This document summarizes the somatosensory system, including touch, proprioception, and pain. It describes the different types of sensory afferents and mechanoreceptors for tactile information and proprioception. The central pathways for somatic sensation including the dorsal column-medial lemniscal system and trigeminothalamic system are discussed. Finally, it covers nociception and pain pathways, peripheral and central sensitization, and modulation of pain perception.

1. Somatosensory System:
Touch, Proprioception, and Pain
Farzana Z Ali
HBY 554 – Principles of Neuroscience
Spring 2013

2. Learning Objectives for Part I
Part I: Touch and Proprioception
•Classify somatic sensory afferents based on their
distinct functional properties
•Characterize mechanoreceptors specialized for
tactile information and proprioception
•Understand the central pathways conveying tactile
and proprioceptive information from body and face
•Learn about cortical maps and plasticity

3. Basic Senses
Somatic sensation
Vision
Audition
Vestibular sensation
Chemical senses
Energy from
mechanical
forces
Afferent
sensory signals
Central
neurons
(qualitative +
quantitative +
location)

4. Somatic Sensory System
Somatic - “of the body”
•Relating to body
•Pertaining to body wall in contrast to viscera
•Distinguished from germplasm or psyche
•Touch, pressure, vibration, limb position, heat, cold
and pain

5. Subsystems
Tactile
•Cutaneous mechanoreceptors
•Fine touch, vibration, and pressure
Proprioceptive
•Muscles, tendons, and joints
•Position of limbs and other body parts in space
Nociceptive
•Painful stimuli, changes in temperature and
coarse touch

6. Somatic Sensory Afferents
Action potentials:
Somatic sensory afferent fibers
 Cell bodies in
Dorsal root ganglia ( pseudo-unipolar),
Cranial nerve ganglia

7. Sensory Transduction
Energy of stimulus
 Electric signal
• Δ Permeability of cation channels
• Depolarizing current
• Threshold to generate AP

8. Dermatomes
• Dorsal root gangilon 
associated spinal nerve
• Overlap substantially
(injury)
• Pain sensation more
precise

9. Axon Diameter
•IA: Largest diameter sensory afferents (muscles)
•Aβ: Slightly smaller diameter fibers (touch)
•Aδ, C: Smaller diameter fibers (pain and temperature)

10. Receptive field
• Sig. Δ in rate of AP
• Smaller receptive fields
 Smaller branching
 Dense innervation
Two-point discrimination
• ↑ receptive fields
• Somatic acuity

11. Temporal Dynamics, Channels and Filters
Channels in sensory afferents
• Quality of stimulation
• Filter properties of
encapsulating receptors
Stimulus movement, clothes
Spatial attributes of the stimulus:
size and shape

12. Mechanoreceptors for Tactile Information
slow, form + texture, fingertips
spatial resolution
rapid, dense, ↑ sensitive, ↑ fields, grip
rapid, filter ν, ↓threshold, ↑ fields, vibrations
slow, stretching, internal, position/conformation
details
coarse
APs

13. Mechanoreceptors for Proprioception
Muscle spindles: length
Group IA:
Velocity + direction
Group II:
Sustained, static position
Golgi tendons: tension
Group Ib:
Branched in collagen
fibers to form tendons
Joint receptors: finger position

14. Central Pathways
Cerebral cortex
Third order neurons in thalamus
Second-order neurons in brainstem nuclei
First order neurons in dorsal root and cranial nerve ganglia

15. DorsalColumn-MedialLemniscalSystem
TactileInformationfromtheBody

16. Trigeminothalamic System
Trigeminal nerve trigeminal brainstem complex:
i) principal, ii) spinal, iii) mesencephalic
Low threshold mechanoreceptors 
principle nucleus  cross midline
trigeminal lemniscus /trigeminothalamic tract
 VPM SI and SII

17. Proprioceptive Pathways for Body
• Upper: dorsal columns
 medulla nuclei
 cross the midline
 medial lemniscus  VPL
• Lower: medulla
 outside gracilis
 decussate and join
medial lemniscus  VPL

18. Proprioceptive Pathway for Face
• Cell bodies in mesencephalic trigeminal nucleus
• Peripheral processes innervating muscle spindles
+ Golgi tendon organs
• Facial musculature + central processes
• Brainstem nuclei for reflex control
• Thalamus  somatosensory cortex
• Exact pathway not known

19. Hemispatial Neglect
Parton A, Malhotra P, and Husain M. Hemispatial neglect. Journal of neurology, neurosurgery, and psychiatry 75: 13-21, 2004.
• Unilateral brain damage
• Cerebral infarction or
hemorrhage
• Fail to be aware of or
acknowledge items
• Right inferior parietal lobe
or nearby temporo-
parietal junction

20. Somatic Sensory Portion of Thalamus
• Ascending: spinal cord and brain stem
Ventral Posterior Complex of thalamus
• VPL  medial lemniscus
from posterior head + body
• VPM trigeminal lemniscus
from face
• Muscle spindle/ Golgi tendons

