1. The Bodily Senses Ch. 22, “Principles of Neural Science”, 5th Ed.
陳韋達 MD PhD
臺北榮總神經醫學中心 主治醫師
國立陽明大學醫學系/腦科所 副教授
哈佛大學醫學院麻省總醫院生醫影像中心 進修

2. Bodily senses Bodily senses = somatic sensation Sensory information
Large variety of receptors
Distributed throughout the body
Sensory information
Nerves transmit information from the receptors by frequency modulation of electrical signals (action potentials)
Reference

3. Dorsal root ganglion neurons
DRG contains the cell bodies of sensory neurons (afferent spinal nerves)
The axons of the DRG neurons are known as afferents
All somatosensory information from limbs and trunk are transmitted via DRG
Stimulus transmission from sensory receptor to CNS
Primary afferent neuron has two branches to
Periphery
Spinal cord

4. The sensory receptors Located at the terminal of the sensory neuron
Molecular specialization that transforms one type of energy into action potentials
4 major modalities (distinct system of receptors and pathways to the brain)
Discriminative touch (size, shape, texture, movement across skin)
Thermal receptors (warm, cold)
Nociceptors (tissue damage, inflammation, chemical irritation, pain, itch)
Proprioception (joint position)
Reference

5. Classification of sensory fibers
C (IV)
Aδ (III)
Aβ (II)1:6
Aα (I)
1:6

6. Classification of sensory receptors

7. Nerve endings and fiber types
Bare nerve endings
Thermal and painful (nociception) sensations
Encapsulated nerve endings
Touch and proprioception
Deformation of receptive surface
Large diameter myelinated axons (rapid conduction)
Mechanoreceptors (touch, greatest density in glabrous skin [hairless], finger tips, lips)
Proprioceptors (joint position)
Small diameter myelinated and unmyelinated (slow conduction)
Thermal receptors
Nociceptors
Reference

8. Measurement of nerve conduction velocities
Reference

9. Measurement of nerve conduction velocities

10. Mechanoreceptors Specialized organs surrounding the nerve endings
Sensitive to displacement/ deformation
4 major types in glabrous skin
Superficial location (located below skin ridges)
Meissner corpuscle – in glabrous skin
Rapidly adapting, fluid filled structure, sense flutter of small areas
Merkel disk receptor - in glabrous skin and hairy skin
Slowly adapting, sense sustained pressure, salient bumps, sharp edges
Deep subcutaneous location (less numerous)
Pacinian corpuscle
Similar to Meissner corpuscle, rapid indentation, minute vibration, frictional displacement, small irregularities (edges/ corners)
Ruffini ending
Slowly adapting, links folds in skin at the joints, sense stretch and bending, shape of grasped objects, global properties of objects, wide area of skin

11. Location of touch receptors
Aα (I)
Aβ (II)1:6

12. Mechanoreceptors
Deep receptors sense deformation of a wider skin area that extends beyond the overlying ridges
Nerve fibers to superficial layers branch off to several nearby sensory receptors
Nerve fibers in subcutaneous layers only innervate one receptor
4 types of mechanosensitivity
Gentle touch of skin (well-localized)
Vibration (frequency and amplitude)
Texture (discrimination with fine spatial detail, two-point discrimination)
Shape of objects grasped
Reference

13. Receptive field and adaptation of touch receptors

14. Activation of mechanoreceptors

15. Two-point discrimination
Min distance as which 2 stimuli can be resolved as distinct
Determine if one or more points are stimulated
Spatial resolution depends on the RF size/ receptor density
Spatial resolution of stimuli varies across the body
Smallest receptive fields in fingers, lips, and tongue

16. Roles of mechanoreceptors in lifting
Lifting and object
Grasp, force increase, object lifted, vertical gravitational pull, force decrease, release
Grasp and release
Meissner c. : contact/ release; increased grasp force
Pacinian c. : transient pressure at start/ stop
Grip force
Merkel disks: continuous firing/ proportional with force
Gravitational pull
Ruffini endings: slowly adapting, sense stretch

17. Nociceptors mediate mechanical pain
Aδ (III)
Aδ (III)

18. Itch sensation A distinctive cutaneous sensation
Transducers are coupled to histamine receptor
Mediated by fibers with very low conduction velocities (0.5 m/s)
C (IV)

19. Thermal receptors normal skin 32C Cold pain < 15 C (C)TR
normal
skin 32C
Cold pain
< 15 C (C)
cold  cool (Aδ)
Heat pain
> 45 C (Aδ)
warm (C) heat
TRP= transient receptor potential
A = Ankyrin; M = melastatin; V=vanilloid
*TRPV4 also play a role in touch.

20. Proprioceptors measure joint positions
Aα (I)
Aβ (II)1:6

21. Distribution of dermatomes
Trigeminal (V1/V2/V3)
Cervical C1-C8
Thoracic T1-T12
Lumbar L1-L5
Sacral S1-S5
Coccygeal (single)

22. Spinal gray matter

23. Dorsal horn (lamina I-VI)
I, II, V
I, II IV

24. Somatotopical organization of spinal cord
Light touch vs. deep touch

25. Somatosensory ascending pathways
Dorsal column-medial lemniscal system
Touch and proprioception from limbs and trunk
Ascends ipsilateral side and cross over to contralateral side in medulla
Somtatotopically organized from spinal to cortical level
Anterolateral (spinothalamic) system
Pain, thermal and visceral sensation from limbs/trunk
Cross over to contralateral side in spinal cord
Branches: spinoreticular & spinalmesencephalic tracts

26. Pain Touch Thermal proprioception Anterolateral system
3 neuron
Pain
Touch
Thermal
proprioception
Anterolateral system
Dorsal column system
2 neuron
1 neuron

27. Homunculus