Sensory Systems

1. Visual

2. Auditory

3. Somatosensory

4. Gustatory

5. Olfactory

acousticvestibular

cutaneousproprioceptive

chemical (flavor)

Distal senses

Proximal senses

SomatosensorySystems

cutaneous

proprioceptive

Adequate Stimulus

A stimulus of a quality and of sufficient intensity to excite a sensory receptor.

Adequate Stimuli for Somatosensation

Thermal (infrared radiation, contact)

Touch (light touch, pressure, vibration)

Pain and Itch (chemical, thermal, mechanical)

Proprioception (mechanical; stretch or pressure)

epicriticlocationvibrationtextureshape

protopathicpaintemperatureitch and tickle

Cutaneoussubsystems

Receptive field

That part of the periphery to which a cell responds.

Meissner’s Merkel’s

Pacinian Ruffini’s Free nerve ending

60 hz vibration

Stretch200 hz vibration

Pressure

Pain

Summation of responses of different receptors (spatial summation).

Coding of intensity by increased rate (temporal summation).

Epricritic, or non-pain

Somatosensation

As in the retina, receptive fields vary in size.

Smaller receptive fields = greater acuitytwo-point discrimination

Center-surround organization of cutaneousreceptive fields results in lateral inhibition.

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 orthinly myelinated Aδ fibers

Cutaneousclassified byconductionvelocity

Proprioceptiveclassified byaxon diameter

Substance Effect

Potassium activation Bradykinin activation Histamine activation Prostaglandins sensitization Substance P sensitization

Gate control theory ofpain control

Transcutaneous electrical nerve stimulation may act via gate control

Referred Pain

CN V and VII

Parallel Processing in theSomatosensory System

Lemniscal System (non-pain;epicritic)

Extralemniscal System (pain;protopathic)Spinothalamic pathways

NeospinothalamicPaleospinothalamic

Spinomesencephalic

Neospinothalamic Paleospinothalamic Spinomesencephalic

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)