21. Somatotopic Map
• Homunculus - “Little man”
• Face and hands
> torso and proximal limbs
• Manipulation,
facial expression
and speech
• Cervical spinal cord
• Receptor density

22. Cortical Plasticity
• Reorganization of cortical circuits
• Peripheral lesions
• Unresponsive responsive to stimulation of
neighboring regions of skin
• Central representation of remaining digits
• Functional remapping
• Thalamus and brainstem

23. Learning Objectives for Part II
Part II: Pain
• Learn about nociceptive receptors and signal transduction
• Distinguish pain pathways from mechanosensory pathways
• Understand the pathway for visceral pain
• Characterize peripheral and central sensitization
• Gain insight into different aspects of the modulation of pain
perception

24. Nociception
• Nocere – “to hurt”
• Nociceptors
• Aδ group: myelinated, dangerously intense mechanical/ both intense
mechanical and thermal stimuli
• C fiber group: unmyelinated, majority respond to thermal,
mechanical and chemical stimuli

25. Nociceptor Afferents
• Aδ mechanosensitive, Aδ mechanothermal, polymodal C fibers
• Sharp first pain: Aδ activated when stimulus intensity ↑
• More delayed, diffuse, and longer-lasting second pain: small-
diameter C fibers activated when simulation ↑ even farther

26. Signal Transduction
• Aδ and C: VR-1 or TRPV1 45°C + capsaicin+ acid
• Aδ: VRL-1 or TRPV2  ↑ threshold, 52°C, not capsaicin
• TRP channel family: resembles voltage gated K+ / cyclic
nucleotide gated
• Influx of Na+ and Ca2+ initiates AP generation

27. Anterolateral System
Brainstem and thalamus in
anterolateral/ventrolateral quadrant
of contralateral half of spinal cord
Cross midline
Second-order neurons in Rexed’s
laminae 1 and 5
Gray matter of dorsal horn
Dorsolateral tract of Lissauer
Sensory neurons in dorsal root
ganglia

28. LateralSpinothalamicTract

29. Anterolateral vs.
Dorsal Column Medial Lemniscal
• Medial lemniscus enter spinal cord, ipsilateral dorsal column
medulla synapse on dorsal column nuclei cross midline
 ascend to contralateral thalamus
• Anterolateral system information crosses in spinal cord
• Unilateral spinal cord lesion

30. Parallel Pain Pathways
Sensory discriminative:
• Location, intensity, quality
• VPL/ VPM 
somatosensory cortex
• Small receptive fields
Affective-motivational:
• Unpleasant feeling,
fear and anxiety
• Autonomic activation

31. Visceral Pain
Referred pain
Punctate

32. Pain from the Face
Discriminative: projections
to the contralateral ventral
posterior medial nucleus via
trigeminothalamic tract
primary and secondary
somatosensory cortex
Affective/motivational:
targets in reticular formation
and midbrain + midline
nuclei of thalamus which
supplies cingulate and
insular regions of cortex

33. Peripheral Sensitization
Sensitization: Neuronal sensitivity
Hyperalgesia: Slightly painful
Significantly painful
↑sensitivity to temperature
after a sunburn
Peripheral sensitization:
• “Inflammatory soup”
• Vasodilation, swelling, histamine
• Protect, promote healing and
guard against infection
• ↑ blood flow and migration of white blood cells

34. Central Sensitization
Allodynia: Innocuous stimulus to the skin
• ↑ Activity in nociceptive afferents AP in dorsal horn
neurons ↑ pain sensitivity
Windup: ↑ Discharge rate of dorsal horn neurons from
repeated ↓ ν activation of nociceptive afferents
• ∑ All slow synaptic potentials in dorsal horn neurons
sustained depolarization
• Voltage dependent L-type calcium channels
• Removing Mg block of NMDA receptor (glutamate)

35. Phantom Pain
Neuropathic pain:
• Afferent fibers or central pathways damaged
Amputation of extremity:
• Illusion that missing limb is still present
• Functional reorganization of somatotopic maps
in primary somatosensory cortex
• Neurons lost their original inputs from remote limb
 tactile stimulation of other body parts

36. Placebo Effect
•Placebo - “I will please”
•Physiological response following the administration
of a pharmacologically inert remedy
1. Sedative: >2/3 reportedly felt drowsy
2. Stimulant: Majority ↓ tired
•1/3 headaches, dizziness, tingling extremities,
staggering gait
•Not “imagining” it
•Acupuncture anesthesia, analgesia through hypnosis

37. Pain Modulation
• Electrical/ pharmacological
stimulation  midbrain
• Descending pain modulating
pathways
+ spinal trigeminal nucleus
• Regulated transmission of
information to higher centers
• Periaqueductal gray of midbrain
• Descending pathways arise from brainstem sites

38. Gate Theory of Pain
• Local interneurons b/w
mechanoreceptive
afferent and neural circuit
within dorsal horn 
nociceptive information to
higher centers
• Rub the site of injury after
stubbing a toe
• ↓ Sensational sharp pain
by activating ↓ threshold
mechanoreceptors