1. Ascending Sensory Pathways
Dorsal Column-Medial Lemniscus system
fine touch
position sense
Anterolateral system
temperature
coarse touch
pain
James Bisley

2. Dorsal Column-Medial Lemniscus system
Conveys mechanosensory information from the periphery to the cortex
Cutaneous Mechanoreceptors (fine touch)
Proprioception & Kinesthesia (position)

3. Fine touch
Pain
Temperature
Coarse touch
afferent
receptor

4. Position sense
Kinesthesia is the “awareness” of body position and movement
Proprioception is the “sub-concious” information used in the feed-back control of posture and precise movements.

5. Position sense Position sense information comes from: Muscle spindles
Golgi tendon organs
Joint receptors
Cutaneous mechanoreceptive afferents
Efference copy

6. Proprioceptors Motor unit Muscle spindle Golgi tendon organ
(controlled by efferent)
Muscle spindle
Golgi tendon organ
Joint receptor

7. Afferent fiber information
Fiber type
Class
Diameter
(μm)
Conduction Vel. (m/s)
Types of receptors Aα Ia Ib
13-20
80-120
Primary muscle spindle
Golgi tendon organ Aβ II
6-12
35-75
Skin mechanoreceptors
Secondary muscle spindle Aδ
III
1-5
5-30
Coarse touch, temperature, and pain C
(no myelin) IV
0.5-2

8. Dorsal Column-Medial Lemniscus system
Some terminology
We use the terms first, second and third order neurons to describe the steps of the pathway to cortex.
First order
neuron
Second
order
neuron
Third
order
neuron
receptor

9. Dorsal Column-Medial Lemniscus system
Afferents have their cell bodies in the DORSAL ROOT GANGLIA.
Called pseudo-unipolar neurons.

10. Dorsal Column-Medial Lemniscus system
The DRG axons enter through the dorsal horn of the spinal cord
Spinal reflexes, Clarke’s Nucleus, etc

11. Dorsal Column-Medial Lemniscus system
Fibers that convey information from lower limbs and body (below spinal segment T6) travel ipsilaterally along the GRACILE TRACT.

12. Dorsal Column-Medial Lemniscus system
Fibers that convey information from upper limbs and body (above spinal segment T6) travel ipsilaterally along the CUNEATE TRACT.
GRACILE TRACT
There is a topographic representation of the body in the dorsal columns

13. Dorsal Column-Medial Lemniscus system
Fibers in the Gracile Tract have their first synapse in the GRACILE NUCLEUS.
Fibers in the Cuneate Tract have their first synapse in the CUNEATE NUCLEUS.
There is a topographic representation of the body in the dorsal column nuclei
Caudal medulla

14. Dorsal Column-Medial Lemniscus system
Axons from the second order neurons form the INTERNAL ARCUATE FIBERS in the caudal medulla, which decussates becoming the contralateral MEDIAL LEMNISCUS.
There is a topographic representation of the body in the medial lemniscus
Caudal medulla

15. Dorsal Column-Medial Lemniscus system
The representation of the body shifts as the medial lemniscus runs rostrally.

16. Dorsal Column-Medial Lemniscus system
The axons of the second order neurons terminate in the VENTRAL POSTERIOR LATERAL NUCLEUS of the thalamus (VPL).
There is a topographic representation of the body in the VPL (lower extremities are lateral)

17. Dorsal Column-Medial Lemniscus system
What about the face?
Gracile
Cuneate

18. Dorsal Column-Medial Lemniscus system
What about the face?
Pseudo-unipolar neurons have their cell bodies in the TRIGEMINAL GANGLION.
Except for Proprioception
Pseudo-unipolar neurons have their cell bodies in the MESENCEPHALIC NUCLEUS inside the CNS.
Mid-pons

19. Dorsal Column-Medial Lemniscus system
What about the face?
Pseudo-unipolar neurons have their cell bodies in the TRIGEMINAL GANGLION.
Except for Proprioception
Pseudo-unipolar neurons have their cell bodies in the MESENCEPHALIC NUCLEUS inside the CNS.

