pressure and movement of skin
• Specialized receptors that respond to particular
  types of inputs
• Pacinian heavy pressure, rapid vibration
  (300Hz)
• Meissners light pressure, slow vibration (50
  Hz)
• Use mechanically gated ion channels

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Pacinian Corpuscle
Nerve axon
Saline bath
Fluid filled layers
Extracellular recording
Mechanical Stimulus
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First Node Of Ranvier
axon
myelin
If receptor potential is large enough APs
produced
Receptor currents flow passively within axon
Mechanical stimulus Opens ion channels
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Pacinian Corpuscle

• Example of a rapidly adapting receptor
• Only gives an on and an off response
• Epithelial layers filter out steady pressure but
  transfer rapid changes in pressure

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Receptive Field

• The region of the skin in which a stimulus evokes
  a response in a single sensory neuron
• Discrimination depends on the density of
  receptors
• Fingers 1-4 mm
• Thigh 45 mm

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skin
dendrites
Sensory neuron
AP generated in one neuron
dendrites
Sensory neuron
AP generated in two different neurons
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• Good Luck!