1. Anatomical Substrates of Somatic Sensation
John H. Martin, Ph.D.
Center for Neurobiology & Behavior
Columbia University CPS

2. The 2 principal somatic sensory systems:
Dorsal column-medial lemniscal system
Anterolateral system

3. Dorsal Column-Medial Lemniscal System
•Mediates mechanical sensations
-touch, limb position sense, vibration sense
•Well established; clinical & experimental

4. Anterolateral System •Mediates protective sensations -Pain
-Temperature (cold & warmth)
-Itch
•Not as definitively established as is the DC-ML system for touch
Why?
•Pain w/o tissue trauma
•Trauma w/o pain
•Cultural; pathological

5. Perspective:
Peripheral somatic sensory receptors are sensitive to different stimulus qualities
Mechanical
Thermal (warm, cold)
Noxious (mechanical, thermal, polymodal)
Different receptor classes provide input to different somatic sensory pathways
Differential thalamic and cortical localization

6. Dorsal column-medial lemniscal system: Mechanical sensations
1° somatic sensory
cortex
3. Thalamus:
Ventral posterior
nucleus
Medial lemniscus
2. Dorsal column
nuclei
Dorsal column
1. Mechanoreceptor
Dorsal root
ganglion
Peripheral nerve

7. Anterolateral system: Pain, Thermal, Itch
1° somatic sensory
cortex
3. Thalamus:
Ventral posterior…
Insular cortex
Cingulate
cortex
& Medial dorsal nuclei.
1. Nociceptor Thermoreceptor Itch/histamine
Peripheral axon
Dorsal root
ganglion
2. Anterolateral system:
•Spinothalamic tract
•Spinoreticular tract
•Spinomesencephalic tract
Dorsal horn

8. Mechanoreceptors are encapsulated
Merkel’s receptor
Meissner’s
corpuscle
Pacinian
corpuscle

9. Nociceptors, thermoreceptors, & itch receptors are bare nerve endings
Merkel’s receptor
Meissner’s
corpuscle
Pacinian
corpuscle

10. Dorsal root ganglion neuron
PNS CNS
Receptive
Conductive - 1° afferent fiber
Output
Integrative

11. Key Characteristics of DRG Neurons
Peripheral receptive field
Spatial area within which stimulation activates the sensory neuron

12. Receptive field

13. CNS Neurons have Receptive Fields

14. Key Characteristics of DRG Neurons
Peripheral receptive field
Spatial area within which stimulation activates the sensory neuron
Response to constant stimulation
Slow adaptation
Off On
Rapid adaptation

15. Mechano- receptor receptive fields
Rapidly adapting
Slowly adapting
Mechano- receptor receptive fields
Meissner's
Merkel
Pacinian
Ruffini

16. Mechano- receptor receptive fields
Rapidly adapting
Slowly adapting
Mechano- receptor receptive fields
Meissner's
Merkel
Pacinian
Ruffini

17. Nociceptors, thermoreceptors, & itch receptors are bare nerve endings
Merkel’s receptor
Meissner’s
corpuscle
Pacinian
corpuscle

18. Nociceptors respond to noxious stimuli
High force; NOT NOXIOUS; NO RESPONSE
Blunt probe
Low force; NOXIOUS; RESPONSE
Pin
Serrated forceps
Low force; VERY NOXIOUS; LARGE RESPONSE

19. Peripheral nerve cross section
Unmyelinated
Small myelinated
Large myelinated

20. Fiber Histogram: Sensory axon innervating the skin
IV (C)
III (A-d)
II (A-b)

21. Fiber Histogram: Sensory axon innervating a muscle
Protective
IV (C)
Mechanoreceptors
III (A-d)
I (A-)
II (A-b)

22. Sensory Dorsal root
Motor Ventral root
Spinal nerve SC

23. Dermatomes
Area of skin innervated by all sensory fibers w/in single dorsal root

24. Dermatomes overlap Dermatome facts: Skin
Pain dermatomes overlap < touch
Dermatomal boundaries vary
Dermatome facts:
Peripheral nerve
Dorsal
root
Ventral
root