20. Dorsal Column-Medial Lemniscus system
What about the face?
Axons project to second order neurons in the PRINCIPAL (SENSORY) NUCLEUS OF THE TRIGEMINAL COMPLEX in mid-pons.
Mid-pons
There is a topographic representation of the face in the principal (sensory) nucleus

21. Dorsal Column-Medial Lemniscus system
What about the face?
Axons of the second order neurons decussate and join the TRIGEMINOTHALAMIC TRACT (which runs adjacent to the medial lemniscus).
Mid-pons

22. Dorsal Column-Medial Lemniscus system
What about the face?
The axons of the second order neurons terminate in the VENTRAL POSTERIOR MEDIAL NUCLEUS of the thalamus (VPM).
Mid-pons
There is a topographic representation of the face in the VPM

23. Dorsal Column-Medial Lemniscus system
Neurons in the VP complex project to PRIMARY SOMATIC- SENSORY CORTEX via the POSTERIOR LIMB of the INTERNAL CAPSULE.
Mid-pons
The whole body is represented in the ventral posterior complex.

24. Dorsal Column-Medial Lemniscus system
Neurons in the VP complex project to PRIMARY SOMATIC- SENSORY CORTEX via the POSTERIOR LIMB of the INTERNAL CAPSULE.

25. Dorsal Column-Medial Lemniscus system
Area 3a
Primarily proprioception input
Area 3b
Primarily tactile input
Area 1
Primarily tactile input, but receptive fields usually cover several digits
Area 2
Combination of tactile and proprioception. Hand configuration & stimulus shape are both important

26. Dorsal Column-Medial Lemniscus system
The whole body is represented in each area of SI
Owl Monkey

27. Dorsal Column-Medial Lemniscus system
The somatosensory homunculus

28. Anterolateral system
Conveys pain, temperature and coarse touch information from the periphery to the cortex
Fiber type
Class
Diameter
(μm)
Conduction Vel. (m/s)
Types of receptors Aα Ia Ib
13-20
80-120
Primary muscle spindle
Golgi tendon organ Aβ II
6-12
35-75
Skin mechanoreceptors
Secondary muscle spindle Aδ
III
1-5
5-30
Coarse touch, temperature, and pain C
(no myelin) IV
0.5-2

29. Central Pain Pathways: Sensory discriminative component
As with the tactile system, the cell bodies are located in the DORSAL ROOT GANGLIA.
Pseudo-unipolar neurons.

30. Central Pain Pathways: Sensory discriminative component
The DRG axons enter through the dorsal horn of the spinal cord
Upon entering, the axons branch into ascending and decending collaterals forming the DORSOLATERAL TRACT of LISSAUER.

31. Central Pain Pathways: Sensory discriminative component
The axons run up or down several spinal cord segments in Lassauer’s tract before synapsing in the gray matter of the dorsal horn.

32. Central Pain Pathways: Sensory discriminative component
The second order neurons decussate immediately and form the SPINOTHALAMIC TRACT (aka the anterolateral tract).
anterior white commissure

33. Central Pain Pathways: Sensory discriminative component
The second order neurons decussate immediately and form the SPINOTHALAMIC TRACT (aka the anterolateral tract).
There is a topographic representation of the body in the spinothalamic tract
anterior white commissure

34. Central Pain Pathways: Sensory discriminative component
The second order neurons decussate immediately and form the SPINOTHALAMIC TRACT (aka the anterolateral tract).

35. Central Pain Pathways: Sensory discriminative component
The second order neurons decussate immediately and form the SPINOTHALAMIC TRACT (aka the anterolateral tract).

36. Central Pain Pathways: Sensory discriminative component
The second order neurons decussate immediately and form the SPINOTHALAMIC TRACT (aka the anterolateral tract).

