• Today
– Sensory receptors
• General properties
– Skin receptors
Sensory receptors
•
•
•
•
•
Vision
Taste
Smell
Hearing
Touch
Light
Chemical
Mechanical
• Internal
–
–
–
–
Proprioception - limb position and movement
Baroreceptors - blood pressure
Osmoreceptors - osmolarity
Chemoreceptors – chemical concentration
Sensory receptors
General Properties:
1. Specialized structures to receive sensory
information
2. Amplification and Transduction of
sensory stimuli to electrical activity
3. Coding of information by the number and
frequency of APs
Types of Sensory Neurons
Sensory receptor
Spike
Initiating
Zone
Action Potentials
Action Potentials
Spike
Initiating
Zone
Sensory receptor
Action Potentials
Chemical synapse
Sensory Reception Cascade
Sensory Energy
Stimulus reaches receptor cells
Activation of receptors
Protein interactions &
Second messengers
Ion channels open or close
Generates a receptor current
Changes in Vm
Spread to spike
initiating zone
Graded
Events
Changes in the amount
of neurotransmitter release
Electrical Energy Influence number and frequency of APs
in sensory neuron
All or none
APs
Common Physiological Properties
•
Dynamic Range
– The range of stimulus intensity the sensory
system can respond to
•
Adaptation
1. Phasic receptors
•
Fire APs for only one part of stimulus
2. Tonic receptors
•
Fire APs for duration of stimulus
Frequency of APs (Hz)
Dynamic Range
Upper limit set by refractory period
1
Threshold for
detection
10
100
1000
10,000
Log Stimulus Intensity
Sensory receptor
responds in this range
Sensory receptor
can’t respond further
How to overcome limited range?
Frequency of APs (Hz)
• 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
1
10
100
1000
10,000
Log Stimulus Intensity
Receptors of the skin
Free nerve endings
pain & temperature
Pacinian Corpuscle
Meissner’s Corpuscle
Ruffini’s Corpuscle
Deep touch
Light touch
stretching
• 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
Pain
Painful heat
Acids
Mechanical damage
Substances released from Damaged cells:
ATP
Bradykinin
Substance P
Na+
Ca++
Free nerve ending
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)
Meissner’s – light pressure, slow vibration (50 Hz)
Use mechanically gated ion channels
Pacinian Corpuscle
Nerve axon
Saline bath
Fluid filled layers
Extracellular recording
Mechanical Stimulus
First Node
Of Ranvier
axon
Mechanical stimulus
Opens ion channels
myelin
Receptor currents
flow passively within axon
If receptor potential is
large enough APs
produced
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
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
skin
dendrites
Sensory neuron
AP generated in one neuron
dendrites
Sensory neuron
AP generated in two different neurons
• Good Luck!