1. Dept. Cell and Neurobiology and Neurology
SENSORY PATHWAYS
Thomas H. McNeill, PhD
Dept. Cell and Neurobiology and Neurology
February 23, 2017

2. LEARNING OBJECTIVES FOR SENSORY PATHWAYS
List the general principles of organization of all sensory pathways.
Nam e the major pathways that convey sensations from the neck, trunk, limbs and face, and know the sensory modalities conveyed by each.
Trace the origin and course of the sensory pathways with emphasis on the level of their decussation.
Learn the somatotopic organization within sensory pathways.
Identify the connections that mediate corneal reflex and jaw jerk reflex.

3. Principles of Organization for All Sensory Pathways
Sensory pathways that carry information from the neck, trunk and limbs are separated from those that carry sensory information from the face.
Sensory pathways for discriminative touch, two point discrimination, vibration and proprioception are separated from those that carry pain, temperature and crude touch.
All somatosensory pathways are composed of a three neuron chain that begins at the sensory receptor and ends in the primary sensory cortex.

4. Principles of Organization (con’t)
Sensory pathways that carry information from the neck, trunk and limbs are separated from those that carry sensory information from the face.
Somatosensory information carried by individual spinal nerves from the skin form a dermatome pattern on the body wall.
Somatosensory information from the face is carried by one of the three subdivisions of the trigeminal nerve (CNV).
dermatome map
CNV innervation pattern of face

5. All somatosensory information from skeletal muscle, joints and skin is carried back to the spinal cord via the dorsal and ventral rami of spinal nerves and enters the dorsal horn via a dorsal root

6. Dorsal Column-Medial Lemniscus (DC-ML) pathway
2. Sensory pathways from the body wall for discriminative touch, two point discrimination, vibration and proprioception are separated from those that carry pain, temperature and crude touch in the spinal cord.
Two Major Somatosensory Pathways
Dorsal Column-Medial Lemniscus (DC-ML) pathway
Fine touch, proprioception and vibration
SpinoThalamic Tract (STT)
Pain: (including referred pain from the internal organs via sympathetics)
Temperature: hot and cold
Crude touch

7. ***PRINCIPLES OF ORGANIZATION:
3. General sensory pathways are organized as a chain of 3 neurons from the receptor to the sensory cortex.
Sensations from the left side
Right primary sensory cortex
***PRINCIPLES OF ORGANIZATION:
Neuron 1: is in the PNS in the dorsal root ganglion (DRG) or sensory ganglion of a cranial nerve
Neuron 2 is in the CNS: spinal cord/brain stem
Axons of neuron 2 cross the midline , i.e, decussate, conveying the sensations to the side contralateral to their origin.
Neuron 3 is in a sensory nucleus in the thalamus
Decussation/crossing
Thalamus
DRG
DC/ML
STT
Decussation/crossing
Decussation

8. Summary of 3 Neuron Chain for Sensory Pathways
Summary Table of 3 Neuron Chains that form the Sensory Pathways
Summary of 3 Neuron Chain for Sensory Pathways
Neuron 1
Neuron 1 Axons
Neuron 2
Pathway of Neuron 2
Neuron 3
DC/ML
DRG
Fas. Gracilis
Fas. Cuneatus
Nuc. Gracilis
Nuc. Cuneatus ML
VPL (thalamus)
STT
N/A
Nuc. proprius TL
Trigeminal ganglion
Main Sensory Nuc of V
VPM (thalamus)
TTT
Spinal tract of V
Spinal Nuc. of V

9. Primary sensory cortex
Pathway Tracing
Primary sensory cortex
Thalamus

10. Dorsal Column-Medial Lemniscus (DC-ML) pathway
Receptors for discriminative touch, vibration, proprioception from neck, upper limb, trunk and lower limb
Thalamus
Neuron 3
VPL nuc.
Thalamus
Midbrain
Pons
Medial Lemniscus
Medulla
F. Cuneatus
F. Gracilis
Neuron 2
Neuron 1
DRG
Dorsal roots C1-T5
Dorsal column
Dorsal roots T6-S2
DRG

11. DC-ML PATHWAY
– axons of the dorsal column ascend ipsilateral to neuron 2 in the nuc. Gracilis and Cuneatus. Axons of neuron 2 cross contralateral to the ML in the brainstem
Thalamus
medulla
Neuron 3
VPL nu.
Thalamus
Midbrain
Pons
Medial Lemniscus
Nuc.G
Medulla
Nuc.C
Neuron 1
Neuron 2
DRG
Dorsal roots C1-T5
Dorsal column
Dorsal roots T6-S2
Neuron 2
DRG