37. Central Pain Pathways: Sensory discriminative component
The second order neurons decussate immediately and form the SPINOTHALAMIC TRACT (aka the anterolateral tract).

38. Central Pain Pathways: Sensory discriminative component
Neurons in the spinothalamic tract terminate in the VENTRAL POSTERIOR LATERAL NUCLEUS (VPL) of the Thalamus.
Just like the tactile system
There is a topographic representation of the body in the VPL (lower extremities are lateral)

39. Some simple differences between the pathways
Dorsal column
Anterolateral X X
Test the pathway
Light touch
Vibration
2-point discrimination
Sense of position
Test the pathway
Pain
Temperature
Coarse touch

40. Central Pain Pathways: Sensory discriminative component
What about the face?
Pseudo-unipolar neurons have their cell bodies in the TRIGEMINAL GANGLION and ganglia associated with nerves VII (Facial), IX (Glosso- pharyngeal) & X (Vagus).
Anterolateral tract

41. Central Pain Pathways: Sensory discriminative component
After entering the brain stem, the fibers descend in the SPINAL TRIGEMINAL TRACT to the medulla, where they synapse onto neurons in the SPINAL NUCLEUS of the TRIGEMINAL COMPLEX (primarily the pars caudalis).
Anterolateral tract
There is a topographic representation of the head in the pars caudalis

42. Central Pain Pathways: Sensory discriminative component
Axons from the second order neurons decussate immediately and then join the ascending anterolateral tract in the brain stem.

43. Central Pain Pathways: Sensory discriminative component
Axons from the second order neurons terminate in the VENTRAL POSTERIOR MEDIAL NUCLEUS (VPM) of the Thalamus.
Anterolateral tract
There is a topographic representation of the face in the VPM

44. Central Pain Pathways: Sensory discriminative component
The whole body & all somatic senses are represented in the ventral posterior complex.
Neurons in the VP complex carrying pain information project to PRIMARY and SECONDARY SOMATIC- SENSORY CORTEX.

45. Central Pain Pathways: Sensory discriminative component
Cortex
localization of pain
Sub-cortical
perception of pain
Paleospinothalamic pathways
suffering component of pain (reduced by benzodiazepines)

46. Central Pain Pathways: Descending Control of Pain
The same holds true for the pars caudalis of the spinal nucleus of the trigeminal complex
Stimulation of PAG results in analgesia.

47. Central Pain Pathways: Descending Control of Pain
In the dorsal horn or the pars caudalis
Opioids play a role in the descending control of pain
Endogenous opioid

48. Central Pain Pathways: Local Control of Pain
Interaction between dorsal column and anterolateral systems regulates pain perception. This is why rubbing a wound after sharp pain helps a bit.
Cutaneous mechanoreceptor
Cutaneous nociceptor
Stimulation of dorsal columns can antidromically induce analgesia

49. Central Pain Pathways:
Local & Descending Control of Pain
What you should know
Aα and Aβ fibers excite interneurons that reduce the transmission of pain information
Descending fibers excite interneurons that reduce the transmission of pain information

50. Dorsal Column-Medial Lemniscus system
Information content
Fine touch, vibration and sense of position
The path to cortex
Locations & projections of 1st, 2nd & 3rd order neurons
Where decussation occurs
Differences between DRG inputs & Vth nerve inputs
Basic arrangement of topography throughout the system
The organization of somatosensory cortex
4 areas
Basic arrangement of topography

51. Anterolateral system Information content The path to cortex
Coarse touch, temperature & pain
The path to cortex
Locations & projections of 1st, 2nd & 3rd order neurons
Where decussation occurs
Differences between DRG inputs & Vth nerve inputs
Basic arrangement of topography throughout the system
The path from cortex
Main areas involved in descending control of pain
2 ways that pain can be modulated in dorsal horn