Somatosensory Systems cutaneous proprioceptive
Adequate Stimuli Thermal (infrared radiation, contact) Touch (light touch, pressure, vibration) Pain and Itch (chemical, thermal, mechanical) Proprioception (mechanical; stretch or pressure)
epicritic location vibration texture shape protopathic pain temperature itch and tickle Cutaneous subsystems
Epricritic, or non-pain Somatosensation
Meissner’sMerkel’s Pacinian Riffini’s Free nerve ending 60 hz vibration Stretch 200 hz vibration Pressure Pain
As in the retina, receptive fields vary in size. Smaller receptive fields = greater acuity two-point discrimination
Center-surround organization of cutaneous receptive fields results in lateral inhibition, as in retina Serves to enhance contrast
Protopathic, or pain Somatosensation
Free nerve endings that respond to: mechanical stimuli thermal stimuli chemical stimuli, or all three (polymodal receptors) Pain Receptors Called Nociceptors
Free nerve endings of unmyelinated C fibers or thinly myelinated A δ fibers
Cutaneous classified by conduction velocity Proprioceptive classified by axon diameter
SubstanceEffect Potassiumactivation Bradykininactivation Histamineactivation Prostaglandinssensitization Substance Psensitization
Gate control theory of pain control
Referred Pain
CN V and VII
Parallel Processing in the Somatosensory System Lemniscal System (non-pain; epicritic) Extralemniscal System (pain; protopathic) Spinothalamic pathways Neospinothalamic Paleospinothalamic Spinomesencephalic
Neospinothalamic PaleospinothalamicSpinomesencephalic
Neospinothalamic Pathway
Paleospinothalamic Pathway
Spinomesencephalic Pathway
Descending control of pain
Sensory System Summary 1. Sensory systems detect change over space (lateral inhibition to enhance contrast) over time (rapidly adapting) 2. Detect “features” 4. Parallel pathways 5. Hierarchical processing 6. Topographical organization 7. Non-uniform receptive fields 8. Extreme sensitivity, wide dynamic range 9. Non-linear response 3. Structures are laminated (cells in layers)