12. Neuron 3 VPL nu. Neuron 1 Neuron 2
SENSORY PATHWAYS EXHIBIT SOMATOTOPY AT ALL LEVELS
Somatotopy = Localization of different body parts in sensory pathways.
Thalamus
Neuron 3
VPL nu.
Thalamus
Midbrain
Pons
Medial Lemniscus
Medulla
Neuron 1
Neuron 2
DRG
Dorsal roots C1-T5
Dorsal column
Dorsal roots T6-S2
L T A
DRG

13. Somatotopy of the Sensory Axons from the VPL is Maintained in the Posterior Limb of the Internal Capsule and Cortex X
Horizontal section at the level of the internal capsule
Coronal section of primary sensory cortex

14. Sensory information remains somatotopically organized in the primary sensory cortex (SSI) (Broadman’s areas 3,1,2)
Sensory information related to intensity is sent to somatosensory area II (SSll) in the wall of the Sylvian fissure
Clinical note: Different parts of the sensory cortex receive blood supply from different cerebral arteries. This is important for understanding the clinical symptoms of stroke.
sensory homunculus.

15. Multimodal association cortex in the Inferior parietal lobule
Information from the primary sensory cortex projects to the general sensory association cortex in the superior parietal lobule (Broadman’s area 5 & 7).
This association cortex is responsible interpreting the information for object identification without the aid of vision:
Stereognosis – the ability to identify an object by the sense of touch
Graphesthesia - the ability to recognize writing on the skin
Postcentral gyrus
General sensory association cortex:
- Superior parietal lobule
Multimodal association cortex in the Inferior parietal lobule

16. Loss of discriminative touch, two-point discrimination, tactile localization, with retention of “crude” touch, pain and temp. carried by STT
Loss of vibration sense
Loss of proprioception with sensory ataxia, loss of balance and ataxic gait, and a positive Romberg sign
Abnormal sensations: Paresthesias/Dysthesias (tingling, numbeness, tightness etc.)
Astereognosis and agraphesthesia with DC/ML lesion or a cortex lesion alone
Pathology or lesions of the DC-ML pathway results in specific sensory deficits in the effected dermatomes

17. LOCATING THE LESION
Lesions/pathology at Neuron 1 or in the spinal cord results in ipsilateral loss of sensations.
Lesions/pathology of the pathway after the crossing over results in contralateral loss of sensations.
Thalamus
Neuron 3
VPL nu.
Midbrain
Pons
Medial Lemniscus
Medulla
Neuron 1
Neuron 2
DRG
Dorsal roots C1-T5
Dorsal column
Dorsal roots T6-S2
DRG

18. SPINOTHALAMIC TRACT (STT)
Pain: (including referred pain from the internal organs via sympathetics)
Temperature: hot and cold
Crude touch

19. Spinothalamic Tract (STT)
Axons of the from the DRG enter the dorsal horn and synapse on Neuron 2 of the chain (nucleus proprius)
Nucleus Proprius
DRG
Neuron 3
VPL nuc. Thalamus
STT
STT
Receptors for crude touch, pain and temperature from neck, upper limb, trunk and lower limbs.
Neuron 1
Dorsal roots C1-T5
DRG
STT
Dorsal roots T6-S2
Neuron 2
DRG
Dorsal horn
(Nucleus Proprius)

20. The course of pain and temperature fibers in the spinal cord differ from the DC/ML pathway in two ways
STT
Nu. Proprius
White commissure
Axons of the nucleus proprius in the dorsal horn cross in the white commissure anterior to the central canal.
Crossing occurs obliquely through 1-2 spinal segments above the entry of the dorsal roots.

21. Axons of the STT ascend contralateral in the spinal cord and in the brain stem
Axons of the DC/ML ascend ipsilateral in the spinal cord and in the brain stem
midline
Neuron 3
DRG
VPL nuc. Thalamus
contralateral
STT
STT
Neuron 1
Dorsal roots C1-T5
STT
DRG
STT
Dorsal roots T6-S2
Neuron 2
DRG
Dorsal horn
(Nucleus Proprius)

22. Axons of STT synapse with Neuron 3 in the VPL
Axons of VPL neurons enter the dorsal part of the posterior limb of the internal capsule and synapse in the primary sensory cortex along with axons from the DC/ML pathway.
VPL Nu
VPL
Internal capsule

23. Somatotopy of the Sensory Axons from the VPL is Maintained in the Posterior Limb of the Internal Capsule and Cortex X
Horizontal section at the level of the internal capsule
Coronal section of primary sensory cortex

24. LOCATION OF THE LESION?
Lesions/pathology at Neuron 1 or 2 results in ipsilateral loss of sensations.
Lesions/pathology in STT after the crossing results in contralateral loss of sensations.
Neuron 3
VPL nu. Thalamus
STT
Neuron 1
Dorsal roots C1-T5
DRG
STT
Dorsal roots T6-S2
Neuron 2
DRG
Dorsal horn

25. Pathology/lesions of the STT pathway results in specific sensory deficits in the effected dermatomes.
Loss of pain sensation or analgesia, or reduced sensation, hypalgesia,
Loss of pain and temperature sensations, with retention of all other sensations related to touch.
Abnormal sensations: Paresthesias/Dysthesias (spontaneous shooting pain, burning sensation)
Clinical Condition of enhanced pain
Allodynia -painful sensations evoked by minor stimuli such as touch that would not normally cause pain.

26. Effects of oblique crossing of the STT pathway on location of the lesion
Lesions of the STT pathway after the crossing (as shown at T6) produce loss of sensations in the dermatomes on the contralateral side of the body beginning approx. 1-2 segments below the level of the lesion (T8)

27. Central cord lesions such as syringomyelia will result in bilateral pain and temp. in the dermatomes innervated by the affected spinal cord segments.
Larger lesions may also effect adjacent structures in the dorsal and ventral horns.
STT
Nu. Proprius
White commissure
STT

28. SENSORY PATHWAYS FROM THE FACE

29. Principles of Organization
Somatosensory pathways that carry information from the face and anterior scalp are separated from those that carry sensory information from the neck, trunk and limbs..
Somatosensory pathways for discriminative touch, two point discrimination, vibration and proprioception are separated from those that carry pain, temperature and crude touch.
All sensory pathways are composed of a three neuron chain that begins at the sensory receptor and terminates in the primary sensory cortex.

30. V1 V2 V3 Trigeminal ganglion
1. Somatosensory pathways from the face are carried by the three subdivisions of the trigeminal nerve (CNV) (posterior part of the scalp is innervated by spinal nerves) 2. Sensory cell bodies are located in the trigeminal ganglia
Trigeminal ganglion V1 V2 V3
Ophthalmic 3
Maxillary
Mandibular

31. 2. Somatosensory pathways for discriminative touch, two point discrimination, vibration and proprioception are separated from those that carry pain, temperature and crude touch
Somatosensory Pathways (face)
Trigeminal Lemniscus
Fine touch, conscious proprioception and vibration
Trigeminal-Thalamic Tract (TTT)
Pain
Temperature: hot and cold
Crude touch
Mesencephalic Trigeminal Tract
proprioception of jaw movement

32. 3. Trigeminal sensory pathways are composed of a three neuron chain that begins at the sensory receptor and ends in the face area of the primary sensory cortex.
Neuron 1 – Trigeminal ganglion
Neuron 2 – Trigeminal nuclei
Neuron 3 – VPM of thalamus
Neuron 3
Neuron 2
Neuron 1

33. Trigeminal Nuclear Complex
The trigeminal nuclear complex can be divided into three parts.
Each part has its own pathway and carries a specific type of sensory information.
Components of the trigeminal nuclear complex are:
Chief sensory nucleus of V
Processes discriminative touch, vibration, proprioception
Spinal nucleus and tract of V
Processes pain and temperature
Mesencephalic nucleus of V
Processes proprioception from the muscles of mastication

34. SOMATOSENSORY PATHWAY 1: TRIGEMINAL LEMNISCUS (TL)
Face area, Primary sensory cortex
Processes sensory information for discriminative touch, proprioception, vibration.
Axons of the TL pathway cross in the pons and ascend contralateral in the midbrain
In the midbrain the TL joins the trigeminothalamic tract (TTT)
Sensory pathways for touch, proprioception and vibration from the face are only found in the pons and midbrain
VPM
Neuron 3
TTT/TL
Post. limb. Int. Cap.
Neuron 2 TL
Chief sensory nuc. V
Trigeminal ganglion
TTT
Neuron 1

35. SOMATOSENSORY PATHWAYS 2: TRIGEMINALTHALAMIC TRACT (TTT)
Face area, Primary sensory cortex
Processes sensory information pain, temperature and crude touch
Axons of the TTT pathway cross in medulla and ascend contralateral in the midbrain next to the medial lemniscus
In the midbrain the TTT is joined by the TL
Sensory pathways for pain and temperature begin in the medulla
VPM
Neuron 3
Post. limb. Int. Cap.
Trigeminal ganglion
Neuron 1
TTT
V Spinal tract & nucleus
Neuron 2

36. Comparison Clinical Defciets Associated with Lesions of the TTT and TL Pathways
Dorsolateral Lesions (PICA) affecting the spinal tract of V in the medulla will result in a loss of pain and temperature sensations in the ipsilateral half of the face
Medial lesions in the lower medulla will effect will spare the face because of remaining input above the lesion site
Lesions of the TTT in the lower pons (basilar art.) will lead to loss of sensations in the contralateral half of the face
Lesions of the TTT/TL at and above the upper pons (PCA) will lead to a loss of all sensations in the contralateral half of the face
Face area, Primary sensory cortex
VPM
Neuron 3
Post. limb. Int. Cap.
TTT/TL TL 4
Neuron 1 3
Trigeminal ganglion
TTT 1 2
V Spinal tract & nucleus
Neuron 2

37. Sensory axons from the VPM are located at the genu of the posterior limb of the internal capsule and are somatotopically distributed in the cortex
Horizontal section at the level of the internal capsule
Coronal section of primary sensory cortex

38. Pathology/lesions of the trigeminal pathways results in specific sensory deficits in the face
Lesions affecting the V nerve will result in a loss of all sensations in the ipsilateral half of the face depending on the branch affected.
Lesions affecting the spinal tract of V in the medulla will result in a loss of pain and temperature sensations in the ipsilateral half of the face. This is one part of a syndrome called the Wallenberg or Lateral Medullary Syndrome that we will discuss in the next lecture.
Lesions of the TTT at and above the upper pons will lead to a loss of all sensations in the contralateral half of the face.
In the pons the TTT lies close to the DC-ML and STT pathways so that a small lesion is likely to affect all three tracts. Involvement of DC-ML , STT and TTT will cause a loss of all sensations in the contralateral half of the neck, trunk, limbs and face. This type of sensory loss of all modalities on one half of the body including the face is referred to as “Hemianesthesia”.

39. Trigeminal Nerve Reflexes
Corneal Blink Reflex – evolutionary reflex used to protect eye from foreign body or bright light
Ex. Touching the right cornea
Sensory afferents from the right cornea travel in the right ophthalmic nerve (V1) to reach the right spinal or chief sensory nucleus of V.
Axons of the trigeminal nuclei project bilaterally to synapse in the motor nuclei of CN VII in lower pons.
Touching one cornea activates both VII motor nuclei that supply the orbicularis oculi muscles causing both eyes to close.

40. Mesencephalic Trigeminal Tract - proprioception of jaw movement
Mesencephalic Nuc. V
The neurons of the mesencephalic nucleus are the only first-order primary sensory neurons within the CNS
Propriocetive sensations from the muscles of mastication and TMJ via mandibular nerve terminate in the mesencephalic nucleus of V
Central processes of these cells synapse on the neurons of the motor nucleus of V which innervate the muscles of mastication
Motor Nuc. of V
Muscles of mastication
Mandibular nerve
Muscles of mastication

41. Jaw-Jerk Reflex – used to test the integrity the trigeminal nerve complex
Sensory information from neuromuscular spindles in the muscles of mastication responding to stretch, conveyed by the mandibular nerve to the mesencephalic nucleus. This is likely to be a bilateral input (only one side is shown).
Neurons of the mesencephalic nucleus then sends axons bilaterally to the motor nuclei of V.
Neurons in in the somatomotor part of CNV sends axons via the mandibular nerve to the muscles of mastication causing the jaw to close.

42. Summary of 3 Neuron Chain for Sensory Pathways
Summary Table of 3 Neuron Chains that form the Sensory Pathways
Summary of 3 Neuron Chain for Sensory Pathways
Neuron 1
Neuron 1 Axons
Neuron 2
Pathway of Neuron 2
Neuron 3
DC/ML
DRG
Fas. Gracilis
Fas. Cuneatus
Nuc. Gracilis
Nuc. Cuneatus ML
VPL (thalamus)
STT
N/A
Nuc. proprius TL
Trigeminal ganglion
Main Sensory Nuc of V
VPM (thalamus)
TTT
Spinal tract of V
Spinal Nuc. of V